Nutrient availability and species evenness influence productivity during early stages of a tropical forest restoration

Background/Question/Methods

Restorations often aim to maximize species richness and community productivity. However, even in species-rich tropical systems, limited numbers of species may be available from local growers.  In these cases, manipulating evenness might best maximize diversity. Though a positive relationship between richness and productivity is well-documented, fewer studies address the relationship of evenness to productivity. Where species abundances are equal, complementarity among species may result in efficient resource use and higher community-level productivity.  Under fertilization, enhanced productivity due to complementarity may diminish. Furthermore, in uneven communities, average productivity depends on the dominant species’ traits. Differences in the identity of the dominant species among local communities thus increases beta diversity and functional diversity at the landscape scale. We asked whether evenness influences the growth of tree seedlings in a tropical forest restoration, and whether community productivity depends on nutrient availability. We altered the relative abundance of tree seedlings in 16 plots within a nonnative bamboo plantation in the lowland wet tropics of Costa Rica.  Richness and composition were consistent across plots, although the identity of the dominant species varied among low-evenness plots.  Half of the plots were fertilized five times over two wet seasons; trees were measured after 18 months of growth.  

Results/Conclusions

After 18 months, seedlings grew most in fertilized, high evenness plots.  A significant interactive effect between fertilizer and evenness on community-level biomass was driven in part by the fact that not all species responded to the nutrient addition.  In fertilized treatments, early successional species showed the most dramatic growth response, while shade-tolerant, mid-late successional species accrued significantly less biomass.  Thus, the reduced overall response to fertilizer that we observed in low evenness plots was likely due to cumulative species-specific responses.  Specifically, in low-evenness plots, positive community-level responses to fertilizer in plots dominated by early successional trees were offset by the plots dominated by late-successional trees which produced less biomass on average.  Our results suggest that at an early stage of restoration, maximizing evenness at a local scale can increase biomass production, particularly when trees are fertilized.  However, altering the identity of the dominant species in uneven communities at a local scale may result in greater beta- and functional-diversity at a regional scale.  The degree to which a practitioner manipulates species evenness and nutrients should depend on whether local or regional diversity is the restoration goal; in either case, species selection will influence productivity.