DATE │ TIME EQUATION │LINCOLN NOON
─────────┬──┼───────────────┼──────┬──────
│ │ _Sun slow_: +│ 26m.│ 49s.
January │ 1│ 3m.│ 47s.│ 12:31│ P.M.
„ │ 6│ 6│ 7│ :33│
„ │11│ 8│ 12│ :35│
„ │16│ 10│ 3│ :37│
„ │21│ 11│ 35│ :38│
„ │26│ 12│ 48│ :40│
„ │31│ 13│ 41│ :40│
February │10│ 14│ 27│ :41│
„ │20│ 13│ 56│ :41│
March │ 2│ 12│ 18│ :39│
„ │ 7│ 11│ 10│ :38│
„ │12│ 9│ 53│ :37│
„ │17│ 8│ 29│ :36│
„ │22│ 6│ 59│ :34│
„ │27│ 5│ 27│ :32│
April │ 1│ 3│ 55│ :31│
„ │ 6│ 2│ 27│ :29│
„ │11│ 1│ 3│ :28│
│ │ _Sun fast_: −│ │
„ │16│ 0│ 13│ :27│
„ │21│ 1│ 20│ :25│
„ │26│ 2│ 16│ :24│
May │ 1│ 3│ 0│ :24│
„ │16│ 3│ 48│ :23│
„ │31│ 2│ 33│ :24│
June │ 5│ 1│ 45│ :25│
„ │10│ 0│ 49│ :26│
│ │ _Sun slow_: +│ │
„ │15│ 0│ 13│ :27│
„ │20│ 1│ 18│ :28│
„ │25│ 2│ 22│ :29│
„ │30│ 3│ 22│ :30│
│ │ _Sun slow_: +│ 26m.│ 49s.
July │ 5│ 4m.│ 19s.│ 12:31│ P.M.
„ │10│ 5│ 7│ :32│
„ │20│ 6│ 6│ :33│
August │ 4│ 5│ 53│ :33│
„ │14│ 4│ 30│ :31│
„ │19│ 3│ 28│ :30│
„ │24│ 2│ 13│ :29│
„ │29│ 0│ 48│ :28│
│ │ _Sun fast_: −│ │
September│ 3│ 0│ 45│ :26│
„ │ 8│ 2│ 25│ :24│
„ │13│ 4│ 9│ :23│
„ │18│ 5│ 55│ :21│
„ │23│ 7│ 41│ :19│
„ │28│ 9│ 23│ :17│
October │ 3│ 10│ 59│ :16│
„ │ 8│ 12│ 26│ :14│
„ │13│ 13│ 43│ :13│
„ │18│ 14│ 48│ :12│
„ │23│ 15│ 37│ :11│
November │ 2│ 16│ 20│ :10│
„ │12│ 15│ 45│ :11│
„ │17│ 14│ 54│ :12│
„ │22│ 13│ 44│ :13│
„ │27│ 12│ 14│ :15│
December │ 2│ 10│ 25│ :16│
„ │ 7│ 8│ 21│ :18│
„ │12│ 6│ 5│ :21│
„ │17│ 3│ 41│ :23│
„ │22│ 1│ 12│ :26│
│ │ _Sun slow_: +│ │
„ │27│ 1│ 17│ :28│
─────────┴──┴───────┴───────┴──────┴──────
=87. Place.= The effect of latitude upon the sun’s altitude, and the
consequent light intensity have been discussed in the pages which
precede. Latitude has also a profound influence upon the duration of
daylight, but the importance of the latter apart from intensity is not
altogether clear. The variation of intensity due to altitude has been
greatly overestimated; it is practically certain, for example, that the
dwarf habit of alpine plants is not to be ascribed to intense
illumination, since the latter increases but slightly with the altitude.
It has been demonstrated astronomically that about 20 per cent of a
vertical ray of sunlight is absorbed by the atmosphere by the time it
reaches sea level. At the summit of Pike’s Peak, which is 14,000 feet
(4,267 meters) high, the barometric pressure is 17 inches, and the
absorption is approximately 11 per cent. In other words, the light at
sea level is 80 per cent of that which enters the earth’s atmosphere; on
the summit of Pike’s Peak it is 89 per cent. As the effect of the sun’s
altitude is the same in both places, the table of curves on page 57 will
apply to both. Taking into account the difference in absorption, the
maximum intensity at sea level and at 14,000 feet on the fortieth
parallel is .98 and 1.09 respectively. The minimum intensities between
8:00 A.M. and 4:00 P.M. of the growing period are .64 and .71
respectively. The correctness of these figures has been demonstrated by
photometer readings, which have given almost exactly the same results.
Such slight variations are quite insufficient to produce an appreciable
adjustment, particularly in structure. They are far within the efficient
difference, and Reinke[5] has found, moreover, that photosynthetic
activity in _Elodea_ is not increased beyond the normal in sunlight
sixty times concentrated. In consequence, it is entirely unnecessary to
take account of different altitudes in obtaining light values.
The slope of a habitat exerts a considerable effect upon the intensity
of the incident light. If the angle between the slope and the sun’s ray
be 90°, a square meter of surface will receive the maximum intensity, 1.
At an angle of 10°, the same area receives but .17 of the light. This
relation between angle and intensity is shown in the table which
follows. The influence of the light, however, is felt by the leaf, not
by the slope. Since there is no connection between the position of the
leaf and the slope of the habitat, the latter may be ignored. In
consequence, it is unnecessary to make allowances for the direction of a
slope, viz., whether north, east, south, or west, in so far as light
values are concerned. The angle which a leaf makes with its stem
determines the angle of incidence, and hence the amount of light
received by the leaf surface. This is relatively unimportant for two
reasons. This angle changes hourly and daily with the altitude of the
sun, and the intensity constantly swings from one extreme to the other.
Moreover, the extremes 1.00 and 0.17, even if constant, are hardly
sufficient to produce a measurable result. When the angle of the leaf
approaches 90°, there is the well-known differentiation of leaf surfaces
and of chlorenchym, but this has no relation to the angle of incidence.
_Table of Intensity at Various Angles_
ANGLE INTENSITY
90 1.00
80 .98
70 .94
60 .87
50 .77
40 .64
30 .50
20 .34
10 .17
In the sunlight, it makes no difference at what height a light reading
is taken. In forest and thicket as well as in some herbaceous
formations, the intensity of the light, if there is any difference, is
greatest just beneath the foliage of the facies. In forests especially,
the light is increasingly diffuse toward the ground, particularly where
layers intervene. In woodland formations, moreover, the exact spot in
which a reading is made must be carefully chosen, unless the foliage is
so dense that the shade is uniform. A very satisfactory plan is to take
readings in two or more spots where the shade appears to be typical, and
to make a check reading in a “sunfleck,” a spot where sunlight shows
through. In forests and thickets, the sunflecks are fleeting, and the
light value is practically that of the shade. In passing into open
woodland and thicket, the sunflecks increase in size and permanence,
until finally they exceed the shade areas in amount and become typical
of the formation.
_Reflected and Absorbed Light_
=88. The fate of incident light.= The light present in a habitat and
incident upon a leaf is not all available for photosynthesis. Part is
reflected or screened out by the epidermis, and a certain amount passes
through the chlorenchym, except in very thick leaves. The light absorbed
is by far the greatest in the majority of species. Many plants with
dense coatings of hairs reflect or withhold more light than they absorb,
and the amount of light reflected by a thick cuticule is likewise great.
As light is imponderable, the actual amount absorbed or reflected by the
leaf can not be determined. It is possible, however, to express this in
terms of the total amount received, by means of readings with solio
paper, and the knowledge thus obtained is of great importance in
interpreting the modifications of certain types of leaves. For example,
a leaf with a densely hairy epidermis may receive light of the full
intensity, 1; the amount reflected or screened out by the hairs may be
95 per cent of this, the amount absorbed 5 per cent, and that
transmitted, nil. In the majority of cases, however, the absorbed light
is considerably more than the amount reflected or transmitted.
[Illustration: Fig. 14. Leaf print: exposed 10 m., 11 A.M. August 20.
The leaves are from sun and shade forms of _Bursa bursa-pastoris_, _Rosa
sayii_, _Thalictrum sparsiflorum_, and _Machaeranthera aspera_. In each
the shade leaf prints more deeply.]
=89. Methods of determination.= If results are to be of value, reflected
and transmitted light must be determined in the habitat of the plant
simultaneously with the total light which a leaf receives. An
approximation of the light reflected from a leaf surface is secured by
placing the photometer so that the light reflected is thrown upon the
solio strip. A much more satisfactory method, however, is to determine
it in connection with the amount of light transmitted through the
epidermis. This is done by stripping a piece of epidermis from the upper
surface of the leaf and placing it over the slit in the photometer for
an exposure. An exposure in the full light of the habitat is made
simultaneously with another photometer, or immediately afterward upon
the same strip. When the epidermis is not too dense, both exposures are
permitted to reach the same tint, and the relation between them is
precisely that of their lengths of exposure. Ordinarily the two
exposures are made absolutely simultaneous by placing the epidermis over
half of the opening, leaving the other half to record the full light
value, and the results, or _epidermis prints_, are referred to a
multiple standard. The difference between the two values thus obtained
represents the amount of reflected light together with that screened by
the epidermis. The amount of light transmitted through the leaf may be
measured in the same way by using the leaf itself in place of the
epidermis alone. The time of exposure is necessarily long, however, and
it has been found practicable to obtain leaf prints by exposing the leaf
in a printing frame, upon solio paper, at the same time that the
epidermis print is made. In a few species both the upper and lower
epidermis can be removed and the amount of light absorbed determined
directly by exposing the strip covered with the chlorenchym. Generally,
however, this must be computed by subtracting the sum of the per cents
of reflected and transmitted light from 100 per cent, which represents
the total light.
[Illustration: Fig. 15. Leaf print: exposure as before. Sun and shade
leaves of _Achillea lanulosa_, _Capnoides aureum_, _Antennaria
umbrinella_, _Galium boreale_, and _Potentilla propinqua_.]
=90. Leaf and epidermis prints.= In diphotic leaves the screening effect
of the lower epidermis may be ignored. Isophotic sun leaves, i. e.,
those nearly upright in position or found above light-colored,
reflecting soils, are usually strongly illuminated on both sides, and
the absorbed light can be obtained only by measuring the screening
effect of both epiderms. Shade leaves and submerged leaves often contain
chloroplasts in the epidermis, and the above method can not be applied
to them. In fact, in habitats where the light is quite diffuse,
practically all incident light is absorbed. The rare exceptions are
those shade leaves with a distinct bloom. In addition to their use in
obtaining the amount of light absorbed, both leaf and epidermis prints
are extremely interesting for the direct comparison of light relations
in the leaves of species belonging to different habitats. The relative
screening value of the upper and lower epidermis, or of the
corresponding epiderms of two ecads or two species, is readily
ascertained by exposing the two side by side in sunshine, over the slit
in the photometer. For leaf prints fresh leaves are desirable, though
nearly the same results can be obtained from leaves dried under
pressure. The leaves are grouped as desired on the glass of a printing
frame, and covered with a sheet of solio. They are then exposed to full
sunlight, preferably at meridian, and the prints evaluated by means of
the multiple standard. This method is especially useful in the
comparison of ecads of one species. These differences due to transmitted
light are very graphic, and can easily be preserved by “toning” the
print in the usual way.
_Expression of Results_
=91. Light records.= The actual photographic records obtained by
photometer and selagraph can at most be kept but a few months, unless
they are toned or fixed. “Toning” modifies the color of the exposure
materially, and changes its intensity so that it can not be compared
with readings not fixed. It would involve a great deal of inconvenience
to make all comparisons by means of toned strips and standard, even if
it were not for the fact that it is practically impossible to obtain
exactly the same shade in lots toned at different times. The field
record, if carefully and neatly made, may well take the place of a
permanent one. The form is the following:
═══════╤══════════╤═════════╤═══════╤════════╤════════╤══════════╤═══════
Day │ Hour │Formation│Station│Altitude│Exposure│ Group │Height
│ │ │ │ │ │ │
│ │ │ │ │ │ │
───────┼──────────┼─────────┼───────┼────────┼────────┼──────────┼───────
14/9/04│ 12:00 M. │ Spruce │ Milky │2600 m. │N.E. 20°│Opulaster │1 foot
│ │ │ Way │ │ │ │
„ │12:05 P.M.│ Spruce │ Moss │2500 m. │ Level │Streptopus│ „
│ │ │ Glen │ │ │ │
„ │12:15 P.M.│ Brook │Grotto │2500 m. │ E. 3° │ Filix │Surface
│ │ b’nk │ │ │ │ │
───────┴──────────┴─────────┴───────┴────────┴────────┴──────────┴───────
═══════╤════╤════════╤════════╤═════╤═════╤═════════╤════════╤════════
Day │No. │ Length │Standard│Light│Base │Reflected│Transm’d│Absorbed
│ │ of │ │value│value│ light │ light │ light
│ │exposure│ │ │ │ │ │
───────┼────┼────────┼────────┼─────┼─────┼─────────┼────────┼────────
14/9/04│2:10│ 160 s. │ 3 s │.019 │ │ │ │
│ │ │ │ │ │ │ │
„ │2:12│ 240 s. │ 3 s. │.012 │ │ │ │
│ │ │ │ │ │ │ │
„ │2:13│ 360 s. │ 3 s. │.008 │ │ │ │
│ │ │ │ │ │ │ │
───────┴────┴────────┴────────┴─────┴─────┴─────────┴────────┴────────
=92. Light sums, means, and curves.= Owing to the fact that the
selagraph has not yet been used in the field, no endeavor has been made
to determine the light value for every hour of the day in different
habitats. Consequently there has been no attempt to compute light sums
and means. Photometer readings have sufficed to interpret the effect of
light in the structure of the formation, and of the individual, but they
have not been sufficiently frequent for use in ascertaining sums and
means. The latter are much less valuable than the extremes, especially
when the relative duration of these is indicated. Means, however, are
readily obtained from the continuous records. Light sums are probably
impracticable, as the factor is not one that can be expressed in
absolute terms. The various kinds and combinations of light curves are
essentially the same as for humidity. The level curve through a series
of habitats is the most illuminating, but the day curve of hour
variations is of considerable value. The curve of daily duration, based
upon full sunlight, is also of especial importance for plants, and
stations which receive both sun and shade during the day.
TEMPERATURE
=93.= In consequence of its indirect action, temperature does not have a
striking effect upon the form and structure of the plant, as is the case
with water and light. Notwithstanding, it is a factor of fundamental
importance. This is especially evident in the character and distribution
of vegetation. It is also seen in the germination and growth of plants,
in the length of season, and in the important influence of temperature
upon humidity, and hence upon water-content. Because of its intimate
relation with the comfort of mankind, the determination of temperature
values has received more attention than that of any other factor, and
excellent simple and recording instruments are numerous. For plants, it
is also necessary to employ instruments for measuring soil temperatures.
The latter unquestionably have much less meaning for the plant than the
temperatures of the air, but they have a direct influence upon the
imbibition of water, and upon germination.
_Thermometers_
=94. Air thermometers.= The accurate measurement of temperature requires
standard thermometers. Reasonably accurate instruments may be
standardized by determining their error, but they are extremely
unsatisfactory in practice, since they result in a serious waste of
time. Accurate thermometers which read to the degree are entirely
serviceable as a rule, but instruments which read to a fraction of a
degree are often very much to be desired. The writer has found the
“cylindrical bulb thermometer, Centigrade scale” of H. J. Green, to be
an exceedingly satisfactory instrument. The best numbers for general use
are 247 and 251, which read from –15° to 50° C. and are graduated
in .2°. They are respectively 9 and 12 inches long, and cost $2.75 and
$3.50. These instruments are delicate and require careful handling, but
even in class work this has proved to be an advantage rather than
otherwise. In making readings of air temperatures with such
thermometers, constant precautions must be taken to expose the bulb
directly to the wind and to keep it away from the hand and person.
=95. Soil thermometers.= The thermometer described above has been used
extensively for soil temperatures. The determination of the latter is
conveniently combined with the taking of soil samples, by using the hole
for a temperature reading. When carefully covered, these holes can be
used from day to day throughout the season without appreciable error,
even in gravel soils. Repeated tests of this have been made by
simultaneous readings in permanent and newly made holes, and the results
have always been the same. It has even been found that the error is
usually less than 1 degree when the hole is left uncovered, if it is
more than 9 inches deep. A slight source of error lies in the fact that
the thermometer must be raised to make the reading. With a little
practice, however, the top of the column of mercury may be raised to the
surface and read before the change of temperature can react upon it.
This is especially important in very moist or wet soils where the bulb
becomes coated with a film of moisture. This evaporates when the bulb is
brought into the air, and after a moment or two the mercury slowly
falls.
[Illustration: Fig. 16. Soil thermometer]
Regular soil thermometers are indispensable when readings are desired at
depths greater than 12–18 inches. They possess several disadvantages
which restrict their use almost wholly to permanent stations. It is
scarcely possible to carry them on field trips, and the time required to
place them in the soil renders them practically useless for single
readings. Moreover, the instruments are expensive, ranging in price from
$7 for the two-foot thermometer, to $19 for the eight-foot one. When it
is recognized that deep-seated temperatures are extremely constant and
that the slight fluctuations affect, as a rule, only the relatively
stable shrubs and trees, it is evident that such temperatures are of
restricted importance. Still, in any habitat, they must be ascertained
before they can well be ignored, though it is unwise to spend much time
and energy in their determination. Soil thermometers of the form
illustrated may be obtained from H. J. Green, Brooklyn.
=96. Maximum-minimum thermometers.= These are used for determining the
range of temperature within a given period, usually a day. Since they
are much cheaper than thermographs, they can replace these in part,
although they merely indicate the maximum and minimum temperatures for
the day, and do not register the time when each occurs. The maximum is a
mercurial thermometer with a constriction in the tube just above the
bulb; this allows the mercury to pass out as it expands, but prevents it
from running back, thus registering the maximum temperature. The minimum
thermometer contains alcohol. The column carries a tiny dumbbell-shaped
marker which moves down with it, but will not rise as the liquid
expands. This is due to the fact that the fluid expands too slowly to
carry the marker upward, while the surface tension causes it to be drawn
downward as the fluid contracts. The minimum temperature is indicated by
the upper end of the marker. In setting up the thermometers, they are
attached by special thumbscrews to a support which holds them in an
oblique position. The minimum is placed in a special holder above the
maximum which rests on a pin that is used also for screwing the
pivot-screw into position. The support is screwed tightly to the
cross-piece of a post, or in forest formations it is fastened directly
to a board nailed upon a tree trunk. A shelter has not been used in
ecological work, although it is the rule in meteorological observations.
The minimum thermometer is set for registering by raising the free end,
so that the marker runs to the end of the column. The mercury of the
maximum is driven back into the bulb by whirling it rapidly on the
pivot-screw after the pin has been taken out. This must be done with
care in order that the bulb may not be broken. As soon as the instrument
comes to rest, it is raised and the pin replaced, great care being taken
to lift it no higher than is necessary. When the night maximum is
sought, the thermometer should be whirled several times in order to
drive the column sufficiently low. Usually, in such cases, a record is
made of this point to make sure that the maximum read is the actual one.
If the pivot-screw is kept well oiled, less force will be required to
drive the mercury back. In practice, the thermometers have been observed
at 6:00 A.M. and 6:00 P.M. each day, thus permitting the reading of the
maximum-minimum for both day and night. Pairs of maximum-minimum
thermometers are to be obtained from H. J. Green, 1191 Bedford Ave.,
Brooklyn, or Julien P. Friez, Baltimore, Maryland, at a cost of $8.25.
[Illustration: Fig. 17. Maximum-minimum thermometer.]
[Illustration: Fig. 18. Terrestrial radiation thermometer.]
[Illustration: Fig. 19. Draper thermograph.]
=97. Radiation thermometers.= These are used to determine the radiation
in the air, and from the soil, i. e., for solar and terrestrial
radiation. The latter alone has been employed in the study of habitats,
chiefly for the purpose of ascertaining the difference in the cooling of
different soils at night. The terrestrial radiation thermometer is
merely a special form of minimum thermometer, so arranged in a support
that the bulb can be placed directly above the soil or plant to be
studied. It is otherwise operated exactly like the minimum thermometer,
and the reading gives the minimum temperature which the air above the
plant or soil reaches, _not_ the amount of radiation. As a consequence,
these instruments are valuable only where read in connection with a pair
of maximum-minimum thermometers in the air, or when read in a series of
instruments placed above different soils or plants.
=98. Thermographs.= Two types of standard instruments are in general use
for obtaining continuous records of air temperatures. These are the
Draper thermograph, made by the Draper Manufacturing Company, 152 Front
St., New York city ($25 and $30), and the Richard thermograph sold by
Julien P. Friez, Baltimore ($50). After careful trial had demonstrated
that they were equally accurate, the matter of cost was considered
decisive, and the Draper thermograph has been used exclusively in the
writer’s own work. This instrument closely resembles the psychrograph
manufactured by the same company. It is made in two sizes, of which the
larger one is the more satisfactory on account of the greater distance
between the lines of the recording disk. The thermometric part consists
of two bimetallic strips, the contraction and expansion of which are
communicated to a hand carrying a pen. The latter traces a line on the
record sheet which is attached to a metal disk made to revolve by an
eight-day clock. In practice the thermograph is set up in the shelter
which contains the psychrograph, and in exactly the same manner. The
clock is wound, the record put in place, and the pen inked in the same
way also. The proper position of the pen is determined by making a
careful thermometer reading under the shelter, and then regulating the
pen hand by means of the screws at the base of it. A similar test
reading is also made each week, when the clock is rewound. A record
sheet may be left in position for three weeks, the pen being filled each
week with a different ink. The fixed order of using the inks is red,
blue, and green as already indicated.
[Illustration: Fig. 20. Shelter for thermograph.]
Owing to the fact that they are practically stationary, soil
thermographs are of slight value, except at base stations. Here, the
facts that they are expensive, that the soil temperatures are of
relatively little importance, and that they can be determined as easily,
or nearly so, by simple thermometers, make the use of such instruments
altogether unnecessary, if not, indeed, undesirable. In a perfectly
equipped research station, they undoubtedly have their use, but at
ordinary stations, and in the case of private investigators, their value
is in no wise commensurate with their cost.
_Readings_
=99. Time.= The hourly and daily fluctuations of the temperature of the
air render frequent readings desirable. It is this variation, indeed,
which makes single readings, or even series of them, inconclusive, and
renders the use of a recording instrument almost imperative in the base
station at least. Undoubtedly, a set of simultaneous readings at
different heights in one station, or at the same height in different
stations, especially if made at the maximum, have much value for
comparison, but their full significance is seen only when they are
referred to a continuous base record. Such series, moreover, furnish
good results for purposes of instruction. In research work, however, it
has been found imperative to have thermographs in habitats of widely
different character. With these as bases, it is possible to eke them out
with considerable satisfaction by means of maximum-minimum thermometers
in less different habitats, or in different parts of the same habitat.
Naturally these are less satisfactory, and are used only when expense
sets a limit to the number of thermographs. In a careful analysis of a
single habitat, more can be gained by one base thermograph supplemented
by three pairs of maximum-minimum thermometers in dissimilar areas of
the habitat than by two thermographs, and the cost is the same.
[Illustration: Fig. 21. Richard thermograph.]
=100. Place and height.= For general air temperatures, thermograph and
thermometer readings are made at a height of 3 feet (1 meter). Soil
temperatures are regularly taken at the surface and at a depth of 1
foot. When a complete series of simultaneous readings is made in one
station, the levels are 6 feet and 3 feet in the air, the surface of the
soil, and 5, 10, and 15 inches in the soil. When sun and shade occur
side by side in the same formation, as is true of many thickets and
forests, surface readings are regularly made in both. Similarly,
valuable results are obtained by making simultaneous readings on the
bare soil, on dead cover, and upon a leaf, while the influence of cover
is readily ascertained by readings upon it and beneath it. A full series
of station readings made at the same time upon north, east, south, and
west slopes is of great importance in studying the effects of exposure.
_Expression of Results_
=101. Temperature records.= Neither field nor permanent form is required
for thermographic records, other than the record sheet itself, which
contains all the necessary information in a fairly convenient form.
Although the temperature of a particular hour and day can not be read at
a mere glance, it can be obtained so easily that it is a waste of time
to make a tabular copy of each record sheet. For thermometer readings,
either single or in series, the following form is used:
═══════╤═════════╤═════════╤════════╤════════╤════════╤═══════════
Day │ Hour │Formation│Station │Altitude│Exposure│ Community
───────┼─────────┼─────────┼────────┼────────┼────────┼───────────
„ │ „ │ „ │ „ │ „ │ „ │ „
───────┼─────────┼─────────┼────────┼────────┼────────┼───────────
17/8/04│6:30 A.M.│ Spruce │ Jack │2550 m. │N.E. 5° │Mertensiare
│ │ │ Brook │ │ │
„ │ „ │ Half │Hiawatha│2550 m. │N.E. 7° │ Asterare
│ │ gravel │ │ │ │
„ │6:30 P.M.│ Spruce │ Jack │2550 m. │N.E. 5° │Mertensiare
│ │ │ Brook │ │ │
„ │ „ │ Half │Hiawatha│2550 m. │N.E. 7° │ Asterare
│ │ gravel │ │ │ │
───────┴─────────┴─────────┴────────┴────────┴────────┴───────────
═══════╤═════════╤═══════════════════╤═══════════╤══════╤════
Day │ Hour │POSITION OF READING│Thermograph│ Sky │Wind
───────┼─────────┼──────┬─────┬──────┼───────────┼──────┼────
„ │ „ │3 feet│Surf.│12 in.│ „ │ „ │ „
───────┼─────────┼──────┼─────┼──────┼───────────┼──────┼────
17/8/04│6:30 A.M.│ 9° │ 9° │ 9.8° │ 10° │Clear │ 0
│ │ │ │ │ │ │
„ │ „ │11.2° │11.2°│14.8° │ 10° │Clear │ 0
│ │ │ │ │ │ │
„ │6:30 P.M.│11.4° │11.4°│ 9.8° │ 11° │Cloudy│ 0
│ │ │ │ │ │ │
„ │ „ │ 12° │13.8°│16.4° │ 11° │Cloudy│ 0
│ │ │ │ │ │ │
───────┴─────────┴──────┴─────┴──────┴───────────┴──────┴────
=102. Temperature sums and means.= The amount of heat, i. e., the number
of calories received within a given time by a definite area of plant
surface, can be determined by means of a calorimeter. From this the
temperature sum of a particular period may be obtained by simple
addition. In the present condition of our knowledge, it is impossible to
establish any exact connection between such results and the functional
or growth effect that can be traced directly to heat. As a consequence,
temperature sums do not at present contribute anything of value to an
understanding of the relation between cause and effect. The mean daily
temperature is readily obtained by averaging twenty-four
hour-temperatures recorded by the thermograph. The method employed by
Meyen[6], of deriving the mean directly from the maximum and minimum for
the day, is not accurate; from a large number of computations, the error
is always more than two degrees. On the other hand, the mean obtained by
averaging the maximum and minimum for the day and night has been found
to deviate less than 1 degree from the mean proper. This fact greatly
increases the value of maximum-minimum instruments if they are read
daily at 6:00 A.M. and 6:00 P.M.
=103. Temperature curves.= The kinds and combinations of temperature
curves are almost without number. The simple curves of most interest are
those for a series of stations or habitats, based upon the level of
three feet, or the surface, or the daily mean. The curves for each
station representing the different heights and depths and the season
curve of the daily means for a habitat are also of much importance. One
of the most illuminating combinations is that which groups together the
various level curves for a series of habitats. Other valuable
combinations are obtained by grouping the curves of daily means of
different habitats for the season, or the various station curves.
=104. Plant temperatures.= The direct effects of temperature as seen in
nutrition and growth can be ascertained only by determining the
temperature of plant tissues. The temperatures of the air and of the
soil surface have an important effect upon humidity, and water-content,
and through them upon the plant, but heat can influence assimilation,
for example, only in so far as it is absorbed by the assimilating
tissue. The temperatures of the leaf, as the most active nutritive organ
of the plant, are especially important. While it is a well-known fact
that internal temperatures follow those of the air and soil closely,
though with varying rapidity of response, this holds less for leaves
than for stems and roots. Owing to the very obvious difficulties,
practically nothing has yet been done in this important field. A few
preliminary results have been obtained at Minnehaha, which serve to show
the need for such readings. Gravel slide rosettes in an air temperature
of 24° C. and a surface temperature of 40° C. gave the following surface
readings: _Parmelia_, 40°, _Eriogonum_, 38.6°, _Arctostaphylus_, 35°,
_Thlaspi_, 31.8°, and _Senecio_, 31°. The leaf of _Eriogonum flavum_,
which is smooth above and densely hairy below, indicated a temperature
of 31.8° when rolled closely about the thermometer bulb with the smooth
surface out, and 28° when the hairy surface was outside. The surface
readings of the same leaf were .5°–1° higher when made upon the upper
smooth surface. This immediately suggests that the lower surface may be
modified to protect the leaf from the great heat of the gravel, which
often reaches 50° C. (122° F.).
PRECIPITATION
=105. General relations.= As the factor which exerts the most important
control upon water-content and humidity, rainfall must be carefully
considered by the ecologist. It is such an obvious factor, and is
usually spoken of in such general terms that the need of following it
accurately is not evident at once. When it is recognized that the
fluctuations of water-content are directly traceable to it, it becomes
clear that its determination is as important as that of any indirect
factor. This does not mean, however, that the amount of yearly rainfall
is to be taken from the records of the nearest weather station, and the
factor dismissed. Like other instruments, the rain gauge must be kept at
the base station of the area under study, and when this is extensive or
diverse, additional instruments should be put into commission. While the
different parts of the same general climatic region may receive
practically the same amount of precipitation during the year, it is not
necessarily true that the rainfall of any particular storm is equally
distributed, especially in the mountains. Nothing less than an exact
knowledge of the amount of rain that falls in the different areas will
make it possible to tell how much of the water-content found at any
particular time in these represents merely the chance differences of
precipitation.
The forms of precipitation are rain, dew, hail, snow, and frost. Of
these, hail is too infrequent to be taken into account, while frost
usually occurs only at the extremes of the growing season, and in its
effect is rather to be reckoned with temperature. Snow rarely falls
except during the period of rest, and, while it plays an important part
as cover, it is merely one of several factors that determine the
water-content of the soil at the beginning of spring. The influence of
dew is not clearly understood. It is almost always too slight in amount
and too fleeting to affect the water-content of the soil. It seems
probable that it may serve by its own evaporation to decrease in some
degree the water loss from the soil, and from bedewed plants. If,
however, the dew is largely formed by the water of the soil and of the
plant, as is thought by some, then it is negligible as a reinforcement
of water-content. From the above, it is evident that rainfall alone
exerts a profound effect upon the habitat, and it is with its
measurement that the ecologist is chiefly concerned.
[Illustration: Fig. 22. Rain gauge showing construction.]
=106. The rain gauge=, as the illustration shows, is a cylindrical
vessel with a funnel-shaped receiver at the top, which is 8 inches in
diameter. The receiver fits closely upon a narrower brass vessel or
measuring tube in which the rain collects. The ratio of surface between
receiver and tube is 10 to 1. For readings covering a general area, the
rain-gauge is placed in the open, away from buildings or other
obstructions, and is sunken in the ground sufficiently to keep it
upright. In localities where winds are strong, it is usually braced at
the sides also or supported by a wooden frame. In measuring the amount
of rain in the measuring tube, the depth is divided by ten in order to
ascertain the actual rainfall. The depth is measured by inserting the
measuring-rod through the hole in the funnel until it touches the
bottom. It is left for a second or so, quickly withdrawn, and the limit
of the wetted portion noted. In the case of standard rods, the actual
rainfall is read directly in hundredths, so that the division by ten is
unnecessary. After each reading, the measuring tube is carefully
drained, replaced, and the receiver put in position. No regular time for
making readings is necessary. During a rainy period, it is customary to
make a measurement each day, but it has been found more satisfactory for
ecological purposes to measure each shower, and to record its duration.
These two facts furnish a ready clue to the relative amount of run-off
in each fall of rain. The measurement of snowfall is often made merely
by determining its depth. For comparison with rainfall, the rain gauge
with receiver and tube withdrawn is used. The snow which falls is
melted, poured into the measuring tube, and measured in the ordinary
way. The U. S. Weather Bureau standard rain gauge, with measuring stick,
may be obtained of H. J. Green, or of J. P. Friez for $5.25.
=107. Precipitation records.= From the periodic character of
precipitation, rainfall sums, means, and curves have little importance
in the careful study of the habitat. The rainfall curve for the growing
season is an aid in explaining the curve of water-content, and the mean
rainfall of a region gives some idea of its vegetation, though even here
the matter of its distribution is of primary importance. The rain and
snow charts published by the U. S. Weather Bureau furnish data of some
importance for the general study of vegetation, but it is evident that
they can play little part in a system which is founded upon the habitat.
Precipitation records, for reasons of brevity and convenience, are
united with wind records, and the form will be found under the
discussion of this factor.
WIND
[Illustration: Fig. 23. Simple anemometer.]
=108. Value of readings.= On account of its direct effect upon humidity,
and its consequent influence upon water-content, the part which wind
plays in a habitat can not be ignored in a thorough investigation. It is
an important element in exposure, and accordingly has a marked
mechanical effect upon the vegetation of exposed habitats, alpine
slopes, seacoasts, plains, etc. Owing to its inconstancy and its extreme
variation in velocity, single wind readings are absolutely without
value. When read in series, anemometers give some information upon the
comparative air movement in different habitats, but the chance of error
is great, except when the breeze is steady. Anemographs alone give real
satisfaction. Accurate results, however, are not obtainable without a
series of two or more in different habitats, and it is still an open
question whether the results obtained justify the expense. For a
completely equipped base station, anemometer, anemograph, and wind vane
are desirable instruments, but the study of the habitat has by no means
reached the stage of precision in which their general use is necessary.
[Illustration: Fig. 24. Standard anemometer.]
=109. The anemometer= in its simplest form is adapted only to readings
made under direct observation, as a sudden change in the direction of
the wind reverses the movement of the indicator needle. This simple wind
gauge, shown in figure 23, has been used for instructional purposes, and
to a slight extent, also, in ascertaining the effect of cover. In
constant winds, successive single readings are found to have value, but,
ordinarily, the observations must be simultaneous. Careful tests of this
simple instrument show that it is essentially accurate. It may be
obtained from the C.H. Stoelting Company, 31 W. Randolph St., Chicago,
for $25. The standard anemometer (Fig. 24) is practically a recording
instrument up to 1,000 miles, but as the dials run on without any
indication of the total number of revolutions, it must be visited and
read each day. This renders its use difficult for habitats which are
some distance apart. When exact determinations of wind values become
necessary, the most successful method is to establish a series of three
standard anemometers. One of these should be placed upon the most
exposed part of a typically open habitat, the second in the most
protected part of the same habitat, while the third is located in the
midst of a representative forest formation. If the two habitats are
close together, the daily visits can be made without serious
inconvenience. The reading of the registering dials requires detailed
explanation, and for this the reader is referred to the printed
directions which accompany the instrument. In setting up the anemometer
it must be borne in mind that the ecologist desires the wind velocity
for a particular habitat. In consequence, the precautions which the
meteorologist takes to place the instrument at a certain height and well
away from surrounding obstructions do not hold here. Standard
anemometers are furnished by H. J. Green, and J. P. Friez for $25 each.
The anemograph is an anemometer electrically connected with an automatic
register. It is the only instrument adapted to continuous weekly records
in different habitats, but the price, $75 ($25 for the anemometer and
$50 for the register) is practically prohibitive, at least until a
complete series of ecographs for other factors has been obtained.
=110. Records.= The following form is used as a combined record for
precipitation and wind:
═══════╤══════════╤═════════╤════════╤════════╤════════╤═════════
Day │ Time │Formation│Station │Altitude│Exposure│Community
───────┼──────────┼─────────┼────────┼────────┼────────┼─────────
„ │ „ │ „ │ „ │ „ │ „ │ „
───────┼──────────┼─────────┼────────┼────────┼────────┼─────────
29/8/04│ 6:30 P.M.│ Half │Hiawatha│2550 m. │N.E. 17°│Asterare
│ │ gravel │ │ │ │
31/8/04│ 5:45 P.M.│ „ │ „ │ „ │ „ │ „
2/9/04│ 4:00 P.M.│ „ │ „ │ „ │ „ │ „
3/9/04│10:00 P.M.│ „ │ „ │ „ │ „ │ „
───────┴──────────┴─────────┴────────┴────────┴────────┴─────────
═══════╤══════════╤═══════════════╤════╤═════════════════════════╤════
Day │ Time │ RAINFALL │Base│ WIND │Base
───────┼──────────┼──────┬────────┼────┼────────┬──────┬─────────┼────
„ │ „ │Inches│Duration│ „ │Velocity│Heig’t│Direction│ „
───────┼──────────┼──────┼────────┼────┼────────┼──────┼─────────┼────
29/8/04│ 6:30 P.M.│ 1 │8 hours │ │ 5 │3 ft. │ N. W. │
│ │ │ │ │ │ │ │
31/8/04│ 5:45 P.M.│Trace │10 min. │ │ 12 │ „ │ „ │
2/9/04│ 4:00 P.M.│ .2 │2 hours │ │ 7 │ „ │ W. │
3/9/04│10:00 P.M.│Trace │ │ │ 18 │ „ │ „ │
───────┴──────────┴──────┴────────┴────┴────────┴──────┴─────────┴────
SOIL
=111. Soil as a factor.= In determining the value of the soil as a
factor in a particular habitat, it must be clearly recognized that its
importance lies solely in the control which it exerts upon water-content
and nutrient-content. The former is directly connected with the texture
or fineness of the soil, the latter with its chemical nature.
Accordingly, the structure of the soil and its chemical composition are
the fundamental points of attack. These are not at all of equal value,
however. Water is both a food, and a solvent for the nutrient salts of
the soil. Furthermore, the per cent of soluble salts, as determined in
mechanical analyses, is practically the same for all ordinary soils.
Indeed, the variations for the same soil types are as great as for
entirely different types. For these reasons, soluble salt-content may be
ignored except where it is readily seen to be excessive, as in alkaline
soils; and determinations of chemical composition are necessary only in
those soils which contain salts or acids to an injurious degree, e. g.,
alkaline soils, peat bogs, humus swamps, etc. The structure of the soil,
on the other hand, in the usual absence of excessive amounts of solutes,
absolutely controls the fate of the water that enters the ground, in
addition to its influence upon the run-off. It determines the amount of
gravitation water lost by percolation, as well as the water that can be
raised by capillarity. The resultant of these, the total soil water or
holard, is hence an effect of structure, while the size and compactness
of the particles are conclusive factors in controlling the chresard. It
must be recognized, however, that these are all factors which enable us
to interpret the amount of holard or chresard found in a particular
soil. They have no direct important effect upon the plant, but influence
it only in so far as they affect the water present.
=112. The value of soil surveys.= The full appreciation of the
preeminent value of water-content, particularly of the chresard, greatly
simplifies the ecological study of soils. The ecologist is primarily
concerned with soil water only in its relation to the plant, and while a
fair knowledge of soil structure is essential to a proper understanding
of this, he has little concern with the detailed study of the problems
of soil physics. For the sake of a proper balance of values, he must
avoid the tendency noted elsewhere of ignoring the claims of the plant,
and of studying the soil simply as the seat of certain physical
phenomena. Accordingly, it is felt that mechanical and chemical
analyses, determinations of soluble salt-content, etc., have much less
value than has been commonly supposed. The usual methods of soil survey,
which pay little or no attention to water-content, and none at all to
available water, are practically valueless for ecological research. This
statement does not indicate a failure to appreciate the importance of
the usual soil methods for many agricultural problems, such as the use
of fertilizers, conservation of moisture, etc., though even here to
focus the work upon water-content would give much more fundamental and
serviceable results. For these reasons, slight attention will be paid to
methods of mechanical and chemical analysis. In their stead is given a
brief statement of the origin, structure, and character of soils with
especial reference to water-content.
=113. The origin of soils.= Rocks form soils in consequence of
weathering, under the influence of physical and biotic factors.
Weathering consists of two processes, disintegration, by which the rock
is broken into component particles of various sizes, and decomposition,
in which the rock or its fragments are resolved into minute particles in
consequence of the chemical disaggregation of its minerals, or of some
other chemical change. These processes are usually concomitant,
although, as a rule, one is more evident than the other. The relation
between them is dependent upon the character of the rock and the forces
which act upon it. Hard rocks, i. e., igneous and metamorphic ones, as a
rule disintegrate more rapidly than they decompose; sedimentary rocks,
on the other hand, tend to decompose more rapidly than they
disintegrate. In many cases the two processes go hand in hand. This
difference is the basis for the distinction, first proposed by Thurmann,
between those rocks which weather with difficulty and those which
weather readily. The former were called dysgeogenous, the latter
eugeogenous. Thurmann restricted the application of the first term to
those rocks which produce little soil, but it seems more logical to
apply dysgeogenous to those in which disintegration is markedly in
excess of decomposition, and eugeogenous to those rocks that break down
rather readily into fine soils. With respect to the general character of
the soil formed, rocks are _pelogenous_, clay-producing, _psammogenous_,
sand-forming, or _pelopsammogenous_, producing mixed clay and sand. The
first two are divided into _perpelic_, _hemipelic_, _oligopelic_,
_perpsammic_, etc., with reference to the readiness with which they are
weathered, but this distinction is not a very practicable one. The
grouping of soils into silicious, calcareous, argillaceous, etc., with
reference to the chemical nature of the original rock, is of no value to
the ecologist, apart from the general clue to the physical properties
which it furnishes.
=114. The structure of soils.= The water capacity of a soil is a direct
result of the fineness of the particles. Since the water is held as a
thin surface film by each particle or group of them, it follows that the
amount of water increases with the water-holding surface. The latter
increases as the particles become finer and more numerous, and thus
produce a greater aggregate surface. The upward and downward movements
of water in the soil are likewise in immediate connection with the size
of particles. The upward or capillary movement increases as the
particles become finer, thus making the irregular capillary spaces
between them smaller, and magnifying the pull exerted. On the contrary,
the downward movement of gravitation water, i. e., percolation, is
retarded by a decrease in the size of the soil grains and hastened by an
increase. Hence, the two properties, capillarity and porosity, are
direct expressions of the structure of the soil, i. e., of its texture
or fineness. Capillarity, however, increases the water-content of the
upper layers permeated by the roots of the plant, while porosity
decreases it. On the basis of these properties alone, soils would fall
into two groups, capillary soils and porous soils, the former
fine-grained and of high water-content, the latter coarse-grained and
with relatively little water. A third factor, however, of great
importance must be taken into account. This is the pull exerted upon
each water film by the soil particle itself. This pull apparently
increases in strength as the film grows thinner, and explains why it
finally becomes impossible for the root-hairs to draw moisture from the
soil. This property, like capillarity, is most pronounced in
fine-grained soils, such as clays, and is least evident in the coarser
sands and gravels. It seems to furnish the direct explanation of
non-available water, and, in consequence, to indicate that the chresard
is an immediate result of soil texture.
[Illustration: Fig. 25. Sieves for soil analysis.]
=115. Mechanical analysis.= From the above it is evident that, with the
same rainfall, coarse soils will be relatively dry, and fine soils
correspondingly moist. However, this difference in holard is somewhat
counterbalanced by the fact that the chresard is much greater in the
former than in the latter. The basis of these relations can be obtained
only from a study of the texture of the soil. The usual method of doing
this is by mechanical analysis. This is far from satisfactory, since the
use of the sieves often brings about the disaggregation of groups of
particles which act as units in the soil. Furthermore, the analysis
affords no exact evidence of the compactness of the soil in nature, and
tests of capillarity and porosity made with soil samples out of position
are open to serious error. Nevertheless, mechanical analyses furnish
results of some value by making it possible to compare soils upon the
basis of texture. For ecological purposes, minute analyses are
undesirable; their value in any work is doubtful. A separation of soil
into gravel, sand, and silt-clay is sufficient, since the relative
proportion of these will explain the holard and chresard of the soil
concerned. The latter are also affected in rich soils, especially of
forests, by the organic matter present. If this is in a finely divided
condition, the amount is determined by calcining. When a definite layer
of leafmold is present, as in forests and thickets, its water-value is
found separately, since its power of retaining water is altogether out
of proportion to its weight.
=116. Kinds of soils.= It is very doubtful whether it is worth while to
attempt to distinguish soils upon the basis of mechanical analysis.
Unquestionably, the most satisfactory method is to distinguish them with
respect to holard and chresard, and to regard texture as of secondary
importance. A series of soil classes which comprise various soil types
has been proposed by the U. S. Bureau[7] of Soils as follows: (1) stony
loam, (2) gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine
sand, (7) sandy loam, (8) fine sandy loam, (9) loam, (10) shale loam,
(11) silt loam, (12) clay loam, (13) clay, (14) adobe. These are based
entirely upon mechanical analyses, and in some cases are too closely
related to be useful. The line between them can nowhere be sharply
drawn. Indeed, the variation within one class is so great that soils
have frequently been referred to the wrong group. Thus, Cassadaga sand
(gravel 22 per cent, sand 43 per cent, silt 21 per cent, clay 10 per
cent) is more closely related to Oxnard sandy loam (26–37–18–12) and to
Afton fine sandy loam (28–43–18–8) than to Coral sand (61–29–3–4),
Galveston sand (6–91–1–1), or Salt Lake sand (84–15–1–0). Elsinore sandy
loam (8–38–35–10) is much nearer to Hanford fine sandy loam (9–36–33–14)
than to Billings sandy loam (1–60–22–11) or to Utuado sandy loam
(48–23–19–8). The soil types are much more confused, and for ecological
purposes at least are entirely valueless. Lake Charles fine sandy loam
has the composition, 1–34–52–9; Vernon fine sandy loam, 1–37–54–7, while
many other so-called types show nearly the same degree of identity.
=117. The chemical nature of soils.= The effect of alkaline and acid
substances in the soil upon water-content and the activities of the
plant is far from being well understood. It is generally recognized that
salts and acids tend to inhibit the absorptive power of the root-hairs.
In the case of saline soils, this inhibitive effect seems to be
established, but the action of acids in bogs and swamps is still an open
question. It is probable that the influence of organic acid has been
overestimated, and that the curious anomaly of a structural xerophyte in
a swamp is to be explained by the stability of the ancestral type and by
the law of extremes. Apart from the effect which excessive amounts of
acids and salts may have in reducing the chresard, the chemical
character of the soil is powerless to produce structural modification in
the plant. Since Thurmann’s researches there has been no real support of
the contention that the chemical properties of the soil, not its
physical nature, are the decisive factors in the distribution and
adaptation of plants. It is not sufficient that the vegetation of a
silicious soil differs from that of a calcareous one. A soil can modify
the plants upon it only though its water-content, or the solutes it
contains. Hence, the chemical composition of the original rock is
immaterial, except in so far as it modifies these two factors. Humus,
moreover, while an important factor in growth, has no formative
influence beyond that which it exerts through water-content.
PHYSIOGRAPHY
=118. Factors.= The physiographic factors of a definite habitat are
altitude, exposure, slope, and surface. In addition, topography is a
general though less tangible factor of regions, while the dynamic forces
of weathering, erosion and sedimentation play a fundamental role in the
change of habitats. It is evident, however, that these, except where
they affect the destruction of vegetation directly, can operate upon the
plant only through more direct factors, such as water, light, and
temperature. While they are themselves not susceptible of measurement,
they can often be expressed in terms of determinable factors, i. e.,
slope, exposure, and surface. Fundamentally, they constitute the forces
which change one habitat into another, and, in consequence, are really
to be considered as the factors which produce succession. The static
features of physiography, altitude, etc., lend themselves readily to
determination by means of precise instruments. These factors, though by
no means negligible, are remote, and consequently their mere measurement
is insufficient to indicate the nature or extent of their influence upon
the plant. It is necessary to determine also the manner and degree in
which they affect other factors, a task yet to be done. Readings of
altitude, slope, and exposure are so easily made that the student must
carefully avoid the tendency to let them stand at their own value, which
is slight. Instead, they should be made the starting point for
ascertaining the differences which they produce in water-content,
humidity wind, and temperature.
_Altitude_
=119. Analysis into factors.= Of all physiographic features, altitude is
the most difficult to resolve into simple factors. Because of general
geographic relations, it has a certain connection with rainfall, but
this is vague and inconstant. Obviously, in its influence upon the
plant, altitude is really pressure, and in consequence its effect is
exerted upon the climatic and not the edaphic factors of the habitat.
Theoretically, the decrease of air pressure in the increased altitude
directly affects humidity, light, and temperature. Actually, while there
is unquestionably a decrease in the absorption of the light and heat
rays owing to the fact that they traverse less atmosphere, which is at
the same time less dense, this seems to be negligible. Photometric
readings at elevations of 6,000 and 14,000 feet have so far failed to
show more than slight differences, which are altogether too small to be
efficient. The effect upon humidity is greater, but the degree is
uncertain. Continuous psychrographic records at different elevations for
a full season, at least, will be necessary to determine this, since the
psychrometric readings so far made, while referred to a base
psychrograph, are too scattered to be conclusive. Finally, the length of
the season, itself a composite, is directly dependent upon the altitude.
This relation, though obscure, rests chiefly upon the rarefaction of the
air which prevents the accumulation of heat in both the soil and the
air.
[Illustration: Fig. 26. Aneroid barometer.]
=120. The barometer.= To secure convenience and accuracy in the
determination of altitude, it is necessary to use both a mercurial and
an aneroid barometer. The latter is by far the most serviceable for
field work, but it requires frequent standardizing by means of the
former. The mercurial form is much more accurate and should be read
daily in the base station. It is practically impossible to carry it in
the field, except in the so-called mountain form, which is of great
service in establishing the altitudes of a series of stations. In use
the aneroid barometer may be checked daily by the mercurial standard, or
it may be set at the altitude of the base station, thus giving a direct
reading. After the normal pressure at the base has once been
ascertained, however, the most satisfactory method is to set the aneroid
each day by the standard, at the same time noting the pressure deviation
in feet of elevation (see p. 46). The absolute elevation of the various
stations of a series may be determined either by adding or subtracting
this deviation from the actual reading at the station, or by noting the
change from the base station, and then adding or subtracting this from
the normal of the latter. When it is impossible to check the aneroid by
means of a mercurial barometer, the average of a series of readings made
at different days at one station, especially if taken during settled
weather, will practically eliminate the daily fluctuations, and yield a
result essentially accurate. Even in this event, the accuracy of the
aneroid should be checked as often as possible, since the mechanism may
go wrong at any time. The barograph, while a valuable instrument for
base stations, is not at all necessary. These instruments can be
obtained from all makers of meteorological apparatus, such as H. J.
Green, and J. P. Friez. Aneroid barometers reading to 16,000 feet cost
about $20; the price of the Richards aneroid barograph is $45. Ordinary
observatory barometers cost $30–$40; the standard instrument sells at
$75–$100. The mountain barometer, which is altogether the most
serviceable for the ecologist, ranges from $30–$55, depending upon
accessories, etc.
_Slope_
[Illustration: Fig. 27. Mountain barometer: (_a_) in carrying case;
(_b_) set up for use.]
=121. Concept.= This term is used in the ordinary sense to indicate the
relation of the surface of a habitat to the horizon. Although it is a
complex of factors, or rather influences several factors, these are
readily determinable. The primary effect of slope is seen in the control
of run-off and drainage, and consequently of water-content, although
these are likewise affected by soil texture and by surface. Slope,
moreover, as a concomitant of exposure, has an important bearing upon
light and heat by virtue of determining the angle of incidence, and also
upon wind, and, through it, upon the distribution of snow. At present,
while it can be expressed definitely in degrees, it has not yet been
connected quantitatively with more direct factors. This is, however, not
a difficult task, and it is probable that we shall soon come to express
slope principally in amount of run-off, and of incident heat.
=122. The clinometer.= In the simplest form, this instrument is merely a
semicircle of paper, with each half graduated from 1–90°. It is mounted
on a board and placed base upward, upon a wooden strip, 2 feet long and
2 inches wide, which has a true edge. At the center of the circle is
attached a line and plummet for reading the perpendicular. A more
convenient form is shown in figure 28, which is both clinometer and
compass. This also necessitates the use of a basing strip to eliminate
the inequalities of the surface. The dial face is graduated to show
inches of rise per yard, as well as the number of degrees, but the
latter, as the simpler term, is preferable for ecological work. In
making a reading, the basing strip is placed upon a representative area
of the slope, and pressed down firmly to equalize slight irregularities.
The clinometer is moved slightly along the upper edge, causing the
marker to swing freely. After the latter comes to rest, the instrument
is carefully turned upon its back, when the angle of the slope in
degrees may be read directly. Two or three such readings in different
areas will suffice for the entire habitat, unless it be extremely
irregular. The clinometer with compass may be obtained from the Keuffel
and Esser Company, 111 Madison St., Chicago, Illinois, for $5.
[Illustration: Fig. 28. Combined clinometer and compass.]
=123. The trechometer.= For measuring the effect of slope upon run-off,
a simple instrument called the trechometer (τρέχω, to run off) has been
devised. This consists merely of a metal tank, 3 × 4 × 12 inches,
holding 144 cubic inches of water, with an opening ¼ × 12 inches at the
base in front, closed by a tight-fitting slide. Three metal strips, 2 ×
12 inches, are fastened to the front of the tank in such a way as to
enclose a square foot of soil into which the strips penetrate an inch.
In the front strip is an opening, 1 inch square, provided with a drip
from which the run-off is collected in a measuring vessel. In use, the
instrument is put in position with the metal rim forced down 1 inch into
the soil; the tank is filled, the graduate put in place, and the slide
raised. The run-off for a square foot is the amount of water caught by
the graduate, and is represented in cubic inches per square foot. For
obtaining results which express slope alone, comparisons must be made
upon the same soil, from which all cover, dead and living, has been
removed. They must be as closely together in time as possible, at least
during the same day, as rain or evaporation will cause considerable
error. It is obvious that with the same slope or on a level the
trechometer may also be used to advantage to determine the absorptive
power of soils of different texture. It serves well a similar purpose
when used in different habitats to measure the composite action of
slope, soil, and cover in dividing the rainfall into run-off and
absorbed water.
_Exposure_
=124. Exposure= refers primarily to the direction toward which a slope
faces, i. e., its exposition or insolation with respect to sun and wind.
It is not altogether separable from slope, however, inasmuch as the
angle of the slope has some effect upon the degree of exposure. The
chief influence of exposure is exerted through temperature, since slopes
longest exposed to the sun’s rays receive the most heat. This is
supplemented in an important degree by the fact that a group of rays 1
foot square will occupy this area only on slopes upon which they fall at
right angles. In all other cases the rays are spread over a longer area,
with a consequent reduction in the amount of heat received. This effect
is felt principally in evaporation from the soil, and in soil
temperatures. For the leaf, it is largely if not entirely negligible,
since the angle of incidence is determined by the position of the leaf,
which is the same for each species whether on the level or upon a slope.
On this account, exposure has little or no bearing upon light, except
that the total amount of light received by the aggregate vegetation of a
slope will be greater than for a level area of the same size. The effect
of wind varies with the exposure. It is naturally most pronounced in
those directions from which the prevailing dry or cold winds blow, and
it is greatly emphasized by the fact that the opposite exposure is
correspondingly protected. The influence of wind, especially in
producing evaporation from the plant and the soil, increases with the
slope, since the mutual protection of the plants, or that afforded the
soil by the cover, is much reduced. Finally, the distribution of the
snow by the wind, a matter of considerable importance for early spring
vegetation, is largely determined by exposure.
Exposure is expressed directly in terms of direction, to which is added
the angle of the slope. A good field compass, reading to twelve points,
is sufficient. It should be checked, of course, by the declination of
the needle at the place under observation. A convenient instrument is
the one already mentioned, in which compass and clinometer are combined,
since these are regularly used at the same time.
=125. Surface.= The most important consideration with respect to surface
is the presence or absence of cover, and the character of the latter.
With the exception of snow, cover is, however, a question of vegetation,
living and dead, and consequently is to be referred to the discussion of
biotic factors. The surface of the soil itself often shows
irregularities which must be taken into account. Such are the rocks of
boulder and rock fields, the hummocks of meadows and bogs, the mounds of
prairie dog towns, the innumerable minute gullies and ridges of bad
lands, the raised tufts of sand-hills, etc. The influence of these is
not profound, but they do have an appreciable effect upon the run-off,
temperature, and wind. In many cases, this is distinctly measurable, but
as a rule little more can be done than to indicate that the surface is
even or uneven, and to describe the degree and kind of unevenness.
=126. Record of physiographic factors.= Altitude, slope, exposure, and
surface are essentially constant factors, and are determined once for
all, after a few check readings have been made, except in those
relatively rare habitats in which dynamic forces are very active. The
form of record used is the following:
═══════╤═════════╤════════╤════════════╤════════╤═════╤════════╤═══════
DATE │FORMATION│STATION │ GROUP │ALTITUDE│SLOPE│EXPOSURE│SURFACE
───────┼─────────┼────────┼────────────┼────────┼─────┼────────┼───────
10/7/02│Gravel │Golf │Eriogonare │2700 m. │ 23°│ N.N.W. │Even
│slide │Links │ │ │ │ │
„ │Brook │Jack │Violare │2550 m. │ 5°│ E.N.E. │ „
│bank │Brook │ │ │ │ │
„ │Half │Hiawatha│Achilleare │2600 m. │ 14°│ E. │Uneven
│gravel │ │ │ │ │ │
„ │Spruce │Milky │Opulasterare│2625 m. │ 12°│ N. │Even
│ │Way │ │ │ │ │
───────┴─────────┴────────┴────────────┴────────┴─────┴────────┴───────
=127. Topography.= As heretofore indicated, questions pertaining to the
form and development of the land concern groups of habitats within which
each habitat is the unit of investigation after the manner already laid
down. A knowledge of topography is essential to the accurate mapping of
a region, for which the simple methods of plane table and contour work
are employed, while the geology of the surface is of primary importance
in the study of successions.
BIOTIC FACTORS
=128. Influence and importance.= Biotic factors are animals and plants.
With respect to influence they are usually remote, rarely direct.
Nevertheless, they often play a decisive part in the vegetation. Their
effect is, as a rule, felt directly by the formation rather than the
habitat, but in either case the one reacts upon the other. Such factors
are not themselves susceptible of exact measurement, but their influence
upon the habitat is usually measurable in terms of the physical factors
affected. In the case of biotic factors, it must be distinctly
understood that these are not properly factors of the habitat as a
physical complex, but that they are rather to be considered as reactions
exerted by the effect, or formation, upon the cause or habitat. This is
most especially true of plants.
=129. Animals.= The activities of man fall into two classes: (1) those
that destroy vegetation, and (2) those that modify it. There are rare
instances also where the work of man has changed a new or already
denuded habitat. In the cases where the vegetation is destroyed, the
habitat itself is sufficiently changed to permit the effect to be
measured by physical factor instruments. Otherwise, the influence is
felt only by the formation, as when man makes possible the migration of
weeds, and it can be measured in terms of invasion by the quadrat alone.
It becomes especially evident, then, in the case of man’s activities,
that where they produce a denuded habitat they are to be regarded as
factors in the habitat; when they merely affect the formation, this is
not strictly true. The changes wrought by other animals are essentially
the same as those produced by man. They are not so marked nor so
important, but their relation to habitat and formation is the same. As a
rule, however, they affect the habitat much less than they do the
formation.
=130. Plants.= As a dead cover, vegetation is a factor of the habitat
proper, but it has relatively little importance, since it occurs
regularly during the resting period. Its chief effects are in modifying
soil temperature, and in holding snow and rain, and thereby increasing
the water-content. By its gradual decay, moreover, it not only adds
humus to the soil, but it thereby increases the water-retaining capacity
of the latter also. The cover of living vegetation reacts upon the
habitat in a much more vital fashion, exerting a powerful effect upon
every physical factor of the habitat. The factors thus affected are
distinctly measurable though it is often impossible to determine just
how much of the factor is directly traceable to the vegetation. This is
a simple problem in the case of most aerial factors, especially light,
but it is extremely difficult for soil factors, such as water-content
and soil texture. In the case of all habitats covered with formations,
by far the great majority, it is impossible as well as unnecessary to
separate the physical factors of the habitat proper from the reaction
upon them which the plant covering exerts. Indeed, the great
differentiation of habitats is largely due to the universal principle
that in vegetation the effect or formation always reacts upon the cause
or habitat in such a way as to modify it. As fundamental causes of
succession, the discussion of the various reactions of vegetation is
reserved for another place.
METHODS OF HABITAT INVESTIGATION
=131.= The use of the various instruments previously described depends
largely upon the preponderance of simple instruments or recording ones.
The former necessitate a number of well-trained assistants; the latter
require only a part of the time of one investigator. For the most
satisfactory results, however, an assistant is all but indispensable.
Since simple instruments are most easily obtained because of their
cheapness, and are especially adapted to purposes of instruction, the
method of using them will be described first, and then that of ecograph
batteries.
THE METHOD OF SIMPLE INSTRUMENTS
[Illustration: Fig. 29. Series of stations: I, at Minnehaha; II, at
Lincoln in the prairie formation.]
=132. Choice of stations.= This method is based upon simultaneous
readings by means of simple instruments in a series of habitats, or of
stations in a single habitat. Such readings are necessary for the
variable atmospheric factors, humidity, light, temperature, and wind.
Frequent readings suffice for water-content and precipitation, while
only two or three determinations, enough to check out the error, are
necessary for the constant factors, altitude, slope, exposure, and
surface. An account of the exact procedure employed in class study at
Lincoln and Minnehaha will best serve to illustrate the use of this
method. The series of stations chosen at Lincoln were primarily within a
single formation, for the purpose of determining the physical factor
variation in different areas. One series was located in the
prairie-grass formation (_Koelera-Andropogon-psilium_), and consisted of
the following stations: (1) low prairie, (2) crest of ridge I, (3)
northeast slope of ridge I, (4) grassy ravine, (5) southwest slope of
ridge II, (6) bare crest of ridge II, (7) thicket ravine. The
other series was established in the bur-oak-hickory forest
(_Quercus-Hicoria-hylium_) at the following stations: (1) thicket, (2)
woodland, (3) knoll in forest, (4) depression in forest, (5) level
forest floor, (6) nettle thicket, (7) brook bank. At Minnehaha the
series was primarily one of different formations: (1) the pine
formation (_Pinus-xerohylium_), (2) the gravel slide formation
(_Pseudocymopterus-Mentzelia-chalicium_), (3) east slope of spruce
forest (_Picea-Pseudotsuga-hylium_), (4) ridge in the spruce forest, (5)
north slope of spruce forest, (6) brook bank in forest, (7) the thicket
formation (_Quercus-Cercocarpus-lochmodium_), (8) the aspen formation
(_Populus-hylium_). When permanent or temporary quadrats are
established, they are ordinarily used as regular stations, since this
enables one to refer the physical factor readings to a few definite
individual plants, as well as to the entire formation. The transects in
figure 29 illustrate two of the above series of stations.
=133. Time of readings.= The frequency of simple readings and the times
at which they are made must be regulated largely by opportunity and
convenience. In addition to making readings once or twice a week
throughout the season, the series should be read at least once every day
for a representative week or two. It is also very desirable to have a
series for each hour of a typical day, or of two days, one of which is
clear, the other cloudy. When a single daily reading is made, it should
be taken at or as near meridian as possible. The usual series is the one
obtained by simultaneous observations at the same level in different
stations. An important series is also secured by simultaneous readings
at the various levels of the same station, though it is not necessary to
take this series frequently.
[Illustration: Fig. 30. A denuded station in the aspen formation.]
=134. Details of the method.= After the stations have been selected by a
careful preliminary survey of the habitat or series of habitats, their
location is indicated by a small flag bearing a number, in case there is
no danger of these being disturbed. Otherwise, less conspicuous stakes
are used. The ordinary practice is to visit each station of the series,
and to take readings of water-content, altitude, slope, and exposure. On
the first trip these are all made by the instructor, but after a short
time the determination of each factor may be assigned in rotation to
each of the students. After these constant factors have been read and
recorded, one student is equipped with photometer, thermometer, and
psychrometer, and, if desirable, anemometer, and left at the first
station. At each succeeding station the same plan is followed, so that
at the end of the series the constant factors have all been read, and
there is an observer at each station prepared to make readings of the
variable ones. The task of acquainting the students with the operation
of photometer, psychrometer, etc., can best be done in class or at a
previous field period, as it is evident that they must be familiar with
the instruments before they can use them accurately in the field series.
The details of operation have already been given and need not be
repeated here. The task of obtaining readings at the same moment may be
met by supplying each observer with a watch, which runs exactly with all
the others, or by making observations upon signal. The second means has
been found most successful in practice, since the signal fixes the
attention at the exact moment. The best plan is for the instructor to
occupy a commanding position somewhere near the middle of the series,
and to give the signals by shout or whistle at the proper interval.
Considerable care and experience are necessary to do the last
satisfactorily. Sufficient time must be given for the operation of the
instrument and the making of the record. In addition, a period must be
permitted to elapse which is long enough for every instrument to reach
the proper reading. For example, in a series which contains a gravel
slide and a forest, the thermometer which has just been used for an air
reading will require four or five times as long an interval to respond
to the temperature of the gravel as to that of the cool forest floor. In
such series, the instructor should regularly take his place in the
station where the response is slowest or greatest. He must record the
exact time of each signal, and note any general changes of sky or wind
that produce temporary fluctuations at the time of reading. When the
readings extend over a whole day, the usual plan is to begin at the last
station and take a second series of water-content samples, noting the
exact time in order that the rate of water loss may be determined. A
check series of physiographic factors may be made at this time also, or
this may be left for future visits. While it is unnecessary to take soil
samples oftener than once a day, it is important to make at least one
series at each visit. Sometimes it becomes desirable to know the rate of
water loss in different stations during the day, and in this event,
samples are taken at one or two hour intervals for the entire day.
In making simultaneous readings at the different levels of one station,
the observers are grouped in one spot in such a way that they do not
interfere with the correct reading of each instrument. Readings of this
sort are most valuable in the case of temperature, which shows greater
differences at the various levels. Important differences of humidity and
wind also are readily obtained, and, in layered formations, marked
variations in the amount of light. In the open, the ordinary levels for
temperature are 6 feet, 3 feet, surface, 5, 10, and 15 inches in the
ground, and for wind and humidity, 6 feet, 3 feet, and surface. In
forests the same levels are used for comparison with formations in the
open, but a more desirable series for light especially is secured by
making readings at the height of, or better, just below the various
layers. Series of this sort are likewise made on signal. The best time
of day is that of a period in which the middle station is read near
meridian, since the variation due to time is sufficiently small to
permit fairly accurate comparisons between the readings for the
different stations.
=135. Records.= The form used for recording the observations made by
means of simple instruments is shown below. It is hardly necessary to
state that it may be readily modified to suit the needs of different
investigators. Ordinarily, each sheet is used for the records of one
habitat or series alone, but for convenience sake, the records of two
different series are here combined. The figures given are taken from
records for the prairie and forest formations at Lincoln.
_Koelera-Andropogon-psilium_
April 25, 1901. Clear. South wind.
════╤═════════════════════════════╤═══════════╤═══════════╤══════════════╤═══════════
│ TEMPERATURE │ LIGHT │ HUMIDITY │WATER-CONTENT │ WIND
────┼─────┬─────┬─────┬─────┬─────┼─────┬─────┼─────┬─────┼──────────────┼─────┬─────
TIME│3:20 │3:24 │3:30 │3:35 │3:40 │ │ │3:45 │3:55 │ % │2:40 │3:05
P.M.│ │ │ │ │ │ │ │ │ │ │ │
────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┬────┬────┼─────┼─────
STA-│1½ m.│Surf.│ 5 │ 10 │ 15 │1½ m.│Surf.│1½ m.│Surf.│ 5 │ 10 │ 15 │1½ m.│Surf.
TION│ │ │ │ │ │ │ │ │ │ │ │ │ │
────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┼────┼────┼─────┼─────
1. │27.8 │29.6 │17.8 │15.8 │10.9 │ │ │ 57 │ 59 │17.7│14.6│17.4│ 740│ 280
2. │26.5 │31.3 │18.3 │16.6 │12.8 │ │ │ 59 │ 59 │17.9│12.2│14.3│ 1100│ 510
3. │26.9 │28.5 │18.2 │14.2 │13.5 │ │ │ 58 │ 59 │16.5│16.9│19 │ 980│ 520
4. │26.2 │30 │16.8 │13.2 │11.6 │ │ │ 63 │ 66 │24.4│21.3│24.8│ 920│ 460
5. │28 │32.4 │18.6 │14.6 │14.2 │ │ │ 59 │ 60 │10.7│17 │17.2│ 1080│ 490
6. │28 │40.8 │23.8 │16 │15 │ │ │ 51 │ 51 │ 5 │8.3 │10.3│ 1010│ 410
7. │26.4 │30.8 │16 │13 │11 │ │ │ 68 │ 70 │27 │24.3│21.4│ 680│ 52
────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴────┴────┴────┴─────┴─────
_Quercus-Hicoria-hylium_
April 20, 1901. Clear. Southeast wind.
════╤═════════════════════════════╤═══════════╤═══════════╤══════════════╤═══════════
│ TEMPERATURE │ LIGHT │ HUMIDITY │WATER-CONTENT │ WIND
────┼─────┬─────┬─────┬─────┬─────┼─────┬─────┼─────┬─────┼──────────────┼─────┬─────
TIME│10:40│10:46│10:50│10:55│11:00│12:00│12:05│11:10│11:20│ % │11:30│11:45
A.M.│ │ │ │ │ │ │ │ │ │ │ │
────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┬────┬────┼─────┼─────
STA-│1½ m.│Surf.│ 5 │ 10 │ 15 │1½ m.│Surf.│1½ m.│Surf.│ 5 │ 10 │ 15 │1½ m.│Surf.
TION│ │ │ │ │ │ │ │ │ │ │ │ │ │
────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┼────┼────┼─────┼─────
1. │16 │25 │ 9.6│8.4 │7.8 │ .08 │ .06 │ 73 │ 81 │24.2│19.2│19.5│ 298│ 0
2. │16.2 │30.5 │ 8.5│8.4 │7.8 │ .11 │ .09 │ 73 │ 86 │22 │22.5│19.4│ 375│ 2
3. │16.2 │17.8 │ 7.6│7.8 │8 │ .08 │ .06 │ 73 │ 95 │22.1│20.4│21.6│ 640│ 6
4. │15.6 │26.2 │ 10.6│8.4 │8.2 │ .06 │ .03 │ 81 │ 95 │25.4│23.1│22.4│ 275│ 12
5. │17.6 │25.4 │ 7.6│7.4 │7.2 │ .03 │ .02 │ 90 │ 95 │27.2│19.8│18.8│ 178│ 2
6. │16.2 │20.2 │ 8.4│7 │6.2 │ .02 │ .01 │ 82 │ 90 │27.6│20.8│18.8│ 115│ 4
7. │15.8 │17.2 │ 6.4│6.4 │6.1 │ .05 │ .04 │ 82 │ 90 │23.8│19 │19.3│ 60│ 0
────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴────┴────┴────┴─────┴─────
METHOD OF ECOGRAPH BATTERIES
=136.= A battery of recording instruments consists of a selagraph, a
psychrograph, and a thermograph, to which an anemograph is added when
possible. As stated before, the determination of water-content by the
geotome method is more satisfactory than by any automatic instrument yet
devised. When the base station is located where the sunlight is
unobstructed, which should be the case whenever possible, it is
unnecessary to include a selagraph in those batteries placed in
similarly exposed stations, since the light values will be the same. As
a rule, batteries are established within different zones or different
habitats, except where a highly diversified habitat is made the subject
of special inquiry. Such a restriction arises from the fact that
expense, care of operation, etc., place a limit upon the number of
batteries, and, in such case, the task of primary importance is to
establish the physical character of representative habitats. For these
reasons, the first series of thermographs established in 1903 was
located with respect to altitude, the instruments being placed at
Manitou 2,000 m., Minnehaha, 2,600 m., and Mount Garfield 3,800 m. In
1904, the stations established for the record of temperature and
humidity were situated with respect to habitats representing the four
formations: gravel slide, half gravel slide, spruce forest, and brook
bank.
The batteries are located and set up according to the directions already
given. A 2–meter quadrat with the battery as the middle is staked and
mapped. Within this, all readings of water-content, soil temperature,
and physiographic factors are made. Altitude, slope, exposure, and cover
are recorded when each battery is located, and a soil sample is taken
for mechanical analysis. When the position of the batteries permits it,
water-content readings should be made frequently, once or twice a week
at least. In addition, a complete series of samples should be taken
daily for a period sufficient to indicate the ordinary extremes of
water-content.
The ecograph battery of each habitat constitutes a standard to which the
results obtained by simple instruments may be referred with accuracy. It
not only does this, but it also serves as a basis for interpreting the
readings of simple instruments in distant habitats of the same
character. In this way a few batteries judiciously placed make possible
the exact physical investigation of a large number of habitats, covering
a considerable area. The only limit, indeed, upon this method is that
placed by time. The proportionate use of batteries and of simple
instruments must be largely determined by the conditions which confront
the investigator. It is obvious that, where expense is not a decisive
factor, the gain in time and in completeness of results is enormously in
favor of the battery. There is an additional value in the automatic and
continuous record which can not be overlooked. When the use of
instruments in the study of habitat and formation becomes universal, the
importance of the ecograph will be immeasurably enhanced. It will be
possible to secure duplicate records of batteries located in the most
remote and diverse regions, from the equator to the poles, and
comparative phytogeography upon a scientific basis will for the first
time be possible. This opens an alluring vista of the future when
ecologists the world over will cooperate in such a way that the results
obtained by ecograph batteries anywhere on the globe will permit of
exact comparison.
THE EXPRESSION OF PHYSICAL FACTOR RESULTS
=137. The form of results.= It is almost inevitable that the general
adoption of precise methods of measuring the habitat will result in a
common form for expressing the physical character of the latter. An
actual diagnosis of each habitat is not a difficult matter, after the
factors are carefully measured, and will unquestionably lead to very
desirable definiteness and precision. The accurate investigation of the
physical factors of a number of habitats for one growing season
furnishes the necessary material for a diagnosis based upon the mean for
the growing season. Similar results for two or three seasons will yield
a diagnosis as accurate and as final as that of a formation, or, indeed,
as that of many species. The author’s investigations have not yet gone
far enough to warrant proposing a final form for this, but the following
diagnosis is offered as a suggestion:
_Elymus-Muhlenbergia-chalicium._ Habitat: holard 9 per cent, chresard 8
per cent, relative humidity 40 per cent, light 0.6, soil colluvial
gravel (gravel 70 per cent, sand 27 per cent, silt 3 per cent), air
temperature 65°, surface 82°, soil 59°, wind 10 miles, rainfall 8
inches, altitude 2,800 m., slope 23°, exposure south, surface even,
cover open, no active biotic agencies.
The detailed comparison of habitats is made most readily by the graphic
method of curves, which constitute the most desirable form of expression
in connection with the original record upon which they are based. Factor
means are particularly desirable for diagnostic purposes, and they
furnish valuable curves also. Factor sums are impracticable at present,
and it seems doubtful that they will ever be of much value. It is by no
means impossible, however, that a more detailed and exact knowledge of
the physiology of adaptation, coupled with methods of precision in the
habitat, will render them necessary.
_Factor Records_
=138.= Experience has shown that the practice of making hasty and often
formless records in the field is unwise and is apt to be inaccurate as
well. The time saved in the field is more than counterbalanced by that
consumed in copying the results into the permanent form. The danger of
error in field notes rapidly taken is very grave, and the chance of
confusion and the waste of time in deciphering them are great. Moreover,
the task of checking a copy with the original, which is absolutely
necessary for accuracy, involves a further expenditure of time and
energy. For these reasons the field record should be made in permanent
form. Definite record sheets are used, and the invariable rule is made
that all readings are to be noted in ink at the time and spot where they
are taken. On a long journey, or in the face of many observations, the
tendency to take notes or to record observations rapidly is very great,
but this will correct itself after a few attempts to use such notes. The
record forms for various factors have been indicated in the proper
place, as well as the one for simultaneous readings. Ecograph sheets are
carefully filed, and constitute permanent records. With a little
practice they may be read almost as easily as tables, and any attempt to
put them into tabular form is a mere waste of time. For purposes of
study and of publication, it often becomes necessary to bring together
all the results obtained for a particular habitat, both by simple
instruments and by ecographs. The form of record used for this is
essentially that already indicated for simultaneous readings on page 92,
since general features and constant factors can not well be included in
the table. Record sheets of this type have been printed at a cost of $5
per thousand, and the various factor records can be obtained at about
the same rate. The size of sheet used is 9½ × 7¾ inches. The record book
is the usual notebook cover, which has been found neither too large nor
too small. It is protected from dirt and rain by a covering of oilcloth
which overlaps the edges. Record books should be carefully labeled, and
each one should contain a single year’s records.
_Factor Curves_
=139. Plotting.= The paper employed is divided into centimeter squares
which are subdivided into 2–millimeter units. For ordinary curves the
size of sheet is 9½ × 7¾ inches, which makes it possible for curve
sheets to be filed in the record book. Tablets containing 60 of these
sheets can be obtained for 20 cents each from the Central School Supply
House, Chicago. For curves longer than 9 inches special sizes of sheets
must be used. On account of their inconvenience large sheets are avoided
whenever possible. This can usually be accomplished by increasing the
numerical value of the intervals. The inks employed in plotting are the
waterproof inks of Chas. Higgins & Co., Brooklyn, New York. These are
made in ten or more colors, black, violet, indigo, blue, green, yellow,
orange, brown, brick red, carmine, and scarlet, and cost 25 cents per
bottle. In addition to being waterproof, they make it possible to
combine curves in all conceivable ways without destroying their
identity. Furthermore, it is a great advantage to use the same color
invariably for the same kind of curve: thus, it has been the practice to
indicate the 3–foot, surface, 5, 10, and 15–inch temperature curves by
violet, green, yellow, blue, and carmine respectively. A fine-pointed
pen, such as the Spencerian No. 1, is most satisfactory for inking;
drawing pens, such as Gillott’s Crowquill, are too finely pointed for
ordinary use.
In plotting a curve, it is first necessary to determine the value of the
interval, and the extreme range of the curve or combination. For
example, in the case of temperature, it is most convenient to assign a
value of 1° Centigrade to each centimeter, since the thermometers used
read to one-fifth of a degree, which corresponds exactly to the
2–millimeter units of each square. The length of the sheet permits a
range of 22 degrees Centigrade, and the actual limits must be determined
for the particular results to be employed. For the same region, it is
very desirable that the unit interval and the range be the same, in
order that all curve sheets may admit of direct comparison. Indeed, it
is greatly to be hoped that in the future ecologists will agree to a
uniform system of curve-plotting, cartography, etc., as the geographers
are beginning to do in the construction of maps. The major intervals are
written, or, better, typewritten, at both sides of the sheet, and the
time or space intervals are indicated at the top. Each curve sheet is
properly labeled, and essential data indicated. The readings are taken
from the field record, and their proper positions indicated by a dot.
These are connected first by a pencil line, the curves being made abrupt
rather than flowing; and the line, after having been carefully checked,
is traced in ink.
=140. Kinds of curves.= Curves are named both with reference to the
factor concerned and the position or sequence of the readings. The
factors which lend themselves most readily to this method of
representation are the variable ones, water-content, humidity, light,
temperature, and wind, and corresponding curves are distinguished.
Altitude and slope may likewise be shown by means of curves, but the use
of cross section or contour lines serves the same purpose and is more
natural. With regard to time and position, curves are distinguished as
level, station, and point curves. A level curve is one based upon
readings made at the same level through a series of stations or of
habitats, e. g., the level curve of surface temperature. The station
curve represents the various levels or points at which readings are made
in a single station. The point curve has for a basis the hourly or daily
variation of a factor at a particular point or level in a station. All
of these may be simple curves, when established upon a single reading
for a series, or mean curves when they are based upon the mean of a
number of readings. Curves which show the extremes of a factor, i. e.,
the maximum and minimum, are also extremely valuable, though a
combination of the two for comparison is preferable.
=141. Combinations of curves= are invaluable for bringing similar curves
together, and permitting ready comparison of them. For this, and also
because they save space, they are regularly employed to the almost
complete exclusion of single curves. Combinations are made simply by
tracing the curves to be compared upon the same sheet, it being
understood that dissimilar curves, e. g., level and station, can not be
combined. Colored inks are an absolute necessity in combining; the
primary principle underlying their use is that curves that approach
closely or cross each other must be traced in inks that contrast
sharply. As elsewhere stated, it has been made the invariable rule to
use the same color for the same level or point. This applies especially
to temperature, but holds also for humidity, light, wind, and
water-content, so that the color always indicates the level. For the
same reason, it is applied to a combination of point curves for one
station, though it is inapplicable to a series of point curves when
these lie in the same level. Light readings above 6 feet and
water-content readings below 15 inches necessitate the use of additional
colors.
Combinations may be made of the curves of a single factor for purposes
of comparison, or they may consist of curves of different factors in
order to aid in interpreting or indicating their relation to each other.
Curves of the same factor may be combined to form various series. The
level series consists of all the level curves for the stations under
observation, e. g., the six levels for temperature, three levels for
wind, etc. Similarly, the station series is a combination of all the
station curves, and a corresponding arrangement may be made for point
curves with reference either to station or to level. An extremely
valuable combination of curves is that of the holard and chresard for a
series of stations. The most important combinations of the curves of
different factors are naturally those based upon factors intimately
related to each other or to the plant. The grouping of water-content and
humidity curves is of great value, especially when the transpiration
curve is added. Light and temperature curves make an interesting
combination, while a humidity, temperature, and wind series is of much
aid in tracing the connection between these factors. Finally, it is
altogether feasible to arrange the curves of water-content, humidity,
light, temperature, and wind upon the same sheet in such fashion as to
give a graphic representation of the whole physical nature of a single
habitat or a series. In all combinations of curves representing
different factors, it must be borne in mind that the position of a curve
does not represent a definite value with reference to the others, since
some are based upon per cents, others upon degrees, etc. The comparison
must be based upon the character of the curves, but even then it is an
important aid. An instructive grouping has been employed where series of
readings on the same day, or on two successive days in forest and in
prairie have yielded the usual level series of curves. The series for
the two habitats are arranged on the same page, one at the right and the
other at the left, and permit direct comparison of corresponding level
or factor curves, both with respect to position and character.
=142. The amplitude of all the curves= described above is determined by
the unit values of the factors concerned, while the length is dependent
upon the number of stations, points, or times. The value assigned the
latter upon the plotting paper is purely arbitrary, but it is most
convenient to fix this at the centimeter square. The unit value for
temperature is 1° Centigrade per square, each subdivision of the latter
representing 0.2, and the range being 22 degrees. For water-content
curves, each square represents a value of 2 per cent, the smaller square
being 0.4 per cent, and the range 2–48 per cent. The unit value for
humidity is taken as 5 per cent, making each small square 1 per cent,
and giving room on the sheet for the entire range from 1–100 per cent.
Owing to the anemometer used, curves of wind velocity have been based
upon the number of feet per minute. One hundred feet is taken as the
unit value, and the range is from 0–2200 feet. The unit value for the
curve of light intensity is .005. Each small square is .001, which
permits a range from .001 to .01 on one sheet. Consequently, when it is
desired to plot the curve of a series of habitats with a range in
intensity greater than this it is necessary to use a double sheet. This
is the usual device when the range of curves is too great, except where
the excess is slight. In this case the curve is left open at the top,
and the value which the crest attains is indicated. All curves in
combination are labeled at the beginning or left to indicate the level,
station, or point, and at the end or right to show the time, or day, if
this is not the basis of the curve or series.
The discussion that precedes deals exclusively with curves representing
factors determined in the field. It applies with equal force to results
obtained by instruments in control houses. In these, however, all
factors except those directly experimented with, usually water-content
and light, are practically equalized, and the curves based upon them are
used chiefly to show how nearly equal they have become. The important
curves are those of the water-content series, both holard and chresard,
and of the shade tents. Where several houses are differentiated with
respect to temperature or humidity, curve series of both these factors
are necessary.
_Factor Means and Sums_
=143.= It has been shown elsewhere that the daily mean of temperature
can be closely approximated from the maximum and minimum of both day and
night. Maximum-minimum instruments for the other factors are lacking,
however, and for light, humidity, and wind these values can only be
obtained from the ecograph which makes it possible to get the exact mean
from the sum of all the hour readings. When it comes to the seasonal
mean, the ecograph is even more necessary, exception being made for
water-content, in which case a number of readings on various days
through the season will suffice. The value of factor means for diagnosis
and for curves has already been sufficiently commented upon, and the
feasibility of factor sums already indicated.