Research methods in ecology

by Frederic E. Clements

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.

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.

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.

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.

Fig. 78. Calthetum (Caltha leptosepala), a consocies of the alpine bog formation.

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.

Fig. 80. Eritrichiare (Eritrichium aretioides), a community of the alpine meadow formation.