Modeling the climate and hydrological controls of the expansion of an invasive grass over southern Arizona

Background/Question/Methods

Climate change has an effect on the resilience of ecosystems and the occurrence of ecological perturbations (e.g. spread of invasive species, wildfires). Changes in vegetation in turn can interrupt regional scale climate patterns and alter the spatial and temporal propagation of ecological disturbances. Understanding the controls of vegetation change are essential for predicting future changes, and for setting
conservation and restoration targets. Vegetation change in transition zones between ecological regions is a significant indicator of future shifts in the composition of neighboring plant communities. The Walnut Gulch Experimental Watershed is in a grassland-shrubland transition zone between the Sonoran and Chihuahuan Desert in Southern Arizona.

During the past decade, at some sites the cover of the invasive Lehmann lovegrass (Eragrostis lehmanniana) drastically increased and the abundance of native vegetation decreased, causing a major decline in biodiversity.

Focusing on a catchment scale (Kendall Site), we used an individual based vegetation model (ECOTONE) and a coupled vegetation-3D surface/subsurface hydrology model (ECOTONE-CATHY) to simulate vegetation change. We set up the models with soil and climatological data (NLDAS and AmeriFlux), incorporated initial conditions of species and biomass distribution and species parameters for the site. Using ECOTONE we tested our hypothesis that a combination of dry years and subsequent wet period caused Lehmann lovegass to have advantage over the natives. In ECOTONE species composition and species distribution of plant communities arise from dynamic interactions of individual plants with species specific traits through intra- and interspecific competition for resources (H2O, nitrogen) and their interaction
with the environment (precipitation and temperature).

Results/Conclusions

Our results indicate that the competitive advantage of Lehmann lovegrass stems from its ability to withstand dryer conditions during establishment and due to its higher seed survival. We are currently using the coupled ECOTONE-CATHY models to evaluate the role of topography and hydrological processes on the patterns of invasion by Lehmann lovegrass. One hypothesis to be tested is that the redistribution of rainfall over the catchment through overland flow controls the spatial distribution of species and biomass, where wetter soil over lowland areas may buffer the effects of climatic control.