Effects of Climate Change on (Semi)-Arid Ecosystems in the Southwestern United States

Climate change is considered amongst the most severe threats to terrestrial and aquatic ecosystems globally. Ecosystems in the southwestern United States have specifically been impacted by intense drought conditions since 2000. Higher temperatures combined with altered precipitation stresses many ecosystems; however, ecosystem specific responses to such stressors may vary. Here, the effects of climate change on semi-arid ecosystems are analyzed for some of the most vulnerable ecosystems in the southwestern United States: lacustrine, riparian, and dryland ecosystems. Lakes and reservoirs in arid environments often serve as drinking water sources and recreational areas where high water quality is essential. Climate change may decrease water quality by shifting phytoplankton community structures to favor bloom forming and toxin producing species. In this chapter, I analyzed phytoplankton community compositions in Lake Mead, Nevada-Arizona, to detect trends in past communities and create predictive models for future communities. Results indicated stable community structures, apart from restricted shallow locations where temperature or phosphorus had increased. This study highlights the current buffering capacity of large, oligotrophic reservoirs to maintain stable phytoplankton communities even in the presence of environmental change, but also highlights potential rapid community shifts once this capacity is passed. Riparian ecosystems are generally believed to be buffered from drought as many trees are phreatophytes with roots extending to groundwater. Recent observations of regional riparian woodland dieback and mortality suggest this ecosystem might be more vulnerable to climate change than previously believed. Understanding drivers of the mortality is important as riparian woodlands harvest high biodiversity, improve water quality, and aid in flood control. In this chapter, I studied riparian woodlands at sites in California, Nevada, Arizona, and New Mexico to construct a conceptual model explaining the timing, regionality, and local occurrences of mortality through a sequence of extreme hydrological events: intensified drought and flooding. Drought can reduce shallow root activity, affecting the ability of riparian trees to deal with high groundwater levels during wet periods. Dryland ecosystems cover approximately 40 percent of the Earth’s surface and contain various vegetation patterns. Patterns may emerge prior to the ecosystem reaching its tipping point, after which rapid shift in ecosystem states occurs which can lead to desertification. It is therefore important to understand drivers of pattern formation. In this chapter, I studied soil moisture as driver of vegetation patterns created by the western harvester ant, Pogonomyrmex occidentalis. Patterns increased soil moisture inside the ant created vegetation pattern by reducing moisture lost through transpiration. The vegetation pattern can disappear following increases in aridity and ant colony mortality, which decreases plant and animal diversity, making the ecosystem more vulnerable to change.