Decomposing recruitment limitation for an avian-dispersed rain forest tree in an anciently fragmented landscape

Abstract

1. Forest fragmentation may intensify recruitment limitation through impacts on seed production, dissemination and seedling establishment processes. In an ancient mosaic of fog-dependent rain forest patches on coastal hilltops of semi-arid Chile (30 °S), we investigated how aridity-driven habitat fragmentation affected the recruitment of the fleshy-fruited, dioecious tree Aextoxicon punctatum (Aextoxicaceae) and the role of birds in seed dispersal across patches. 2. We hypothesized that reduced fragment size enhanced recruitment limitation due to constraints on tree fecundity, avian disperser activity and seedling recruitment, caused primarily by strong edge effects in small fragments. 3. We mapped adult Aextoxicon trees and placed 686 seed traps distributed in 3 small (<0.3 ha) and 3 large patches ( > 0.9 ha) in the forest patch mosaic. To identify seed sources, endocarps of 282 dispersed seeds from seed traps were genotyped and compared with 155 genotyped females using nine microsatellite markers. Seed dispersal curves for different forest patches were constructed using maximum likelihood methods. Further, seed addition experiments were conducted in the six fragments to assess recruitment probabilities. 4. Lower density and basal areas of reproductive female trees, in addition to shorter fruiting periods and lower seed yields, led to strong source limitation in small patches. Three bird species accounted for the bulk of visits to fruiting trees, but were less active in smaller patches. Dispersal curves were strongly leptokurtic with maximum dispersal distances related to the shape of patches. Important proportions (5–40%) of genotyped seeds in all patches were immigrants. However, seeds arrived mainly at patch edges or below reproductive trees where germination and survival are reduced. Fewer seedlings originated from experimentally added seeds in small patches subjected to greater edge effects. 5. Synthesis. In summary, we provide evidence for source, dispersal and establishment limitation of this dioecious tree in an aridity-driven fragmented landscape. Small fragment size and edge effects had negative impacts on fecundity and seedling establishment. Although bird-mediated seed dispersal favoured immigration between patches, recruitment from such seeds will be unlikely because of low quality of deposition sites.