Climate change and the carbon cycle: A view from arid-semiarid grassland ecosystems

Background/Question/Methods

Regional climate models for the southwestern United States project increased rainfall variability including more extreme precipitation events and prolonged droughts. Precipitation and temperature are also the primary drivers of fire in the southwest, but the interaction of fire and increased climate variability in southwestern grasslands is largely unknown. We examined the effects of climate regime and fire on aboveground net primary productivity (ANPP) and soil respiration (Rs) in Chihuahuan Desert grassland. We present results from two on-going climate change experiments at the Sevilleta LTER. The first is the Monsoon Rainfall Manipulation Experiment (MRME) where treatments include ambient rainfall, ambient rainfall + four 5 mm rainfall events and ambient rainfall + one 20 mm rainfall event per month. The second is the Nighttime Warming Experiment (NWE) where treatments include, + 2^oC nighttime temperatures, increased nitrogen deposition, winter rainfall additions, and all treatment combinations. Both experimental sites were burned in 2009 from a lightening caused wildfire. We hypothesized that: 1) ANPP and Rs would increase in summer rainfall plots, 2) ANPP would increase in nitrogen addition plots 3) ANPP would decrease in warmed plots, and 3) ANPP would not be significantly affected by winter rainfall additions. 

Results/Conclusions

At MRME, small frequent and large infrequent rainfall additions significantly increased ANPP and Rs prior to the fire while nitrogen additions had no effect on ANPP. After the fire, ANPP and Rs at MRME decreased and there were no significant rainfall treatment effects on ANPP for two years following the burn. For all years at the NWE site, mean ANPP was highest in nitrogen addition plots (117 g m^-2) and lowest in warmed plots (89 g m^-2). When compared to the control, ANPP was higher in plots that received winter rainfall amendments and this trend continued after the fire in plots that received nitrogen additions. Treatment affects varied by plant functional type: Bouteloua gracilis responded to nitrogen additions pre- and post-fire where the subshrub Gutierrezia sarothrae has not recovered to pre-fire abundance. Following fire, all treatment effects on ANPP at the NWE site were muted in a dry year. Results indicate that fire and climate variability may alter ANPP, Rs and species composition which could impact local ecosystem processes. Understanding these changes is important for estimates of carbon uptake and ecosystem response to climate change scenarios.