Page:EB1911 - Volume 21.djvu/793

Rh an association of Quercus sessiliflora may be referred to as a Quercetum sessiliflorae.

A plant formation is a group of associations occupying habitats which are in essentials identical with each other. Thus, associations of Agropyrum (Triticum) junceum, of Carex arenaria, of Ammophila (Psamma) arenaria, and of other plants occur on sand dunes: the associations are related by the general identity of the habitat conditions, namely, the physiological dryness and the loose soil; but they are separated by differences in floristic composition, especially by different dominant species, and by minor differences of the common habitat. The whole set of associations on the sand dunes constitutes a plant formation.

The plant formation may be designated in technical language by the termination -ion added to a stem denoting the habitat. Thus, a sand dune formation may be termed an Arenarion. The associational term, in the genitive, may be added to the formational term to indicate the relationship of the formation and the association; thus, a plant association of Ammophila arenaria belonging to the plant formation of the sand dunes may be designated an Arenarion Ammophilae-arenariae (cf. Moss, op. cit. 1910: 43).

The question of universal names for vegetation units is bound up with that of the universality or otherwise of particular formations. “Remote regions which are floristically distinct . . . may possess areas physically almost identical and yet be covered by different formations” (Clements, 1905: 203). For example, the sand dunes of North America and those of western Europe are widely separated in geographical position and therefore in floristic composition, yet they are related by common physical factors. This relationship may be indicated by the addition of some prefix to the formational name. For example, an Arenarion in one climatic or geographical region might be termed an -Arenarion and one in a different region a -Arenarion, and so on (Moss, loc. cit.).

It is, however, frequently desirable to consider such allied formations as a single group. Such a group of formations may be designated a plant federation: and this term may be defined as a group of formations, which are characterized by common edaphic factors of the habitat, and which occur in any geographical region. Thus, different geographical or climatic regions are characterized by salt marshes. The latter all agree in their edaphic characteristics; but they differ climatically and in floristic composition. The salt marshes of a given region constitute a single plant formation: the salt marsh formations of the whole world constitute a plant federation.

Again, it is possible to arrange plant associations into groups related by a common plant form. Thus woodland associations may be classified as deciduous forests, coniferous forests, sclerophyllous forests, &c. These, in a general way, are the “formations” of Warming, and (in part) the “climatic formations” of Schimper. Thus the various reed-swamps of the whole world constitute a “formation” in Warming's sense (1909: 187).

There is much difference of opinion among ecologists and plant geographers as to which of these points of view is the most fundamental. Among British authorities, it is now customary to adopt the position of Clements, who states (1905: 292) that “the connexion 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,” and that (1905: 18) “habitats are inseparable from the formations which they bear” (cf. Moss, 1910).

From the standpoint of plant communities, it is convenient to divide the earth's surface into (1) tropical districts; sub-tropical and warm temperate districts; (3) temperate districts; (4) cold temperate and frigid districts.

1. Tropical Districts.—The vegetation of tropical districts hu been subdivided by Schimper (1903:260, et seq.) as follows:— (i.) Tropical woodland: (a) rain forest, (b) monsoon forest, (c) savana forest, (d) thorn forest. (ii) Tropical grassland: (a) savana, (b) steppe. (iii.) Tropical desert: (a) scrub, (b) succulent plants, (c) perennial herbs.

Schimper regards the minor divisions as groups of “climatic formations”; and he also distinguishes certain tropical “edaphic formations,” such as mangrove swamps. He states that rain forests and high monsoon forests in the tropics occur when the average rainfall is over 70 in. (178 cm.) per annum, and that tropical thorn forest may prevail when the mean annual rainfall is below 35 in.

A tropical rain forest exhibits great variety both of species of plant and of plant forms. There is great diversity in the trees and masses of tangled lianes, and a wealth of flowers in the leafy forest crown. Humboldt points out that whilst temperate forests frequently furnish pure associations, such uniformity of association is usually absent from the tropics. Some tropical forests exhibit dense foliage from the forest floor to the topmost leafy layer; and the traveler finds the mass of foliage almost impenetrable. Other tropical forests afford a free passage and a clear outlook. It is obvious that tropical forests will eventually be subdivided into plant associations; but the difficulties of determining the relative abundance of the species of plants in the upper layers of tropical rain and monsoon forests are very great. One of the best known results of the great struggle for light which takes place in tropical forests is the number of epiphytic plants which grow on the high branches of the trees.

The leaves of the trees are frequently of leathery consistency, very glossy, usually evergreen, entire or nearly so, and seldom hairy; and thus they agree closely with the leaves of sclerophyllous forest generally.

Monsoon forests are characteristic of localities with a seasonal rainfall. The trees usually lose their foliage during the dry season and renew it during the monsoon rains. With a less abundant rainfall, savana forest and thorn forest occur. Less precipitation induces tropical grassland, which, according to Schimper (1903: 346) is of the savana type; but Warming (1909: 327) thinks that all grassland in the tropics is artificial. Still greater drought induces desert vegetation; but, as deserts are more characteristic of subtropical districts, they are discussed later on.

Mangrove swamps, or tropical tidal forests, occur in saline or brackish swamps on flat, muddy shores in the tropics; and, being almost independent of atmospheric precipitations, Schimper regards them as “edaphic formations.” However, they are climatic communities in the sense that they occur only in hot districts. Cases such as this illustrate the difficulty of regarding the distinction between “climatic formations” and “edaphic formations” as absolute. The plants exhibit markedly xerophilous structures; and many of the fruits and seeds of the mangrove trees and shrubs are provided with devices to enable them to float and with curious pneumatophores or “prop roots.” The latter serve as supports and also as a means of supplying air to the parts buried in the mud. The seedlings of characteristic species of Rhizophoraceae germinate on the trees, and probably perform some assimilatory work by means of the hypocotyl.

Other tropical “edaphic formations” occur on sandy shores, where the creeping Ipomoea biloba (Pes-caprae) and trees of Barringtonia form characteristic plant associations.

The succession of associations on new soils of a tropical shore has recently been described by Ernst.

2. Warm Temperate and Subtropical Districts.—In subtropical and warm temperate districts, characterized by mild and rainy winters and hot and dry summers, we find two types of forests. First, there are forests of evergreen trees, with thick, leathery leaves. Such forests are known as sclerophyllous forests, and they occur in the Mediterranean region, in south-west Africa, in south and south-west Australia, in central Chile, and in western California. In the Mediterranean district, forests of this type are sometimes dominated by the Cork Oak (Quercus Suber), sometimes by the Holm Oak (Q. Ilex). When these forests become degenerate, maquis and garigues respectively are produced. Maquis and garigues are characterized by the abundance of shrubs and undershrubs, especially by shrubby Leguminous plants, and by species Cistus and Lavandula. Secondly, there are forests of coniferous trees. In the Mediterranean region, even at comparatively low altitudes, forests occur of the maritime pine (Pinus maritima) and of the Aleppo pine (P. halepensis); and these forests are also related to maquis and garigues respectively in the same way as the evergreen oaks. The occurrence of forests of this type in the Mediterranean and in Arctic regions, whose dominant species belong to the same genus (Pinus) and to the same plant form, renders it