METHODS OF INVESTIGATION AND RECORD
=198. The need of exact methods.= The use of instruments in the study of
the habitat has made it evident that the loose methods of descriptive
ecology were altogether inadequate to the accurate investigation of the
formation. This feeling has been heightened by the recognition of the
fact that vegetation exhibits both development and structure, and is, in
consequence, open to exact methods of inquiry. In the search for
feasible methods, it was quickly seen that the quadrat, first[20] used
for determining the abundance of species, furnished the key to the
problem. Accordingly, the principle underlying it, viz., that of
intimate detailed study and record, was developed and extended in such a
way as to give rise to a number of methods of precision. These have been
applied in the field for several years with signal success, and they are
here described in the conviction that they constitute a satisfactory
system, if not, indeed, the only one for the exact study of formations.
There has been a growing appreciation of the fact that the superficial
methods of descriptive ecology made it impossible to build upon such a
foundation, and they, indeed, were making actual progress in the field
of ecology more and more difficult. Ecologists have now begun to see
clearly that precise methods are as indispensable in the habitat as they
are to the study of the structure and modification of the plant. For
some reason, however, they have been slow to perceive that accuracy in
the investigation of the cause, the habitat, is a fruitless task unless
it be followed by corresponding exactness in the study of the effect,
the formation. After having urged the fundamental necessity of
instrumental methods, for six or seven years, both in season and out of
season, the writer does not feel called upon to further plead the cause
of the quadrat. The final acceptance of the instrument was inevitable if
progress were to be made in the habitat, and it is just as obvious that
the quadrat must be accepted if the study of the habitat is to bear
fruit in the interpretation of the formation. The use of the quadrat
does not mean that the general methods of descriptive ecology are all to
be discarded, whether they have value or not. The statement that quadrat
methods are indispensable signifies merely that they must be used for
research work in the development and structure of vegetation. They are
not necessary in reconnaissance, nor do they displace general methods of
real value. The use of the latter in even a supplementary way will
gradually be discontinued, however, as fields become smaller by reason
of increase in the number of workers, and as the need for precise
methods becomes more universally felt.
The quadrat constitutes the initial concept from which all the methods
have grown. In itself, it has given rise to a variety of quadrats
applicable to the most fundamental problems of vegetation. From it have
come, on the one hand, the migration circle, and on the other, the
transect. The latter in turn has yielded the ecotone chart, and the
layer chart. All of these are based upon direct and detailed contact
with vegetation itself, and permit accurate recording of all the results
obtained.
QUADRATS
=199. Uses.= In its simplest form, the quadrat, as the name implies, is
merely a square area of varying size marked off in a formation for the
purpose of obtaining accurate information as to the number and grouping
of the plants present. As indicated above, it was first used for
determining the abundance of the various species of a formation. This
made it possible to ascertain the relative rank of the species of layers
and formations, and enabled one for the first time to gain some idea of
the minute structure of a bit of vegetation. The results were at once
applied to the task of establishing a numerical basis for abundance, and
of working out a new system of abundance to correspond. The quadrat
method was also used to determine the character of seasonal aspects, and
to yield a knowledge of the exact differences in diverse areas of the
same formation. Incidentally, the determinations of abundance were made
the basis of an actual census of certain alpine formations. This, while
it was extremely interesting to find that a square mile of alpine meadow
contained approximately 1,500,000,000 plants, was confessedly destitute
of ecological value. The most important applications of the quadrat idea
were made by Clements[21] in the chart, the permanent and the denuded
quadrats. The development of these was due to the fact that zones or
formations permit of comparison upon floristic as well as physical
grounds, and that a detailed record of their structure is necessary for
this purpose. Similar comparisons are necessary for the consocies,
zones, and patches of the same formation, and the quadrat becomes an
indispensable means for studying alternation and zonation. For the
investigation of invasion year by year, and especially for succession,
the method of permanent quadrats is imperative, and the denuded quadrat
an invaluable aid. Changes, which would otherwise be incompletely
observed and imperfectly recorded, are followed in the minutest detail
and recorded with perfect accuracy.
=200. Possible objections.= The use of the quadrat has led to the
criticism that it is needlessly detailed and thorough, and that, after
all, the space covered is but a minute part of the entire formation. The
first objection is one that has also been urged against the use of
instruments of precision in the habitat. It is always brought forward by
those who have not used instruments, and as witnesses they are of
necessity incompetent. No one who is familiar with the instrument or the
quadrat by actual practice has felt that the methods based upon them
were too thorough. In no case has the writer ever listed or mapped a
quadrat without discovering some new fact or relation, or clearing up an
old question. It can not be denied that quadrat methods require both
time and patience, but this is true of any kind of research work that is
at all worth while. Every ecologist, moreover, that has the interests of
his field at heart and deprecates the present slipshod work, will
appreciate the necessity of methods which seem like drudgery to the mere
dabbler.
The second objection, that the quadrat is at best but a small bit of the
area under investigation, seems at first to be a valid one. It can not
be gainsaid that the actual space studied is insignificant as compared
with the whole formation; still, it must be obvious that even a single
quadrat can add at least some facts of value, which can never be
obtained by the best of general methods. Furthermore, if the formation
be an actual and not an imaginary one, a single quadrat will be in some
measure representative. In the more homogeneous ones, it will have much
the same value that a type specimen bears to the species established
upon it. In formations which are less uniform, its value is
correspondingly reduced, so that in formations which show marked zones,
consocies, or patches, it becomes necessary to locate a quadrat in each.
In the matter of representation alone, the graphic method of the quadrat
map with its close-focus detail photograph, is far superior to anything
that can be obtained by the ordinary description and photograph.
Finally, the scientific study and recording of succession, and
particularly of competition, is an impossibility without the aid of the
permanent and denuded quadrat. The stoutest champion of the practice of
walking through a formation, and jotting down impressions, can not avoid
their use if he would attack these problems, and, once familiar with the
quadrat, his objections to the drudgery of thoroughness will soon
vanish.
_Kinds of Quadrats and Their Use_
=201. Size and kinds.= The unit size of quadrat is the meter, and when
the term is used without qualification, it refers to the meter quadrat.
To make them strictly comparable, and exactly divisible, unit quadrats
are always grouped in squares; thus a major quadrat is a square of four
units, and a perquadrat one of sixteen units, or four meters square.
Quadrats of greater size are necessary in woodland and forest, where the
rule, however, is that the woody plants alone are recorded for the whole
quadrat, the herbaceous growth being listed or mapped for but one or two
representative units. For special purposes, quadrats of 3, 5, 6, etc.,
meters may be used, but they are much less convenient. Quadrats are
further distinguished with respect to their use. A list quadrat is one
in which the plants are merely listed and the number of individuals of
each species indicated. Chart quadrats are those in which the area
concerned is accurately mapped on plotting paper. Both list and chart
quadrats are rendered permanent by careful labeling, so that their
changes can be followed from year to year. The greater value of the
chart causes practically all permanent quadrats to be of this type, and
for the same reason only permanent chart quadrats are converted into
denuded ones.
=202. Tapes and stakes.= The lines for marking out quadrats are made of
strong white tape, ⅝ inches wide. This is doubled and sewed firmly at
both edges. Under moderate stretching, the tape is carefully marked off
into decimeters, and eyelets 5 mm. in diameter are set in at each end
and at the marks. This can readily be done by any shoemaker at slight
expense. The usual lengths are one and two meters, as these are most
frequently used, and they can also be easily combined to make larger
quadrats. The tapes are slightly longer than one meter in order that the
distance between the end eyelets may be exact. The tapes of the larger
forest quadrats should be divided into lengths of one meter, as these
permit ready plotting and also make it possible to interpolate a meter
quadrat for the study of the undergrowth at any point. The intervals of
the tape are numbered from left to right, as conspicuously and clearly
as possible. For this a waterproof ink or paint is very desirable. For
holding the tapes in position, hatpins, nails, and meat-skewers have
been used with more or less satisfaction. The ideal stake, however, is
one which holds the tape close to the ground, and can be readily moved.
It is merely a stout wire, 3 mm. in diameter and 8 inches long, looped
at the top, sharpened at the tip, and with a small ring of solder 3
inches from the tip.
=203. Locating quadrats.= In staking a quadrat, the end tapes are
invariably placed so that the numbers read from left to right, and the
side tapes so that they read down. In mapping, a fifth tape is stretched
parallel to the top, and as each decimeter strip is marked, the outer
tape is shifted to delimit the new strip. Indeed, the side tapes can be
placed alone, and the plotting tapes moved down one at a time as the
mapping proceeds, but it is usually more satisfactory to locate the
quadrat exactly and to square it first, a task most easily done by
enclosing the whole quadrat, and then using a fifth tape. In the case of
list quadrats in open vegetation, the measuring strip is unnecessary,
but as a rule it facilitates counting, as well as mapping.
[Illustration: Fig. 50. Mapping a major quadrat on Mount Garfield at
3,600 m.]
_The List Quadrat_
=204. Description.= This, as the simplest form of quadrat, is employed
primarily to ascertain the abundance of species in a formation or during
a particular aspect of it. Since this can be obtained readily from the
chart, the list quadrat has fallen more and more into disuse, except
where it is desired to determine abundance alone, or to aid in deciding
whether a chart is really representative. The size depends almost wholly
upon the nature of the vegetation. When the number of trees is to be
determined, a quadrat of 10 or 50 meters is necessary. In ordinary
herbaceous formations, the usual size is 2 meters, while the meter
quadrat is used when the plants are especially small and crowded, as in
alpine meadows. The location of the quadrat is based upon the general
rule, but since its especial task is the determination of the greatest
variable in vegetation, viz., number, it is necessary to use more
quadrats, and to place them in areas which show the greatest differences
in the mixture of species. For example, it was found that a half dozen
list quadrats, when carefully located in the prairie formation, gave
results almost identical with those obtainable from a larger number.
With a little experience, the various degrees of mixture can be picked
out superficially, and the corresponding number of quadrats established.
If a single list quadrat is to be made for a formation or station, such
a time should be selected as will make it possible to cover the greatest
number of plants. Fortunately, this usually falls near the middle of the
summer, when the remains of spring plants are still in evidence, and the
autumn ones are sufficiently developed to be recognizable. In taking the
census of different aspects, the quadrat should be made as near the
middle of the period as is possible.
=205. Manner of use.= In listing a quadrat, i. e., counting the
individuals of each species, the plan followed is to list the smaller,
less conspicuous plants first, since they are apt to be tramped down. As
a rule, the outside tapes and the taller species afford sufficient
landmarks. When this is not the case, the measure tape is used, and the
individuals of all species are checked as they are found, while in the
first method one species, rarely two, is taken at a time. In cases of
peculiar difficulty, it may be permissible to pull or break plants as
they are counted, but ordinarily this can and should be avoided.
Clusters, and bunches of stems from the same root are counted as single
plants, and the number of stems indicated by an exponent. In the case of
bunch grasses, each bunch counts as one plant.
=206. Table of abundance.= The species are arranged in the final list in
the order of their numerical importance, and are divided into groups
which correspond to the different degrees of abundance. The latter are
arranged in two series, based upon the fact that association is by
groups or by individuals. The table of abundance, based upon a 2–meter
quadrat rather than upon the 5–meter one, by means of which the earlier
results were obtained, is as follows:
Social exclusive, no other species of
vascular plants present
social inclusive, above 100
gr^1 gregarious^1 100–50 copious^1 cop^1
gr^2 gregarious^2 50–25 copious^2 cop^2
gr^3 gregarious^3 25–10 copious^3 cop^3
sg subgregarious 10–5 subcopious sc
vg vixgregarious 5–1 sparse sp
It is obvious that the above outline is faulty inasmuch as it takes no
account of the height and width of the individuals. This is a serious
defect, and it constitutes one of the many reasons why the list quadrat
should be replaced by the chart quadrat. The prairie formation affords
an unusually striking illustration of this. A single quadrat may be
filled by ten plants of _Psoralea floribunda_, and at the same time
contain 22,000 plants of _Festuca octoflora_. Yet the former is
conspicuous and controlling, the latter plays an altogether
insignificant role. This difference is readily shown by comparing a
plant of each. The one is 3 × 3 feet, the other 3 × ¼ inch. Such figures
furnish a valuable check upon mere number, but make the brief, graphic
designation of abundance difficult. An attempt has been made to solve
this problem by roughly determining the space occupied by the plant, by
means of the formula, height (π_R_^2) × abundance. This would give
_Psoralea_ a value of 210, and _Festuca_ one of 1.6, which much more
nearly represents their real importance in the formation. Abundance or
numerical value is a floristic concept entirely, and has little place in
ecology unless checked in the way indicated. The whole problem,
ecologically, depends upon an intimate knowledge of competition, and its
solution in consequence is at present impossible.
_The Chart Quadrat_
=207. Description and use.= The detailed labor required in mapping makes
it advisable to use the meter quadrat. An additional reason of much
importance is furnished by the desirability of securing a detail
photograph of the quadrat. This is impossible with field cameras, which
should not exceed 6½ x 8½ inches, and are indeed most serviceable in the
4 × 5 size, if the area be larger. In open formations, the major quadrat
of 2 meters can be used if necessary, but this is very rarely the case.
Forest quadrats of ten meters square are easily charted, but detail
photographs can not be made of them. Larger quadrats are impracticable;
they can be counted but not mapped to advantage. The location of the
chart quadrat must be decided by the structure to be studied. Its
greatest service is in connection with zones and societies of the same
formation, which can be easily compared in the chart form. In fact, the
chart quadrat may well be regarded as the fundamental method for inquiry
into zonation and alternation. It is an important aid in delimiting
areas from the contiguous formations, and in determining the
relationships of mixed formations. It is also used to record the
character of the different aspects, but this is done more satisfactorily
by the permanent quadrat.
=208. The chart= used is a decimeter square, and the scale is
consequently 10 : 1. It is outlined on centimeter plotting paper, and
the centimeter squares are numbered at the edges to correspond to the
intervals of the quadrat, i. e., the top and bottom lines are numbered
from left to right, and the side lines from top to bottom. These
outlines are ruled in quantity and used as needed, or the forms can be
furnished by the printer. In practice, a special quadrat book the size
of the chart has been used. The need of a second book may be avoided by
outlining two charts on the plotting sheet, and filing the latter in the
field record book. In the few cases where 2–meter quadrats are
desirable, four charts are used, care being taken to label them so that
they can be combined whenever necessary. Ten-meter quadrats are recorded
on the decimeter chart also, each meter interval corresponding to a
centimeter, i. e., the scale is 100 : 1.
[Illustration: Fig. 51. Permanent chart quadrat, _Andosacile_,
_Carex-Campanulacoryphium_.]
[Illustration: Fig. 52. Chart of the quadrat shown in figure 51. Legend:
a, _Androsace chamaejasme_; c, _Carex rupestris_; t, _Tetraneuris
lanata_; p, _Potentilla rubricaulis_; as, _Arenaria sajanensis_; ar,
_Artemisia scopulorum_; ag, _Agropyrum scribneri_; sa, _Silene acaulis_;
st, _Sieversia turbinata_; d, _Dasyphora fruticosa_; al, _Allium
reticulatum_; o, _Oreoxis alpina_.]
=209. Mapping= is invariably begun at the upper left-hand corner of the
chart, and is carried across the strip marked off by the plotting tape,
decimeter by decimeter. As soon as this strip is completed, a second one
is formed by moving the top tape to a position one decimeter below the
plotting tape, which then becomes the upper one. This is repeated until
the last strip is reached. Little difficulty is experienced in locating
each plant exactly, as the decimeter interval is small, and the
centimeter square which corresponds is divided into twenty-five tiny
squares. Each plant is put in whenever possible, but mats, turfs, and
mosses are merely outlined in mass if the individuals are not
distinguishable. This holds true of all large rosettes and mats, even
when they are single plants. Symbols were formerly used for indicating
the various species. They have the advantage of requiring little space
on the chart, and the disadvantage of necessitating constant reference
to the legend. They are at present replaced by initials. By this plan,
the decapitalized first letter of the generic name is used if no other
genus found in the quadrat begins with the same letter. If, however, two
or more genera begin with _a_, for example _Agropyrum_, _Anemone_, and
_Allium_, the most abundant one is indicated by _a_, and the others by
the first two letters, as _an_, _al_. In case two species of the same
genus are present, the species initial is used in connection with that
for the genus, as _ac_ and _ar_ for _Agropyrum caninum_ and _Agropyrum
richardsonii_ respectively. It is rarely necessary to exceed two letters
for any species. Plants which regularly have several stems from the same
root are indicated by the initial and an exponent as _a_^3. Seedlings
are represented by a line drawn through the letter. Usually the chart
sheet affords sufficient space below the chart for the legend. When the
list of species is long, the back of the sheet is used.
=210. Factors and photographs.= Each chart is numbered, and the
formation, station, and date indicated. The constant factors, altitude,
slope, and exposure are ascertained and recorded on the sheet. The
variable factors are read in each quadrat whenever possible, and in
addition to being preserved in the record book, are noted on the chart
sheet along with the base reading in the formation for the same time.
This facilitates the interpretation of the differences found when two or
more charts are compared. Chart quadrats are regularly photographed. For
this purpose a long focus 4 × 5 camera with a telephoto lens is used. At
the proper distance this will make a view of the same size as the chart,
thus making possible an exact comparison of the two. The chart and
photograph serve as mutual checks, as well as complements, since the
former shows number, position, and arrangement, and the latter, height,
form, position, and arrangement. The view is usually made by placing the
camera directly in front of the middle of the lower tape, at such a
distance that the side tapes fall just within the limits of the ground
glass. The swing is always used in order that the focus may be uniformly
sharp. Surface views of the quadrat can be taken by means of a device
which permits the camera to hang downward from the tripod, or by means
of a tripod with a swinging platform. Such views are especially valuable
for the study of competition, since they give a clear idea of the spread
and density of the various plants. They are difficult to make unless the
vegetation is low and nearly uniform in height. The usual photograph is
much more serviceable in regular quadrat work.
_The Permanent Quadrat_
=211. Description and uses.= As stated heretofore, either list or chart
quadrats may be rendered permanent in order that they may be followed
from season to season or from year to year. As a matter of fact,
however, an area which is to be studied repeatedly really demands
charting, and in practice chart quadrats alone are made permanent. This
is done simply by driving a labeled stake at one corner of the quadrat,
and locating the latter definitely in relation to a conspicuous
landmark. When one is in residence for several years, practically all
chart quadrats are converted into permanent ones, since the work already
done in the chart quadrat is so much accomplished towards the permanent
one. This is not necessary when one wishes merely to compare different
areas of stable formations. As a rule, however, some change is
constantly being wrought by invasion or competition, and the amount and
direction of this can only be revealed by the permanent quadrat. The
latter has a fundamental value for all kinds of invasion, but it is
absolutely indispensable in studying complete invasion or succession,
and in discovering and recording the gradual effects of competition. It
is in the detailed investigation of these dynamic phenomena that the
paramount importance of the quadrat is most evident. If the experience
of several years be taken as conclusive, no other method is capable of
revealing the minute changes as they are occurring.
[Illustration: Fig. 53. Permanent quadrat, _Polygonile_ (_Polygonum
bistortoides_) Ruxton Park; mapped and photographed July 22, denuded
September 8, 1903.]
The permanent quadrat is regularly 1 meter square, a size determined
both by the exigencies of charting and photographing. When ecograph
batteries are used, the quadrat is located as close to the latter as is
possible. Otherwise, the quadrat itself should constitute a station for
making factor observations. This connection is absolutely essential,
since the quadrat is used expressly to determine the structural changes,
which are produced by physical factors, and the reaction of vegetation
upon them. Permanent quadrats are established in different formations or
stages of a succession to trace the invasion of new species and the
dropping out of old ones in response to competition. They serve to
distinguish the proper formation, which represents a particular stage of
development, from the mixed formations which precede and follow, and
also to determine the exact course as well as the rapidity of the change
that follows each reaction. When applied to different examples of the
same stage, and to all the different stages of a succession, the whole
development of the latter may be minutely traced and definitely
recorded. The importance of following the changes from aspect to aspect
is much less, since these are periodical rather than dynamic. They are
an essential feature of structure, however, and it has been the practice
to make at least one series of aspect charts from each permanent
quadrat.
For tracing the invasion and competition of lichens and mosses, which
play a primary role in initial formations, a subquadrat is used. The
size varies, but it is usually smaller than the quadrat, although the
latter is entirely available in the case of the large foliose lichens.
For the crustose and smaller foliose forms, a subquadrat 2 decimeters
square is used, and for the larger forms and tufted mosses, one of 5
decimeters. In the case of ground forms, tapes are employed, and the
quadrat is permanently staked. On rocks and cliffs, where moss and
lichen stages are most common, tapes are impracticable, and the quadrat
is permanently outlined with paint. Charts of lichen quadrats are made
to the usual scale of 10 : 1.
=212. Manner of use.= Permanent quadrats are mapped and photographed in
exactly the same way as chart quadrats. As soon as this has been done, a
labeled stake is driven at the upper left-hand corner, so that its edge
indicates the exact position of the quadrat stake, and a smaller one is
placed at the opposite corner to facilitate the task of setting the
tapes accurately in later readings. The label stake bears merely the
number of the quadrat and the date when it was first established. It is
firmly fixed and allowed to project just enough to enable it to be
located readily. Its position requires careful landmarking when the
quadrat is to be visited year by year. In forest formations, this is
readily done by blazing, but in grassland it is necessary to have
recourse to compass and pacing, or to erect an artificial landmark.
After several charts have been made, a permanent quadrat attains a high
value, and every precaution must be taken to prevent losing its exact
location. At the second reading of a quadrat, whether in the succeeding
aspect or year, the tapes are placed with reference to the stakes, and a
chart and photograph are made in the usual manner. These are labeled and
dated like the original ones, but they are numbered to indicate both the
quadrat and the series, e. g., 15^2 indicates the second chart, and
photograph made of quadrat 15. The date indicates whether the readings
are by the aspect or the year, though this may be shown also in the name
of the series itself. It is clearly an advantage to have the two
successive charts of a quadrat upon the same sheet, and to file all the
charts and photographs of the same permanent quadrat together, and in
the proper order.
Since much of the value of a permanent quadrat depends upon its use as a
station for observing physical factors, it is unprofitable to establish
a large number. The results of invasion and competition can be
ascertained by the quadrat alone, but these should be merely preliminary
to seeking for their causes. Clearly, a quadrat should be established
for each battery of instruments, while additional ones should be located
only in so far as they can be visited often enough to give an insight
into the factors that control them. In view of the fact that the most
important factors, water-content and light, are less variable than
humidity, temperature, and wind, it will suffice if visits are made once
a week. This is especially true when it is possible to refer the more
variable factors to the continuous records of a base station. While all
the results determined for permanent quadrats are preserved in the field
record, a record of them is also kept on the reverse of the chart sheet
for convenience in interpreting the different charts.
_The Denuded Quadrat_
=213. Description.= This is primarily a permanent quadrat from which the
plant covering has been removed, after it has been charted and
photographed. What is practically the same thing is obtained by
establishing a permanent quadrat in a new soil, or in one recently laid
bare and not yet reclothed with plants. These, however, are merely
permanent quadrats, in which the first chart and photograph furnish a
record of the habitat alone. They are of great importance in succession,
and will be more fully discussed under experimental vegetation. The
denuded quadrat is of the usual size, 1 meter, though the smaller lichen
quadrats are also denuded. The location is subject to the conditions
already indicated, especially with reference to physical factors. The
denuded quadrat, however, is particularly adapted to the study of
invasion and the resulting competition. Consequently, when migration is
markedly from one direction, a series of denuded quadrats throws a flood
of light upon the actual steps in invasion. Denuding is a valuable aid
in succession, but it must be clearly recognized that, while permanent
quadrats register the exact course of the succession, denuded ones can
merely furnish facts as to the probable courses of stages not now in
evidence.
[Illustration: Fig. 54. Denuded quadrat; this is the quadrat shown in
figure 53; photographed September 7, 1904.]
=214. Methods of denuding and recording.= Permanent quadrats may be
denuded at any time during the time they are under observation. The best
results, however, are to be obtained by establishing the two side by
side, or at least close together. In this way, they are mutually
supplementary, and furnish the most evidence possible with regard to the
procedure of invasion and competition. Another advantage is found in
that the same observations of climatic factors will do for both, though
water-content and soil temperatures are necessarily different. A quadrat
which is to be denuded is first mapped, photographed, and labeled
exactly like a permanent quadrat. The vegetation is then destroyed. This
is usually done by removal, though it may also be burnt, destroyed by
flooding, or in some other manner. The method will depend upon the use
which the quadrat is to serve. If it is to throw light upon the
vegetation of an area in which denudation has affected the surface
alone, the aerial parts only are removed by paring the surface with a
spade. When the disturbance is to be more profound, the upper
seed-bearing layer is removed, and the underground parts dug up. In the
interpretation of a secondary succession, the denuding cause is made use
of in a fashion as nearly natural as possible. Ordinarily, the plants
are removed just below the top of the ground by a spade, leaving the
underground parts undisturbed. This method has yielded very interesting
results.
Quadrats have been denuded in the fall after the majority of the plants
have completed their growth. This is largely owing to the fact that
other field work is less pressing at this time. Denudation can be done
as well in the spring, though the invasion will be slower in this case,
since the seeds which have accumulated will be partly or entirely
removed. During the first season the denuded quadrat should be mapped
every month, and, if the invasion be rapid, photographed also. In open
formations, especially those of a xerophytic nature, a single chart and
photograph made at the end of the season are sufficient. In a few cases
of this sort, indeed, no invaders have appeared until the second year.
Beginning with the second season, a single record taken near the close
of the growing period will suffice. Denuded quadrats are labeled, dated,
and filed exactly as other permanent quadrats, but it should be noted
that the first member of the chart and photograph series is that which
records the original vegetation of the area denuded.
=215. Physical factors.= When denuded quadrats are single, their
physical factors must be observed in the usual way. If they are
associated with permanent ones, the ordinary readings are made for the
latter, and those factors which are affected by exposing the soil are
alone taken for the denuded area. These are the water-content, soil and
surface temperatures, and in some stations at least the humidity near
the surface. As everywhere, water-content is the most important, but the
temperature at or near the surface has a marked effect upon germination.
Because of its bearing upon the latter, the surface water-content is
usually determined also. This has been done by taking a surface sample 2
inches square and 1 inch deep. Denuded quadrats naturally show
considerable differences from year to year as the action of the invaders
becomes more pronounced. To this fact is due much of their value as aids
in interpreting succession.
_Aquatic Quadrats_
=216. Scope.= The preceding discussion of quadrat methods is based
wholly upon their use in terrestrial formations. Wet meadow and dry bog
are the wettest places in which quadrats have been used. It is clear,
however, that with certain necessary modifications, quadrats can be used
as successfully, though not as conveniently, in many water formations as
in land ones. The tapes need to be raised above the surface of the water
by longer stakes, and photographs often taken from a boat, but otherwise
the usual methods apply, at any rate for bogs and shallow bodies of
water. In lakes or streams the tapes might be attached to buoys or
floats. The determination of factors is made as usual. Permanent
quadrats are feasible in many cases at least, and denuded quadrats are
not altogether impossible.
TRANSECTS
=217. The transect= is essentially a cross section through the
vegetation of a station, a formation, or a series of formations. It is
designed primarily to show the order of arrangement of species in zones
and societies, but it also serves as a record of the heterogeneity of
any area. In the form of the layer transect, it furnishes a graphic
method of representing the spatial relations of the species in layered
formations, e. g., forests, ponds, and lakes. It is merely a logical
extension of the idea underlying the quadrat, and the transect is,
indeed, little more than an elongated quadrat. An important difference,
however, lies in the fact that the former normally traverses areas more
or less unlike, while the latter is always located in a homogeneous one.
Furthermore, the transect is plotted with especial reference to the
topography. With respect to dimension, transects are classified as line,
layer, and belt transects, and the latter may also be permanent or
denuded.
_The Line Transect_
=218. Description and method.= A simple transect is sometimes made by
establishing the points between which it is to be run, and then
recording the plants pace by pace along this line. This is satisfactory
where the striking changes in structure are desired. A more accurate
method is ordinarily used, since it gives detailed results, and at the
same time brings out the more general features. For this, use is made of
a tape of proper length which is divided into decimeters. Tapes of 10,
50, and 100 meters are used, and if they are furnished with eyelets,
transects of intermediate lengths may be run with them. When longer
transects are desired, as in the case of forest formations, tapes of 500
or 1,000 meters should be used with eyelets a meter apart. The transect
is located in the area to be studied by running the tape from one
landmark to another, fastening it here and there by means of quadrat
stakes. Previous to this, the shortest distance between landmarks is
ascertained when the transect runs through a depression or upon a level
surface. In the case of an elevation, the height is ascertained by a
barometer, the length and angle of the two slopes obtained, and the
length of the base line determined from these data. The field record of
the arrangement of the plants is made entirely without reference to the
surface line. The vertical lines on the centimeter sheet are taken to
correspond with the tape, and the individual which touches the latter on
either side is recorded to the right or left respectively and within the
proper square. The species are indicated as for quadrats. A single row
on either side may be taken alone, but the double series serves as a
desirable check. After the record is made, the topography of the
transect is drawn carefully to scale. This drawing is made upon the
scale of 100 : 1 for transects of 10 meters or less, and of 1000 : 1 for
those that are longer. The combination of this drawing with the line
series of plants can not be made advantageously in the field. For the
shorter transects, meter sizes of centimeter plotting paper can often be
used to advantage. In this event, the topographic line is drawn to the
scale of 10 : 1 and the series of plants transferred directly to it. In
the case of transects between 10 and 100 meters, the scale of the
drawing is increased from 1000 : 1 to 100 : 1, so that each decimeter of
the original series is compressed into a centimeter. For the longest
transects, corresponding reductions must be made, but in these it will
be remembered that the series is plotted by meter instead of decimeter.
[Illustration: Fig. 55. Line transect running east and west in the
_Picea-Pinus-hylium_, showing the relation of the herbaceous layer to
the _Carex-Catha-helium_, invading along the brook; ecotones at _e_.]
=219. The location and size= of line transects are determined by the
purpose for which they are designed. Short transects are valuable for
detail, but they can be used to advantage only where changes in
arrangement are taking place rapidly. They are especially adapted to the
study of minute alternations and to the zonation of small ponds,
streams, ditches, roads, blowouts, etc. Longer transects can not furnish
the same detail, on account of the amount of time necessary, but they
are invaluable for the zonation and alternation of larger areas, such as
the consocies, formation, and formation series. They are of particular
importance for the record of zonation, since they afford a clue to the
topographic symmetry of the area. The location of a transect depends
upon the area to be studied, though it should always run through a
portion as typical as possible. The general direction is ascertained by
means of the compass, and when there is a measurable difference in
elevation it is taken by the barometer or otherwise.
The points at which ecotones cross the transect are carefully indicated
upon the chart. They serve as stations for simultaneous readings of
physical factors, though in the majority of cases water-content readings
alone will determine the reason for the ecotone. Photographs of line
transects should be made while the tape is in position, in order that
the superficies of the series may be as evident as possible.
_The Belt Transect_
=220. Details.= This differs from the line transect in that it is wider,
and consequently affords a more accurate record of the arrangement of
plants. While both give the actual facts of distribution, the line
transect necessarily ignores the minor lateral deviations in position.
These are brought out in a strip of some width, and the belt transect
thus gives a more correct view of the variations which result from
competition in an area physically homogeneous. The width of such
transects depends upon the length, and the character of the vegetation.
The standard width is one decimeter in herbaceous formations, and one
meter in the long transects which are used in woodlands. In open
vegetation, especially in the initial stages of successions, the width
may often be increased to advantage, but ordinarily the amount of work
necessary to run a belt transect of some length limits the width to one
decimeter.
The location of a belt transect, the choice of landmarks, the
determination of direction and elevation are made exactly as for the
line transect. The topographic map is made in precisely the same way
also, the scale used depending upon the length. Two tapes, however, are
employed, and these are placed so that they mark off a strip just one
decimeter wide. Every few meters, or oftener if need be, they are
checked by a decimeter rule, and fixed firmly in place by quadrat
stakes. The arrangement of the plants is recorded as for the line
transect, except that the record covers a decimeter strip just as in
quadrat work. Accordingly, an interval of a centimeter is left on the
sheet between the successive portions of the strip, in order that the
latter may be put together without confusion when the topographic map
and the plant series are combined. The record should invariably start in
the upper left-hand corner and read down. The map and the centimeter
strip recording the plants of the transect are combined on a common
scale as already indicated for the line transect.
The ecotones of zones are shown on belt transects by single cross lines,
and those of consocies by parallel cross lines. In taking photographs of
the transect, it is desirable to use guidons to mark these points
clearly. The same device may also be used to indicate the course of the
transect, when the tapes are completely hidden by the plants. Physical
factor readings should always be taken, and, as before, they are best
made at the intersections of the ecotones.
_The Permanent Transect_
=221. Advantages.= Both line and belt transects, after they have been
recorded, should be rendered permanent, in order that they may serve to
indicate the changes of a heterogeneous area from year to year in the
same detailed fashion that the permanent quadrat does for homogeneous
ones. For historical as well as for physical reasons, the ecotones of
zones and of consocies are subject to change from year to year, and the
amount and direction of this change can only be ascertained from annual
records made in exactly the same spot. By means of the permanent
transect alone the very origin of such areas can be followed from one
stage to another of the succession. Moreover, the transect is equally
valuable with the quadrat in making it possible to follow every step of
the minute changes wrought by competition.
=222. Details.= The transect is made permanent by blazing the landmarks
at either end, if these already exist, or by erecting them when it is
necessary. A label stake is driven at each end, on which is painted the
number and date of the transect and its length. Each stake should also
indicate the exact direction in which the other lies. The position of
the ecotone is indicated by smaller stakes bearing the number of the
transect and the date when the ecotone was found at that point. These
are left in place, and in a few years show very graphically the change
in position of the zones. For the first season, permanent transects
afford results of great value when recorded for each aspect, but after
this an annual visit will suffice. The details of mapping, plotting,
etc., are identical with those indicated above, with the addition that
all charts and photographs must bear the number of the reading as well
as that of the transect. Physical factor observations are taken as often
as the charts are made, and the results noted on the back of the chart
sheet for purposes of ready comparison.
_The Denuded Transect_
=223.= The denuded transect bears exactly the same relation to a
permanent one as that which exists between the denuded and the permanent
quadrat. While the permanent transect records the actual mutations due
to changing physical factors or to competition, the denuded transect
throws needed light upon the mobility and ecesis of the various species,
and upon the nature of the competition between them. Denuded transects
may be established wherever it seems desirable, after the strip has been
properly charted and photographed. The most valuable results, however,
are secured by locating each one alongside of a permanent one. The best
plan is to locate and chart two permanent transects a meter apart. A
single view is then made of the two. One of them is denuded together
with a strip 2 decimeters on either side, resulting in a denuded
transect 5 decimeters wide. In charting this during succeeding years,
the entire width may well be plotted as long as the vegetation is open,
but after it has again become well established, it is necessary to save
time by confining one’s attention to the central decimeter strip.
Photographs can be made either of the permanent and denuded transects
singly, or of the two together. The latter method has certain obvious
advantages. Climatic factor readings can be made for both transects in
common, but all those factors which are affected by the exposure of the
soil surface must be observed in each.
_The Layer Transect_
=224.= This is a modification of the line transect, by means of which
the vertical relations of plants are also shown, especially the tendency
to form layers which is so regular a feature of forest formations. Owing
to the difficulty of charting in three planes, belt transects do not
lend themselves to this purpose. Because of the greater complexity,
layer transects can rarely exceed ten meters in length except in those
formations where layering is little or not at all developed. The
simplest method is to establish a line transect in the ordinary way, and
then to record the height of each plant as its position is noted. This
is done by means of a measuring stick ruled in decimeters, which can be
moved from interval to interval along the tape, or better, by two such
sticks connected by tapes a meter long at every five decimeters of the
sticks. These should be two meters high for woodland, and one meter for
grassland. Layer transects often run on even surfaces, but if this is
not the case, the usual data for a topographic map should be taken. The
final chart is constructed on the scale of 10:1, the height of each
plant being indicated by a vertical line equal to .1 of the observed
height. A photograph of a representative meter of the transect is taken
when the measuring sticks and rods indicated above are in position.
Physical factor readings, principally of light, but often also of
humidity, temperature, and wind are made at the height of the various
layers when these are present.
ECOTONE CHARTS
=225.= The contour lines of zones and consocies are of the utmost
importance in recording the structure of vegetation. They do not permit
such accuracy as do quadrats and transects, but this is hardly to be
considered a disadvantage in view of the fact that ecotones are rarely
sharply defined. In establishing the ecotones of zonation, the width and
the length of the base, i. e., the area of excess or deficiency, or as
much of it as is to be considered, are determined. This base may be
road, ditch, pool, lake, or stream, or the peak or crest of a hill,
ridge, or mountain. When the zonation is bilateral, meter tapes are run
at right angles to the base, at proper intervals, and the points and the
distances where the ecotones cross are noted. In the case of radial
zones, the tapes are run in the four cardinal directions, and if the
base be large, in the four intermediate ones also, the intersections
being likewise noted. From the data thus obtained, the zones may be
outlined with a fair degree of accuracy. If the series be an extensive
one, it is charted to the scale of 100:1; in cases of small areas,
however, the scale of 10:1 will give better results. Whenever the zones
show clearly enough to warrant, a photograph is also taken.
Water-content readings are of paramount importance in the interpretation
of zones. Samples should be taken at all intersections, and the
resulting values indicated at the corresponding points upon the chart.
When the zones are broken up into alternating patches in consequence of
asymmetry in the topography, the ecotones of the latter are traced in a
similar fashion from the center of each as a base, the absolute position
of which is ultimately determined with reference to the ecotone lines
already established.
THE MIGRATION CIRCLE
=226. Purpose.= The migration circle is designed to record the invasion
of species, since it operates outward from an individual or a group of
plants as a center. As migration takes place to a certain degree in all
directions, a circle is better adapted to the purpose than the quadrat.
From the very nature of invasion, migration circles should always he
permanent in order that the yearly advance may be accurately noted.
Circles of this character are important aids in the study of any
vegetation, except, perhaps, one that has practically become stabilized.
Their great value, however, is found in succession, where it is
necessary to trace the movement of new individuals away from the
original invaders as centers of colonization.
=227. Location and method.= The size of the migration circle is largely
controlled by the density of the vegetation, and in some degree by the
height of the species also, since this determines the trajectory of the
disseminule. In close formations, a circle of 1–, rarely of 5–meter
radius can best be used, but in the more open initial stages of
succession a radius of 5, 10, or, in exceptional cases such as open
woodland, even 25 meters, affords the best results. The location should
always be made with a plant or group of plants of the species to be
studied as a center. This migration circle differs from the quadrat in
that it is used to show the movement of one, rarely two or three
species, and not the position of all the plants within it. The center is
permanently fixed by driving a labeled stake with the number of the
circle and the data. Two tapes the length of the radius are used for
recording. These are provided with the usual eyelets, 5 decimeters
apart, and are fastened on a peg in the top of the central stake so that
they move readily. At the outer ends they are staked 5 decimeters apart
by a tape of this length when the radius is 1 meter, and 1 meter when
the radius is 5 or 10 meters. The record forms must be especially
prepared on blank sheets about 9 inches square. The scale is 10:1 for
circles of 1 meter, and 100:1 for those of 5– and 10–meter radius. In
the former, concentric circles are drawn about the center at intervals
of 5 decimeters, and radii are drawn to the circumference at the same
interval. In the larger circles, the intervals are 1 meter. Each segment
of the circle is read by means of the two tapes, and the position
indicated with reference to the concentric lines and radii. When but one
species is read, a tiny circle is used to denote the position of each
plant. If more than one is used, the symbols are those already indicated
for the quadrat. One tape is left in place and the other with the
segment tape is shifted to a new position, and the resulting segment is
read as before. The exact position of the base radius is fixed by a
label stake, in order that the segments of successive years may exactly
correspond. The record sheet is labeled, dated, and filed. By folding at
one edge, it may be filed in the regular field book.
=228. The denuded circle= is established in the same way as a permanent
one. The original position of the individuals of the species under
consideration may be recorded or not, depending upon the use to be made
of the results. The safest plan is first to read the circle in the usual
way, and then to denude it. The latter should be done in such a way as
to remove all the disseminules from the surface in so far as possible.
It is essential also that this be done before the seeds are mature and
begin to be scattered. The central plant or cluster is of course not
removed. In special cases, all the plants of the species are allowed to
remain to serve as centers of colonization. The successive yearly
readings of the denuded area are made exactly as for a permanent circle.
Permanent and denuded circles, like quadrats, should always be
established near each other so that they permit of ready comparison
under similar conditions.
=229. Photographs= of migration circles furnish the most detail when the
camera is placed just behind the central group in such a way as to show
its relation to the other individuals or clusters of the circle. In the
denuded circle, or when the plants stand out conspicuously from the bulk
of the vegetation, it is not necessary to use guidons, but in other
cases the latter greatly increase the value of the picture. Factor
readings are less important for migration circles than for quadrats and
transects. The factors of principal importance are those that deal with
migration and ecesis, i. e., wind, water-content, and soil temperatures.
The former may be determined for both circles in common, but the
conditions that affect ecesis must be observed separately for each.
CARTOGRAPHY
=230. Value of cartographic methods.= Chart, map, and photograph are
records indispensable to the systematic study of vegetation. They serve
not merely to preserve the facts ascertained, and to permit their ready
comparison, but they also put a premium upon accurate methods, and
consequently bring to light many points otherwise overlooked. For
ecology, they have the value which drawings possess in taxonomy, in that
they make clear at a glance what pages of description fail to indicate.
They are the fundamental material of comparative phytogeography, and in
all careful vegetational study their use is no longer optional but
obligatory. Hence it is obvious that cartographic methods should be
clear and simple, and that they should be uniform, so that charts and
maps of widely separated formations may be directly compared without
difficulty. It is not to be expected that uniform methods will come into
general use immediately, but a proper appreciation of the obligation
that rests upon every ecologist to make his results both easily
comprehensible and usable will serve to produce this very necessary
result. In the treatment that follows, as elsewhere, no attempt is made
to describe the general cartographic methods used by other ecologists,
notably Flahault. The methods employed by the author form a complete
system, which has proved valuable, and for various reasons it alone is
discussed here.
=231. Standard scale.= The question of the scale to which charts and
maps are to be made is of primary importance. The general principle is
that the ratio between area and drawing should be as small as possible.
Moreover, charts and maps of the same character should always be drawn
to the same scale, unless a good reason to the contrary exists. The
ideal scale is 1:1, which is manifestly an impossibility. This is
approached most nearly in the quadrat chart where the scale is 10:1.
Charts of definite areas are made on a scale as large as possible, while
maps of formations, regions, etc., are necessarily drawn upon a very
small scale. General maps designed to show the distribution of species
and formations, or the vegetation of continents, are usually not drawn
with reference to a scale at all. While it is manifestly impossible to
use the same scale for charts and maps, it is feasible and desirable
that they be constructed upon scales readily convertible into each
other. This is most satisfactorily accomplished by means of the decimal
system, and the various type scales are 10:1, 100:1, 1000:1, etc. The
first two or three scales are used for charts of quadrats, transects,
and circles; the remaining ones are employed in making maps of large
areas. No attempt has been made to draw an absolute line between charts
and maps, but an endeavor is made to restrict the term chart to the
record of the number and position of plants, while maps deal with the
arrangement and location of formational areas. It is hardly necessary to
point out the reasons why all charts and maps should be based upon the
decimal system of scales. Experience will furnish the very best of
arguments.
=232. Color scheme.= The first requisite for the graphic representation
of formations, regions, etc., is that each class of formations be
invariably indicated by the same color. It is also necessary that the
colors and shades be easily distinguishable, and it is at least
desirable that they be referred to the different classes in some
consistent sequence. Uniformity in all these points is greatly to be
desired at the hands of all ecologists. Here, as in the case of the
standard scale, uniformity will be found the more desirable the more
impossible it is made by ignoring it. In the use of color to represent
regions and provinces, on maps too small to indicate formations, the
color of each division is represented by the color of its dominant
formation; thus the prairie province is colored ochroleucus on account
of the color used to represent prairie formations, the boreal-subalpine
zone atrovirens on account of the typical coniferous forests, etc. No
endeavor has been made to take account of the various types of
formations, e. g., the different coniferous forests, as this is a
problem to be worked out for more local maps in various shades of dark
green, etc. The following color scheme which has been based upon the
points made above is proposed as a satisfactory solution of the problem.
The color standard used is that of Saccardo’s Chromotaxia.
I. Hydrophytic Formations: _blue_
1. Marine: _cyaneus_
2. Brackish: _ardesiacus_
3. Freshwater: _caeruleus_
4. Swamps and marshes: _caesius_
II. Mesophytic Formations
A. Forest formations: _green_
1. Coniferous forests: _atrovirens_
2. Broadleaved evergreen forests: _viridis_
3. Deciduous forests: _flavovirens_
B. Grassland formations: _yellow_
1. Meadows: _melleus_
2. Prairies: _ochroleucus_
C. Culture and waste formations: _red_
1. Fields: _ruber_
2. Groves and orchards: _atropurpureus_
3. Wastes: _purpureus_
III. Xerophytic Formations: _brown_
1. Deserts: _isabellinus_
2. Plains and steppes: _avellaneus_
3. Saline formations: _umbrinus_
4. Arctic-alpine formations: _testaceus_
=233. Formation and vegetation maps= are detailed maps of a single
formation or a series of them, showing the formational limits, and when
the scale is not too small, the ecotones of zones and consocies. In the
cases where the topography is level, as sometimes happens in mapping
single formations, the chain and pedometer must be used to ascertain the
size of the different areas. Indeed in all mapping of vegetation, the
methods of surveying are directly applicable. Over large areas, however,
it is not necessary that limits be drawn with mathematical accuracy, and
for the purposes of the ecologist, the plane table and camera are
satisfactory substitutes for the surveyor’s transit, at least in the
present aspect of the subject. When the formation or group of formations
is commanded by an elevation of some height, the latter is used as a
base. A plane table is established upon it and the topographical and
vegetational features are recorded in the usual way. This map is usually
supplemented by a series of views from the same base. Indeed it has come
to be recognized that a complete series of photographs of this kind give
a more valuable record than the plane table, and that the construction
of an accurate map from them is an easy matter. Since the camera saves
much time and energy also, it is used almost exclusively to furnish the
data for map making. In hilly, and especially in mountainous regions,
the photographic method is indispensable. Its application is extremely
simple. A central hill or mountain is selected, and from it a series of
views is taken so that the edge of one exactly meets the edge of the
other. This is an extremely important matter, and demands much nicety of
judgment. The camera is kept in the same spot, and after each exposure
it is turned as the operator looks through it until a landmark at one
edge just passes from view at the other. As soon as the new position is
determined, the tripod screw is turned to hold the box firmly in
position. In case of a slight jar, the exact position should again be
obtained. If the series is accurately made, the resulting prints will
give a complete panoramic view of the region, without overlap or
omission. For this purpose, a 6½ × 8½ camera is desirable, since the
topographic and vegetational features are larger and stand out more
distinctly. A large camera requires fewer changes of position, and hence
saves time and reduces the chance of error. A 4 × 5 camera serves the
purpose sufficiently well, though it requires a little more care in
operation on account of the greater number of exposures necessary. This
may be avoided in some degree by the use of a wide-angle lens if the
depth of the area is not too great. Whatever camera may be used, a
telephoto lens is a very desirable adjunct, since it enables one to
choose between three different sizes of the view without changing the
position of the camera. To avoid possible confusion, the exposures are
always made from right to left, and the plates are used in the numerical
order of their holders. For the same reason the landmarks are described
and numbered in their proper order. The prints obtained are mounted on a
card in sequence. The view map may be preserved in this form, or it may
be reduced or enlarged by making a copy to the size desired. Outline
maps of topography may be traced from the resulting negative, and the
formations filled in by means of the proper colors. The most
satisfactory method, however, is to have the original views or the copy
printed “light” and to color the formations just as they appear there,
with all the wealth of topographic and vegetational detail. If a
detailed topographic map alone is desired, this is traced directly from
the large copy.
=234. Continental maps.= A method of determining the general outlines of
regions, provinces, and vegetational zones as a preliminary to their
detailed study has been used successfully for several years.[22] This is
based upon provincial and continental maps on which are traced the
geographical areas of the species of genera typical of the various
formations. Detail topographic maps of the prairie province and the
North American continent have been used for this purpose. A number of
the facies of extensive and representative formations of the different
portions of the continent are selected and grouped according to genera.
One map is devoted to each genus, unless the number of species is large.
In this case a number of maps are used, since the limits are apt to
become confused. The range of each species is determined from all the
reliable sources, and a corresponding line is drawn upon the map to
delimit its geographical area. The limits of the area of each species
are drawn in a different color, and the name of the species printed in
the same color in the legend. Although this work has as yet been done
only for the trees of North America, and for the grasses and principal
species of the prairie province, it promises to constitute a final
method for the limitation of vegetational divisions. It is clear that if
the original data concerning ranges are accurate, the increasing study
of formations will do little more than rectify the detailed course of
the limiting line, since in most cases facies and formations coincide in
distribution. The limiting line or ecotone of a zone or province is a
composite obtained from the limits of certain representative facies and
principal species, and checked by the limits of species typical of the
contiguous vegetations. Thus, the boreal-subalpine zone is clearly
outlined by combining the limits of _Populus tremuloides_, _Larix
americana_, _Pinus banksiana_, _Abies balsamea_, _Picea mariana_, _Picea
canadensis_, and _Betula papyracea_, and checking the results by the
areal limits of the hardwoods and grasses to the southward.
PHOTOGRAPHY
=235.= The camera is an indispensable instrument for the ecologist.
Although it has too often been employed to give an air of thoroughness
to work of no ecological value, it is as important for recording the
structure of vegetation as the automatic instrument is for the study of
the habitat. No ecologist is equipped for systematic field investigation
until he is provided with a good camera and has become skilful in its
use. For this reason, it is felt that a few hints concerning
photographic methods and their application in ecology may not be out of
place. No written advice can take the place of experience, but certain
elementary suggestions and cautions will greatly shorten the
apprenticeship of one who does not have the good fortune to be taught by
a professional photographer. To the student of ecology, the camera is
not a toy. It must be understood and operated with as much thoroughness
as any other instrument, and when this is done, the results will be
equally certain and desirable.
[Illustration: Fig. 56. 4 × 5 long focus “Korona” camera (series V).]
[Illustration: Fig. 57. 5 × 7 long focus “Premo” camera.]
=236. The camera and its accessories.= Although two cameras are
desirable whenever it is possible to obtain them, a single one will meet
all the requirements of field work. This should be 4 × 5 inches in size,
since it is much more convenient and will do all the work that a larger
camera can. In the comparatively few cases in which larger views are
needed, the 4 × 5 negatives can be readily enlarged. The smaller
instrument is less expensive in operation because of the cheapness of
the plates, and it gives a negative of the proper size for lantern
slides and for reproduction. A 6½ × 8½ camera is valuable in special
cases, such as making a series of photographs for maps. In the writer’s
own experience, the 6½ × 8½ camera, although used exclusively at first,
has been almost completely supplanted by the 4 × 5. The best field
camera is of the folding type with a good stout box. It must be what is
known technically as a long focus instrument, which enables small
objects to be taken natural size and permits the use of a telephoto
lens. It should be provided with a swing and also a reversible back by
which the position of the plates can be changed instantly. The lens must
be of the telephoto pattern, which makes it possible to use the front or
back lens either alone or in combination. The chief advantage of this is
that the image, when distant, may be made of three different sizes
without changing the position of the camera. Generally speaking, the
high-priced rapid lenses are the best, since it is exceptional to get
the desired length of exposure in vegetation, on account of the ease
with which the plants move in the wind. Before buying such a lens it is
desirable to test its rapidity and depth of focus, since it is not
necessarily better than some of the lenses furnished with good cameras.
The lens should be provided with an iris diaphragm capable of being
stopped down to 128 or 256. The shutters furnished with the ordinary
lenses are satisfactory, since “snap-shots,” i. e., instantaneous
exposures, are practically never possible for plants. The automatic
shutter of the “Premo” camera is an especially convenient form. All
shutters should be carefully tested before using to determine the exact
time value of the exposures indicated. It is not uncommon for the
exposure at 1 second, or at other points, to have a value quite
different from the one indicated. When this is the case, it is evident
that it can not be known too soon. The camera should have at least a
half-dozen double plate-holders. These are numbered consecutively so
that the figure uppermost when the holder is in the camera will indicate
the number of the plate exposed. A carrying case is desirable on a long
trip when all the plate-holders must be taken, but ordinarily it is a
disadvantage, since the camera box will carry two or three holders. The
camera cloth should be as small and light as possible, and at the same
time opaque. The most satisfactory one for the field is the rubber
cloth. The tripod should be a happy combination of lightness and
stability, a condition more nearly reached by the aluminum tripod than
by any other. It should have not less than three joints in order to
facilitate the use of the long focus upon objects near the ground.
[Illustration: Fig. 58. 5 × 7 “Korona” Royal camera.]
=237. Choice of a camera.= There is not a great deal of choice between
the moderate-priced cameras of the various makers. A field camera is
restricted to certain special uses, and hence is more serviceable when
attachments useful only in portraiture or instantaneous work are absent.
Even the ray filter, which has some value in the indoor photography of
flowers, is useless in the field on account of the long exposure
required. From considerable experience, “Premo” and “Korona” cameras
have been found to be very satisfactory instruments. Doubtless the same
statement would be found true of all the standard makes, but they have
not been used by the writer. “Premo” cameras are made by the Rochester
Optical Co., Rochester, N. Y., and “Korona” cameras by the
Gundlach-Manhattan Optical Co., Rochester, N. Y. When two or more
cameras are used, the best results can be obtained if they are of the
same make, since the details of operation are then the same. The reduced
liability of making a blunder is often offset by the fact that a
different pattern will permit of a wider range of use. Any standard
brand of plates will produce good negatives when skilfully used; at
least, this has been proved in the case of the Cramer, Hammer, Seed, and
Stanley brands. Every professional photographer has his favorite brand
of plate, but the ecologist will do well to give the various kinds a
thorough trial, and then to invariably use the one which gives him the
best results. Thus, while it seems to be less popular with the
profession than the others mentioned, the writer has obtained at least
as satisfactory results with the Stanley plate as with the others, and
consequently now uses it exclusively, since it is cheaper. The one
important point is to make a final choice only after personal
experience, and then to always use plates of the same brand, and
preferably of the same rate of speed.
=238. The use of the camera.= To the ecologist, objects to be
photographed fall into two categories, viz., those that move, and those
that do not move. For practical purposes, areas sufficiently distant to
render the movement imperceptible belong to the latter, as well as
those, such as rock lichens, many fungi, etc., which can not be stirred
by ordinary winds. The treatment accorded the two is essentially
different. A fundamental rule of ecological photography is that detail
must receive the first emphasis. The ecological view should be a picture
as well as a map, however, but when one must be sacrificed, artistic
effect must yield to clearness, and accuracy, i. e., technically
speaking, contrast must give way to detail. Leaving apart the necessity
of securing a sharp focus, which holds for all work, detail or
definition depends directly upon the aperture of the diaphragm. Detail
is increased by decreasing the size of the aperture. This in turn
increases the length of time necessary for a proper exposure, and
consequently the danger that the plant will be moved in the midst of the
exposure. When the movement is negligible, the invariable rule should be
to reduce the aperture to its smallest size, and to expose for a
corresponding time. In all cases where the plants are close enough to
show even a slight blurring on account of the action of the wind, the
time of exposure must be reduced, in the hope that a short period of
quiet will suffice for it. This reduction in time must be compensated by
increasing the aperture of the diaphragm, and hence the amount of light
which strikes the plate. The proper balance between the two is a matter
of considerable nicety. It depends much upon the vagaries of the wind,
and can readily be determined only after considerable experience.
Although regions naturally differ somewhat in the nature of their winds,
much experience in prairie and mountain regions warrants the primary
rule that views of vegetation and plants subject to movement are not to
be attempted on windy or cloudy days when it can possibly be avoided.
Even on reconnaissance, a poor picture is no better than none at all,
while in resident work a time will come sooner or later which will
permit the making of a view satisfactory in all respects. There may be
occasional instances when one is rewarded for keeping the camera trained
on a particular spot for hours, and for wasting several plates in the
hope that still moments will prove to be of the requisite duration. As a
regular procedure, however, this has nothing to commend it.
Various methods have been tried to reduce or eliminate the trouble
caused by the wind. Canvas screens have been used for this purpose with
some benefit. When the picture is worth the trouble, a tent may be
erected to afford a very efficient protection. This is too prodigal of
time and energy, however, to be practicable under the usual conditions.
Flashlight exposures on still nights are sometimes feasible, but the
disadvantages connected with them are too great to bring them into
general use. The best procedure is to bide one’s time, and to take
quadrats, transects, and other detail areas, as well as many plant
groups, at a time that promises to be most favorable. Single plants can
often be moved in the field so that they are protected from the wind, or
so that they are more strongly lighted. Slender, or feathery plants are
usually very difficult to handle out of doors. The best plan is to
photograph them in a room that is well and evenly lighted, or, best of
all, in a stable, roomy tent.
=239. The sequence of details.= No photographer ever escapes blunders
entirely. At the outset of his work, the ecologist must fully realize
this, and accordingly plan a method of operating the camera which will
reduce the chance of mistake to a minimum. The usual blunders which
every one makes sooner or later, such as making two exposures on one
plate, drawing the slide before closing the shutter, allowing the light
to strike the plate through the slit in the holder, etc., can be all but
absolutely avoided by a fixed order of doing things. This order will
naturally not be the same for different persons; it is necessary merely
that each have his own invariable sequence. The following one will serve
as an illustration. As a preliminary, the plate-holders are filled,
after having been carefully dusted, and the slides are uniformly
replaced with the black edge inward. It is a wise precaution to again
see that all the slides are in this position before leaving the dark
room. This will ensure that a black edge outward always means that the
plate has been exposed. The tripod is first set up and placed in what
seems about the proper position. The camera is next attached to it, and
the front and back opened. The bellows is pulled out, a short distance
for views, and a longer one for detail pictures, and fastened. It is
necessary to move the diaphragm index to the largest aperture and to
open the shutter at “time.” The next steps are to orient the view or
object, and to bring it into sharp focus upon the ground glass. The
first is accomplished by moving the entire instrument, changing the
position of the tripod legs, swinging the camera upon the tripod, or by
raising or lowering the lens front. It is often desirable also to change
the position of the object on the plate by use of the reversible back.
In views with much distance, the foreground is brought into sharp focus.
In close views, especially of quadrats, the swing is used to increase
the distance for the foreground, and the focus is made upon the center.
After focusing, the shutter is closed, the indicator set at the time
desired, and the diaphragm “stopped down” as far as possible.
Plate-holder 1 is slipped into place, care being taken not to move the
camera by a sudden jar. The camera cloth is dropped above the holder and
allowed to hang down over the slide end. The slide is drawn and put on
top of the instrument, the black edge always up. The exposure is made
and the slide replaced _with the black edge outward_. This point should
receive the most critical attention, as a blunder here will often cause
the loss of two negatives. The plate-holder is returned to the
receptacle, or merely placed in the back of the camera, which is then
closed. The number of the plate, the name of the view or object, the
condition of the light, the length of exposure, and the aperture of the
diaphragm, as well as the date, are recorded in a notebook for this
purpose. The shutter is then opened at “time,” the diaphragm thrown wide
open, and the front of the camera closed. When distances are short, the
camera is often carried upon the tripod. As a rule, however, it is
usually removed, and the tripod folded. In making subsequent pictures,
the plates should always be used in their numerical order.
=240. The time of exposure= is obviously the most critical task in the
manipulation of a camera. The time necessary for a proper exposure
varies with the season, the hour, the condition of the sky, the light
intensity of the formation, the color and size of the area to be
photographed, and, finally, of course, with the aperture of the
diaphragm. Fortunately for the ecologist, the variation in light
intensity during the season, and even during the greater part of the
day, is not great, and can ordinarily be ignored. The beginner will make
the most progress by determining the exposure demanded by his instrument
for taking a general view in full sunlight and with the smallest stop of
the diaphragm. In standard cameras with lenses of ordinary rapidity,
this is usually about one second. This will serve as a basis from which
all other exposures may be reckoned until one has worked through a wide
range of conditions and can recall just what time each view requires. On
completely cloudy days the time required is five to ten times that
necessary on a clear day; filmy clouds and haze necessitate an exposure
of two or three seconds. The more open forest formations demand an
exposure of about five to ten seconds on a sunny day, while the deeper
ones require two or three times as long. A close view requires more time
than a distant one, since the light-reflecting surface is much smaller.
Quadrats require two or three seconds, and individual groups frequently
take a longer time. The color of the vegetation plays an important part
also: a dark green spruce forest requires twice as long an exposure as
the aspen forest, and a grassland quadrat takes more time than one
located in a gravel slide. In this connection, it is hardly necessary to
point out that the lighted side of objects should always be taken, never
the shaded one. The exposures indicated above are based upon the
smallest stop. The reasons for using this whenever possible have already
been given. When a larger stop is necessary, the exposure is decreased
to correspond; for example, a quadrat that takes three to four seconds
at 256 can be taken at 64 in one second. As a rule, the sun should not
be in front of the camera, but, when necessary, views can be made in
this position if the sun is prevented from shining directly into the
lens.
=241. Developing= is as important as exposing. Indeed, it may well be
considered more important, since a properly exposed plate may be spoiled
in developing, while an under-exposure or over-exposure may be saved.
Owing to the ease with which plants move in the wind, the ecologist is
obliged to reconcile himself to many under-exposures, which can be
converted into good negatives only by skilful developing. Every base
station should have a good dark room, equipped with running water when
possible, a good ruby lantern, and the proper trays and chemicals.
Prepared developing solutions are alluring because of their convenience,
but after an extended trial of several kinds, the writer has reached the
conviction that pyrogallic acid, or “pyro,” is by far the most
satisfactory in working with vegetation. Of almost innumerable formulae,
the following gives excellent satisfaction and is convenient to use.
I. II.
500 cc. water 500 cc. water
30 grams sodium sulphite 5 grams pyrogallic acid
30 grams sodium carbonate
For developing, equal parts of I and II are mixed, and a few drops of a
10 per cent solution of potassium bromide added, unless there is reason
to suspect that the plate has been seriously underexposed. The fixing
bath is a concentrated solution of sodium hyposulphite, “hypo,” to which
a few drops of acetic acid are added. It should be replaced every week
or two, depending upon how much it is used. A tray of water is kept at
hand for bringing out the detail in underexposed negatives, and a second
tray is used for washing. The “pyro” and the bromide solution should
always be within reach, the former for accelerating, and the latter for
retarding the development of unsatisfactory plates.
The image will begin to show on a properly exposed plate within one to
three minutes after it has been put in the developer. If the image
appears almost instantly, and then recedes quickly, the plate is badly
overexposed, and should be thrown away. In case it “comes up” less
quickly, indicating that it is not greatly overexposed, it can be saved
by the addition of more bromide. When the image does not show till the
end of five to ten minutes, the plate has been underexposed. It is then
necessary to add more “pyro,” taking care not to pour it on the plate,
and, after the image appears with its striking contrast, to leave the
plate in water until as much detail as possible is brought out in the
shadows. In the case of a normal exposure, when greater detail is
desired, the negative is left for some time in water, and when contrast
is sought more “pyro” is used. Negatives with unusual detail lack
“snap”; they are “flat,” and fail to make artistic pictures. Contrast,
on the other hand, often obscures detail, and the best results can only
be obtained by a happy combination of the two. The most important maxim
in developing is that the process shall be continued until the image has
become indistinct. The universal tendency of the beginner is to remove
the negative the moment the outlines grow dimmer, and the result is a
thin, lifeless negative. It is almost impossible to develop too far, if
the image is not allowed to disappear. Negatives of this sort are
“thick,” and though they print more slowly, produce brilliant pictures.
A large quantity of the developing solution is used with single plates
in small trays, and is allowed to act without rocking the tray. Much
time is saved, however, by developing several plates together, and to
avoid using a large quantity of the solution, the tray is gently rocked
from time to time. This movement is particularly necessary at the
beginning, in order that the plates may be covered evenly, and at once.
Fifty cubic centimeters of the solution will develop three or four 6½ ×
8½ plates, and twice as many 4 × 5’s. After the developer has once been
used, it is kept for several days to restrain overexposed plates. As
soon as the plate is developed, it is rinsed in water, and placed in the
fixing fluid, until the white opaqueness is entirely removed. The “hypo”
is then washed out by immersing the negatives for one to two hours in
running water. If the latter can not be secured, the water in which they
are placed should be changed frequently. The negatives are then
air-dried within doors, in a place free from dust. Finally, they are
filed away in negative envelopes, each bearing the name and number of
the negative, and preferably also, the time and other exposure data.
=242. Finishing.= On account of the time demanded by other field tasks,
it has not been found desirable to make and finish prints in the field.
This, with the making of lantern slides, enlargements, etc., may well be
turned over to a professional photographer. It is the custom to make a
proof of each negative to meet the casual needs that arise in the field.
For this purpose, solio “seconds” are used, since they are both cheap
and satisfactory. When an urgent demand for a finished print does arise,
it is met by using “velox” paper, which can be exposed in the dark room,
and then developed and fixed exactly like a plate. Two standard papers
for views are “solio” and “platina.” The former gives brown tones, and
is used for contrast and brilliancy, hence it is especially good for
printing from negatives that have too much detail and too little
contrast. “Platina,” on the contrary, yields soft gray tones, and
softens contrasts.
FORMATION AND SUCCESSION HERBARIA
=243. Concept and purpose.= A formation herbarium is a collection of
exsiccati, in which the species are arranged with respect to their
position in the formation, instead of being grouped in genera and
families. Its primary purpose is to furnish a record of the constitution
and the structure of a formation or a series of formations. At the same
time, it affords the basal material for developing the subject of
comparative phytogeography. It is impossible for one ecologist to visit
many remote regions, to say nothing of spending a period sufficient for
obtaining even a fair knowledge of the vegetation. He can at the best
acquire an acquaintance with but few regions at first hand. In
consequence, a method that brings a vegetation to him, with its
structure carefully wrought out by years of study, is of the highest
value. Time, as well as distance, sets a narrow limit to the number of
formations which one man can investigate critically in a lifetime. It is
no longer possible for a botanist to explore vast regions, and to bring
back results which have anything more than a very general value. This
fact, far from restricting the comparative study of vegetation, will
serve to make it more accurate and systematic. The exact results of
numerous resident investigators, expressed in formation herbaria, with
the proper series of quadrat maps and photographs, will be worked over
by men who are themselves specially acquainted with a particular
vegetation. Comparisons will be founded upon a definite basis, and the
relationship of various vegetations can then be expressed in precise
rather than general terms. It is hardly too sweeping to assert that
accurate work in the field of comparative phytogeography can be done
only in this fashion. The value of formation herbaria in class work is
evident. On account of the limitations of time and distance, classes can
touch but few formations, and these at every time except the growing
period. For these reasons, an accurate and complete formational record
that can be consulted or studied at any time is almost indispensable to
class study in the development and structure of formations.
=244. Details of collecting.= Formational collections, unlike the
ordinary sets of exsiccati, can not be made upon the first visit to a
region, or by a single journey through it. The determination of
formation limits, and of developmental stages, of aspects, layers,
abundance, etc., must necessarily precede, a work which alone takes
several years. Moreover, collecting itself requires more than one year
in a region containing numerous formations. This is exemplified by the
_Herbaria Formationum Coloradensium_.[23] The preliminary study for this
was made from 1896–1899, the collecting was done chiefly in 1900 and
1901, while additional numbers were added in 1902–3. For the purposes of
the formation herbarium, specimens should be collected and pressed in
such fashion as to show all the ecological features possible. Plants
must be collected both in flower and in fruit, with the underground
parts as perfect as may be. Seedlings and rosettes should be included
whenever present. In pressing, one or two leaves should be arranged with
the lower side uppermost to admit of the ready comparison of both
surfaces. Opened flowers are valuable for flower biology, while seeds
and fruits are desirable for showing migration contrivances. The ferns,
mosses, and lichens of the formation should be fully represented,
together with the more important fungi and algae. The number of
photographs taken for each herbarium should be limited only by
considerations of time and expense. The ideal series consists of a
general view of each formation, showing its physiographic setting,
nearer views of each of its aspects, detail views of its consocies,
societies, and layers, and flower portraits of all the constituent
species. Such a series can only be obtained by residence through a long
term of years, and in most cases general and aspect views, with
portraits of the facies and a few of the striking principal species,
must suffice. Quadrat and transect charts, together with formational
maps, are extremely desirable, and, indeed, all but indispensable.
=245. Arrangement.= The arrangement of species within each formation
herbarium is based upon the structure of the vegetation. The primary
groupings are made with reference to time of appearance and abundance;
when definite zones, associations, or layers are present, they must
likewise be taken into account. In the Colorado collection, the first
division is into three aspects based upon the period of flowering
(_aspectus vernalis_, _aestivalis_, _autumnalis_). Within each aspect,
the species are arranged with respect to abundance in the groups,
facies, principal species, and secondary species. Each group is placed
in an ordinary manila cover, which bears a printed label indicating the
aspect and the group. The species labels give, in addition to the name,
date, and place of collection, the phyad or vegetation form, the
geographical area, the rank of the species, the aspect, and the
formation. To these may well be added data concerning migration
contrivances, seed production, pollination, period of flowering, etc.
The photographs are mounted on the usual herbarium sheets, and placed in
the proper order in the various groups, and a similar disposition is
made of quadrat and transect charts, and such physical factor summaries
as seem desirable.
=246. Succession herbaria.= The arrangement of formation herbaria may
follow the classification of formations with respect to character,
region, or development. The first is the most convenient for purposes of
instruction, and has distinct advantages in permitting a close
comparison of the vegetation of different habitats. The second basis,
which is the one used in the _Herbaria Formationum Coloradensium_, is
peculiarly adapted to mountain vegetation in which the zones are usually
very distinct. The arrangement of herbaria in a developmental series,
however, is the most logical and the most illuminating, since the
structure of the ultimate formations is not only made plain, but the
stages in their development are also laid bare. Such succession herbaria
are the natural outgrowth of formational ones. Indeed, the latter should
be made merely the starting point for these in all regions where the
causes which bring about successions are active. Where weathering is
still an important factor, as in mountains, the initial and intermediate
formations which lead to the final grassland or forest are often in
evidence. After a formation herbarium of each stage has been made in the
way indicated, a succession herbarium is obtained merely by arranging
the various herbaria in the sequence of the developmental stages. Thus,
in the Colorado collection, the subalpine formations are arranged
according to altitude in the following series: (1) the pine formation,
(2) the gravel slide formation, (3) the half gravel slide formation, (4)
the aspen formation, (5) the balsam-spruce formation, (6) the
spruce-pine formation, (7) the meadow thicket formation, (8) the brook
bank formation. Of these, five belong to the same succession, and it is
possible to indicate the development of the spruce-pine forest by
arranging these five formations in their proper order in a succession
herbarium, as follows: (1) the gravel slide formation, (2) the half
gravel slide formation, (3) the pine formation, (4) the balsam-spruce
formation, (5) the spruce-pine formation.
DEVELOPMENT AND STRUCTURE
=247. Vegetation an organism.= The plant formation is an organic unit.
It exhibits activities or changes which result in development,
structure, and reproduction. These changes are progressive, or periodic,
and, in some degree, rhythmic, and there can be no objection to
regarding them as functions of vegetation. According to this point of
view, the formation is a complex organism, which possesses functions and
structure, and passes through a cycle of development similar to that of
the plant. This concept may seem strange at first, owing to the fact
that the common understanding of function and structure is based upon
the individual plant alone. Since the formation, like the plant, is
subject to changes caused by the habitat, and since these changes are
recorded in its structure, it is evident that the terms, function and
structure, are as applicable to the one as to the other. It is merely
necessary to bear in mind that the functions of plants and of formations
are absolutely different activities, which have no more in common than
do the two structures, leaf and zone.
=248. Vegetation essentially dynamic.= As an organism, the formation is
undergoing constant change. Constructive or destructive forces are
necessarily at work; the former, as in the plant, predominate until
maturity, when the latter prevail. Consequently, it no longer seems
fruitful to classify the phenomena of vegetation as dynamic or static.
The emphasis which has been placed upon dynamic aspects of vegetation
has served a useful purpose by calling attention to the development of
the latter. Although it is a quarter of a century since Hult, and more
than a half century since Steenstrup, by far the greater number of
ecological studies still ignore the problem of development. This
condition, however, can be remedied more easily by insisting upon an
exact understanding of the nature of the formation than in any other
way. It is entirely superfluous to speak of dynamic and static effects
in the plant, and the use of these terms with reference to the formation
becomes equally unnecessary as soon as the latter is looked upon as an
organism. The proper investigation of a formation can no more overlook
development than structure, so closely are the two interwoven. Future
research must rest squarely upon this fact.
=249. Functions and structures.= The functions of a formation are
association, invasion, and succession: the second may be resolved into
migration and ecesis, and the third, perhaps, into reaction and
competition. Formational structures comprise zones, layers, consocies,
societies, etc., all of which may be referred to zonation, or to
alternation. The term association has been used in both an active and a
passive sense. In the former, it applies to the inevitable grouping
together of plants, by means of reproduction and immobility. Passively,
it refers to the actual groupings which result in this way, and in this
sense it is practically synonymous with vegetation. Invasion is the
function of movement, and of occupying or taking possession; with
association, it constitutes the two fundamental activities of
vegetation. It is the essential part of succession, but the latter is so
distinctive, because of the intimate relation of competition and
reaction, that clearness is gained by treating it as a separate function
which is especially concerned with development. Association, zonation,
and alternation are structural phenomena, which are in large part the
immediate product of habitat and function, and in a considerable degree,
also, the result of ancestral or historical facts. It is a difficult
matter to determine in what measure the last factor enters, but it is
one that must always be taken into account, particularly when the
physical factors of the habitat are inadequate to explain the structures
observed. Structurally, association regularly includes both zonation and
alternation. As there are certain typical instances in which it exhibits
neither, the treatment will be clearer if each is considered separately.
ASSOCIATION
=250. Concept.= The principle of association is the fundamental law of
vegetation. Indeed, association is vegetation, for the individual passes
into vegetation, strictly speaking, at the moment when other individuals
of the same kind or of different kinds become grouped with it. It is
then (and the same statement necessarily holds for vegetation) the
coming together and the staying together of individuals and, ultimately,
of species. A concrete instance will illustrate this fact. In the
development of the blowout formation of the Nebraska sand-hills
(_Redfieldia-Muhlenbergia-anemium_), association begins only when the
first plant of _Redfieldia flexuosa_ is joined by other plants that have
sprung from it, or have wandered in over the margin of the blowout.
Henceforth, whatever changes the blowout formation may undergo,
association is a settled characteristic of it until some new and
overwhelming physical catastrophe shall destroy the associated
individuals. It will readily be seen that association does not depend
upon particular individuals, for these pass and others take their place,
but that it does depend essentially upon number of individuals.
Association involves the idea of the relation of plants to the soil, as
well as that of plants to each other. It is synonymous with vegetation
only when the two relations are represented, since there may be
association such as that of a parasite with its host, which does not
constitute vegetation. But it will be seen that the relation of the
parasite to the host is practically identical with the relation of the
plant to the soil or stratum, and the two concepts mentioned above
become merged in such a case. From this it follows that association
results in vegetation only when the two ideas are distinct. The concept
of association contains a fact that is everywhere significant of
vegetation, namely, the likeness or unlikeness of the individuals which
are associated. In the case of parasite and host, this unlikeness is
marked; in vegetation, all degrees of similarity obtain. As will be
evident when the causes which lead to association are considered,
alternate similarity and dissimilarity of the constituent individuals or
species is subordinate as a feature of vegetation only to the primary
fact of association.
Since association contains two distinct, though related, ideas, it is of
necessity ambiguous. It is very desirable that this be avoided, in order
that each concept may be clearly delimited. For this reason, the act or
process of grouping individuals is termed _aggregation_, while the word
association is restricted to the condition or state of being grouped
together. In a word, aggregation is functional, association is
structural; the one is the result of the other. This distinction makes
clear the difference between association in the active and passive
sense, and falls in with the need of keeping function and structure in
the foreground.
=251. Causes.= In considering the causes which produce association, it
is necessary to call in evidence the primary facts of the process in
concrete examples of this principle. These facts are so bound up in the
nature of vegetal organisms that they are the veriest axioms.
Reproduction gives rise immediately to potential, and ultimately, in the
great majority of cases, to actual association. The degree and
permanence of the association are then determined by the immobility of
the individuals as expressed in terms of attachment to each other or to
the stratum, such as sheath, thallus, haustoria, holdfasts, rhizoids,
roots, etc. The range of immobility is very great. In terrestrial
plants, mobility is confined almost entirely to the period when the
individual lies dormant in the seed, spore, or propagative part, which
is alone mobile. In aquatic spermatophytes, the same is true of all
attached forms, while free floating plants such as _Lemna_ are mobile in
a high degree, especially during the vegetative period. Among the algae
and hydrophilous fungi, attached forms are mobile only in the spore or
propagative condition, while the motile forms of the plancton typify the
extreme development of mobility. The immediate result of reproduction in
an immobile species is to produce association of like individuals, while
in the case of a mobile species reproduction may or may not lead
immediately to association. We may lay down the general principle that
immobility tends to maintain the association of the individuals of the
same generation, i. e., the association of like forms, while mobility
tends to separate the similar individuals of one generation and to bring
unlike forms together. With the mobile algae, separation of the members
of each generation is the rule, unless the individuals come to be
associated in a thallus, or are grouped in contact with the substratum.
Flowering plants that are relatively immobile, especially in the seed
state, drop their seeds beneath and about the parent plants, and in
consequence dense association of the new plants is the rule. In very
many cases, however, this primitive tendency is largely or completely
negatived by the presence of special dissemination contrivances, which
are nearly, if not quite, as effective for many terrestrial plants as
the free floating habit is for algae. From this point, the whole
question of mobility belongs to migration, just as the adjustment
between the parent plants and their offspring, or between plants
established and the mobile plants to be established, belongs to
competition.
If association were determined by reproduction and immobility alone, it
would exhibit areas dissimilar in the mass of individuals, as well as
areas dissimilar in the kinds of individuals. Some areas would be
occupied by plants of a single species, others by plants of several or
many species. This tendency of association to show differences is,
however, greatly emphasized by the fact that vegetation is fundamentally
attached to and dependent upon a surface that exhibits the most extreme
physical differences. For this reason, new differences in association
appear, due not only to the morphological differentiation of vegetation
forms, but also to the changes in the degree and manner of association
produced directly by the different habitats. Association might then be
defined as a grouping together of plant individuals, of parents and
progeny, which is initiated by reproduction and immobility, and
determined by environment. It is a resultant of differences and
similarities. In consequence, association in its largest expression,
vegetation, is essentially heterogeneous, while in those areas which
possess physical or biological definiteness, habitats and vegetation
centers, it is relatively homogeneous. This fundamental peculiarity has
given us the concept of the formation, an area of vegetation, or a
particular association, which is homogeneous within itself, and at the
same time essentially different from contiguous areas, though falling
into a phylogenetic series with some and a biological series with
others. From its nature, the plant formation is to be considered the
logical unit of vegetation, though it is not, of course, the simplest
example of association.
=252. Aggregation.= As indicated under the causes of association, the
process by which groups of individuals are formed depends entirely upon
reproduction and migration. In short, aggregation is merely a corollary
of movement. The simplest example of this process occurs in forms like
_Gloeocapsa_, _Tetraspora_, and others, where the plants resulting from
fission are held together by means of a sheath. Though called a colony,
such a group of individuals is a family in the ordinary sense.
Practically the same grouping results in the case of terrestrial plants,
especially spermatophytes, when the seeds of a plant mature and fall to
the ground about it. The relation in both instances is essentially that
of parent and offspring, although the parent soon disappears in the case
of annuals, while among the algae its existence is regularly terminated
by fission. The size and the density of the family group are determined
by the number of seeds produced, and by their mobility. These are
further affected by the height and branching of the plant, and by the
position of the seeds upon it. The disseminules of immobile species fall
directly beneath the parent, and the resulting group is both uniform and
definite. A similar arrangement is caused likewise by offshoots. An
increase in mobility brings about a decrease of aggregation, since the
disseminules are carried away from the parent plant. Perfectly mobile
forms rarely produce family groups for this reason. It is evident,
however, that mobile perennials sometimes arrange themselves in similar
fashion in consequence of propagation by underground parts.
Consequently, it is possible to state the law of single aggregation,
viz., that immobility promotes the grouping of parent and offspring, and
mobility hinders it.
If all species were immobile, the family group would be characteristic
of vegetation. Since the great majority are more or less mobile,
aggregates of this sort are the exception rather than the rule. Mobility
not only decreases the number of offspring in the family group, but it
also spreads disseminules broadcast to enter dissimilar groups. It leads
directly to mixed aggregation, by which individuals of one or more
species invade the family group. Once established, the newcomers tend
also to produce simple groups, thus causing an arrangement corresponding
essentially to a community. Such collections of family groups are
extremely variable in size and definition. This arises in part from the
nature of simple aggregation, and in part from the varying mobility of
different species. Mobility alone often produces similar communities by
bringing together the disseminules of different plants, each of which
then becomes the center of a mixed group. In the case of permobile
species, several disseminules of each may be brought together. The
resulting area, though larger, is practically the same. At present, it
is difficult to formulate the law for this method of grouping. It may be
stated provisionally as follows: mixed aggregation is the direct result
of mobility, and the greater the mobility the more heterogeneous the
mixture.
The constitution of all the major areas of a formation is to be
explained upon the basis of aggregation by the two methods described.
The relative importance of family groups and communities differs for
every formation, and the exact procedure in each can be obtained only by
the detailed study of quadrats. The problem is further complicated by
competition and reaction, particularly in closed vegetation. For this
reason, aggregation can be studied most satisfactorily in a new or
denuded area, where these processes are not yet in evidence.
_Kinds of Association_
=253. Categories.= In the analysis of association, it must be kept
clearly in mind that the concrete examples from which all
generalizations must be drawn are often in very different stages of
development, and are of correspondingly different ages. For this reason
it has seemed best to consider the primary relations of association in
general in this place, leaving the treatment of the effects of invasion,
succession, alternation, and zonation to be taken up under these topics.
Various categories of association may be distinguished, according to the
dominant physical factor concerned or the point of view taken. These
will fall into two series, as we consider the relation of plant to plant
with reference to some object or characteristic, or the grouping of
plants together in response to some dominant factor. In the first series
may be placed association with reference to substratum, to the ground
(occupation), and to invasion; in the second belong light and
water-content association. It should be noted that these are all actual
associations in nature, and not concepts such as the vegetation form,
within which plants from widely different associations may be
classified. Naturally, it does not follow that it is not logical or
valuable to group together those plants, such as hydrophytes,
sciophytes, hysterophytes, etc., which have a common relation to some
factor, but belong to different formations.
=254. Stratum association.= Plants manifest independent or dependent
association with reference to the stratum to which they are attached and
from which they derive food or support. Independent association is
exhibited by those holophytic species of a formation which are entirely
independent of each other with respect to mechanical support or
nutrition. It is characteristic of the greater number of the constituent
species of formations. Dependent association is manifested in the
relation between host and parasite, stratum and epiphyte, support and
liane. Warming[24] has distinguished six kinds of associations:
parasitism, helotism, mutualism, epiphytism, lianism, and commensalism.
Commensalism corresponds to the primary principle of association which
has given rise to vegetation. Homogeneous commensalism is the term
applied to social exclusive plants, in which the patch is composed of a
single species. Such association is extremely rare in nature, and if the
most minute forms be considered, probably never occurs. On the other
hand, heterogeneous commensalism, in which individuals of more than one
species are present, is everywhere typical of vegetation. Warming
regards saprophytism merely as a specialized kind of parasitism, an
opinion that may well be defended. Helotism, however, is also a mere
modification of parasitism, if it is not indeed parasitism pure and
simple. Mutualism is an altogether vague concept, including parasites,
epiphytes, and endophytes of doubtful physiological relation. Pound and
Clements[25] treated lianes, parasites, and saprophytes as vegetation
forms, relating herbaceous creepers and twiners to the lianes, and
dividing the fungi and lichens into nine groups. Whatever the value of
these divisions may be from the standpoint of vegetation forms, they
represent the same relation between plant and nutritive stratum, and
with respect to association should be merged in one group. Schimper[26]
was the first to perceive the essential similarity of all such groups
from the standpoint of association. He terms these plant societies
(_Genossenschaften_), retaining the four groups already established,
lianae, epiphyta, saprophyta, and parasiticae. It is evident that
dependent association comprises extremely divergent forms, from the
slightly clinging herb, such as _Galium_, to the most intense parasite.
The distinction, however, is a clear one, if restricted to that relation
between plants in which one acts as a mechanical support or stratum or
as a nutritive host for the other.
=255. Ground association.= The first division of formations into open
and closed was made by Engler and Drude.[27] Open formations were
defined as those having incomplete stability and heterogeneous
composition, while closed formations have a more definite uniform stamp.
What is true of formations is equally true of vegetation, so that
association may be regarded as open or closed with reference to the
density and thoroughness with which the plants occupy the ground. In
open association, the ground is slightly or partially occupied, readily
permitting the entrance of new plants without the displacement of those
already present. Such an arrangement is characteristic of the early
stages of a formation, or of a succession of formations. It produces
unstable open formations, which arise, usually after denudation, in
sand-hills, blowouts, gravel slides, dunes, flood plains, burned areas,
etc. In closed association, occupation of the ground is complete, and
the invasion of new species can occur only through displacement. Closed
association results in stable, closed formations, such as forest,
thicket, meadow, and prairie. As open association characterizes the
early stages of a succession of formations, so closed association is
peculiar to the later or last stages of all such successions. In short,
open formations represent certain phases of the development of
vegetation, while closed formations correspond to the relatively final
structural conditions. It is a fundamental principle of association that
every succession from denudation, or from newly formed soils, begins
with open formations and ends with a closed formation. The causes
leading up to open and closed association are intimately connected with
development, and hence are considered under invasion and succession.
=256. Species guild association.= Drude has distinguished a kind
of association peculiar to invasion, in which there is a
successive or concomitant movement of certain species of a
formation into another formation or region, resulting in species
guilds (_Artengenossenschaften_). The association in this case is
largely one of community of origin or area, and of concomitant
migration. It is especially characteristic of areas adjacent to
formational and regional limits. Fundamentally, it is merely the
grouping of plants which are invading at the same time, and
consequently it differs only in degree from what occurs in every
invasion where more than a single individual is concerned.
Accordingly, this type of association has little more than
historical interest. This must not be construed to mean that it
does not occur, but that it differs in no essential from the
ordinary grouping of invaders.
=257. Light association.= The constituent species of formations show two
fundamentally different groupings with respect to light. In the one
case, the individuals are on the same level, or nearly so, in such a way
that each has direct access to sunlight. Such an arrangement is
characteristic of most grassland and herbaceous formations. In the case
of desert formations, there is often considerable difference in the
height of the plants, but the distance between them is so great as to
admit of direct illumination of all. This arrangement may be termed
coordinate association. In forests, thickets, and many herbaceous
wastes, the height and density of certain species enable them to
dominate the formation. In a dense forest, the trees receive practically
all the light incident upon the formation, and the shrubs, herbs, fungi,
and algae of lower habit and inferior position must adapt themselves to
the diffuse light which passes through or between the leaves. The same
is equally true of dense thickets and wastes, except that the vertical
distance is less, and the diffuseness of the light is correspondingly
modified. In these formations, the dominant trees, shrubs, or herbs, the
facies, constitute a primary or superior layer. The degree of
subordinate association, as a result of which inferior layers will
arise, is entirely determined by the density of the facies. In open
woodlands, which are really mixed formations of woodland and grassland,
the intervals, and usually the spaces beneath the trees also, are
covered with poophytes, showing an absence of subordination due to
light. This is the prevailing condition in the pine formation (_Pinus
ponderosa-xerohylium_) of the ridges and foot-hills of western Nebraska.
When, however, the trees stand sufficiently close that their shadows
meet or overlap throughout the day, the increasing diffuseness begins to
cause modification and rearrangement of the individuals. By photometric
methods, the light in a forest is found to be least diffuse just below
the facies, while the diffuseness increases markedly in passing to the
ground. The taller, stronger individuals are consequently in a position
to assimilate more vigorously, and to become still taller and stronger
as a result. Just as these have taken up a position inferior to that of
the facies, so the shorter or weaker species must come to occupy a still
more subordinate position. This results, not only because the light is
primarily weaker nearer the ground, but also because the taller plants
interpose as a second screen. The complete working out of this
arrangement with reference to light produces typical subordinate
association, which finds its characteristic expression in the layering
of forests and thickets. Layers tend to appear as soon as open woodland
or thicket begins to pass into denser conditions, and up to a certain
point, at which they disappear, they become the more numerous and the
more marked, the denser the forest.
In the Otowanie woods near Lincoln (_Quercus-Hicoria-hylium_), layering
usually begins at a light value of .1 (1 = normal sunshine in the open).
Thornber[28] has found the same value to obtain in the thickets of the
Missouri bluffs. In these, again, layers disappear at a value of .005,
the extreme diffuseness making assimilation impossible except for
occasional mosses and algae. A number of herbaceous plants are present
in the spring, but these are all prevernal or vernal bloomers, which are
safely past flowering before shade conditions become extreme. In the
_Fraxinus-Catalpa-alsium_, all inferior holophytic vegetation disappears
between the light value of .004 and that of .003. The spruce-pine
formation (_Picea-Pinus-hylium_) of the Rocky mountains, with a light
value of .01, usually contains but a few scattered herbs, mostly
evergreen; in some cases there are no subordinate plants other than
mosses and hysterophytes. The lodge-pole pine formation (_Pinus
murrayana-hylium_), with light values often less than .005, is nearly or
quite destitute of all but hysterophytic undergrowth. Such extremely
dense formations are examples of coordinate association merely, since
the formation is reduced to a single superior layer, in which the
individuals of the facies bear the same spatial relation to incident
light. In layered formations, in addition to the subordinate relation of
other species to the facies, there is, of course, a kind of coordinate
association manifested in each layer.
=258. Water-content association.= Schouw[29] was the first to give
definite expression to the value of the water-content of the soil for
the grouping of plants. He established four groups: (1) water plants,
(2) swamp plants, (3) plants of moist meadows, (4) plants of dry soils.
The first he termed hydrophytes, introducing the term halophytes to
include all saline plants. Thurmann[30] recognized the fundamental
influence of water-content upon association, and further perceived that
the amount of water present was determined primarily by the physical
nature of the soil. He distinguished plants which grow in soils that
retain water as _hygrophilous_, and those found upon soils that lose
water readily as _xerophilous_. Those which seemed to grow indifferently
upon either were termed _ubiquitous_. The latter correspond in some
measure to mesophytes, but they are really plants possessing a
considerable range of adaptability, and do not properly constitute a
natural group. Warming[31] proposed the term mesophytes to include all
the plants intermediate between hydrophytes and xerophytes. He
recognized the paramount value of water-content association as the basis
of ecology, and upon this made a logical and systematic treatise out of
the scattered results of many workers. Schimper[32] placed the study of
vegetation upon a new basis by drawing a distinction between physical
and physiological water-content, and by pointing out that the last alone
is to be taken into account in the study of plant life, and hence of
plant geography. Accepting the easily demonstrable fact that an excess
of salts in the soil water, as well as cold, tends greatly to diminish
the available water of the soil, i. e., the chresard, it is at once seen
why saline and arctic plants are as truly xerophytic as those that grow
on rocks or in desert sands. An anomalous case which, however, physical
factor records have explained fully, is presented by many plants growing
in alpine gravel slides, strands, blowouts, sandbars, etc., in which the
water-content is considerable, but the water loss excessive, on account
of extreme heat or reduced air pressure. The effect of these conditions
is to produce a plant xerophytic as to its aerial parts, and mesophytic
or even hydrophytic as to subterranean parts. Such plants may, from
their twofold nature, be termed _dissophytes_; they are especially
characteristic of dysgeogenous soils in alpine regions where
transpiration reaches a maximum, but are doubtless to be found in all
gravel and sand habitats with high water-content. With these
corrections, the concept of water-content association, which owes much
to both Warming and Schimper, but is largely to be credited to Thurmann,
becomes completely and fundamentally applicable to all vegetation.