Traditionally, it has long been assumed that this process of succession is initiated by green plants and that heterotrophs (in effect, eaters of organic matter) move in later. This view has recently been challenged by Hodkinson et al. (2002), who have listed many instances worldwide where an initial heterotrophic stage, of variable duration, has been described as occurring on fresh substrates devoid of green vegetation. These include newly exposed glacial moraine, shingle beaches, cooled lava flows and ash fields, whose surfaces often teem with moving organisms including active predators and detritivores. Moreover, heterotrophic protozoa and nematodes are often abundant in the interstices between the rock fragments. All these organisms, together with the dead organic matter which provides them with their food, have reached these new habitats before the development of the green plants which will later come to dominate them. It seems that as a general rule community assembly by autotrophs is preceded by this previously little recognized heterotrophic stage in which primary and secondary consumers, including predators and scavengers, form simple communities which conserve mineral nutrients, particularly nitrogen, thus facilitating the later growth of green plants.
Early theories of succession viewed it as a process that was ordered, directional and deterministic (that is, following a set order). Plant communities developed on bare ground under particular climatological, soil and hydrolo-gical conditions, passing through a series of stages (seres) which eventually resulted in a climax vegetation suitable for the environment concerned. This process was largely controlled by competition for light and was very frequently dominated by the largest and tallest plants present. If these were trees, the final stage would be a woodland or forest. More modern views do not envisage such a set order of stages or a fixed endpoint, but simply a probabilistic process largely driven by the availability of propagules and competition between species, interacting with variable environmental conditions. Box 9.2 also indicates that succession may often commence with heterotrophs. This less deterministic process, which has many possible pathways and final vegetation types rather than a set climax, is termed community assembly, a phrase now tending to replace the older term succession. In either theory the green plants play a most important role by providing shade, enriching the soil with humus and, together with the associated animals, modifying the often harsh conditions originally present (facilitation). In some cases the later-successional (or primary) trees do not invade until the early pioneers create suitable conditions. However, in many developing forests (even in primary successions), all tree species colonize early on (when the ground is bare and there is less competition)
and the changing forest is a reflection of changing patterns of dominance (Walker et al, 1986 give a good example). This is driven by a combination of conditions changing to suit different species (e.g. British oaks grow faster with some organic matter in the soil) and differences in growth rates (e.g. oaks grow slower than birches and so take longer to dominate). At the same time, the original pioneer plants tend to be suppressed by later dominants as they are shaded out. An outline of the tolerance classes used by foresters, based largely on the ability oftree species to survive shade while growing in the understorey, is given in Box 9.3. Associated communities of insects, birds, mammals and soil fungi also change, particularly when a woodland or forest develops. Recent work concerning the influence of large animals upon the form and species balance of former forests, considered in Section 5.7, has also changed views on the processes operating in truly natural (old-growth) forests.
Philosophical approaches to vegetation change are important, influencing both the design of research and the interpretation of results. Pickett and McDonnell (1989), for instance, claim that community dynamics 'emphasizes process rather than the end point, accommodates the richness of causes of succession and motivates diverse research approaches'. They also point out that the dynamics which lead to succession are often allogenic (controlled by outside factors) rather than autogenic (governed by biotic interactions and changes within the community).
The variability of succession can be illustrated by examining what happens in gaps created by one or several trees falling over. In deterministic theory, the vegetation that develops in a gap will be a miniature succession, following the same stages that the original forest might have gone through. Drawing on his earlier work, A. S. Watt (1947) gave a classic example for a beech woodland in southern England (Fig. 9.7). Young beech exerts a shade so heavy that there is
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