Recruitment subsidies support tree subpopulations in non-preferred tropical forest habitats

P>1. A large share of tree species in tropical forests exhibit preference for certain habitats, resulting in strong abundance differences across habitats. While the occurrence of such habitat preference is now documented for over a thousand species, little is known about the underlying mechanisms. How are subpopulations in non-preferred habitats able to survive? Theoretical studies have suggested that these subpopulations are maintained by source-sink dynamics -'recruitment subsidies' from high-density to low-density subpopulations - but empirical tests are lacking. 2. Here, we evaluate the role of recruitment subsidies in maintaining subpopulations of a rainforest tree in non-preferred habitats. Our study species Scaphium borneense (Sterculiaceae) shows strong habitat preference for drier ridges at Lambir Hills, Malaysia, where it occurs at ninefold higher densities than in the wetter valleys. On slopes, Scaphium occurs at intermediate densities. 3. We estimated source-sink exchange between subpopulations in three habitats in a 52-ha plot, using nearest-neighbour analyses. We found evidence for strong recruitment subsidies to the non-preferred habitats: 83-91% of recruits (of 1 cm diameter) in the valley had likely mother trees in another habitat. For slope recruits this was 54-78%. 4. We then used 10-year demographic data to construct multi-state matrix models that included the dynamics within, and exchange of recruits between subpopulations. We found that blocking recruitment subsidies to valley and slope subpopulations led to strong reductions in subpopulation growth rates over 100 years (lambda(100)). By contrast, lambda(100) of the ridge population was hardly affected by blocking recruitment exchange. 5. Elasticity analysis confirmed the importance of recruitment subsidies for lambda(100) in valley and slope subpopulations: elasticity of recruitment subsidies to these subpopulations was three to five times larger than local recruitment. Again, the reverse pattern was found for the preferred habitat, where elasticity for recruitment from other habitats was very low. 6.Synthesis. Our results show that recruitment subsidies can be crucial for maintaining subpopulations of tropical tree species in non-preferred habitats. To the extent that such source-sink dynamics are common among tropical tree species, this mechanism may play a role in maintaining high tree diversity in tropical forests.