vegetation moves constantly and gradually toward stabilization. Each
successive stage modifies the physical factors, and dominates the
habitat more and more, in such a way that the latter seems to respond to
the formation rather than this to the habitat. The more advanced the
succession, i. e., the degree of stabilization, the greater the climatic
or physiographic change necessary to disturb it, with the result that
such disturbances are much more frequent in the earlier stages than in
the later development. Constant, gradual movement toward a stable
formation is characteristic of continuous succession. Contrasted with
this is intermittent succession, in which the succession swings for a
time in one direction, from xerophytic to mesophytic for example, and
then moves in the opposite direction, often passing through the same
stages. This phenomenon usually is characteristic only of the less
stable stages, and is generally produced by a climatic swing, in which a
series of hot or dry years is followed by one of cold or wet years, or
the reverse. The same effect upon a vast scale is produced by alternate
elevation and subsidence, but these operate through such great periods
of time that one can not trace, but can only conjecture their effects. A
normal continuous succession frequently changes its direction of
movement, or its type, in transition regions or in areas where the
outposts of a new flora are rapidly advancing, as in wide mesophytic
valleys that run down into or traverse plains. Here the change is often
sudden, and grass and desert formations are replaced by thickets and
forests, resulting in abrupt succession. Species guilds are typical
examples of this. More rarely, a stage foreign to the succession will be
interpolated, replacing a normal stage, or slipping in between two such,
though finally disappearing before the next regular formation. This may
be distinguished as interpolated succession.
The apparent terminus of all stabilization is the forest, on account of
the thoroughness with which it controls the habitat. A close examination
of vegetation, however, will show that its stable terms are dependent in
the first degree upon the character of the region in which the formation
is indigenous. It is obviously impossible that successions in desert
lands, in polar barrens, or upon alpine stretches should terminate in
forest stages. In these, grassland must be the ultimate condition,
except in those extreme habitats, alpine and polar, where mosses and
lichens represent the highest type of existing vegetation. Forests are
ultimate for all successions in habitats belonging to a region generally
wooded, while grassland represents the terminus of prairie and plains
successions as well as of many arctic-alpine ones.
_CAUSES AND REACTIONS_
=315.= The initial cause of a succession must be sought in a physical
change in the habitat; its continuance depends upon the reaction which
each stage of vegetation exerts upon the physical factors which
constitute the habitat. A single exception to this is found in anomalous
successions, where the change of formation often hinges upon the
appearance of remote or foreign disseminules. The causes which initiate
successions have already been considered; they may be summarized as
follows: (1) weathering, (2) erosion, (3) elevation, (4) subsidence, (5)
climatic changes, (6) artificial changes. The effect of succeeding
stages of vegetation upon a new or denuded habitat usually finds
expression in a change of the habitat with respect to a particular
factor, and in a definite direction. Often, there is a primary reaction,
and one or more secondary ones, which are corollaries of it. Rarely,
there are two or more coordinate reactions. The general ways in which
vegetation reacts upon the habitat are the following: (1) by preventing
weathering, (2) by binding aeolian soils, (3) by reducing run-off and
preventing erosion, (4) by filling with silt and plant remains, (5) by
enriching the soil, (6) by exhausting the soil, (7) by accumulating
humus, (8) by modifying atmospheric factors. The direction of the
movement of a succession is the immediate result of its reaction. From
the fundamental nature of vegetation, it must be expressed in terms of
water-content. The reaction is often so great that the habitat undergoes
a profound change in the course of the succession, changing from
hydrophytic to mesophytic or xerophytic, or the reverse. This is
characteristic of newly formed or exposed soils. Such successions are
_xerotropic_, _mesotropic_, or _hydrotropic_, according to the ultimate
condition of the habitat. When the reaction is less marked, the type of
habitat does not change materially, and the successions are
_xerostatic_, _mesostatic_, or _hydrostatic_, depending upon the
water-content. Such conditions obtain for the most part only in denuded
habitats.
=316. Succession by preventing weathering.= Reactions of this nature
occur especially in alpine and boreal regions, in the earlier stages of
lichen-moss successions. They are typical of igneous and metamorphic
rocks in which disintegration regularly precedes decomposition. The
influence of the vegetation is best seen in the lichen stages, where the
crustose forms make a compact layer, which diminishes the effect of the
atmospheric factors producing disintegration. In alpine regions
especially, this protection is so perfect that the crustose lichens may
almost be regarded as the last stage of a succession. There are no
recorded observations which bear upon this point, but it seems certain
that the pioneer rock lichens, _Lecanora_, _Lecidea_, _Biatora_,
_Buellia_, and _Acarospora_, cover alpine rocks for decades, if not for
centuries. Ultimately, however, the slow decomposition of the rock
surface beneath the thallus has its effect. Tiny furrows and pockets are
formed, in which water accumulates to carry on its ceaseless work, and
the compact crustose covering is finally ruptured, permitting the
entrance of foliose forms. The latter, like the mosses, doubtless
protect rock surfaces, especially those of the softer rocks, in a slight
degree against the influence of weathering, but this is more than offset
by their activity in hastening decomposition, and thus preparing a field
for invasion. Rocks and boulders (_petria_, _petrodia_, _phellia_)
furnish the best examples of this reaction; cliffs (_cremnia_) usually
have a lichen covering on their faces, while the forces which produce
disintegration operate from above or below.
[Illustration: Fig. 66. Thicket formation
(_Quercus-Holodiscus-driodium_), stage V of the talus succession.]
=317. Succession by binding aeolian soils.= Dunes (_thinia_) are classic
examples of the reaction of pioneer vegetation upon habitats of
wind-borne sand. The initial formations in such places consist
exclusively of sand-binders, plants with masses of fibrous roots, and
usually also with strong rootstalks, long, erect leaves, and a vigorous
apical growth. They are almost exclusively perennial grasses and sedges,
possessing the unique property of pushing up rapidly through a covering
of sand. They react by fixing the sand with their roots, thus preventing
its blowing about, and also by catching the shifting particles among
their culms and leaves, forming a tiny area of stabilization, in which
the next generation can establish a foothold. The gradual accumulation
of vegetable detritus serves also to enrich the soil, and makes possible
the advent of species requiring better nourishment. Blowouts (_anemia_)
are almost exact duplicates of dunes in so far as the steps of
revegetation are concerned; while one is a hollow, and the other a hill,
in both the reaction operates upon a wind-swept slope. Sand-hills
(_amathia_) and deserts (_eremia_) show similar though less marked
reactions, except where they exhibit typical inland dunes. Sand-binders,
while usually classed as xerophytic or halophytic, are in reality
dissophytes. Their roots grow more or less superficially in moist sand,
and are morphologically mesophytic while their leaves bear the stamp of
xerophytes. The direction of movement in successions of this kind is
normally from xerophytes to mesophytes, i. e., it is mesotropic. In
sand-hills and deserts, the succession operates wholly within the
xerophytic (dissophytic) series. Along seacoasts, the mesophytic
terminus is regularly forest, except where forests are remote, when it
is grassland.
=318. Succession by reducing run-off and erosion.= All bare or denuded
habitats that have an appreciable slope are subject to erosion by
surface water. The rapidity and degree of erosion depend upon the amount
of rainfall, the inclination of the slope, and the structure of the
surface soil. Regions of excessive rainfall, even where the slope is
slight, show great, though somewhat uniform erosion; hill and mountain
are deeply eroded even when the rainfall is small. Slopes consisting of
compact eugeogenous soils, notwithstanding the marked adhesion of the
particles, are much eroded where the rainfall is great, on account of
the excessive run-off. Porous dysgeogenous soils, on the contrary,
absorb most of the rainfall; the run-off is small and erosion slight,
except where the slope is great, a rare condition on account of the
imperfect cohesion of the particles. In compact soils, the plants of the
initial formations not merely break the impact of the raindrops, but,
what is much more important, they delay the downward movement of the
water, and produce numberless tiny streams. The delayed water is largely
absorbed by the soil, and the reduction of the run-off prevents the
formation of rills of sufficient size to cause erosion. As in dunes,
such plants are usually perennial grasses, though composites are
frequent; the root system is, however, more deeply seated, and a main or
tap root is often present. On sand and gravel slopes, the loose texture
of the soil results generally in the production of sand-binders with
fibrous roots. Unlike dunes, such slopes exhibit a large number of mats
and rosettes with tap-roots, which are effective in preventing the
slipping or washing of the sand, and run little danger of being covered,
as is the case with duneformers. In both instances, each pioneer plant
serves as a center of comparative stabilization for the establishment of
its own offspring, and of such invaders as find their way in. From the
nature of these, slopes almost invariably pass through grassland stages
before finding their termini in thickets or forests. Bad lands (_tiria_)
furnish the most striking examples of eroded habitats. The rainfall in
the bad lands of Nebraska and South Dakota is small (300 mm.); yet the
steepness of the slope and the compactness of the soil render erosion so
extreme that it is all but impossible for plants to obtain a foothold.
Their reaction is practically negligible, and the vegetation passes the
pioneer stages only in the relatively stable valleys. Mountain slopes
(_ancia_), and ridges and hills (_lophia_) are readily eroded in new or
denuded areas. This is especially true of hill and mountain regions
which have been stripped of their forest or thicket cover by fires,
lumbering, cultivation, or grazing. Where the erosion is slight, the
resulting succession may show initial xerophytic stages, or it may be
completely mesostatic. Excessively eroded habitats are xerostatic, as in
the case of bad lands, or, more frequently, they are mesotropic, passing
first through a long series of xerophytic formations. Sandbars
(_cheradia_, _syrtidia_) should be considered here, though they are
eroded by currents and waves, and not by run-off. They are fixed and
built up by sand-binding grasses and sedges, usually of a hydrophytic
nature, and pass ultimately into mesophytic forest.
=319. Succession by filling with silt and plant remains.= All aquatic
habitats into which silt, wash, or other detritus is borne by streams,
currents, floods, waves, or tides are slowly shallowed by the action of
the water plants present. These not only check the movement of the
water, thus greatly decreasing its carrying power, and causing the
deposition of a part or all of its load, but they also retain and fix
the particles deposited. In accordance with the rule, each plant becomes
the center of a stabilizing area, which rises faster than the rest of
the floor, producing the well-known hummocks of lagoons and swamps. All
aquatics produce this reaction. It is more pronounced in submerged and
amphibious forms than in floating ones, and it takes place more rapidly
with greatly branched or dissected plants than with others. In pools
(_tiphia_) and lakes (_limnia_), debouching streams and surface waters
deposit their loads in consequence of the check exerted by the still
water and the marginal vegetation, and delta-like marshes are quickly
built up by filling. Springs (_crenia_) likewise form marshes where they
gush forth in sands, the removal of which is impeded by vegetation. The
flood plains and deltas of rivers show a similar reaction. The heavily
laden flood waters are checked by the vegetation of meadows and marshes,
and deposit most of their load. The banks of streams (_ochthia_) and of
ditches (_taphria_) are often built up in the same fashion by the action
of the marginal vegetation upon the current. The presence of marginal
vegetation often determines the checking or deflecting of the current in
such a way as to initiate meanders, while natural levees owe their
origin to it, in part at least. Along low seacoasts, waves and tides
hasten the deposit of river-borne detritus, causing the water to spread
over the lowlands and form swamps. They often throw back also the
sediment that has been deposited in the sea, the marsh vegetation acting
as a filter in both cases. Successions of the kind indicated above are
regularly mesotropic. Where the soil is sandy, and the filling-up
process sufficiently great, or where salts or humus occur in excess,
xerophytic formations result. In certain cases, these successions appear
to be permanently hydrostatic, changing merely from floating or
submerged to amphibious conditions, but this is probably due to the
slowness of the reaction. As a rule, the accumulation of plant remains
is relatively slight, and plays an unimportant part in the reaction. In
peat bogs and other extensive swamps, the amount of organic matter is
excessive, and plays an important role in the building up of the swamp
bed.
[Illustration: Fig. 67. Pine forest formation (_Pinus-xerohylium_),
stage VI of the talus succession.]
=320. Succession by enriching the soil.= This reaction occurs to some
degree in the great majority of all successions. The relatively
insignificant lichens and mosses produce this result upon the most
barren rocks, while the higher forms of later stages, grasses, herbs,
shrubs, and trees, exhibit it in marked progression. The reaction
consists chiefly in the incorporation of the decomposed remains of each
generation and each stage in the soil. A very important part is played
by the mechanical and chemical action of the roots in breaking up the
soil particles, and in changing them into soluble substances.
Mycorrhizae, bacterial nodules, and especially soil bacteria play a
large part in increasing the nutrition-content of the soil, but the
extent to which they are effective in succession is completely unknown.
The changes in the color, texture, and food value of the soil in passing
from the initial to ultimate stages of a normal succession are well
known, and have led many to think them the efficient reactions of such
successions. It seems almost certain, however, that this is merely a
concomitant, and that, even in anomalous successions where facies
replace each other without obvious reasons, the reactions are concerned
more with water-content, light, and humidity than with the food-content
of the soil.
=321. Succession by exhausting the soil.= This is a reaction not at all
understood as yet in nature. A number of phenomena, such as the “fairy
rings” of mushrooms and other fungi, the peripheral growth and central
decay of lichens, _Lecanora_, _Placodium_, _Parmelia_, and of matforming
grasses, such as _Muhlenbergia_, and the circular advance of the
rootstalk plants, indicate that certain plants at least withdraw much of
the available supply of some essential soil element, and are forced to
move away from the exhausted area. It is probable that the constant
shifting of the individuals of a formation year after year, a phenomenon
to be discussed under alternation, has some connection with this. It
will be impossible to establish such a relation, however, until the
facts are exactly determined by the method of quadrat statistics. So far
as native formations are concerned, there can not be the slightest
question that prairies and forests have existed over the same area for
centuries without impoverishing the soil in the least degree, a
conclusion which is even more certain for the open vegetation of deserts
and plains. With culture formations, the case is quite different. The
exhaustion of the soil by continuous or intensive cultivation is a
matter of common experience in all lands settled for a long period.
Calcium, phosphorus, and nitrogen compounds especially are used up by
crops, and must be supplied artificially. The reason for this difference
in reaction between native and culture formations seems evident. In
harvesting, not merely the grain, but the stems and leaves, and in
gardening often the root also, are removed, so that the plant makes
little or no return to the soil. In nature, annual plants return to the
ground every year all the solid matter of roots, stems, leaves, and
fruits, with the exception of the relatively small number of seeds that
germinate. Perennial herbs return everything but the persistent
underground parts. Shrubs and trees replace annually an immense amount
of material used in leaves and fruits, and sooner or later, by the
gradual decay of the individuals or by the destruction of the whole
formation, they restore all that they have taken from the soil. This
balance is further maintained to an important degree by the activity of
the roots, which take from the deep-seated layers of the soil the crude
materials necessary for the formation of leaves and fruits. Upon the
fall and decay of these, their materials are incorporated with the upper
layers of the formation floor, from which they may be absorbed by the
undergrowth, or find their way again into the layers permeated by the
tree roots. From the universal occurrence of weeds in cultivated
regions, the pioneers in impoverished or exhausted fields are uniformly
ruderal plants. As is well known, the seed production and ecesis of
these forms are such that they take possession quickly and completely,
while their demands upon the soil are of such a nature that the most
sterile field can rapidly be covered by a vigorous growth of weeds. As
indicated elsewhere, ruderal formations ultimately yield to the native
vegetation, though in regions so completely given over to culture that
native formations are lacking or remote, it is probable that successions
reach their final stage within the group of ruderal plants.
[Illustration: Fig. 68. Spruce forest formation
(_Picea-Pseudotsuga-hylium_), stage VII, the ultimate stage of the talus
succession.]
=322. Succession by the accumulation of humus.= This is the
characteristic reaction of peat bogs and cypress swamps (_oxodia_), in
which the accumulation of vegetable matter is enormous. The plant
remains decompose slowly and incompletely under the water, giving rise
to the various humic acids. These possess remarkable antiseptic
qualities, and have an injurious effect upon protoplasm. They affect the
absorption of water by the root-hairs, though this is also influenced by
poor aeration. The same acids are found in practically all inland
marshes and swamps, but the quantity of decomposing vegetation in many
is not great enough to produce an efficient reaction. Formations of this
type usually start as freshwater swamps. The succession is apparently
hydrostatic, but no thorough study of its stages has as yet been made.
=323. Succession by modifying atmospheric factors.= All layered
formations, forests, thickets, many meadows and wastes, etc., show
reactions of this nature, and are in fact largely or exclusively
determined by them. The reaction is a complex one, though it is clear
that light is the most efficient of the modified factors, and that
humidity, temperature, and wind, while strongly affected, play
subordinate parts. In normal successions, the effect of shade, i. e.,
diffuse light, enters with the appearance of bushes or shrubs, and
becomes more and more pronounced in the ultimate forest stages. The
reaction is exerted chiefly by the facies, but the effect of this is to
cause increasing diffuseness in each successively lower layer, in direct
ratio with the increased branching and leaf expansion of the plants in
the layer just above. In the ultimate stage of many forests, especially
where the facies are reduced to one, the reaction of the primary layer
is so intense as to preclude all undergrowth. Anomalous successions
often owe their origin to the fact that certain trees react in such a
way as to cause conditions in which they produce seedlings with
increasing difficulty, and thus offer a field favorable to the ecesis of
those species capable of enduring the dense shade. Successions of this
kind are almost invariably mesostatic, as it is altogether exceptional
that layered formations are either xerophytic or hydrophytic.
_LAWS OF SUCCESSION_
=324.= The investigation of succession has so far been neither
sufficiently thorough nor systematic to permit the postulation of
definite laws. Enough has been done, however, to warrant the formulation
of a number of rules, which apply to the successions studied, and afford
a convenient method for the critical investigation of all successions
upon the basis of initial causes, and reactions. Warming has already
brought together a few such rules, and an attempt is here made to reduce
the phenomena of succession, including its causes and effects, to a
tentative system. At present it is difficult to make a thoroughly
satisfactory classification of such rules, and they are here arranged in
general conformity with the procedure in succession.
I. Causation. The initial cause of a succession is the formation
or appearance of a new habitat, or the efficient change of an
existing one.
II. Reaction. Each stage reacts upon the habitat in such a way as
to produce physical conditions more or less unfavorable to its
permanence, but advantageous to the invaders of the next stage.
III. Proximity and mobility.
(1) The pioneers of a succession are those species nearest at
hand that are the most mobile.
(2) The number of migrants from any formation into a habitat
varies inversely as the square of the distance.
(3) The pioneer species are regularly derived from different
formations, as the latter nearly always contain permobile
species capable of effective ecesis.
(4) The plants of the initial stages are normally algae and
fungi, with minute spores, composites, and grasses, which
possess permobile fruits, or ruderal plants, on account of
their great seed production.
IV. Ecesis.
(1) All the migrants into a new, denuded, or greatly modified
habitat are sorted by ecesis into three groups: (1) those that
are unable to germinate or grow, and soon die; (2) those that
grow normally under the conditions present; (3) those that pass
through one or more of the earlier stages in a dormant state to
appear at a later stage of the succession.
(2) Wherever ruderal vegetation is present, it contributes a
large number of the pioneer species of each succession, on
account of the thorough ecesis. In other regions this part is
played by subruderal native species.
(3) Annuals and biennials are characteristic of the early stages
of secondary successions, on account of their great seed
production and ready ecesis.
(4) In layered formations, heliophytes appear before sciophytes;
they ultimately yield to the latter, except where they are able
to maintain a position in the primary layer.
(5) Excessive seed production and slight mobility lead to the
imperfect ecesis of individuals in dense stands, and in
consequence usually produce great instability.
(6) Each pioneer produces about itself a tiny area of ecesis and
stabilization for its own offspring, for the disseminules of
its fellows, or of invaders.
(7) Species propagating by offshoots, or producing relatively
immobile disseminules in small number, usually show effective
ecesis, as the offspring appear within the area of the reaction
of the parent forms.
V. Stabilization.
(1) Stabilization is the universal tendency of vegetation.
(2) The ultimate stage of a succession is determined by the
dominant vegetation of the region. Lichen formations are often
ultimate in polar and niveal zones; grassland is the final
vegetation for plains and alpine stretches, and for much
prairie, while forest is the last stage for mesophytic midlands
and lowlands, as well as for subalpine regions.
(3) Grassland or forest is the usual terminus of a succession;
they predominate in lands physiographically mature.
(4) The limit of a succession is determined in large part by the
progressive increase in occupation, which makes the entrance of
invaders more and more difficult.
(5) Stabilization proceeds radiately from the pioneer plants or
masses. The movement of offshoots is away from the parent mass,
and the chances of ecesis are greatest near its edges, in a
narrow area in which the reaction is still felt, and the
occupation is not exclusive.
VI. General laws.
(1) The stages, or formations, of a succession are distinguished
as initial (_prodophytia_), intermediate (_ptenophytia_), and
ultimate (_aiphytia_).
(2) Initial formations are open, ultimate formations are closed.
(3) The number of species is small in the initial stages; it
attains a maximum in intermediate stages; and again decreases
in the ultimate formation, on account of the dominance of a few
species.
(4) The normal sequence of vegetation forms in succession is: (1)
algae, fungi, mosses; (2) annuals and biennials; (3) perennial
herbs; (4) bushes and shrubs; (5) trees.
(5) The number of species and of individuals in each stage
increases constantly up to a maximum, after which it gradually
decreases before the forms of the next stage. The interval
between two maxima is occupied by a mixed formation.
(6) A secondary succession does not begin with the initial stage
of the primary one which it replaces, but usually at a much
later stage.
(7) At present, successions are generally mesotropic, grassland
and forest being the ultimate stages, though many are
xerostatic or hydrostatic. If erosion continue until the sea
level is reached, the ultimate vegetation of the globe will be
hydrophytic. Should the heat of the sun decrease greatly before
this time, the last vegetation will be xerophytic, i. e.,
crymophytic.
(8) The operation of succession was essentially the same during
the geological past as it is to-day. From the nature of their
vegetation forms, the record deals largely with the ultimate
stages of such successions.
_CLASSIFICATION AND NOMENCLATURE_
=325. Basis.= New or denuded habitats arise the world over by the
operation of the same or similar causes, and they are revegetated in
consequence of the same reactions. Similar habitats produce similar
successions. The vegetation forms and their sequence are usually
identical, and the genera are frequently the same, or corresponding in
regions not entirely unrelated. The species are derived from the
adjacent vegetation, and, except in alpine and coast regions, are
normally different. The primary groups of successions are determined by
essential identity of habitat or cause, e. g., aeolian successions,
erosion successions, burn successions, etc. When they have been more
generally investigated, it will be possible to distinguish subordinate
groups of successions, in which the degree of relationship is indicated
by the similarity of vegetation forms, the number of common genera, etc.
For example, burn successions in the Ural and in the Rocky mountains
show almost complete similarity in the matter of vegetation forms and
their sequence, and have the majority of their genera in common. A
natural classification of successions will divide them first of all into
normal and anomalous. The former fall into two classes, primary and
secondary, and these are subdivided into a number of groups, based upon
the cause which initiates the succession.
[Illustration: Fig. 69. Aspen forest formation (_Populus-hylium_), the
typical stage of burn successions in the Rocky mountains; it is
sometimes an anomalous stage in primary successions, interpolated in
place of the thicket formation.]
=326. Nomenclature.= The need of short distinctive names of
international value for plant formations is obvious; it has become
imperative that successions also should be distinguished critically and
designated clearly. From the very nature of the case, it is impossible
to designate each formation or succession by a single Greek or Latin
term, as habitats of the same character will show in different parts of
the world a vegetation taxonomically very different. It may some day be
possible to use a binomial or trinomial for this purpose, somewhat after
the fashion of taxonomy, in which the habitat name will represent the
generic idea as applied to formations, and a term drawn from the
floristic impress the specific idea. Such an attempt would be futile or
valueless at the present time; it could not possibly meet with success
until there is more uniformity in the concept of the formation, and
until there has been much accurate and thorough investigation of actual
formations, a task as yet barely begun. At present, it seems most
feasible as well as scientific to designate all formations occupying
similar habitats by a name drawn from the character of the latter, such
as a meadow formation, _poium_, a forest formation, _hylium_, a desert
formation, _eremium_, etc. A particular formation is best designated by
using the generic name of one or two of its most important species in
conjunction with its habitat term, as _Spartina-Elymus-poium_,
_Picea-Pinus-hylium_, _Cereus-Yucca-eremium_, etc. Apparently a somewhat
similar nomenclature is adapted to successions. The cause which produces
a new habitat may well furnish the basis for the name of the general
groups of successions, as _pyrium_ (literally, a place or a habitat
burned over), a burn succession, _tribium_, an erosion succession, etc.
A burn succession consists of a sequence of certain formations in one
part of the world, and of a series of quite different ones,
floristically, in another. A particular burn succession should be
designated by using the names of a characteristic facies of the initial
and ultimate stages in connection with the general term, e. g.,
_Bryum-Picea-pyrium_, etc. A trinomial constructed in this way
represents the desirable mean between definition and brevity. Greater
definiteness is possible only at the expense of brevity, while to
shorten the name would entirely destroy its precision. The following
classification of successions is proposed, based upon the plan outlined
above. The termination _-ium_ (εῖον) has been used throughout in the
construction of names for successions, largely for reasons of euphony.
If it should become desirable to distinguish the names of formations and
successions by the termination, the locative suffix _-on_ (-ών) should
be used for the latter. The terms given below would then be _hypson_,
_rhyson_, _hedon_, _sphyron_, _prochoson_, _pnoon_, _pagon_, _tribon_,
_clyson_, _repon_, _olisthon_, _xerasion_, _theron_, _broton_, _pyron_,
_ecballon_, _camnon_, _ocheton_, _ardon_.
I. Normal successions: cyriodochae (κύριος, regular, δοχή, ἡ,
succession)
_a._ Primary successions: protodochae (πρῶτος, first, primary)
1. By elevation: hypsium (ὔψος, το, height, elevation, -εῖον, place)
2. By volcanic action: rhysium (ῥυσίς ἡ, flowing, especially of
fire)
3. In residuary soils: hedium (ἔδος, τό, a sitting base)
4. In colluvial soils: sphyrium (σφύρον, τό, ankle, talus)
5. In alluvial soils: prochosium (πρόχωσις, ἡ, a deposition of mud)
6. In aeolian soils: pnoium (πνοή, ἡ, blowing, blast)
7. In glacial soils: pagium (πάγος, ὁ, that which becomes solid, i.
e., a glacier)
_b._ Secondary successions: hepodochae (ἕπω, to follow)
8. In eroded soils: tribium (τρίβω, wear or rub away)
9. In flooded soils: clysium (κλύσις, ὁ, a drenching, flooding)
10. By subsidence: repium (ῥέπω, incline downwards, sink)
11. In landslips: olisthium (ὄλισθος, ὁ, slip)
12. In drained and dried out soils: xerasium (ξηρασία, ἡ, drought)
13. By animal agencies: therium (θήρ, ὁ, wild animal)
14. By human agency: brotium (βροτός, ὁ, a mortal)
_a._ Burns: pyrium (πῦρ, τό, fire)
_b._ Lumbering: ecballium (ἐκβάλλω, cut down forests)
_c._ Cultivation: camnium (κάμνω, cultivate)
_d._ Drainage: ochetium (ὀχετός, ὁ, drain)
_e._ Irrigation: ardium (ἄρδω, irrigate)
II. Anomalous successions: xenodochae (ξένος, strange, unusual)
=327. Illustrations.= The following series will illustrate the
application of this system of nomenclature to particular successions,
and their stages, or formations.
Thlaspi-Picea-sphyrium: pennycress-spruce talus succession
Thlaspi-Eriogonum-chalicium: pennycress-eriogonum gravel slide
formation
Elymus-Gilia-chalicium: wildrye-gilia half gravel slide formation
Quercus-Holodiscus-driodium: oak-fringewood dry thicket formation
Pinus-xerohylium: pine dry forest formation
Picea-Pseudotsuga-hylium: spruce-balsam forest formation
Bryum-Picea-pyrium: moss-spruce burn succession
Bryum-telmatium: moss meadow formation
Aster-Chamaenerium-poium: aster-fireweed meadow formation
Deschampsia-Carex-poium: hairgrass-sedge meadow formation
Salix-Betula-helodrium: willow-birch meadow thicket formation
Populus-hylium: aspen forest formation
Picea-hylium: spruce forest formation
Lecanora-Carex-hedium: lichen-carex residuary succession
Lecanora-Gyrophora-petrium: crustose lichen rock formation
Parmelia-Cetraria-chalicium: foliose lichen gravel slide formation
Paronychia-Silene-chalicium: nailwort-campion gravel slide formation
Carex-Campanula-coryphium: sedge-bluebell alpine meadow formation
Eragrostis-Helianthus-xerasium: eragrostis-sunflower drainage
succession
Eragrostis-Polygonum-telmatium: eragrostis-heartsease wet meadow
formation
Helianthus-Ambrosia-chledium: sunflower-ragweed waste formation
_INVESTIGATION OF SUCCESSION_
=328. General rules.= The study of succession must proceed along two
fundamental lines of inquiry: it is necessary to investigate
quantitatively the physical factors of the initial stages and the
reactions produced by the subsequent stages. This should be done by
automatic instruments for humidity, light, temperature, and wind, in
order that a continuous record may be obtained. Water-content is taken
daily or even less frequently, while soil properties, and physiographic
factors, altitude, slope, surface, and exposure are determined once for
all. It is equally needful to determine the development and structure of
each stage with particular reference to the adjacent formations, to the
stage that has just preceded, and the one that is to follow. For this,
the use of the permanent quadrat is imperative, as the sequence and
structure of the stages can be understood only by a minute study of the
shifting and rearrangement of the individuals. Permanent migration
circles are indispensable for tracing movement away from the pioneer
areas by which each stage reaches its maximum. Denuded quadrats are a
material aid in that they furnish important evidence with respect to
migration and ecesis, By means of them, it is possible to determine the
probable development of stages which reach back a decade or more into
the past. In the examination of successions, since cause and effect are
so intimately connected in each reaction, it is especially important
that general and superficial observations upon structure and sequence be
replaced by precise records, and that vague conjectures as to causes and
reactions be supplanted by the accurate determination of the physical
factors which underlie them.
[Illustration: Fig. 70. Alternating gravel slides on Mounts Cameron and
Palsgrove, from the comparison of which the initial development of the
talus succession has been reconstructed.]
=329. Method of alternating stages.= The period of time through which a
primary succession operates is usually too great to make a complete
study possible within a single lifetime. Secondary successions run their
course much more quickly, and a decade will sometimes suffice for
stabilization, though even here the period is normally longer. The
longest and most complex succession, however, may be accurately studied
in a region, where several examples of the same succession occur in
different stages of development. In the same region, the physical
factors of one example of a particular succession are essentially
identical with those of another example in the same stage. If one is in
an initial stage, and the other in an intermediate condition, the
development of the former makes it possible to reestablish more or less
completely the life history of the latter. The same connection may be
made between intermediate and ultimate stages, and it is thus possible
to determine with considerable accuracy and within a few years the
sequence of stages in a succession that requires a century or more for
its complete development. In the Rocky mountains, gravel slides (talus
slopes) are remarkably frequent. They occur in all stages of
development, and the alternating slides of different ages furnish an
almost perfect record of this succession. This method lacks the absolute
finality which can be obtained by following a succession in one spot
from its inception to final stabilization, but it is alone feasible for
long successions, i. e., those extending over a score or more of years.
When it comes to be universally recognized as a plain duty for each
investigator to leave an exact and complete record in quadrat maps and
quadrat photographs of the stages studied by him, it will be a simple
task for the botanists of one generation to finish the investigations of
succession begun by their predecessors.
=330. The relict method= of studying succession is next in importance to
the method of alternating areas. The two in fact are supplementary, and
should be used together whenever relicts are present. This method is
based upon the law of successive maxima, viz., the number of species and
of individuals in each stage constantly increases up to a certain
maximum, after which it gradually decreases before the forms of the next
stage. In accordance with this, secondary species usually disappear
first, principal species next, and facies last of all. There are notable
exceptions to this, however, and the safest plan is to use the relict
method only when principal species or facies are left as evidence. An
additional reason for this is that secondary species are more likely to
be common to two or more formations. In the majority of cases, the
relict is not modified, and is readily recognized as belonging properly
to a previous stage. This is true of herbs in all the stages of
grassland, and in the initial ones of forest succession. The herbs and
shrubs of earlier stages, which persist in the final forest stages, are
necessarily modified, often in such a degree as to become distinct
ecads, or species. The facies of the stages which precede the ultimate
forest are rarely modified. The application of the relict method,
together with the modification just described, is nicely illustrated by
the balsam-spruce formation at Minnehaha. Of the initial gravel slide
stage, the relicts are _Vagnera stellata_ and _Galium boreale_, the one
modified into _Vagnera leptopetala_, and the other into _G. boreale
hylocolum_. The thicket stage is represented by _Holodiscus dumosa_,
greatly changed in form and branching, and in the shape and structure of
the leaf. The most striking relict of the aspen formation is the facies
itself, _Populus tremuloides_. The tall slender trunks of dead aspens
are found in practically every balsam-spruce forest. In many places,
living trees are still found, with small, straggling crowns, which are
vainly trying to outgrow the surrounding conifers. Of the aspen
undergrowth, _Rosa sayii_, _Helianthella parryi_, _Frasera speciosa_,
_Zygadenus elegans_, _Castilleia confusa_, _Gentiana acuta_, and
_Solidago orophila_ remain more or less modified by the diffuse light.
It is still a question whether the aspen stage passes directly into the
balsam-spruce forest, or whether a pine forest intervenes. The presence
of both _Pinus ponderosa_ and _P. flexilis_, which are scattered more or
less uniformly through the formation, furnishes strong evidence for the
latter view.
[Illustration: Fig. 71. Relict spruces and aspens, showing the character
of the succession immediately preceding the burn succession now
developing.]
The lifetime of forest and thicket stages of successions is ascertained
by counting the annual rings of the stumps of facies. This is a
perfectly feasible method for many woodland formations where stumps
already abound or where a fire has occurred, and it is but rarely
necessary to cut down trees for this purpose. When trees or shrubs are
present as relicts, the same method is used to determine the length of
time taken by the development of the corresponding stages.
THE STRUCTURE OF THE FORMATION
=331.= Since all the structures exhibited by formations, such as zones,
layers, consocies, etc., are to be referred to zonation or alternation,
these principles are first considered in detail. This, then, constitutes
the basis for a consideration of the structure of a normal formation,
with special reference to the different parts that compose it. The
investigation of formational structure, since the latter is the result
of aggregation, invasion, and succession, is accomplished by
instruments, quadrats, etc., in the manner already indicated under
development, and no further discussion of it is necessary here.
ZONATION
=332. Concept.= The recognition of vegetation zones dates from
Tournefort[40], who found that, while the plants of Armenia occupied the
foot of Mount Ararat, the vegetation of the slopes above contained many
species of southern Europe. Still higher appeared a flora similar to
that of Sweden, and on the summit grew arctic plants, such as those of
Lapland.
As the historical summary shows, the concept of zonation is the oldest
in phytogeography. Notwithstanding this, it has never been clearly
defined, nor has there been any detailed investigation of the phenomenon
itself, or of the causes which produce it. Zones are so common, and
often so clearly marked, that they invite study, but no serious attempt
has heretofore been made to analyze zonation, or to formulate a definite
method of investigating it. Zonation is the practically universal
response of plants to the quantitative distribution of physical factors
in nature. In almost all habitats, one or more of the physical factors
present decreases gradually in passing away from the point of greatest
intensity. The result is that the plants of the habitat arrange
themselves in belts about this point, their position being determined by
their relation to the factor concerned. Close investigation will show
that there is hardly a formation that is entirely without zonation,
though in many cases the zones are incomplete or obscure for various
reasons. Zonation is as characteristic of vegetation as a whole as it is
of its unit, the formation, a fact long ago recognized in temperature
zones. A continental climate, however, often results in the interruption
of these, with the consequence that these belts of vegetation are not
always continuous.
_CAUSES OF ZONATION_
=333. Growth.= The causes that produce zones are either biological or
physical: the first have to do with some characteristic of the plant,
the second with the physical features of the habitat. Biological causes
arise from the method of growth, from the manner of dissemination, or
from the reaction of the species upon the habitat. The formation of
circles as a result of radial growth is a well-known occurrence with
certain plants, but it is much more common than is supposed. In the case
of agarics, this phenomenon has long been known under the name of
“fairy-rings.” It is found in a large number of moulds, and is
characteristic of early stages of the mycelium of the powdery mildews.
It occurs in nearly all maculicole fungi, and is exhibited by certain
xylogenous fungi, such as _Hysterographium_. Among the foliose lichens,
it is a common occurrence with the rock forms of _Parmelia_,
_Placodium_, _Physcia_, and _Lecanora_, and with the earth forms of
_Parmelia_ and _Peltigera_. The thalloid liverworts show a similar
radial growth. The flowering plants, and many mosses also, furnish good
examples of this sort of growth in those species which simulate the form
of the mycelium or thallus. These are the species that form mats, turfs,
or carpets. Alpine mat formers, such as _Silene acaulis_, _Paronychia
pulvinata_, _Arenaria sajanesis_, etc., are typical examples.
Xerophytic, turf-forming species of _Muhlenbergia_, _Sporobolus_,
_Bouteloua_, _Festuca_, _Poa_, and other grasses form striking ring-like
mats, while creeping species of _Euphorbia_, _Portulaca_, _Amarantus_,
etc., produce circular areas. Rosettes, bunch-grasses, and many ordinary
rootstalk plants spread rapidly by runners and rhizomes. The direction
of growth is often indeterminate in these also, and is in consequence
more or less bilateral or unilateral. Growth results in zonation only
when the older central portions of the individual or mass die away,
leaving an ever-widening belt of younger plants or parts. This
phenomenon is doubtless due in part to the greater age of the central
portion, but seems to arise chiefly from the demands made by the young
and actively growing parts upon the water of the soil. There may
possibly be an exhaustion of nutritive content, as in the case of the
fungi, but this seems improbable for the reason that young plants of the
same and other species thrive in these areas. It must not be inferred
that these miniature growth zones increase in size until they pass into
zones of formations. Growth contributes its share to the production of
these, but there is no genetic connection between a tiny plant zone and
a zone of vegetation.
Radial and bilateral growth play an important part in formational zones
in so far as they are related to migration. The growth of the runner or
rhizome itself is a very effective means of dissemination, while the
seeding of the plants thus carried away from the central mass is most
effective at the edge of the newly occupied area. This holds with equal
force for plants with a mycelium or a thallus. The circular area becomes
larger year by year. Sooner or later, the younger, more vigorous, and
more completely occupied circumference passes into a more or less
complete zone. This will result from the reaction of the central
individuals upon the habitat, so that they are readily displaced by
invaders, or from their increasing senility and dying out, or from the
invasion of forms which seed more abundantly and successfully. This
result will only be the more marked if the radiating migrants reach a
belt of ground especially favorable to their ecesis. In this connection
it must be carefully noted that vegetation pressure, before which weaker
plants are generally supposed to flee, or by which they are thought to
be forced out into less desirable situations, is little more than a
fanciful term for radial growth and migration. It has been shown under
invasion that disseminules move into vegetation masses, as well as away
from them, the outward movement alone being conspicuous, because it is
only at the margin and beyond that they find the necessary water and
light for growth.
=334. Reactions.= Certain reactions of plants upon habitats produce
zonation. The zones of fungi are doubtless caused by the exhaustion of
the organic matter present, while in lichens and mosses the decrease in
nutritive content has something to do with the disappearance of the
central mass. In the mats of flowering plants, the connection is much
less certain. The reaction of a forest or thicket, or even of a tall
herbaceous layer, is an extremely important factor in the production of
zonation. The factor chiefly concerned here is light. Its intensity is
greatest at the edge of the formation and just below the primary layer;
the light becomes increasingly diffuse toward the center of the forest,
and toward the ground. In response to this, both lateral and vertical
zones appear. The former are more or less incomplete, and are only in
part due to differences in illumination. The vertical zones or layers
are characteristic of forest and thickets, and are caused directly by
differences in light intensity.
[Illustration: Fig. 72. Zones of _Cyperus erythrorrhizus_ produced by
the recession of the shore-line.]
=335. Physical factors.= The physical causes of zonation are by far the
most important. They arise from differences in temperature, water, and
light. In the large, temperature differences are the most important,
producing the great zones of vegetation. In a particular region or
habitat, variations of water-content and humidity are controlling, while
light, as shown above, is important in the reactions of forest and
thicket. Physical factors produce zonation in a habitat or a series of
habitats, when there is either a gradual and cumulative, or an abrupt
change in their intensity. Gradual, slight changes are typical of single
habitats; abrupt, marked changes of a series of habitats. This
modification of a decisive factor tends to operate in all directions
from the place of greatest intensity, producing a characteristic
symmetry of the habitat with reference to the factor concerned. If the
area of greatest amount is linear, the shading-out will take place in
two directions, and the symmetry will be bilateral, a condition well
illustrated by rivers. On the other hand, a central intense area will
shade out in all directions, giving rise to radial symmetry, as in
ponds, lakes, etc. The essential connection between these is evident
where a stream broadens into a lake, or the latter is the source of a
stream, where a mountain ridge breaks up into isolated peaks, or where a
peninsula or landspit is cut into islands. The line that connects the
points of accumulated or abrupt change in the symmetry is a stress line
or _ecotone_. Ecotones are well-marked between formations, particularly
where the medium changes; they are less distinct within formations. It
is obvious that an ecotone separates two different series of zones in
the one case, and merely two distinct zones in the other.
[Illustration: Fig. 73. Regional zones on a spur of Pike’s Peak (3,800
m.); the forest consists of _Picea engelmannii_ and _Pinus aristata_,
the forewold is _Salix pseudolapponum_, and the grassland, alpine meadow
(_Carex-Campanula-coryphium_).]
=336. Physiographic symmetry.= The physical symmetry of a habitat
depends upon the distribution of water in it, and this is profoundly
affected by the soil and the physiography. The influence of
precipitation is slight or lacking, as it is nearly uniform throughout
the habitat; the effects of wind and humidity are more localized.
Differences of soil rarely obtain within a single habitat, though often
occurring in a zoned series. The strikingly zonal structure or
arrangement of habitats is nearly always due to differences in
water-content produced by physiographic factors, slope, exposure,
surface, and altitude. The effect of these upon water-content and
humidity is obvious. Wherever appreciable physiographic differences
occur, there will be central areas of excess and deficiency in
water-content, between which there is a symmetrical modification of this
factor. Peaks are typical examples of areas of deficiency, lakes and
oceans of areas of excess. When these areas are extreme and close to
each other, the resulting zonation will be marked; when they are
moderate, particularly if they are widely separated, the zones produced
are obscure. Asymmetry of a habitat or a region practically does not
exist. Central areas of excess and deficiency may be very large and in
consequence fail to seem symmetrical, or the space between them so great
that the symmetry is not conspicuous, but they are everywhere present,
acting as foci for the intervening areas.
The response of vegetation to habitat is so intimate that physiographic
symmetry everywhere produces vegetational symmetry, which finds its
ready expression in plant zones. The reaction of vegetation upon habitat
causes biological symmetry, typical of growth zones and light zones.
From these facts it is clear that zonation will be regularly
characteristic of the vegetative covering. The zonal arrangement of
formations is usually very evident; the zones of a formation are often
obscured, or, where the latter occupies a uniform central area of excess
or deficiency, they are rudimentary or lacking, as in shallow ponds.
Zones are frequently imperfect, though rarely entirely absent in new
soils, such as talus. They are rendered obscure in several ways. In the
initial stages of a succession, as well as in the transitions between
the various stages, the plant population is so scattered, so transient,
or so dense as to respond not at all to a degree of symmetry which
produces marked zonation in later formations. The alternation of
conspicuous species not only causes great interruption of zones, but
often also completely conceals the zonation of other species, such as
the grasses, which, though of more importance in the formation, have a
lower habit of growth. Furthermore, the ecotones of one factor may run
at right angles to those of another, and the resulting series of zones
mutually obscure each other. Finally, such a physiographic feature as a
hill may have its symmetry interrupted by ridges or ravines, which
deflect the zones downward or upward, or cause them to disappear
altogether, while the shallows or depths of a pond or lake may have the
same effect. An entire absence of zones, i. e., azonation, is
exceptional in vegetation. Almost all cases that seem to exhibit it may
be shown by careful examination to arise in one of the several ways
indicated above.
_KINDS OF ZONATION_
=337.= Two kinds of zonation are distinguished with reference to the
direction in which the controlling factor changes. When this is
horizontal, as with water-content and temperature, zonation will be
lateral; when it is vertical, as in the case of light, the zonation is
vertical. There exists an intimate connection between the two in
forests, where the secondary layer of small trees and shrubs is
continuous with a belt of trees and shrubs around the central nucleus,
and the lower layers of bushes and herbaceous plants with similar zones
still further out. This connection doubtless arises from the fact that
conditions are unfavorable to the facies, outside of the nucleus as well
as beneath it. Floristically, each layer and its corresponding zone are
distinct, as the one consists of shade, the other of sun species.
Lateral zonation is radial when the habitat or physiographic feature is
more or less circular in form, and it is bilateral, when the latter is
elongated or linear. Vertical zonation is unilateral.
=338. Radial zonation= is regularly characteristic of elevations and
depressions. From the form of the earth, it reaches its larger
expression in the girdles of vegetation corresponding to the zones of
temperature. The zones of mountain peaks are likewise due largely to
temperature, though humidity is a very important factor also. Mountain
zones are normally quite perfect. The zonation of islands, hills, etc.,
is due to water-content. In the former, the zones are usually quite
regular and complete; in the latter, they are often incomplete or
obscured. Prairies and steppes are not zoned as units, but are complexes
of more or less zonal hills and ridges. Ponds, lakes, and seas regularly
exhibit complete zones, except in those shallow ponds where the depth is
so slight that what is ordinarily a marginal zone is able to extend over
the entire bottom. The line between an elevation and a depression, i.
e., the edge of the water level, is the most sharply defined of all
ecotones. It separates two series of zones, each of which constitutes a
formation. One of these is regularly hydrophytic, the other is usually
mesophytic. The line between the two can rarely be drawn at the water’s
edge, as this is not a constant, owing to waves, tides, or periodical
rise and fall. There is in consequence a more or less variable
transition zone of amphibious plants, which are, however, to be referred
to the hydrophytic formation. Nearly all forest formations serve as a
center about which are arranged several somewhat complete zones. As a
rule, these merge into a single heterogeneous zone of thickets.
=339. Bilateral zonation= differs from radial only in as much as it
deals with linear elevations and depressions instead of circular ones.
With this difference, the zones of ranges and ridges correspond exactly
to those of peaks and hills, while the same relation is evident between
the zones of streams, and of lakes and ponds. The ecotones are identical
except as to form; they are linear in the one and circular in the other.
Incompleteness is more frequently found in bilateral zonation, though
this is a question of distance or extent, rather than one of symmetry.
=340. Vertical zonation= is peculiar in that there is no primary ecotone
present, on either side of which zones arrange themselves with reference
to the factor concerned. This arises from the fact that the controlling
factor is light, which impinges upon the habitat in such manner as to
shade out in but one direction, i. e., downward. Vertical zones appear
in bodies of water, on account of the absorption of light by the water.
In a general way, it is possible to distinguish bottom, plancton, and
surface zones, consisting almost wholly of algae. There is little
question that minor zones exist, especially in lakes and seas, but these
await further investigation. The most characteristic vertical zones
occur in forests, where the primary layer of trees acts as a screen. The
density of this screen determines the number of zones found beneath it.
In extreme cases the foliage is so dense that the light beneath is
insufficient even for mosses and lichens. As a rule, however, there will
be one or more zones present. In an ordinary deciduous forest, the
layers below the facies are five or six in number: (1) a secondary layer
of small trees and shrubs, (2) a tertiary layer of bushes, (3) an upper
herbaceous layer of tall herbs, (4) a middle herbaceous layer, (5) a
lower herbaceous layer, (6) a ground layer of mosses, lichens, other
fungi, and algae. The upper layers are often discontinuous, the lower
ones are more and more continuous. As a forest becomes denser, its
layers disappear from the upper downward, the ground layer always being
the last to disappear because of its ability to grow in very diffuse
light. A vertically zoned formation shows a complex series of reactions.
The primary layer determines the amount of heat, light, water, wind,
etc., for the subordinate layers in general. Each of these layers then
further determines the amount for those below it, the ground layer being
subject in some degree to the control of every layer above it. This
accounts probably for the definiteness and permanence of this layer. The
degree to which the lower layers influence the upper by reacting upon
the habitat is not known. It is evident that this influence must be
considerable by virtue of their control of the water supply in the upper
soil strata, by virtue of their transpiration, their decomposition, etc.
The ecotone between two formations is never a sharp line, but it is an
area of varying width. The edge of this area which is contiguous to one
formation marks the limit for species of the other. Both formations
disappear in this transition zone, but in opposite directions. The
overlapping which produces such zones arises from the fact that the
physical factors tend to approach each other at the line of contact
between formations, and that many species are more or less adjustable to
conditions not too dissimilar.
=341. Vegetation zones.= As a fundamental expression of progressive
change in the amount of heat and water, zonation is the most important
feature of vegetation. It constitutes the sole basis for the division of
continental as well as insular vegetation. The continent of North
America furnishes striking proof of the truth of this. Conforming to the
gradual decrease of temperature and water-content northward, three
primary belts of vegetation stretch across the continent from east to
west. These are forest, grassland, and polar desert. The first is
further divided into the secondary zones of broad-leaved evergreen,
deciduous, and needle-leaved forests. At right angles to this
temperature-water symmetry lies a symmetry due to water alone, in
accordance with which forest belts touch the oceans, but give way in the
interior to grasslands, and these to deserts. It is at once evident that
the mutual interruption of these two series of zones has produced the
primary features of North America vegetation, i. e., tropical forests
where heat and water are excessive, deserts where either is unusually
deficient, grassland when one is low, the other moderate, and deciduous
and coniferous forests, where the water-content is as least moderate and
the temperature not too low. Such a simple yet fundamental division has
been modified, however, by the disturbing effect which three continental
mountain systems have had upon humidity and upon temperature symmetry.
The two are intimately interwoven. The lowering of temperature due to
altitude produces the precipitation of the wind-borne moisture upon
those slopes which look toward the quarter from which the prevailing
winds blow. A mountain range thus makes an abrupt change in the
symmetry, and renders impossible the gradual change from forest to
grassland and desert. The Appalachian system is not sufficiently high to
produce a pronounced effect, and forests extend far beyond it into the
interior before passing into prairies and plains. On the other hand, the
influence of the Rocky mountains and the Sierra Nevada is very marked.
The latter rise to a great height relatively near the coast, and
condense upon their western slopes nearly all of the moisture brought
from the Pacific. The Rocky mountains have the same effect upon the much
drier winds that blow from the east, and the two systems in consequence
enclose a parched desert. This series of major zones thus becomes,
starting at the east, forest, grassland, desert, and forest, instead of
the more symmetrical series, forest, grassland, desert, grassland,
forest, which would prevail were it not for these barriers. This actual
series of major zones undergoes further interruption by the action of
these mountain systems in deflecting northern isotherms far to the
south. This action is greatest in the high ranges, the Rocky mountains
and the Sierras, and least in the lower Appalachians. Its result is to
carry the polar deserts of the north far southward along the crests of
the mountains, and to extend the boreal coniferous forests much further
south along their slopes. In the Appalachians, this means no more than
the extension of a long tongue of conifers into the mass of deciduous
forests, and the occasional appearance of an isolated peak. In the
western ranges, it produces two symmetrical series of minor mountain
zones, forest, alpine grassland or desert, and forest, to say nothing of
the foot-hill and timber-line zones of thicket.
There seems to be no good reason for distinguishing the zones of
mountains as regions. The term itself is inapplicable, as it has no
reference to zonation, and is used much more frequently as a term of
general application. Its use tends to obscure also the essential
identity of the so-called vertical zones of mountains with the major
continental zones, an identity which can not be insisted upon too
strongly. For the sake of clearness, it is important to distinguish all
belts of vegetation as zones, though it is evident that these are not
all of the same rank. The following division of the vegetation of North
America is based upon the fundamental principles of continental symmetry
and the community of continental and mountain zones.
I. Polar-niveal zone—zona polari-nivalis
II. Arctic-alpine zone—zona arctici-alpina
Arctic province—provincia arctica
Alpine province—provincia alpina
III. Boreal-subalpine zone—zona boreali-subalpina
Alaska province—provincia alaskana
Cordilleran province—provincia cordillerana
Ontario province—provincia ontariensis
IV. Temperate zone—zona temperata
Atlantic province—provincia atlantica
Appalachian province—provincia appalachiana
Nebraska province—provincia nebraskensis
Utah province—provincia utahensis
Coast province—provincia litoralis
Pacific province—provincia pacifica
V. Subtropical zone—zona subtropicalis
Florida province—provincia floridana
Mexican province—provincia mexicana
VI. Tropical zone—zona tropicalis
Antilles province—provincia antilleana
Andean province—provincia andeana
ALTERNATION
=342. Concept.= The term alternation is used to designate that
phenomenon of vegetation, in which a formation recurs at different
places in a region, or a species at separate points in a formation.
Although it is a fundamental feature of vegetation, it has been
recognized but recently.[41]
Alternation is the response of vegetation to the heterogeneity of the
surface of the earth. It is in sharp contrast to zonation, inasmuch as
it is directly caused by asymmetry in the topography. In consequence, it
deals with the subdivisions of zones, arising from physical differences
within the symmetrical area. It deals with vegetation areas of every
rank below that of major zone, with the habitat and geographical areas
of species, and, in a certain way, with the correspondence of vicarious
genera. The breaking up of vegetation into formations is a striking
example of alternation. The same phenomenon occurs in every formation,
producing consocies and minor plant groups, and everywhere giving
variation to its surface and structure. The essential idea involved in
this principle is the recurrence of like formations, consocies, or
groups, which are more or less separated by formations, consocies, or
groups differing from them. It is an exact expression of the primary law
of association that heterogeneity of structure varies directly as the
extent and complexity of the habitat, or the series of habitats.
Vegetation is made up of what are superficially homogeneous formations,
but upon analysis these are seen to contain consocies. The latter,
though more uniform than formations, break up into groups, each of which
still shows a characteristic heterogeneity arising from the varying
number and arrangement of its constituent species.
=343. Causes.= The primary cause of alternation is physical asymmetry,
which is everywhere present within the symmetrical areas which produce
zones. This is influenced so strongly, however, by migration and plant
competition (_phyteris_) that the consideration of this subject will
gain in clearness if these are treated as separate causes. The essential
relation between them must not be lost sight of, however. Migration
carries disseminules into all, or only some of the different areas of a
formation, or into different formations, with little respect to the
physical nature of these. The physical character of these asymmetrical
areas determines that some of these plants shall be established in one
series of places, and some in another, while the competition between the
individuals in the various areas determines the numerical value of each
species as well as its persistence. These three causes are invariably
present in the production of alternating areas, and originally, i. e.,
in new or denuded soils, the sequence is constant, viz., migration,
ecesis in asymmetrical areas, and competition.
With respect to the different portions of an asymmetrical area,
migration will have one of three effects: (1) it will carry disseminules
into both favorable and unfavorable areas, (2) into favorable ones only,
or (3) into unfavorable ones alone. From the radial nature of migration,
the first case is far the most frequent; it is typical of sporostrotes,
and the highly specialized spermatostrotes and carpostrotes. The effect
of migration is uniform here, and alternation arises in consequence of
the selective power of ecesis. It is evident that migration does not
have an even indirect effect, when the disseminules are carried into
none but unfavorable situations. Where the movement is into favorable
places alone, alternation is the immediate result. The intermittent
operation of migration and the presence of barriers are responsible for
the absence of plants in situations favorable to them, and in
consequence bring about a certain alternation between corresponding
species.
The selective operation of physical factors upon the disseminules
carried into the different parts of an asymmetrical area is the usual
cause of alternation. Asymmetry alone is universal within the more
conspicuous structures termed zones, down to the smallest areas which a
group of plants can occupy. The difference between contiguous areas,
particularly within the same habitat, is often small. It sometimes seems
inefficient in the initial stages of a succession when a single species
is present, but even in extreme cases its effect will be recognizable in
the size and density of the individuals. Asymmetry is clearly evident in
vegetation where two symmetrical series cross each other, or when a
symmetry is interrupted by barrier-like elevations or depressions.
Within formations, it arises from differences, often very slight, in
slope, exposure, elevation, from irregularities of surface, differences
in soil structure, or composition, in the amount of cover, and in the
reactions of the living plants. At the last point, it is in direct
connection with plant competition.
=344. Competition.= Much uncertainty, as well as diversity of opinion,
seems still to exist in regard to the precise nature of the competition
between plants that occupy the same area. It has long been admitted that
the phrase, “struggle for existence,” is true of this relation only in
the most figurative sense, but the feeling still prevails that, since
plants live in associations, there must be something mysterious and
vitalistic in their relation. No one has been able to discover anything
of this nature, but nevertheless the impression remains. Such a direct
relation exists only between parasites, epiphytes, and lianes, and the
plants which serve to nourish or support them. In the case of plants
growing on the same stratum, actual competition between plant and plant
does not occur. One individual can affect another only in as much as it
changes the physical factors that influence the latter. Competition is a
question of the reaction of a plant upon the physical factors which
encompass it, and of the effect of these modified factors upon the
adjacent plants. In the exact sense, two plants do not compete with each
other as long as the water-content and nutrition, the heat and light are
in excess of the needs of both. The moment, however, that the roots of
one enter the area from which the other draws its water supply, or the
foliage of one begins to overshade the leaves of the other, the reaction
of the former modifies unfavorably the factors controlling the latter,
and competition is at once initiated. The same relation exists
throughout the process; the stronger, taller, the more branched, or the
better rooted plant reacts upon the habitat, and the latter immediately
exerts an unfavorable effect upon the weaker, shorter, less branched, or
more poorly rooted plant. This action of plant upon habitat and of
habitat upon plant is cumulative, however. An increase in the leaf
surface of a plant not merely reduces the amount of light and heat
available for the plant near it or beneath it, but it also renders
necessary the absorption of more water and other nutritive material, and
correspondingly decreases the amount available. The inevitable result is
that the successful individual prospers more and more, while the less
successful one loses ground in the same degree. As a consequence, the
latter disappears entirely, or it is handicapped to such an extent that
it fails to produce seeds, or these are reduced in number or vitality.
Competition in vegetation furnishes few instances as simple as the
above, but this will serve to make clear the simplest case of ordinary
competition, i. e., that in which the individuals belong to a single
species. The various individuals of one species which grow together in a
patch show relatively slight differences, in height, width, leaf
expanse, or root surface. Still, some will have the largest surfaces for
the impact of water, heat, and light, while others will have the
smallest; the majority, perhaps, will occupy different places between
the extremes. The former will receive more than their share of one or
more factors. The reaction thus produced will operate upon the plants
subject to it inversely as the amount of surface impinged upon. The
usual expression of such competition is seen in the great variation in
height, branching, etc., of the different individuals, and in the
inability of many to produce flowers. This is particularly true of
annuals, and of perennials of the same generation. In the competition
between parents and offspring of the same perennial species, the former
usually have so much the advantage that the younger plants are often
unable to thrive or even germinate, and disappear, leaving a free space
beneath and about the stronger parents. This illustrates the primary law
of competition, viz., that this is closest when the individuals are most
similar. Similar individuals make nearly the same demands upon the
habitat, and adjust themselves least readily to their mutual reactions.
The more unlike plants are, the greater the difference in their needs,
and some are able to adjust themselves to the reactions of others with
little or no disadvantage.
In accordance with the above principles, the competition is closer
between species of like form than between those that are dissimilar.
This similarity must be one of vegetation or habitat form, not one of
systematic position. The latter is in fact of no significance, except
where there is a certain correspondence between the two. Leaf, stem, and
root characters determine the outcome, and those species most alike in
these features will be in close competition, regardless of their
taxonomic similarity or dissimilarity. This is as conclusive of the
competition between the species of the same genus as it is between those
belonging to genera of widely separated families. From this may be
deduced a second principle of competition, viz., the closeness of the
competition between the individuals of different species varies directly
with their similarity in vegetation or habitat form. This principle is
of primary importance in the competition which arises between occupants
and invaders in the different stages of succession. Those invading
species that show the greatest resemblance to occupants in leaf, stem,
and root form experience the greatest difficulty in establishing
themselves. The species, on the contrary, which are so unlike the
occupants that they come in at a clear advantage or disadvantage,
establish themselves readily, in the one case as a result of the
reaction, in the other by taking a subordinate position. This principle
lies at the base of the changes in succession which give a peculiar
stamp to each stage. A reaction sufficient to bring about the
disappearance of one stage can be produced only by the entrance of
invaders so different in form as to materially or entirely change the
impress of the formation. Stabilization results when the entrance of
invaders of such form as to exert an efficient reaction is no longer
possible. In forests, while many vegetation forms can still enter, none
of these produce a reaction sufficient to place the trees at a
disadvantage, and the ultimate forest stage, though it may change in
composition, can not be displaced by another.
It is obvious that the vegetation forms and habitat forms of associated
species are of fundamental importance in determining the course and
result of competition. Identity of vegetation form regularly produces
close competition, and the consequent numerical reduction or
disappearance of one or more species. Dissimilarity, on the other hand,
tends to eliminate competition, and to preserve the advantage of the
superior form. Species of trees compete sharply with each other when
found together; the same is true of shrubs, or rosettes, etc. The
relation of the shrubs to the trees, or of the rosettes to the shrubs of
a formation is one of subordination rather than of competition. The
matter of height and width often enters here also to such a degree that
the tallest herbs compete with the bushes and shrubs, and rosettes with
mats or grasses. The amount and disposition of the leaf surface are
decisive factors in the competition between species of the same
vegetation form, in so far as this is governed by light. In those plants
in which the leaves are usually erect, notably the grasses and sedges,
the competition between the aerial parts is relatively slight, and the
result is determined by the reactions of the underground stems and
roots.
The position of the competing individuals is of the greatest importance.
The distance between the plants affects directly the degree of
competition, while their arrangement, whether in groups according to
species or singly, exerts a marked influence by determining that the
contest shall be between like forms, or unlike forms. Position is
controlled primarily by the relation existing between seed-production
and dissemination. It is of course influenced in large measure by the
initial position taken by the invaders into a nudate area, but this is
itself a result of the same phenomena. The individuals of species with
great seed-production and little or no mobility usually occur in dense
stands. In these, the competition is fierce, for the two reasons of
similarity and density, and the result is that the plants fall far below
the normal in height and width. This is an extreme example of the group
arrangement. When the seed-production is small, the mobility may be
great or little without seriously affecting the result. The individuals
of a species of this kind will be scattered among those of other
species, and the closeness of competition will depend largely upon the
similarity existing between the two. The arrangement in such cases is
sparse. A species with great seed-production and great mobility usually
shows both kinds of arrangement, the position of the individuals and the
competition between them varying accordingly. This is due to the
intermittent action of distributing agents, making it possible for the
seeds to fall directly to the ground during the times that winds, etc.,
are absent. The three types of arrangement indicated above are termed
gregarious, copious, and gregario-copious. They furnish the basis for
the investigation of abundance which deals essentially with the number
and arrangement of the individuals of competing species. The effect of
distance, i. e., the interval between individuals, upon competition is
fundamental. The competition increases as the interval diminishes, and
the reverse.
The view here advanced, i. e., that competition is purely physical in
nature, renders untenable the current conceptions of vegetation
pressure, occupation, etc. Masses of vegetation are thought to force the
weaker species toward the edge, thus initiating an outward or forward
pressure. As has been shown above, no such phenomenon occurs in
vegetation. This movement is nothing but simple migration, followed by
ecesis, and has no connection with “weaker” species, or the development
of a vital pressure. The direction taken by the migrating disseminules
is essentially indeterminate. Migration seems to be outward, or away
from the mass, merely because the ecesis is greater at the edge, where
the increased dissimilarity between plant forms diminishes the
competition. The actual movement is outward, but it takes place through
the normal operation of competition. In this connection, it should be
pointed out that the common view that plants require room is inexact, if
not erroneous. This is difficult of proof, as it is impossible to
distinguish room as such from the factors normally present, light, heat,
water, and nutrient salts, but it seems obvious that the available
amounts of these will determine the space occupied by a plant,
irrespective of the room adjacent plants may allow it. The explanation
of competition upon physical grounds likewise invalidates the view that
plants possess spheres of influence other than the areas within which
they exert a demonstrable reaction upon the physical factors present.
Competition plays a very important role in alternation. It produces
minor examples of alternation in the physical units of an asymmetrical
series. Its greatest influence, however, is exerted in modifying the
effects of asymmetry. The reaction of occupants emphasizes or reduces
the effect of asymmetry, and has a corresponding action upon
alternation. This result of competition is typical of succession, in
which the sequence of stages arises from the interaction of occupant and
invader.
=345. Kinds of alternation.= Alternation involves two ideas, viz., the
alternation of different species or formations with each other, and the
alternation of the same species or formation in similar but separate
situations. This is the evident result of asymmetry, in response to
which contiguous areas are dissimilar and remote ones often similar.
Individuals of the same species or examples of the same formation may be
said to alternate between two or more similar situations, while
different species or formations are said to alternate _with_ each other,
occurring usually in situations different in character. From the nature
of alternation, the two phenomena are invariably found together.
It is possible to distinguish three kinds of alternation: (1) of a
formation, consocies, layer, facies, or species in similar situations;
(2) of similar or corresponding formations, species, etc., in similar
situations; (3) of facies and other species with respect to number. The
last two are merely variations of the first, arising out of slight
differences in the physical factors of the alternating areas, the
adjacent flora, or the course of competition. The alternation of
different examples of the same formation is a significant feature of
greatly diversified areas, such as mountains. It is naturally much less
characteristic of lands physiographically more uniform. A xerophytic
formation will alternate from ridge to ridge, a mesophytic formation
between the intermediate valleys; aquatic vegetation will alternate from
pond to pond, or stream to stream. The appearance of new or denuded
soils upon which successions establish themselves is the most important
cause of the alternation of formations. The weathering of rocks in
different areas of the same region produces in each a sequence of
similar or identical formations. The same statement is true in general
of other causes of succession, such as erosion, flooding, burning,
cultivation, etc., wherever they operate upon areas physically similar
and surrounded by the same type of vegetation. The areas of more or less
heterogeneous formations characterized by major physical differences are
occupied by consocies. In an extensive formation, the same consocies
alternates from one to another of these areas that are similar. When the
formation is interrupted and occurs here and there in separate examples,
a consocies often alternates from one to another of these. A consocies
regularly derives its character from the fact that one or more of the
facies of the formation is more intimately connected with certain areas
of the latter than with others. This explains why the alternations of
consocies and facies are usually identical. Layers sometimes alternate
between different examples of the same forest or thicket formation, when
they are suppressed in some by the diffuseness of the light.
The alternation of species is a typical feature of formations; it is
absent only in those rare cases where the latter consist of a single
species. The areas of a habitat which show minor physical or historical
(i. e., competitive) differences are occupied by groups of individuals
belonging to one or more species responsive to these differences. Each
of these groups will recur in all areas essentially similar, the
intervals being occupied of course by slightly different groups. Such
groups are constituted by gregarious or copious species of restricted
adjustability. Sparse plants likewise alternate, but they necessarily
play a much less conspicuous part. In habitats not too heterogeneous, a
large number of species are sufficiently adjustable to the slight
differences so that they occur throughout the formation. Often, to be
sure, they show a characteristic response, expressed in the size or
number. This is illustrated by the facies and many of the principal
species of the prairie formation. _Festuca_, _Koelera_, _Panicum_, and
_Andropogon_ occur throughout, except in the moist ravines which are
practically meadows. _Astragalus_, _Psoralea_, _Erigeron_, and _Aster_
grow everywhere on slopes and crests, but they are much more abundant in
certain situations. Other plants, _Lomatium_, _Meriolix_, _Anemone_,
_Pentstemon_, etc., recur in similar or identical situations upon
different hills. _Lomatium_ alternates between sandy or sandstone
crests, _Meriolix_ and _Pentstemon_ occur together upon dry upper
slopes, while _Anemone_ alternates between dry slopes and crests.
Owing to the accidents of migration and competition, similar areas
within a habitat are not occupied by the same species, or group of
species. A species found in one area will be replaced in another by a
different one of the same or a different genus. The controlling factors
of the area render imperative an essential identity of vegetation and
habitat form, though in systematic position the plants may be very
diverse. Such genera and species may be termed _corresponding_. The
relation between such plants is essentially alternation; it should,
perhaps, be distinguished from alternation proper as _corresponsive_.
The prairie formation furnishes a good example of this on exposed sandy
crests, upon which _Lomatium_, _Comandra_, and _Pentstemon_ alternate.
Formations exhibit a similar correspondence.
[Illustration: Fig. 74. Numerical alternation of _Pinus_ and
_Pseudotsuga_ upon east and west slopes.]
All species that alternate show a variation in abundance from one area
to another. Frequently, the difference is slight, and may be ignored,
except in determining abundance. Very often, however, the variation is
so great that a facies may be reduced, numerically, to the rank of a
principal species, or one of the latter to a secondary species. This
phenomenon is distinguished as _numerical_ alternation. It arises from
the fact that the similar areas are sufficiently different to affect the
abundance, without producing complete suppression. It is probable that
this result is due almost entirely to competition. _Astragalus
crassicarpus_ grows on all the slopes of the prairie formation, but on
some it has the abundance of a facies, while on others it is represented
by a few scattered individuals. This difference is much more striking in
separate examples of the same formation, particularly when a normal
facies is reduced to the numerical value of a secondary species. This is
a matter of great importance in the study of formations, for it has
doubtless often resulted in mistaking a consocies for a formation.
Alternation furnishes the logical basis for what may be called
comparative phytogeography. The latter is of much broader scope than the
old subject of geographical distribution, for it treats not only of the
distribution of formations and associations as well as of species, but
it also seeks to explain this by means of principles drawn from the
relation between habitat and vegetation. When the latter come to be
fully based upon physical factor investigations, and upon the effects of
migration and competition as shown in alternation, the comparative study
of formations will represent the highest type of phytogeographical
activity.
THE FORMATION IN DETAIL
=346. The rank of the formation.= There have been as many different
opinions in regard to the application of the term formation as there are
concerning the group which is to be called a species. In taxonomy,
however, the concept of the species is purely arbitrary, and agreement
can not be hoped for. In vegetation, on the contrary, the connection
between formation and habitat is so close that any application of the
term to a division greater or smaller than the habitat is both illogical
and unfortunate. As effect and cause, it is inevitable that the unit of
the vegetative covering, the formation, should correspond to the unit of
the earth’s surface, the habitat. This places the formation upon a basis
which can be accurately determined. It is imperative, however, to have a
clear understanding of what constitutes the difference between habitats.
A society is in entire correspondence with the physical factors of its
area, and the same is true of the vegetation of a province.
Nevertheless, many societies usually occur in a single habitat, and a
province contains many habitats. The final test of a habitat is an
efficient difference in one or more of the direct factors,
water-content, humidity, and light, by virtue of which the plant
covering differs in structure and in species from the areas contiguous
to it. A balsam-spruce forest shows within itself certain differences of
physical factors and of structure. The water-content will range from
20–25 per cent, and the light from .02–.003. One portion may consist
chiefly of _Pseudotsuga mucronata_, another of _Picea engelmannii_, and
a third of _Picea parryana_, or these species may be intermingled. If,
however, this forest is compared with the gravel slide, which touches it
on one side, and the meadow thicket, which meets it on another, the
physical factors and the species both demonstrate that it is the forest,
and not its parts, which corresponds to a distinct physical entity, the
habitat. This test of a formation is superfluous in a great many cases,
where the physiognomy of the contiguous areas is conclusive evidence of
their difference. It is evident also that remote regions which are
floristically distinct, such as the prairies and the steppes, may
possess areas physically almost identical and yet be covered by
different formations. This point is further discussed under
classification.
The existing confusion in the matter of formations is due to two causes.
The first arises from the fact that much ecological work has been hasty.
Little or no attention has been given to development, and in consequence
rudimentary and transitory stages of succession have often been
described as formations. Mixed areas in particular have caused trouble.
In the second place, there has been a marked tendency to minimize the
need of thoroughness and training by calling every slightly different
area a formation. A failure to recognize the primary value of
alternation has also contributed materially to this. Alternating facies,
and principal species, when separated from each other, have often been
mistaken for formations. This is a danger that must be fully appreciated
and guarded against. In practically all regions, the same formation is
represented by numerous scattered areas, all showing greater or less
differences arising from alternation. This is especially true of thickly
populated regions where virgin areas are rare. The fact that twenty-five
miles intervene to-day between two small stretches of primitive prairie
is permitted to unduly emphasize their differences. It requires the
study of a number of such examples to counteract this tendency, and to
cause one to see clearly that they must have been at one time merely so
many bits of the prairie formation.
In this connection, the lichen and moss groups which are found on rocks
constitute an interesting problem. It is clear that _Peltigera_ and
_Cladonia_, which grow on the forest floor, and _Evernia_, _Ramalina_,
and _Physcia_, which are found on the trees, are merely constituent
species of the forest formation. The same is true of _Cladonia_,
_Urceolaria_, and _Parmelia_, which are found among the sedges and
grasses of alpine meadows. The physical conditions are essentially those
of the formation, and the lichens themselves are more or less peculiar
to it. This is particularly true of the forest, in which the two strata,
bark and moist shaded soil, are present because of the trees. In the
case of granitic rocks, the circumstances are very different. The
species of lichens found on the rocks are not peculiar to the formation,
but they also occur elsewhere. In the forest, _Parmelia_, _Placodium_,
_Physcia_, _Rinodina_, _Urceolaria_, _Lecanora_, _Lecidea_, etc., occur
on the rocks. In the alpine meadows, the rock groups are composed of
_Parmelia_, _Gyrophora_, _Cetraria_, _Acarospora_, _Lecanora_,
_Lecidea_, _Buellia_, etc. The stratum itself is physically very
different and constitutes a distinct habitat. These groups are really
small formations, which are quite distinct from the surrounding forest
or meadow. This is proven conclusively in many places in the mountains
where areas of the characteristic lichen formations of cliffs are
carried by the fall of rock fragments into forest and meadow, where they
persist without modification. This also shows clearly that the groups on
scattered rocks in the same area are to be regarded as examples of the
same cliff formation, except where the differences are evidently to be
ascribed to development and not to alternation. Where these rock
formations can not be traced to cliffs or magmata with certainty, they
must be considered as antedating the vegetation in which they occur.
Often, indeed, especially in igneous areas, they are relicts of the
initial stage of a primary succession. Finally, they prove their
independence of the forest or meadow formation by initiating a distinct
succession within these. Crustaceous groups or formations yield to
foliose ones, and these in turn give way to formations of mosses,
particularly in the forest where the effect of the diffuse light is
felt. From the above, the following rule of formational limitation is
obtained: any area, which shows an essential difference in physical
character, composition, or development from the surrounding formation is
a distinct formation.
[Illustration: Fig. 75. Relict lichen formation in a spruce forest,
invaded by rock mosses.]
=347. The parts of a formation.= All the parts which make up the
structure of a formation are directly referable to zonation and
alternation, alone or together, or to the interaction of the two. The
principles which underlie this have already been discussed under the
phenomena concerned. It is necessary to point out further that the
structure may be produced in several ways: (1) by zonation alone, (2) by
alternation alone, (3) by zonation as primary and alternation as
secondary, (4) by primary alternation and secondary zonation, (5) by the
interaction of the two, as in layered formations. Though all these
methods occur, the first two are relatively rare, and the resulting
structure comparatively imperfect. The typical structure of formations
can best be made clear by the consideration of a prairie which belongs
to the fourth group, and a forest which represents the last.
[Illustration: Fig. 76. Early (prior) aspect of the alpine meadow
formation (_Carex-Campanula-coryphium_), characterized by _Rydbergia
grandiflora_.]
The major divisions of prairie and forest formations are regularly due
to alternation. There is an inherent tendency to the segregation of
facies, arising out of physical or historical reasons, or from a
combination of both. Not all formations show this, but it is
characteristic of the great majority of them. The primary areas which
thus arise have been called associations: they are naturally subordinate
to the formation. To avoid the confusion which inevitably results from
using the word association in two different senses, it is proposed to
term this primary division of the formation, a _consociation_, or
better, a _consocies_. This term is applied only to an area
characterized by a facies, or less frequently, by two or more facies
uniformly commingled. The consocies of grassland are determined by
grasses, those of forests by trees, etc. From the different position of
the facies in these two types of vegetation such areas are readily seen
at all times in the forest, but they are often concealed in grassland by
the tall-growing principal species of the various aspects. When definite
consocies are present, they are often found to mingle where they touch,
producing miniature transition areas, and, very rarely, they sometimes
leave gaps in which no facies appears.
[Illustration: Fig. 77. Late (serotinal) aspect of the alpine meadow,
characterized by _Campanula petiolata_, _Rydbergia_ in fruit.]
The seasonal changes of a formation, which are called aspects, are
indicated by changes in composition or structure, which ordinarily
correspond to the three seasons, spring, summer, and autumn. The latter
affect the facies relatively little, especially those of woody
vegetation, but they influence the principal species profoundly, causing
a grouping typical of each aspect. For these areas controlled by
principal species, but changing from aspect to aspect, the term
_society_ is proposed. They are prominent features of the majority of
herbaceous formations, where they are often more striking than the
facies. In forests, they occur in the shrubby and herbaceous layers, and
are consequently much less conspicuous than the facies. A close
inspection of the societies formed by principal species shows that they
are far from uniform. Since they usually fail to exhibit distinct parts,
it becomes necessary to approach the question of their structure from a
new standpoint. Such is afforded by aggregation, which yields the
simplest group in vegetation, i. e., that of parent and offspring. This
is so exactly a family in the ordinary sense that there seems to be
ample warrant for violating a canon of terminology by using the word for
this group, in spite of its very different application in taxonomy. It
has already been shown that aggregation further produces a grouping of
families, which may properly be called a _community_. As they are used
here, _family_ and _community_ become equally applicable to the
association of plants, animals, or man. Both families and communities
occur regularly in each society of the formation, and they represent its
two structures. In some cases, all the families are grouped in
communities, two or more of which then form the society. Very
frequently, however, families occur singly, without reference to a
community, and the two then constitute independent parts of the same
area. This is typically the case wherever gregarious species are
present, since these are merely family groups produced by aggregation.
[Illustration: Fig. 78. _Calthetum_ (_Caltha leptosepala_), a consocies
of the alpine bog formation.]
[Illustration: Fig. 79. _Iridile_ (_Iris missouriensis_), a society of
the aspen formation.]
Objection may be made that this analysis of formational structure has
been carried too far, and that some of the structures recognized are
mere interpretations, and not actual facts. Such a criticism will not
come from one who has got beyond the superficial study of formations,
for he will at once recognize that certain probable features of
structure have not been considered. On the other hand, the ecologist or
the botanist who has not made a careful investigation from the
standpoints of development and structure will naturally refrain from
expressing an opinion, until he has obtained an acquaintance at first
hand with the facts. Over-refinement is the usual penalty of intensive
work. The unbiased investigator, however, will not be misled by the
suddenness with which new concepts appear. It seems plausible that the
structure of a formation, if not as definite, is at least nearly as
complex as that of an individual plant. Few botanists will insist that
the refinement of tissues and tissue systems has been carried further
than the differentiation of the plant warrants. Yet, if these had been
defined within a period of a few years rather than slowly recognized
during more than a century, they would have been called seriously in
question. As a matter of fact, the consocies, under the term
association, and the society, under various names, have been recognized
by ecologists for several years. They are definite phenomena of
alternation which can be found anywhere. The family and the community,
though the latter is less distinct in outline, are equally valid
structures, the proof of which anyone can obtain by thorough methods of
study.
=348. Nomenclature of the divisions.= The suffix _-etum_ is used to
designate a consocies of a formation, e. g., _Picetum_, _Caricetum_,
etc. When two or more species characterize the area, the most
important, or more rarely, the two are used. The termination used to
designate a society is _-ile_, as _Asterile_, _Sedile_, _Rosile_. The
suffix which denotes the community is _-are_, and for the family, it
is _-on_, viz., _Giliare_, _Bromare_, _Bidenton_, _Helianthon_, etc.
Layers are indicated by the affix _-anum_, as _Opulasteranum_,
_Verbesina-Rudbeckianum_, etc. It is evident that these suffixes, like
the terms to which they refer, must be used always for the proper
divisions if they are to have any value at all. There has been a
marked tendency, for example, to use _-etum_ in connection with the
names of groups of very different rank. It is hardly necessary to
point out that such a practice does not promote clearness. The
following tabular statement will illustrate the application of both
terms and suffixes:
_Picea-Pseudotsuga-hylium_ formation (_-ium_) _Paronychia-Silene-chalicium_
_Picetum_ consocies (_-etum_) _Paronychietum_
_Opulaster-Ribesanum_ layer (_-anum_)
_Opulasterile_ society (_-ile_) _Androsacile_
_Thalictrare_ community (_-are_) _Festucare_
_Pirolon_ family (_-on_) _Arenarion_
=349. The investigation of a particular formation.= A comprehensive and
thorough study of a formation should be based upon as many examples of
it as are accessible. The example which is at once the most typical and
the most accessible is made the base area. This plan saves time and
energy, reduces the number of instruments that are absolutely necessary,
and establishes a common basis for comparison. The inquiry should be
made along four lines, all fundamental to a proper knowledge of the
formation. These lines are: (1) the determination of the factors of the
habitat, (2) a quadrat and a transect study of the structure of the
formation, (3) a similar investigation of development, (4) a floristic
study of the contiguous formation, with special reference to migration.
The sequence indicated has proven to be the most satisfactory, and is to
be regarded as all but absolutely essential. Naturally, this applies
only to the order in which the various lines are to be taken up, as they
are carried on together when the work is fully under way. Since
instrument and quadrat methods have already been given in detail, it is
unnecessary that they be repeated. Similarly, the questions which
pertain to structure and development and to the surrounding vegetation
are considered in detail in the pages which precede.
[Illustration: Fig. 80. _Eritrichiare_ (_Eritrichium aretioides_), a
community of the alpine meadow formation.]
CLASSIFICATION AND RELATIONSHIP
=350. Bases.= Formations may be grouped with reference to habitat or
kind, development or position. Classification upon the basis of habitat
places together formations which are similar in physiognomy and
structure. Developmental classification is based upon the fact that the
stages of a particular succession are organically connected or related,
though they are normally different in both physiognomy and structure.
Grouping with respect to position is made solely upon occurrence in the
same division of vegetation. The formations thus brought together
usually possess neither similarity of kind or structure, nor do they
have any necessary developmental connection. Habitat and developmental
classification are of fundamental value; regional arrangement is more
superficial in character. All serve, however, to emphasize different
relations, and, while the developmental system expresses the most, they
should all be used to exhibit the vegetation of a region, province, or
zone.
[Illustration: Fig. 81. _Pachylophon_ (_Pachylophus caespitosus_), a
family of the gravel slide formation.]
=351. Habitat classification.= In arranging formations with reference to
habitats, the direct factors, water and light, can alone be used to
advantage. Such a system is fundamental, because it is founded upon
similarity of habitat and of structure. Proposed groupings based upon
nutrition-content, or upon the division of factors into climatic and
edaphic, have elsewhere[42] been shown to be altogether of secondary
importance, if not actually erroneous. The basis of the habitat grouping
is water-content, which is supplemented by light whenever the factor is
decisive. The primary divisions thus obtained are water, forest,
grassland, and desert, which are characterized respectively by
associations of hydrophytes, mesophytes, hylophytes, poophytes, and
xerophytes respectively. Within these, formations are arranged according
to the type of habitat, i. e., pond, meadow, forest, dune, etc. These
divisions comprise all formations which belong to the type by virtue of
their physiognomy and structure. Such formations differ from each other
very considerably or completely in the matter of floristic, i. e.,
component species, but they still belong to the same type. A dune
formation in the interior and one on the coast may not have a single
species in common, and yet they are essentially alike in habitat,
development, and structure.
=352. Nomenclature.= The names of formations are taken from the habitats
which they occupy. Each formation should have a vernacular and a
scientific name. The latter is especially important since it ensures
brevity and uniformity, and obviates the obscurity and confusion that
arise from vernacular terms in many tongues. Scientific names have been
made uniformly from Greek words of proper meaning by the addition of the
suffix _-ium_ (εῖον), which denotes place.[43] The following list gives
the English and the scientific name of the various habitats, and their
corresponding formations, and indicates the primary divisions into which
these fall.
I. Hydrophytia: water plant formations
1. ocean: oceanium: oceanad,[44] oceanophilous, etc.
2. sea: thalassium
surface of the sea: pelagium
deep sea: pontium
3. lake: limnium, limnad
4. pond, pool, tiphium, tiphad
5. stagnant water: stasium: stasad
6. salt marsh: limnodium, limnodad
7. fresh marsh: helium
8. wet meadow: telmatium
9. river: potamium
10. creek: rhoium
11. brook: namatium
12. torrent: rhyacium
13. spring: crenium
14. warm spring: thermium
15. ditch: taphrium
16. sewer: laurium
17. swamp forest: helohylium
18. swamp open woodland: helodium
19. meadow thicket: helodrium
20. bank: ochthium
rock bank: petrochthium
sand bank: ammochthium
mud bank: pelochthium
21. rocky seashore: actium
22. sandy seashore: agium
23. sandbar: cheradium
24. tank: phretium
II. Mesophytia: middle plant formations
_a._ Sciophytia: shade plant formations
26. forest: hylium
27. grove: alsium
28. orchard: dendrium
29. canyon: ancium
30. open woodland: orgadium
31. thicket: lochmium
_b._ Heliophytia: sun plant formations
32. meadow: poium
33. pasture: nomium
34. culture land: agrium
35. waste place: chledium
III. Xerophytia: dry plant formations
36. desert: eremium
37. sand-hills, sandy plain: amathium
38. prairie, plains: psilium
39. dry, open woodland: hylodium
40. dry thicket: driodium
41. dry forest: xerohylium
42. gravel slide: chalicium
43. sandbar: syrtidium
44. sand draw: enaulium
45. blowout: anemium
46. strand: psamathium
47. dune: thinium
48. badlands: tirium
49. hill, ridge: lophium
50. cliff: cremnium
51. rock field: phellium
52. boulder field: petrodium
53. rock, stone: petrium
54. humus marsh: oxodium
55. alkali area: drimium
56. heath, dry meadow: xeropoium
57. moor: sterrhium
58. alpine meadow: coryphium
59. polar barrens: crymium
60. snow: chionium
61. wastes: chersium
Particular formations are indicated by means of floristic
distinctions. Thus, _Populus-hylium_ is the aspen forest as
distinguished from the _Picea-Pseudotsuga-hylium_, or the
balsam-spruce forest; and the _Bulbilis-psilium_, or buffalo-grass
prairie, from the _Bouteloua-Andropogon-psilium_, or grama-bluestem
prairie. Similarly, the aspen formation of the Old World and of the
New may be distinguished as _Populus-tremula-hylium_ and
_Populus-tremuloides-hylium_, respectively. In all formational names,
the facies alone should be used. Frequently, a single facies will
suffice for clearness. As a rule, however, the two most important
facies should be employed; in rare cases only is it necessary to use
the names of three. When it is desirable to refer to two or more
examples of the same formation, a geographical term is added, e. g.,
(1) _Populus-hylium_ (_Crystal Park_), (2) _Populus-hylium_ (_Cabin
Canyon_).
=353. Developmental classification.= This is based upon succession as
the record of development. Upon the basis of development, all the
formations which belong to the same succession are classed together.
They are arranged within each group in the sequence found in the
particular succession. From its nature, developmental classification is
of primary importance in exhibiting the history of vegetational changes.
It has less value than the habitat system for summarizing the essential
structure of a vegetation, inasmuch as it places the emphasis upon
historical rather than structural features. It is evident that both deal
with the same formations, and that the difference is merely one of
viewpoint. The habitat classification is simpler in that it considers
only those formations actually on the ground, while development has
regularly to take into account stages which have disappeared. The groups
of the developmental system, and the arrangement of formations within
them have already been indicated under the nomenclature of succession
(sections 326 and 327).
=354. Regional classification.= The grouping of formations with respect
to the divisions of vegetations is chiefly of geographical value. It
indicates a certain general relationship, but its principal use is to
summarize the structure of the vegetative covering of a region. The
arrangement of formations in the various divisions is made with
reference to the outline of North American vegetation (section 341).
This is naturally based upon the identity of altitude and latitude
zones. In the study of mountain countries, it is often desirable to
group formations with reference to altitude alone. In this case, the
grouping is based upon the following divisions: (1) _bathyphytia_,
lowland plant formations; (2) _mesiophytia_, midland formations; (3)
_pediophytia_, upland formations; (4) _pagophytia_, foot-hill
formations; (5) _orophytia_, subalpine formations; (6) _acrophytia_,
alpine formations; (7) _chionophytia_, niveal formations.