Modeling Tamarisk (Tamarix spp.) Habitat and Climate Change Effects in the Northwestern United States

Tamarisk species are shrubs or small trees considered by some to be among the most aggressively invasive and potentially detrimental exotic plants in the United States. Although extensively studied in the southern and interior west, northwestern (Oregon, Washington, and Idaho) distribution and habitat information for tamarisk is either limited or lacking. We obtained distribution data for the northwest, developed a habitat suitability map, and projected changes in habitat due to climate change in a smaller case study area using downscaled climate data. Results show extensive populations of tamarisk east of the Cascade Mountains. Despite the perceived novelty of tamarisk in the region, naturalized populations were present by the 1920s. Major population centers are limited to the warmest and driest environments in the central Snake River Plain, Columbia Plateau, and Northern Basin and Range. Habitat suitability model results indicate that 21% of the region supports suitable tamarisk habitat. Less than 1% of these areas are occupied by tamarisk; the remainder is highly vulnerable to invasion. Although considerable uncertainty exists regarding future climate change, we project a 2to 10-fold increase in highly suitable tamarisk habitat by the end of the century. Our habitat suitability maps can be used in "what if' exercises as part of planning, detection, restoration, management, and eradication purposes. Nomenclature: Tamarisk, species in the genus Tamarix L., primarily Tamarix chinensis Lour. and Tamarix ramosissima Ledeb. and their hybrids.