Considerable uncertainty and concern exist regarding ways climatic alterations will impact environmental services and ecological resilience in semi-arid savanna ecosystems. Climate-change predictions suggest magnified drought durations and intensities interspersed with extreme rainfall events, increased nitrogen deposition, and altered fire regimes. Moreover, these effects will likely interact in complex and novel ways that may potentially result in unforeseen community assemblages and ecosystem dynamics. We experimentally manipulated fire, drought, and soil fertility in order to assess how both soils and herbaceous plant populations within a semi-arid savanna respond to these variables independently and through interactions. In March 2018, we established 64 5x5m, grass-dominated plots in shallow soils at the Texas A&M Sonora Research Station on the southwestern Edwards Plateau (31°N, 100°W). Prescribed fire (burned or control), drought (rainout shelter excluding 69% of precipitation thereby simulating the 1st percentile extreme drought recorded for the site or ambient), and soil fertility (5 gN/m2 ammonium nitrate addition or control) treatments were assigned to each plot in a full-factorial, completely randomized experimental design. Drought plots were trenched and surface water flow was excluded to a depth of ~12cm. Treatment effects on plant community composition, vegetation structure and biomass, tissue chemistry, and soil chemistry were measured.
Results/Conclusions
Rapid vegetative community responses (<8 months) to the treatments were manifest in the form of drought-reduced ANPP (p<0.05) while the full combination of treatments differentially affected plant community composition (p<0.05). Nitrogen additions significantly interacted with drought treatments to affect plant species diversity (p<0.05). The soil chemistry was heterogeneous pre-treatment and recalcitrant to treatment-induced differences in pH, organic carbon, and nitrogen to date. We acknowledge that the system may respond differently to anthropogenic alterations at larger temporal and spatial scales, but submit that these findings help identify mechanisms of future change. Over a short time-frame, herbaceous communities can be expected to experience compositional and productivity alterations despite soil chemistry remaining unchanged. In the fullness of time, feedbacks that affect both soils and vegetation will need to be carefully assessed and continuously monitored. These results will provide insights into the manner in which fire, drought, and soil fertility interact and may potentially assist in the development of restoration or management efforts designed to enhance the resilience and long-term integrity of semi-arid savanna ecosystems experiencing unprecedented changing climatic conditions. Further, we are partnering with the coordinated Drought-NET experimental effort and hope to contribute to fundamental theories associated with precipitation limitation in terrestrial ecosystems worldwide.