Effect of distance and density on seed/seedling fate of two dipterocarp species

Abstract

We investigated the effects of both seed density and distance from conspecific adult trees on the survivorship of seeds of Shorea laxa and Dipterocarpus tempehes. We examined whether those species support predictions of the escape hypothesis of distance- and density-dependent mortality. We placed seeds on the forest floor using a matrix of distance from the parent tree (5, 20, and 80 m) and seed density (2, 10, and 50 m −2). The fates of seeds and seedlings were recorded 5 days, 2 weeks, and 1 month after the seeds were exposed. Camera-traps at quadrats 5 and 80 m from the parent trees were used to identify mammalian seed and seedling predators and to examine the frequency of their appearance. The proportion of seedlings surviving after 1 month was very small, 32.0 and 4.0% for S. laxa and D. tempehes, respectively. The main cause of mortality for both species was predation and removal by mammals. Total mortality and mortality caused by mammal predation were highest at the greatest distance from the parent tree. Camera-traps revealed that the diversity of mammalian predators was highest 5 m from the parent trees, while the total frequency of mammalian predator appearance was higher at 80 m than at 5 m for S. laxa. Thus, S. laxa and D. tempehes exhibited distance-dependent mortality, but the highest mortality occurred at the greatest distance from the parent trees. This is the inverse of the pattern predicted by escape hypothesis and instead supports the “McCanny pattern”. Those results would be caused by “predator satiation” near the seed-bearing trees where foods were concentrated, while seeds more distant from the parent tree were exhausted by predation.