Gap dynamics

Abstract: Evolutionary hypotheses about how so many species of tropical rainforest trees might have arisen include (a) genetic drift (71), (b) habitat specialization (8) in benign environments, or (c) repeated geographic isolation followed by remixing of species during Pleistocene climatic fluctuations (144). Ecological hypotheses about how these species continue to coexist are often cast into equilibrium or non-equilibrium frameworks: Do tropical forests comprise " . . . sets of highly coevolved niche-differentiated tree species in stable or semistable floristic assemblages," or do they consist of " . . . diffusely coevolved, broadly generalist species which slowly drift in relative abundance within a few large life-history guilds" (67, 102)? In this context, there has been considerable recent interest in the role of adaptations by species to different regeneration sites in structuring plant assemblages in general (83, 84, 86, 141), and tropical tree communities in particular (62, 89, 90, 150). The immediacy of this interest is heightened by rising rates of deforestation throughout the tropics and a critical need for management strategies of the remaining preserves, for ecologically sound harvesting procedures, and for the tools with which to restore degraded forests. Openings in the forest canopy are widely recognized as important for the establishment and growth of rainforest trees (5, 33, 62, 90). Hartshorn (89) suggests that perhaps 75% of the tree species at La Selva Biological Station, Costa Rica are dependent on canopy opening for seed germination or for growth beyond sapling size. Similar statements are found in descriptions of forest dynamics in Queensland, Australia (185), Malaysian dipterocarp forests (195), and West African rain forest (108). Demographic studies demon-

Abstract: In all forests there is a cycle initiated by disturbance. We may arbitrarily recognize gap, building, and mature phases (Watt 1947, Cousens 1974, Whitmore 1975, 1978, 1982), thereby designating forests as spatial mosaics of structural phases which change over time as a result of dynamic processes. Gaps, openings in the forest canopy, drive the forest cycle. Very tiny gaps may be filled by lateral ingrowth of surrounding trees. Usually, though, trees grow up from seedlings to form an immature forest of saplings and poles which grow on and develop into mature trees. The mature phase may enter a fourth degenerate phase as trees become senile, but often is destroyed at a stroke by some external factor. Differences in sizes of gaps result in differences in species composition of the next cycle. Present evidence suggests that, in all forests, tree species fall into one or other of two groups. In small gaps seedlings that became established in the shade of the closed forest are released (i.e., commence height growth). By contrast, a quite different group of species colonizes large gaps. Seeds of these species germinate only in the open, so seedlings occur only after formation of gaps. Swaine and Whitmore (1988), while recognizing that there are no universally accepted terms, propose that these groups be called climax (non-pioneer) and pioneer, respectively. Pioneer species only regenerate in large gaps. When a mature canopy of pioneers enters the degenerate phase, small gaps develop, and these are closed by growth of climax species that became established under them. The next cycle is composed of climax species that, unlike the pioneers, can regenerate in situ. The gap phase is thus the most important part of the growth cycle for the determination of floristic composition. Competition among tree species and their different light requirements at the building and mature phases play a lesser role. Intensive study of forest dynamics since the early 1970s suggests this paradigm is applicable to many forests at all latitudes (Whitmore 1988). There are, however, numerous embellishments, as well as many alternatives, and these lead to the great diversity seen even within a single forest biome. A few examples include: clonal spread occurs in some temperate forests; in some tropical rain forests big gaps may be invaded by woody climbers that arrest the next growth cycle; both pioneer and climax species differ in longevity (hence size) and in autecology (e.g., microsite preferences for seedling establishment).

Abstract: To characterize the disturbance regime of one type of vegetation, study areas in which relatively small-scale disturbance predominates were chosen in several old-growth mesic forests in the eastern United States. Canopy openings covered 9.5% of total land area. New gaps were formed at an average rate of 1% of total land area per year; old gap area closed at a similar rate primarily by sapling height growth. With increased gap size, vegetation within gaps increased in woody species diversity, total basal area, and total number of stems. Stems also showed accelerated growth into larger size classes. As gaps aged, stems grew into larger size classes and basal area increased. Species responses to canopy gaps varied. Some species survived and became established in fairly small gaps (50-100 M2). Although in large gaps (up to 2009 m2 in the present study) these species usually increased in total number of stems and basal area, they declined in importance relative to species which rarely survived in small gaps but grew rapidly in large gaps. The disturbance regimes in the forests studied favored tolerant species but allowed opportunists to persist at low densities.

Abstract: Light regimes beneath closed canopies and tree-fall gaps are compared for five temperate and tropical forests using fish-eye photography of intact forest canopies and a model for calculating light penetration through idealized gaps. Beneath intact canopies, analyses of canopy photographs indicate that sunflecks potentially contribute 37–68% of seasonal total photosynthetically active radiation. In all of the forests, potential sunfleck duration is brief (4–6 min), but the frequency distributions of potential sunfleck duration vary because of differences in canopy geometry and recent disturbance history. Analysis of the photographs reveals that incidence angles for photosynthetically active radiation beneath closed canopies are not generally vertical for any of the forests, but there was considerable variation both among and within sites in the contribution of overhead versus low-angle lighting. Calculations of light penetration through idealized single-tree gaps in old growth Douglas-fir – hemlock forests...