Gap Partitioning among Tropical Rainforest Trees

Abstract

Published observations on adaptations for seed disperal and seedling establishment are consistent with the hypothesis that rainforest trees partition forest clearings as establishment sites for offspring. Gaps vary importantly in two ways. The size of the opening affects the microclimate of the gap and therefore the conditions for seedling establishment. For any individual tree, the frequency of occurrence of gaps of a particular size range affects the probability that its propagules will reach a gap of suitable size for germination and establishment. In most rainforests large gaps (involving the death of several trees) are probably more rare than small gaps (involving single trees or branches). Interspecific competition for establishment sites has resulted in adaptive compromises in the regeneration strategies of each species. Traits that increase the probability of establishing seedlings in gaps of a particular size range appear to lower establishment in gaps outside this size range. I suggest that the coexistence of many rainforest tree species is at least partially due to their partitioning of canopy gaps by size. Therefore the size-class frequercy distribution of gaps peculiar to a given rainforest is expected to influence the types and diversity of species present. Examination of vegetation data from New and Old World rainforests reveals many patterns consistent with this hypothesis. This framework provides a mechanism for predictive and experimental studies of competitive interactions among rainforest trees. MECHANISMS PROPOSED to account for patterns of species richness within animal communities have relied heavily on resource partitioning (cf. Schoener 1974). Similar hypotheses have been less successful in accounting for plant species diversity. Different plants have similar modes of resource acquisition and share the same few essential resources (light, moistLre, minerals). It is not clear how such uniform resources could be partitioned by physiologically similar species in complex communities (e.g. Richards 1969). Although rainforest species exhibit patterns associated with variation in topography or soil (e.g. Ashton 1964a, Grieg-Smith et al. 1967, Poore 1968, Williams et al. 1969, Austin et a/. 1972, Ashton 1977), species with non-random distributions often show no association with edaphic variation (Schulz 1960, Poore 1968), and overlap along edaphic gradients is high between similar species. It remains difficult to account for high diversity in relatively uniform topographic and edaphic environments. In face of high plant species diversity unexplained by resource partitioning, theorists have invoked stochastic or historical processes to account for modern patterns (e.g. Federov 1966, Van Steenis 1969, Prance 1973, Stebbins 1974, and see Ashton 1969 for a discussion). These hypotheses assume that competitive interaction among plant species is of little importance in the determination of relative abundances of species. Here I suggest that a mechanism for resource partitioning among rainforest trees exists in their differential regeneration in treefall gaps of different sizes and spatial distributions. At a superficial level some of these differences are a well-established part of natural history lore (e.g. Richards 1964, Van Steenis 1958, Budowski 1965) and form the basis of sustained-yield forestry systems (e.g. Taylor 1962, Whitmore 1975). Several papers have emphasized that gaps are an important source of environmental heterogeneity in rainforest (Schulz 1960, Whitmore 1975, 1978, Hartshorn 1978). Nevertheless, it is evident from the literature that gap regeneration strategies have not been considered an important component of competitive interactions among trees. Few studies of rainforest vegetation include attention to the nature and distribution of natural gaps or to differential seedling establishment in them. This paper summarizes data on regeneration patterns of trees within the framework of hypotheses that (1) tree species partition gaps of different spatial distributions and sizes and that (2) partitioning occurs because regeneration strategies keyed to gaps of particular size ranges involve adaptive compromises that restrict the competitive success of the species in gaps of differing sizes. High mortality rates of seeds and seedlings (e.g. Liew and Wong 1973) suggest that selection pressures are likely to be particularly strong on factors affecting dispersal of seeds and establishment of seedlings. Gaps as establishment sites for seedlings are critical resources, and gap partitioning provides an important mechanism through which empirically to examine interspecific competitive interactions among tree species. Rainforest spatial structure and species diversity are reviewed in this light. Experimental tests of the relationships between TROPICAL SUCCESSION 47-55 1980 47 This content downloaded from 157.55.39.159 on Sun, 18 Sep 2016 06:29:01 UTC All use subject to http://about.jstor.org