Effects of logging on landscape-level tree diversity across an elevational gradient in Bornean tropical forests

Abstract

Logging has caused a substantial loss of biodiversity and associated ecosystem services. Therefore, it is important to examine how logging affects biodiversity on a landscape scale to plan responsible management of a tropical forest. Although a number of plot-based studies have shown the effect of logging on local tree species richness (alpha diversity), the effect on species turnover along environmental gradients (beta diversity) remains largely unknown. In this study, we evaluated how logging disturbance affects alpha and beta diversity along an elevational gradient on the eastern slope of Mount Trus Madi in Borneo. We further investigated how pioneer and late-successional tree species differed in the habitat range to clarify the mechanism underlying the beta diversity pattern. We selected 90 plots, each with a radius of 20 m, with a range of disturbance intensity (five classes from highly degraded forests to pristine forests) in lower (285–600 m asl) and higher elevation areas (600–1105 m asl). The remaining above-ground biomass, which is an indicator of past disturbance intensity, strongly varied across the plots (5.4–570.6 and 3.1–771.6Mgha−1 in lower and higher elevation areas, respectively). Diameter at breast height (DBH) and species name were recorded for all trees with a DBH larger than 10 cm. We calculated the species number per 20 individual trees for each plot to represent alpha diversity. Beta diversity along the elevational gradient was calculated as the slope of the relationship between standardized compositional dissimilarity (beta deviation) and the elevational difference. Alpha diversity decreased in higher (17.3–12.3 species per 20 trees) and lower areas (16.8–11.3 species per 20 trees) with increasing logging intensity. Beta diversity along the elevational gradient also decreased to almost zero in highly disturbed areas. Pioneer tree species had a wider elevational range than late-successional species. These results suggest that the shift in dominant tree species after logging (from late-successional to pioneer species) was the main driver of the decline in beta diversity along the elevational gradient. We conclude that preserving and restoring beta diversity are important to sustain tropical production forests.