The effect of the spatial scale of recruitment on tree diversity in Afromontane forest fragments

In fragmented landscapes, tree recruitment is critical for forest persistence. We examined the effects of disturbance and environmental factors, isolation distance, and forest area on the spatial scale (grain) of regeneration of tree species in Afromontane forest fragments in South Africa. A species' grain is defined by whether it typically regenerates within its own canopy shadow (fine-grained) or over a larger spatial scale (coarse-grained). Species richness did not differ between small and large forest fragments but there were proportionately more coarse-grained species and fewer fine-grained species in small than in large fragments. While coarse-grained species richness increased with decreasing disturbance and increasing fragment isolation, fine-grained species richness increased with increasing fragment area. Fine-grained species are vulnerable to area-dependent fragmentation pressures. Although they regenerate in their canopy shadow, fine-grained species do not dominate disturbed fragments as expected. While able to survive in small fragments, fine-grained species are potentially dispersal limited and are not good colonisers and depend for their persistence on establishment in large forests. Conversely, because coarse-grained species have effective dispersal mechanisms they can colonise small fragments and are important for the maintenance of tree diversity in fragmented Afromontane forests. Thus, consideration of species grain of regeneration is necessary in conserving Afromontane tree diversity. Fine-grained species are conserved by protecting large forests while coarse-grained species are effectively conserved by maintaining small forest fragments often assumed to be ecologically unviable.