Fine-scale genetic dynamics of a dominant neotropical tree in the threatened Brazilian Atlantic Rainforest

Abstract

We present a case study of the relationship between spatial genetic structure (SGS) and age structure in Protium spruceanum (Burseraceae), an insect-pollinated, mass-fruiting, and secondary bird-dispersed tree, as determined through variation in allozyme loci. Using ten polymorphic loci, we investigated spatial and temporal patterns of a genetic structure within a 40 m × 60 m plot in a small (1.0 ha) fragment of Atlantic Rainforest to investigate the processes shaping the distribution of genetic diversity. Individuals (n = 345) from seedlings to adults were grouped and analyzed in four diameter classes. The results showed a high average level of genetic diversity (H e = 0.438), but genetic diversity parameters did not vary significantly among cohorts. The spatial distribution pattern analysis of individuals showed significant levels of aggregation among small- and medium-diameter classes and random distribution among the highest diameter class, likely due to processes of competitive thinning. There was an association between demographic and SGS at short distances (less than 10 m) which is likely the consequence of restricted seed dispersal. The degree of SGS decreased across small- to large-diameter classes. We inferred that limited seed dispersal and subsequent density-dependent mortality from the family clusters are responsible for the observed changes in fine-scale SGS across different demographic classes.