Research methods in ecology

by Frederic E. Clements

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.