Coexistence and environmental filtering of species-specific biomass in an African savanna.

Biomass density is a key metric of vegetation abundance, but understanding how community assembly processes, such as environmental filtering and competitive exclusion, affect biomass distributions of coexisting species has proven logistically challenging. Here we apply airborne remote sensing to study the ecosystem-scale distribution of species-specific, woody plant biomass and its relation to topographic and hydrologic gradients in a South African savanna. We also spatially analyzed variation in biomass among species to understand patterns of coexistence, mapping the species and biomass over one million trees across 10500 ha. We found the biomass of dominant woody species to be weakly but significantly related to environmental filters, where a combination of 10 topographic and edaphic variables accounted for < 15% of the variance in the biomass of any given species. Distance to nearest stream was the only environmental variable significantly correlated to all species' biomass. Despite an overall negative trend observed between the biomass of species pairs, we found a number of regions where the biomass of two species was similar or equal, and all species pairs exhibited some level of co-occurrence. This suggests that even weak stabilizing mechanisms (e.g., environmental niches) can overcome fitness differences and balance competitive exclusion, enabling coexistence. Future work of repeated measurements of species-specific biomass will provide a novel advance in understanding woody plant community assembly processes in natural ecosystems. Characterizing the species composition of biomass is an important advance in understanding the balance of community assembly processes and its control over current species assemblages.