Pattern and process in the plant community

The plant community may be described from two points of view, for diagnosis and classification, and as a working mechanism. My primary concern is with the second of these. But in as much as the two aspects are not mutually exclusive, a contribution to our understanding of how a community is put together, and how it works, may contain something of value in description for diagnosis. It is now half a century since the study of ecology was injected with the dynamic concept, yet in the vast output of literature stimulated by it there is no record of an attempt to apply dynamic principles to the elucidation of the plant community itself and to formulate laws according to which it maintains and regenerates itself. Pavillard's assessment of the dynamic behaviour of species comes very near it, but is essentially concerned with the 'influence (direct or indirect) of the species on the natural evolution of plant communities' (Braun Blanquet & Pavillard, 1930). As things are, the current descriptions of plant communities provide information of some, but not critical, value to an understanding of them; how the individuals and the species are put together, what determines their relative proportions and their spatial and temporal relations to each other, are for the most part unknown. It is true that certain recent statistical work is stretching out towards that end, but the application of statistical technique, the formulation of laws and their expression in mathematical terms, will be facilitated if an acceptable qualitative statement of the nature of the relations between the components of the community is first presented. Such a statement is now made based on the study of seven communities in greater or less detail, for data of the kind required are seldom recorded. The ultimate parts of the community are the individual plants, but a description of it in terms of the characters of these units and their spatial relations to each other is impracticable at the individual level. It is, however, feasible in terms of the aggregates of individuals and of species which form different kinds of patches; these patches form a mosaic and together constitute the community. Recognition of the patch is fundamental to an understanding of structure as analysed here. In the subsequent analysis evidence is adduced to show that the patches (or phases, as I am calling them) are dynamically related to each other. Out of this arises that orderly change which accounts for the persistence of the pattern in the plant community. But there are also departures from this inherent tendency to orderliness caused by fortuitous obstacles to the normal time sequence. At any given time, therefore, structure is the. resultant of causes which make for order and those that tend to upset it. Both sets of causes must be appreciated. In describing the seven communities I propose in the first examples to emphasize those features which make for orderliness, in the later to content myself with little more than passing reference to these and to dwell specifically upon departures from it; in all examples to bring out special points for the illustration of which particular communities are well suited or for which data happen to be available. For the present the field of inquiry is limited to the plant community, divorced from its context in the sere; all reference to relics from its antecedents and to invaders from the next state is omitted. I am assuming essential uniformity in the fundamental factors of the habitat and essential stability of the community over a reasonable period of time.