Semi-arid savanna herbaceous production and diversity responses to interactive effects of drought, nitrogen deposition, and fire

Question: Savannas are globally widespread and furnish a variety of ecological services through their structural heterogeneity. Unfortunately, those essential ecosystem services are threatened by climate changes including expected increases in duration of drought and nitrogen deposition. The objective of this study was to determine how overall herbaceous production, functional group production and diversity respond in the short-term to interactions between forecasted environmental conditions and prescribed fires. Location: Western Edwards Plateau, Texas (latitude 31°N, longitude 100°W). Methods: We randomly assigned full-factorial treatment combinations of rainout shelters, simulated nitrogen deposition and prescribed fires to field plots in an herbaceous-dominated area of a semi-arid savanna. Aboveground net primary productivity (assessed via destructive harvest) and diversity (using Shannon’s index) were assessed as indicators of ecosystem functioning. Results: Total aboveground net primary production was reduced by fire in the short-term (4 months), and reduced by drought at 8 months, but drought reductions were later overridden by lagged responses to large precipitation events (12 months). Forb production increased in response to nitrogen addition (8 months) and drought (12 months), but decreased as a result of fire (12 months). Live grass production was consistently reduced by drought. Plant species diversity was positively responsive to nitrogen addition, particularly in the absence of drought. Conclusions: Our findings suggest that the concentrated precipitation events that are forecasted to follow extended droughts may aid rapid recovery of drought-induced production decreases. In addition, the small-scale diversity of this semi-arid savanna may be driven more by resource availability than light-competition in the short-term. Managers and ecologists can use these results to help disentangle the ecosystem functioning that may be observed in the presence of future droughts, nitrogen deposition and prescribed fire. Understanding these processes will be key to protecting the integrity of savannas.