Recruitment limitation and seed-mediated gene flow in the dioecious tree Aextoxicon punctatum in relict forest fragments in semiarid Chile

Background/Question/Methods

Fog-inundated coastal hilltops of Fray Jorge National Park (FJNP), in semiarid Chile (30º S), contain a mosaic of rainforest patches which are remnants of a continuous distribution fragmented by increasing aridity during the Pleistocene. Such scenario provides insights into the long-term effects of climate change and habitat fragmentation on tree recruitment and genetic diversity. Patches are dominated by the fleshy-fruited, dioecious tree Aextoxicon punctatum, endemic to southwestern South America. Low seedling density in small patches suggests strong recruitment limitation due to either, seed source limitation, dispersal limitation, or establishment limitation from lack of safe sites. We assessed the role of these three types of limitation on seedling recruitment of Aextoxicon and quantified seed-mediated gene flow in six forest patches ranging from 0.14 to 2.4 ha. All trees were mapped and fruit yield was quantified using 0.25 m² seed traps (N=636). In order to estimate dispersal kernels and gene flow produced by avian seed vectors, we used microsatellite markers to genotype 155 females and to identify 232 seed sources. Establishment limitation was assessed by experimental seed additions.

Results/Conclusions

Aextoxicon fruits were produced from February to July. Differences in peak date and length of fruiting period depended on patch size. Trees in smaller patches produced significantly less fruits per unit area than larger patches. The best fit to seed-to-mother distances was a lognormal probability distribution, showing low probability of deposition beyond 60 meters. Dispersed seeds that remained unassigned to a genotyped female which was present within large patches ranged from 7 to 16%. This percentage provides an estimate of the level of gene flow from outside the patch. We pooled all genotyped seeds from small patches due their low numbers, and estimated that 30% of all seeds trapped arrived from outside these patches. Foreign seeds were found predominantly along the edge of forest fragments, which could reduce their survival probability. The mean number of seedlings from quadrats to which seeds were added was significantly lower in the smaller than larger patches. Results provide strong evidence for source, dispersal, and establishment limitation in smaller forest patches. Seed movement mediated by birds between patches would result in low probability of seedling establishment due to low quality deposition sites along edges.