Regeneration Recruitment and Survival in an Asian Tropical Rain Forest; Implications for Sustainable Management

Abstract

Few studies have been published that focus on monitoring and investigating the long-term patterns of recruitment and survival of seedlings in the understories of tropical forests. Studies for mixed dipterocarp forest, the dominant tropical forest type for South and Southeast Asia, demonstrate seedlings in the existing forest understory (advance regeneration) play the most important role in forming a new forest after disturbance (e.g., logging, windstorms). Our study monitors the regeneration dynamic across a topographic gradient (ridge, mid-slope, and valley) of the understory for a mixed dipterocarp forest in the Sinharaja MAB reserve in southwest Sri Lanka. We recorded seedling recruitment, growth, and mortality by species in replicated plots on multiple forest understory sites over a ten-year period. Seedling density ranged from 26 to 31 m2 with all of it categorized as advance regeneration of canopy tree species. Results show differences in seedling density per species across the topography and over time, with differences driven by changes in species composition from valley to ridge and in recruitment periodicity potentially related to supra-annual variations in climate. Seedling mortality was higher on ridges than valley and midslopes; and disproportionately higher seedling mortality occurs of relatively shade-intolerant late-successional trees species (70% mortality after 8 yr) compared with their more shade-tolerant associates (30% mortality after 8 yr). Our results contradict current selective logging prescriptions for tropical forests that comprise a single cutting cycle and minimum diameter felling for all commercial timber trees. Such harvests give no consideration to timing and manipulation of the forest canopy to favor release of advance regeneration. We recommend careful timing of timber harvests to occur in synchrony to the periodicity of seedling recruitment and silvicultural treatments that both insures their release and that accommodates to differences in seedling composition and density across topography.