EFFECT OF FOREST FRAGMENTATION ON GENETIC DIVERSITY AND MATING SYSTEMIN A TROPICAL TREE, PITHECELLOBIUM ELEGANS

Genetic diversity, population differentiation, and temporal variation in outcrossing rates were examined for Pithecellobium elegans, a Neotr opical rain forest canopy tree. Several forest fragments and a large r eserve (1500 ha) were compared for several population genetic paramete rs. For eight populations samples on the Atlantic coastal plain of Cos ta Rica, allozyme heterozygosity (0.13), polymorphism (35%), and effec tive number of alleles (1.24) were similar to values reported for othe r tropical tree species that occur at similar densities of less than o ne individual per hectare. These measures of genetic variation were lo west in populations of the smallest size, farthest from the reserve, a nd more isolated from other populations. Differentiation among samples collected in small forest fragments and the reserve population accoun ted for 10% of the total genetic variation observed. There was a posit ive relationship between the level of differentiation of populations f rom the reserve population and their distance from the reserve. Though predominantly an annually flowering species, the number of trees in f lower at any one time varied from 80% of observed trees to only 6%. Ou tcrossing rates did not differ for two episodes in which the proportio ns of flowering trees were 33% and 80%. But periods of low density of flowering adults resulted in poor seed crops of failure to set fruit f or many individuals. Population size at many sites will be effectively decreased of the variation in flowering. Fragmentation of what was on ce, a large, continuous forested area is resulting in genetic erosion of small, isolated populations of Pithecellobium elegans.