Climate change in the American Southwest deserts: Opportunities for invasion by exotic grasses?

Background/Question/Methods

Lehmann lovegrass (Eragrostis lehmanniana), a perennial grass originally from South Africa, is a highly invasive species in the Sonoran Desert, but has a limited distribution in the higher elevation Chihuahuan Desert. We hypothesized that an increase in temperature with global warming would increase the susceptibility of the Chihuahuan Desert to invasion by this species, and that invasion success would vary across heterogeneous landscapes that differ in climate and soil properties that interact to affect soil water availability.  Our objectives were: (1) to identify the climatic factors responsible for differences in invasion success between these two hot deserts, and (2) to determine how an increase in temperature as a result of global warming, and either an increase or decrease in precipitation, could influence patterns in invasibility of these deserts. We used a daily time step, multi-layer model of soil water dynamics, SOILWAT, to simulate Lehmann lovegrass seedling establishment at a site in the Sonoran Desert of Arizona (Santa Rita) and in the Chihuahuan Desert of New Mexico (Jornada LTER-USDA). We used long-term precipitation and temperature data from each site, and soil properties of deserts to parameterize SOILWAT for current conditions and for a suite of climate change scenarios.

Results/Conclusions

Under current climatic conditions for both sites, the probability of Lehmann lovegrass seedling establishment is highest on sandy loams, very low on loamy sands, and decreases as sand content decreases. Probabilities of establishment were much larger for the Santa Rita compared with the Jornada. If future air temperature is 3^oC higher and precipitation amount increases, then the probability of seedling establishment will increase with greater increases occurring on sandier soils than on soils high in clay or silt.  Increases in probabilities of establishment are greater for the Jornada than for the Santa Rita. If future air temperature is 3^oC higher but precipitation amount decreases, the probability of establishment will decrease for both sites, and will approach zero at the Jornada.  Our study shows that Chihuahuan Desert landscapes will become more susceptible to invasion by exotic grasses under global warming, but only under increases in precipitation and for sandy sites.  This information will help land managers decide where to allocate limited resources in controlling exotic grasses.