PARENT SESSION
Oral Session #34: Forests: Models of function and structure. Presiding: M. Lechowicz.
Tuesday, August 7, 2001. 1:00 PM to 3:45 PM. Hall of Ideas F.
Twenty years of change and development in a tropical dry forest, Guanacaste, Costa Rica.
ENQUIST, CAROLYN1,2, 1 2
ABSTRACT- Long-term data of tropical tree communities are needed to understand factors that influence species composition and forest dynamics. Initial results are presented of work describing the species abundance, composition and biomass of a tropical dry forest in Costa Rica, where over 30,000 trees were surveyed in 1976 and 1996. Total density per ha decreased by 27%, yielding a forest-wide annual mortality rate of 3.52%. Recruitment limitation is likely a factor as recruitment (3.05%) was less than mortality. Forest biomass decreased by 96.5%, possibly reflecting a 77-year decrease in rainfall. Significant changes in size distribution occurred in 58% of species, with a large reduction in the smaller size classes. There is a life history component to this change as a negative trend between mortality and maximum growth rates was detected, indicating an increase in fast-growing species. Spatial changes in abundance and biomass were not correlated, signaling succession and a response in forest dynamics to shifts in local climate. Many aspects of the forest have not changed and appear to be structured. While there was a 23% change in species composition, overall species richness was invariant. Multivariate analyses indicated there was spatial segregation between most species but the degree of spatial segregation did not differ significantly over time. Because tropical dry forest is more threatened than rainforest, continued monitoring of the long-term plot may be critical to conservation of this biome.
KEY WORDS: tropical forest, tree growth, community dynamics, forest turnover