Research methods in ecology

by Frederic E. Clements

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.

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.

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

Alaska province—provincia alaskana Cordilleran province—provincia cordillerana Ontario province—provincia ontariensis

Atlantic province—provincia atlantica Appalachian province—provincia appalachiana Nebraska province—provincia nebraskensis Utah province—provincia utahensis Coast province—provincia litoralis Pacific province—provincia pacifica

Florida province—provincia floridana Mexican province—provincia mexicana

Antilles province—provincia antilleana Andean province—provincia andeana