Monsoon precipitation extremes and the response of two dominant grassland species across an arid-semiarid ecotone

Background/Question/Methods

Desert grasslands are predicted to be highly responsive to future climate variability. These water-limited ecosystems provide: 1) habitat for organisms that may live near their physiological limits and 2) soil stability that prevents erosion and dust production. While climate models project increased frequency of extreme rainfall events with longer intervening dry periods, research regarding how desert grassland species will respond to changing rainfall patterns is limited. We measured the effect of extreme precipitation events separated by increasing dry intervals on the ecophysiological response of two dominant grassland species, Bouteloua gracilis and B. eriopoda. We hypothesized that 1) less frequent but larger rainfall events would result in less negative pre-dawn water potentials (Ψ_pd) and increased leaf photosynthesis (A_net) in both species and 2) as the interval between rain events increases, B. eriopoda would maintain higher rates of A_net than B. gracilis. In 2010, we established complete rain-out shelters (n=10) in B. gracilis grassland, B. eriopoda grassland and in a B. eriopoda - B. gracilis grassland ecotone. Experimental rainfall treatments included: one 30 mm event added once per month (n=5) or three 10 mm events added three times a month (n=5; July-September). Over the monsoon season, A_net and Ψ_pd were measured. In situ sensors measured soil water content (θ), soil temperature, air temperature, relative humidity and photosynthetically active radiation.

Results/Conclusions

Throughout the monsoon season, 10 mm and 30 mm rainfall events increased soil θ but the magnitude of the response varied across this arid-semiarid grassland. At the B. gracilis and B. eriopoda sites, 30 mm rainfall events significantly increased soil θ and this translated into significantly higher A_net and Ψ_pd within both species. Similarly, 10 mm and 30 mm rainfall events increased soil θ at the B. gracilis – B. eriopoda ecotone, but A_net and Ψ_pd were not significantly different within each species. Rather, B. gracilis maintained significantly higher A_net and Ψ_pd when compared to B. eriopoda. In contrast to the rhizomatous B. eriopoda, the bunchgrass B. gracilis may have a competitive edge in obtaining soil water where these species co-occur. Climate models consistently indicate a future with altered precipitation patterns and an increase in extreme precipitation events. Understanding the spatial and temporal patterns in precipitation is important in arid-semiarid ecosystems, because changes in rainfall distribution alter resource availability, species interactions and plant community composition.