Global Environmental Change in Drylands: Ecohydrological Controls on Plant Communities

Drylands cover much of the terrestrial land surface and many people depend on them as sources of their livelihoods. In the past few decades human activities associated with fossil fuel burning, fertilizer applications, and land use change have dramatically increased atmospheric CO2 concentrations and other trace gases (i.e., NO and NO2) as well as atmospheric temperature, a trend that is expected to continue in the decades to come. Global climate models predict increased precipitation variability at intra-annual, interannual and decadal time scales, especially in dryland regions. These changes in environmental conditions combined with human activities have led and will lead to significant changes in vegetation cover and plant community composition, with important impacts on ecohydrological and geochemical processes, regional climate and the provision of ecosystem services such as livestock grazing, sheltering of the soil surface, and carbon sequestration. Two major shifts in the composition of dryland vegetation are associated with woody plant-grass interactions and the increase in the abundance of plants which conduct Crassulacean Acid Metabolism (CAM). CAM plants feature water storage, nocturnal CO2 uptake, photosynthetic plasticity, and a high water usage efficiency. Primary studies of CAM plants have aimed to engineer CAM modules/genetics into other functional groups (i.e., C3) with the purpose of improving plant water usage efficiency, plant productivity, bioenergy production, and carbon sequestration in a changing environment. However, the ecohydrological controls underlying these vegetation changes (particularly expansion of CAM plants) remain poorly understood. To this end, this dissertation examined the impacts of major global environmental change drivers on woody plant-grass interactions and the competitive relationships between CAM plants and other functional groups (i.e., C3 and C4 plants) in dryland regions. Through the analysis of the ecohydrological controls underlying these vegetation changes, I show that woody plant encroachment can substantially suppress grass production by the effect of lateral root spread and limitations in soil water and light. The work also demonstrated that grass invasions and interannual rainfall fluctuations could act in concert to induce the ecosystem transition from shurblands to the unvegetated state. Instead of these “win-loss” scenarios associated with woody plant-grass interactions, my research also shows that hydraulic lift could be an important mechanism responsible for the coexistence of woody plants and grasses in savannas. In addition to using new mechanistic models integrated with field or satellite data, I conducted greenhouse experiments to show that: i) under CO2 enrichment and drought conditions Cylindropuntia imbricata (a constitute CAM plant) outcompeted Bouteloua eriopoda (C4 grass), with which it coexists in semiarid ecosystems across the Southwestern United States; ii) drought and nitrogen deposition – which have been predicted to increase in the near future – could serve as important drivers of expansion of facultative CAM plants such as Mesembryanthemum crystallinum, which interact with Bromus mollis (C3 grasses) in California’s coastal grasslands; iii) competition with other functional groups may enhance (or suppress, in case of insufficient carbohydrate availability) CAM expression in M. crystallinum, thereby affecting its plasticity and ability to cope with biological stress. Collectively, my research clarifies the effects of major global change drivers and shed light on the ecophysiological and ecohydrological processes responsible for the expansion of CAM plants in drylands around the world, a phenomenon that, to date, has been largely ignored in the environmental science literature. Finally, I developed new mechanistic models to assess the potential links between the two major changes in dryland vegetation observed around the world, namely, woody plants encroachment and expansion of CAM plants. It is found that woody plants could directly and/or indirectly facilitate CAM plants in their access to soil water resources, while the high rate of hydraulic descent performed by woody plants in woody plant-CAM associations could turn the facilitation of CAM plants by woody plants into competition. The novel contribution of my research is to address the key knowledge gaps in dryland vegetation response to global drivers of environmental change. The experimental analyses and process-based models provide an integrated understanding of woody plant encroachment and CAM plant expansion, which are two major changes observed in dryland vegetation around the world.