Rainforests are severely threatened by agricultural expansion, frequently resulting in the fragmentation of formerly extensive tracts of continuous forest. The immediate and longer-term effects of fragmentation on tree regeneration, and on alpha- and beta-diversity, remain poorly understood. Forest area and isolation can drive changes in diversity, and may be key considerations for conserving forest biodiversity in human-modified landscapes. I studied trees, saplings, and seedlings (pre- and post-fragmentation recruits), in 14 forest fragments and 5 continuous forest sites in Sabah, Malaysian Borneo. Local alpha-diversity of seedlings was significantly lower in fragments than in undisturbed continuous forest, and lowest in the smallest fragments, potentially signalling an extinction debt. However, saplings showed no declines in alpha-diversity, suggesting that density dependent mortality and/or year-to-year variation in recruitment may compensate for reductions in seedling richness: low seedling diversity may not necessarily translate into low sapling diversity. Nonetheless, 57-64% of genera in small fragments occurred only as adult trees, with no seedlings present, indicating a recruitment failure of some genera. This contributed to greater distinctiveness (increased beta-diversity) of seedling communities in small fragments, which were diverging from trees in the same fragment, and from seedlings in other fragments. Divergence, which has not yet been observed in mature trees, may continue as seedling cohorts mature, causing fragment communities to follow different trajectories of change. Regeneration of 25 functionally-important dipterocarp species was reduced in fragments by almost half (comparing four fragments and four continuous forest sites), but some dipterocarps were still recruiting seedlings effectively in fragments. Collectively, the research shows that there may be some taxonomic impoverishment within fragments (reduced plot-scale alpha-diversity; possible losses from entire fragments), but that continued recruitment in fragments is resulting in increasingly divergent plant communities (increased beta-diversity). Hence, forest fragments continue to make a valuable contribution to landscape-scale diversity and warrant future protection.
