Impacts in Southwestern Deserts National Academy of Sciences National Academy of Engineering Institute of Medicine National Research Council Impacts in Southwestern Deserts Wildfire and Invasive Species Nonnative grasses becoming established in deserts: Red brome (in the Mojave) Buffelgrass (in the Sonoran) Grasses transform desert into flammable grassland: fire-adapted grasses re-establish quickly, pushing out native species like Saguaro cactus Spread of grasses not directly a result of climate change, but warming may allow them to further spread in the desert and extend to higher elevations. See Ecological Impacts of Climate Change booklet, p. 18 In the past, the Mojave and Sonoran deserts of the southwestern United States were generally considered fireproof. There simply was not enough fuel to carry a fire through the sparse shrubs and cacti. However, fire-adapted invasive grasses are now changing that dynamic. While climate change is not directly implicated in the spread of these grasses, scientists are concerned that warming temperatures will allow them to spread further in the desert Southwest and to extend into higher elevations. Image courtesy T. Esque, USGS.

Impacts in Southwestern Deserts National Academy of Sciences National Academy of Engineering Institute of Medicine National Research Council Impacts in Southwestern Deserts The Piñon Pine: Past a Tipping Point Drought in 2000-2003 stressed a large swath of piñons, leaving them susceptible to infestation by pine bark beetles This example shows how a stressful event can trigger dramatic ecological change when an ecosystem is subject to many interacting stresses See Ecological Impacts of Climate Change booklet, p. 19 The 2000-2003 drought in the Four Corners region was abnormally severe because it combined low precipitation—25–50 percent less than the long-term average—with unusually high temperatures. By the end of 2003, a large number of the piñons in the region were dead. The main cause of death was infestation by the pine bark beetle, which often targets trees that have been weakened by other stresses—in this case, heat and drought. The widespread loss of these pine trees caused a major ecological change over a large area. In general, we do not know the thresholds for such major changes before they occur. This example highlights the threat that a stressful event that would not normally trigger a dramatic ecological change may do so when an ecosystem is subject to many interacting stresses. 2002 2004 Images courtesy D. Allen, USGS.