Abstract
Among factors that might help to explain coexistence of tree species is environmental heterogeneity created by physical and biological processes of the forest environment. The differences in resource availability across rainforest landscapes were investigated by measuring the abiotic resources of light, soil nutrients and soil water availability in relation to forest structure. This study was conducted in canopy gaps and adjacent understories across an elevational gradient from lowland mixed dipterocarp forest (100 m amsl) to submontane mixed dipterocarp forest (1200 m amsl) in southwest Sri Lanka. Middle elevation gap sites (300-700 m amsl) were sub divided into valley, midslope and ridge topographic positions. Eighteen canopy openings caused by tree falls were randomly selected across the elevation gradient. Plots were demarcated in gap centers and in adjacent understories and measurements were taken of forest structure (basal area, canopy height, tree canopy projection area), shade (light sensors- photosynthetic photon flux density or PPFD), and soil nutrients (pH, Al, K, N, P, Mg and Ca) across all sites. Surface soil moisture was measured at bi- weekly intervals for five years across the middle elevation site only. Stand basal area, mean canopy height, gap size and canopy area index (a surrogate for leaf area index) all declined with increase in elevation. As expected, understory PPFD decreased with increases in canopy height, basal area and canopy area index. The size of canopy opening decreased with increase in elevation. Valley sites had significantly greater levels of mean soil water contents compared to midslope and ridge sites. K and Ca in gap and adjacent forest understories increased with increase in elevation. pH increased and Al decreased with elevation but only for forest understory conditions. Results suggest strong differentiation in soil and light resources with elevation that appear to relate to size of disturbance, stature of the forest, and underlying geology and soil weathering environment. This implies that silvicultural practices need to develop and tailor techniques and treatments that can be applied to the forest that emulate and/or account for change in elevation and topographic position. Keywords: Dipterocarpus , gap dynamics, PPFD, Shorea , Sri Lanka. DOI: http://dx.doi.org/10.4038/jnsfsr.v39i4.3881 J.Natn.Sci.Foundation Sri Lanka 2011 39 (4):321-331
Highlights
Many factors have been put forward to explain coexistence of tree species in tropical rain forests
Plots were demarcated in gap centers and in adjacent understories and measurements were taken of forest structure, shade, and soil nutrients across all sites
Some obvious trends in forest structure were observed with increase in elevation from 175 to 1050 m amsl (Figure 1)
Summary
Many factors have been put forward to explain coexistence of tree species in tropical rain forests. In particular variation in seedling characteristics (growth and growth allocation to stems, roots and leaves, leaf area, and mortality) in response to shade and soil resources may have important implications for the coexistence of shade-tolerant tropical tree species (Hall et al, 2002; Bloor & Grubb, 2003; Singhakumara et al, 2003). The long-term persistence of seedlings under closed forest canopies provides a partial explanation for high levels of tree species in mixed dipterocarp forests of Asia (Delissio et al, 2002). Heterogeneity of resource availability within mixed dipterocarp forests promotes differences in horizontal and vertical tree species distribution, which partially explains the coexistence of tree species (Davies et al, 1998)
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