DENSITY-DEPENDENT POPULATION DYNAMICS OF A DOMINANT RAIN FOREST CANOPY TREE

Abstract

Intraspecific, negative density dependence may contribute to the maintenance of diversity by limiting the dominance of common species. Shorea quadrinervis Slooten (Dipterocarpaceae) is one of the dominant canopy trees in a species-rich tropical rain forest in Southeast Asia. We test whether juvenile density and performance and overall population growth rate of S. quadrinervis decline with increasing local abundance of conspecific adult trees. We mapped the 357 S. quadrinervis adults (≥15 cm dbh) in 75 ha at Gunung Palung National Park, Indonesian Borneo, and compared the dynamics of juveniles <1 cm dbh between eight areas (80 m diameter plots) with high densities of conspecific adults and eight containing low densities. Within these 16 areas, we mapped 5215 S. quadrinervis juveniles and followed their growth and survival for two years. Plots with high and low density of S. quadrinervis adults did not differ in understory light, slope, aspect, or soil properties nor in the total density (all species combined) of either adults or juveniles. We evaluated four kinds of evidence. All were consistent with the hypothesis of negatively density-dependent population dynamics: (1) Density and size: Although the density of juveniles 75 cm tall. This suggests positive density-dependent seedling recruitment followed by negative density-dependent juvenile dynamics. (2) Foliar condition: Juveniles had more leaves per unit height where adult density was low. Growth and survival increased more with leaf number than with height. (3) Juvenile dynamics: Growth and survival were higher where adult density was low. Previous work suggests that herbivores may drive this negative density dependence in juvenile performance. (4) Population dynamics: We modeled population growth rate (λ) for each of the 16 areas using separate matrix models parameterized with empirical juvenile transition rates. We found that λ declined with local adult density (r2 = 0.40), and this trend was robust to the inclusion of El Nino conditions. We conclude that replacement of adults in the canopy is less likely in areas where conspecific adults are more abundant, tending to limit the dominance of S. quadrinervis in the canopy.