This study addresses basic ecological processes that influence the nature and diversity of tree regeneration during secondary forest succession in low-elevation tropical forest. It is hypothesized that seedling recruitment during succession is strongly dependent on both resource availability and heterogeneity. Specifically, it is suggested that changes in canopy and gap structure, as successional forests age, increase both light availability and light heterogeneity at ground level. It is expected that species richness of the regeneration pool is enhanced by both overall light availability and the diversity of light environments represented in older, structurally heterogeneous stands. Seedling recruitment is further enhanced by availability of patches of disturbed soil associated with gaps or animal activity. This interdisciplinary approach integrates ecosystem-level studies of the patterns of tropical forest succession with detailed analyses of within-stand processes producing those patterns and studies of physiological and growth responses of representative species. Changes in forest composition and structure with be documented in the species richness of seedlings, and in within-stand environmental heterogeneity in a chronosequence of secondary and old-growth premontane rain forests of northeastern Costa Rica. Detailed measurements of the spatial variation of light availability, soil disturbance, and litter cover in selected forests will be performed along this chronosequence. The influence of distance to seed sources on propagule diversity will be assessed in these stands by sampling soil seed stocks in the study sites and by comparing the diversity of seedlings and saplings as a function of distance from seed sources. Investigators will experimentally examine the effects of resource availability and heterogeneity on seedling establishment and growth by manipulating light availability and soil disturbance in a series of 15-25 year old stands. Light will be manipulated by creating large and small canopy gaps and removing understory shrubs in a replicated single-factor (4 treatments) design in which we will assess the effects on natural seedling recruitment and species richness. Additional manipulative studies in gaps and closed-canopy sites will examine the interactive effects of light and soil disturbance on natural seedling recruitment and the germination and survival of seeds of selected species. Experimental and observational field data will be used to model the effects of stand structure and light availability of seedling abundance and diversity using a spatial, mechanistic, simulation model of forest dynamics. To evaluate the role of species-specific differences in canopy light transmission within secondary forest, a comparative study of canopy structure and light transmission will be conducted for five abundant canopy tree species in the stands. The model will also simulate the effect of spatial heterogeneity on the diversity of seedling regeneration, based on measured ecophysiological and growth responses of selected species. Model inputs will be based on field data from representative stands within the chronosequence. This modelling effort will integrate measured organismal responses in the context of stand-level changes in resource availability in tropical forests. Project results will contribute to basic ecological research on the maintenance of biological diversity in moist tropical forests, with direct applicability to studies of tropical forest restoration, secondary forest management, and sustainable tropical forestry.
