Desertification pathways induced by grass invasion into shrublands: The ecophysiological context

Grasslands and shrublands provide fundamental ecosystem services in arid and semi-arid regions but are highly susceptible to desertification induced by anthropogenic and climatic drivers. A proposed pathway of desertification suggests that in years of high moisture availability, invasive grasses spread rapidly into native shrublands, increasing fire frequency and shrub mortality. This destroys the inherent soil heterogeneity in shrublands and yields invader-dominated grasslands in which drought can cause widespread plant mortality, ultimately resulting in irreversible soil loss through erosion. Climate change could actually amplify the effects of grass invasion on desertification if invasive grasses perform more favorably under altered conditions than the native grass species with which they compete. We sought to determine (i) if ecophysiological mechanisms contribute to invasive grass performance and (ii) how a warmer temperature regime could affect grass ecophysiology and drought induced mortality. We monitored the gas exchange, water relations, biomass accrual, and mortality of both an invasive grass (Pennisetum ciliare, Buffel grass) and a native grass (Heteropogan contortus, Tangle head) under ambient and warmed (+4°C) conditions in the Biosphere 2 facility. The grasses were grown in monotypic and mixed communities under moisture stress conditions simulating a moderate to strong drought. Warming caused the invasive grass to photosynthesize more rapidly and out-compete the native grass more efficiently, indicating that warming could enhance the susceptibility of desert landscapes to grass invasion. Under ambient temperature regimes, both grass species were able to adapt to moisture stress conditions by inducing dormancy, while the combination of drought and warming resulted in complete mortality of both the species. Overall, climate change may provide more opportunities for this land degradation pathway to occur by inducing fluctuations in the rates and distribution of precipitation and by increasing the frequency of droughts in dryland systems.