Delivering adequate water supplies to support expanding human enterprise while maintaining the necessary flow regimes to support desired riparian ecosystems and formally protected wildlife species that depend upon them is increasingly difficult in the arid western United States. Many riparian systems have undergone dramatic alteration over the last 50 - 100 years, exacerbating the conflicts between resource use and biodiversity protection. One of the most visible changes that is in part due to altered flow regimes is the establishment of invasive plant species in riparian ecosystems. The highest priority invasive riparian plant is the Eurasian tree/shrub, tamarisk (or saltcedar, Tamarix spp.) the third most abundant woody species in the western United States riparian zones. Tamarisk control and removal has become a priority of riparian ecosystem management, due in part to its potential negative impacts on stream flow and groundwater recharge. A controversial and potentially effective tamarisk control approach is the release of the tamarisk leaf beetle, Diorhabda carinulata. The beetle has spread throughout virtually the entire upper Colorado River Basin, established major populations at Lake Mead in 2012, and is now poised to expand into the lower Colorado River Basin concordant with documented evolutionary change in beetle developmental stages that enables southern establishment. The beetle feeds exclusively on tamarisk, resulting in periods of defoliation that can last several weeks. These episodic defoliation events result in carbon starvation that in turn reduces leaf production and growth, leading to incremental dieback and in some cases mortality. The actual rate of mortality, however, is highly variable across the landscape. Repeated years of defoliation can result in anywhere between 0% to 100% mortality, with mortality rates of 20-40% being common after three to five years. Superimposed on this direct plant/herbivore relationship is the potential impact of climate change on the future performance of tamarisk, and hence on its effects (both negative and positive) on ecosystem structure and function. Temperature and drought-related mortality of native riparian tree species is not random and some genotypes are more susceptible than others. Common garden studies show that riparian species exhibit substantial genetic variation and source population differences in survival and growth in hotter and drier environments. Tamarisk shows a similar range of responses to environmental stressors, both within Tamarix species and across hybrids formed from multiple Tamarix species. Nevertheless, information on how tamarisk populations and genotypes will respond to a combination of episodic defoliation and climate change is lacking. Therefore, the overarching objectives of this project are to: 1) determine if climate warming coupled with herbivory by the tamarisk leaf beetle synergistically reduce the performance of tamarisk in western North America: 2) assess whether some tamarisk populations are more susceptible (in terms of canopy die back, above ground productivity and plant mortality) to the combination of climate change and herbivory than others: and )3 evaluate how genetic change in D. carinulata will enable southern colonization into the lower Colorado River Basin and extend phenology, thereby changing riparian vegetation in this critical region.