Dynamic structure of wave-regenerated Abies balsamea forests in the north-eastern United States.

In 1947 A. S. Watt began his presidential address to the British Ecological Society by saying, '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.' The intervening decades have not changed the situation much; except for Watt's own paper and a very few others, this field is still largely unexplored. The main difference is that advances in nearly all other areas of ecology have rendered our ignorance about long-term ecosystem dynamics even more striking by contrast. Despite the fact that the concepts of succession and climax have been among the central themes of plant ecology since the late nineteenth century, our knowledge of how ecosystems vary and maintain themselves through time has increased only slightly in the last seventy-five years. Much of this neglect has been due to the rather oppressive influence on the minds of many ecologists of the notion of 'climax'. For years, it was generally assumed that in the absence of disturbance the vegetation on any site would eventually reach a self-reproducing steady-state equilibrium, in which all system properties would be relatively constant through time (cf. Clements 1936). Although discussion of this point focused mainly on species composition, it would seem to follow that general system properties such as net primary productivity, decomposition rates, and soil chemistry would also be constant in such a stable system. In many cases this theoretically stable 'climax' vegetation-type came to be thought of as the 'natural' vegetation for the area; the idea that the frequency of natural disturbance might make this concept of 'stable climax' a totally unrealistic model for natural ecosystems was rarely considered. Thus, ecologists spent much time and effort searching for, describing, and classifying 'climax' ecosystems in this special sense, even though there was often little or no evidence that stable systems of this sort would ever come into existence under natural conditions. In fact, many studies have indicated that natural disturbance plays a far more vital role in ecosystem dynamics than that attributed to it by the classic climax theory. Cooper (1913) found that the native vegetation of Isle Royale was not the homogeneous, all-aged stand that would have been predicted by the classic theory, but rather an irregular matrix of small wind-throw areas of various ages, with regeneration generally restricted to recent wind-throws. It was, he said, 'a mosaic or patchwork which is in a