Measuring the impacts of climate variability on desert plant communities with in situ sensor networks
In the southwestern US, climate change will impact the amount, timing and variability of rainfall during the summer monsoon. These changes are likely to affect plant community dynamics and ecosystem processes, especially across ecotones. In 2012, we established a rainfall manipulation experiment (EDGE-Extreme Drought in Grasslands Experiment) in Chihuahuan Desert grassland (CDG) dominated by black grama (Bouteloua eriopoda) and shortgrass steppe (SGS) dominated by blue grama (B. gracilis) across a grassland ecotone in central New Mexico. EDGE includes two rainfall treatments, chronic drought (~66% reduction in monsoon rainfall) and altered timing of the summer monsoon. To alter precipitation seasonality complete rainout shelters are erected in July and August, and all rainfall occurring during this period is captured, stored, and reapplied during September and October. Thus, this treatment receives the same amount of precipitation as ambient but differs in seasonality. We measured soil moisture, CO2 production and temperature with sensor arrays, and aboveground net primary production (ANPP) and plant species richness in each replicate (n=10) of each treatment at both sites.
No significant pre-treatment differences in ANPP or plant species richness occurred at either site. In 2013 following an above average monsoon, ambient ANPP was 99.4 and 44.3 g m-2 at CDG and SGS, respectively. Chronic drought resulted in a 75% reduction in ANPP at CDG but only a 33% reduction in ANPP at SGS. Shifting the monsoon to later in the growing season resulted in a 50% and 43% reduction in ANPP and soil respiration at CDG and SGS, respectively. In 2014 following an average monsoon with considerable late rains, ANPP completely recovered at SGS and CDG in treatments where the summer monsoon was shifted to later in the season. However, a second year of drought at CDG reduced ANPP to <5 g m-2 with no late season recovery. Soil respiration at both sites was also reduced under drought in 2014. Chronic drought also resulted in a decrease in species richness at CDG, but not at SGS. These results suggest that chronic drought and changes in precipitation seasonality are likely to have variable impacts on grassland communities across this ecotone, but responses are contingent upon the amount and seasonal distribution of precipitation. The ability of black grama grassland to recover under changing precipitation seasonality suggests that CDG may continue to expand northward replacing SGS vegetation along this ecotone under a changing precipitation regime.