The role of plant diversity and facilitation during tropical dry forest restoration

Restoration programmes that promote the functioning of restored ecosystems are in urgent demand. Although several biodiversity and ecosystem functioning (BEF) experiments have demonstrated the importance of functional complementarity in enhancing plant community performance, no biodiversity experiment has yet manipulated facilitation to test its contribution to how the complementarity effect (CE) modulates community performance.We built a restoration experiment manipulating diversity and facilitation potential in a tropical semi-arid forest. We planted 4704 seedlings of 16 native tree species to assemble 147 experimental communities with 45 different compositions comprising 1, 2, 4, 8 or 16 species. Facilitation potential was included in the experimental design by creating a gradient of communities from low to high facilitation potential (based on prior research). We measured functional dispersion and functional identity using species above- and below-ground traits to investigate how they modulate the effects of species diversity and facilitation potential on leaf biomass production, using the additive partition biodiversity effects CE and selection effect (SE).The joint influence of diversity and facilitation potential was tested separately for leaf biomass production and net biodiversity effect using linear mixed models (LMMs). We subsequently ran LMMs including functional dispersion and functional identity. We hypothesised that facilitation potential would increase community productivity and functioning and that functional dispersion and functional identity related to above- and below-ground traits would explain facilitation performance.Facilitation potential positively influenced leaf biomass production as predicted, but unexpectedly, neither of the functional traits were important for modulating the facilitation process. Positive values for CE showed that plants performed better in mixtures in comparison to monocultures. SE negative values, showed that species with below average performance in monocultures, performed better in mixtures. Unexpectedly, CE did not increase as species diversity or facilitation potential increased. The SE was influenced negatively by facilitation potential leading to a more equal distribution of biomass production between species in mixtures.Synthesis. Facilitation improves biomass production in restored communities and increases biomass equitability among plant species and thus ecosystem reliability. To improve restoration success, plant communities should be built using a combination of facilitative species.