Contrasting patterns of local richness of seedlings, saplings and trees may have implications for regeneration in rainforest remnants

Remnants of lowland rainforest remain following deforestation, but the longer-term effects of fragmentation remain poorly understood, partly due to the long generation times of trees. We study rainforest trees in three size classes: seedlings (<1 cm dbh), saplings (1-5 cm dbh) and trees (>5 cm), that broadly reflect pre- and post-fragmentation communities, and we examine the impacts of fragmentation on forest regeneration in Sabah, Malaysian Borneo. We found that seedling richness (measured as the number of genera per plot) in fragments was about 30 percent lower than in plots in undisturbed forest, and about 20 percent lower than in an extensive tract of selectively logged forest, providing evidence of recruitment declines in fragments. Seedling richness was lowest in small, isolated, and disturbed fragments, potentially signalling an extinction debt given that these fragmentation impacts were not observed in trees. Unlike seedlings, saplings showed no declines in richness in fragments, suggesting that density dependent mortality (where rare individuals have a higher survival rate) and/or year-to-year variation in which species are recruiting could potentially compensate for the reductions in seedling richness we observed. Longer-term studies are required to determine whether sporadic or failed recruitment in small fragments will eventually translate into reduced richness of mature trees, or whether the processes that currently retain high sapling richness will continue in fragments.