Monsoon precipitation extremes and soil water dynamics across an arid-semiarid grassland ecotone

Background/Question/Methods

Throughout the southwestern United States climate models predict increased aridity and frequency of extreme rainfall events with longer dry intervening periods. The boundary between arid and semiarid lands represents one of the most sensitive indicators of global environmental change. Yet, few studies across arid to semiarid ecotones address how projected changes in precipitation will affect the ecophysiological response of the dominant species that influence aboveground net primary productivity (ANPP) and species composition. We investigated the effect of altered precipitation regimes on the ecophysioloigcal response of Bouteloua eriopoda and Bouteloua gracilis at a grassland ecotone at the Sevilleta LTER site in central New Mexico, USA. In total, 30 rain-out shelters were constructed to exclude ambient rainfall: ten at a B. gracilis dominated site, ten at a B. eriopoda dominated site and ten at a site where B. gracilis and B. eriopoda co-dominate. Rainfall treatments include: 1) one 30 mm event added once a month and 2) three 10 mm events added three times a month (July-September). In situ soil sensors measured soil volumetric water content (VWC) at 2 cm and 16 cm depths. Leaf-level photosynthesis (A_net) and predawn water potential (Ψ_pd) were measured in association with experimental rainfall events; ANPP was measured in spring and fall when B. eriopoda and B. gracilis had reached peak biomass.

Results/Conclusions

Over the study period, large less frequent rainfall additions significantly increased mean soil VWC on each application date and for several consecutive days thereafter when compared to plots that received small frequent rainfall additions. Although both species responded favorably to rainfall additions, B. eriopoda consistently maintained higher A_net and Ψ_pd (less negative) in response to each application and during the inter-pulse period. This was reflected in a significant increase in ANPP when compared to B. gracilis. Overall, our results suggest that a small number of large events (more extreme events) will increase soil moisture content leading to a positive production B. eriopoda that may shift the boundary between B. eriopoda and B. gracilis dominated grasslands. Long-term data indicate that abundance of B. eriopoda is increasing at a rate greater than that of B. gracilis. Because B. eriopoda is more susceptible to disturbance (fire, grazing, and drought) than B. gracilis, these grasslands may become less resistant to environmental change in the future.