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The Classification of Vegetation and the Concept of Development

A. G. Tansley
Journal of Ecology
Vol. 8, No. 2 (Jun., 1920), pp. 118-149
DOI: 10.2307/2255529
Stable URL: http://www.jstor.org/stable/2255529
Page Count: 32
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The Classification of Vegetation and the Concept of Development
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Abstract

1. One-sided ways of considering and classifying vegetation result from differences of training and interest and from the geographical isolation of different workers. Floristics, life-form, and habitat are all essential objects of synecological study, but none is sufficient to serve as a basis of classification. Classification must be based primarily on the sum of the characteristics of vegetation itself. The units of vegetation are essentially topographical units. 2. The units of vegetation, while differing in many obvious characters from true organisms, are correctly and usefully regarded as quasi-organisms or organic entities. A comparison of plant communities with human communities illustrates this thesis. 3. The organising factors of vegetation are expressed in various ways, and give rise to various grades and kinds of aggregate. The concept of the plant association as representing the primary and fundamental unit of vegetation as we see it in the field has won an increasing degree of acceptance degree of acceptance during the last 20 years. The definition or description of the association presented to the Brussels Congress of 1910 by the reporters forms a good basis of this concept. Both the morphology and the physiology of the association are essential to a full knowledge of its characters, and both involve a study of the development and fate of the association. Within the association, minor units, the consociation and the society, and perhaps the "clan," may be recognised. 4. The individuality of the association is illustrated and emphasised by the intensive study of its morphology. The recognition of the association as the fundamental unit of vegetation implies that in the association the organisation of vegetation reaches its highest expression. This justifies the limitation of the term to mature units in relatively stable equilibrium with their environment, a limitation already implicit in much of the best work on associations. The separation of transitory (developmental) plant communities of equivalent rank as associes, with their subordinate consocies and socies, is useful as expressing the distinction between the mature and the developmental units. The conception of climatic "pre-climaxes" and "stabilised sub-climaxes" is unsuitable as a basis of classification, since it involves speculative interpretation, and departs from the criteria of maturity and stability. Physiographic as well as climatic climaxes (associations) must be recognised. It must be recognised that besides associations and their developmental stages (associes), there exists a great deal of mixed fragmentary vegetation which cannot be classified under the developmental concept, because it shows few of the characters of organic entities, though the smaller units may often be distinguished within it. 5. Succession in general must be distinguished from development, which is a particular kind of succession. Developmental succession may be marked by a series of "climaxes" which are relatively stable and mature, and which differ from one another in habitat, life-form, and floristic composition. These "climaxes" must be recognised as associations. 6. Life-form is unsatisfactory as a criterion of the higher unit, the plant formation, because it brings together communities which are not closely related in nature and separates others which are closely related; and because it proceeds by abstraction and departs from concrete relationships. The formation must be based on the communities which are developmentally related, of which the association is the highest expression. Climatic and physiographic formations may be distinguished in many cases, but a formation may be determined partly by climatic and partly by physiographic factors, and the ultimate criterion of a formation, as of an association, is the vegetation itself. The formation is the whole of the vegetation naturally grouped round one or more associations. 7. Empirical determination of the natural units of vegetation met with in the field is the only sound basis of the classification of vegetation, which should not be confused by theories of causation. The concept of development is essential in grouping the units of vegetation thus recognised. It will be useful to indicate the modifications here suggested of the terminology adopted in Types of British Vegetation ('11) which was mainly based on the concepts developed by Moss ('10). 1. The term association is confined to relatively stable, fully developed communities, and is replaced by associes (Clements, '16) for transitory developmental units. Association thus becomes equivalent to Moss's "chief association" ('10) and Nichol's "permanent association" ('17) while Moss's "subordinate associations" and Nichols's "temporary associations" become "associes." 2. The term consociation is used in Clement's sense for the part of an association dominated by a single dominant species of the association. Consocies is the corresponding unit of an associes. 3. The term formation is applied to a set of communities related developmentally and culminating in one or more associations. It corresponds of course with habitat, but not in the rather artificial sense employed in Types of British Vegetation. Thus English chalk grass-land would not necessarily belong to the same formation as beech forest, though they may be developed side by side on the same chalk down, nor would the great climatic formation of summer deciduous forest be split up into distinct "formations." Climatic and physiographic (edaphic or topographic) formations may be distinguished, but not so sharply as by Nichols ('17), because of the frequent "replacement" of climatic by physiographic factors, which is gradual in the transition region between two climatic regions. The formation must be determined empirically by the actual associations and related communities which are present.

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