Tamarisk in the Pacific Northwest: current distribution and species-environment relationships

Species in the genus Tamarix are considered highly invasive and among the most detrimental exotic plants in the U.S. Primarily associated with the arid southwest in the past, tamarisk is now dominant along Pacific Northwest (PNW, Oregon, Idaho, Washington) rivers and streams. Yet there is presently no tamarisk infestation map for the region and little information regarding species-environment relationships. Recent national scale modeling work by others used data that included only one point for tamarisk in the PNW. Our objectives are to 1) describe the current tamarisk distribution; 2) examine species-environment relationships; 3) explore factors important for early detection and rapid response; and 4) compare results with patterns and hypotheses from the literature. We combined data from multiple sources into a single dataset. Data were visually examined using 1 m natural color aerial photography from NAIP (2004-2006) to validate point locations and collect additional information. We are modeling species-environment relationships with an ecological niche process that does not require absence data, using variables identified as important for explaining occurrence and abundance of tamarisk: climate, hydrology, disturbance, and soils. We found over 2000 point locations in the region, with populations concentrated in arid interior river basins.  Herbarium records document residential plantings in the Columbia River gorge as early as 1893, with natural populations established in the region by 1928. The vast majority of points were along relatively natural (86%), riparian (38.4%), and ephemeral stream (54.3%) areas. Strong relationships with temperature and precipitation were noted. Tamarisk is associated with relatively warm and dry sites (30-yr means: max temp. 17.6±0.70 C, min temp. 3.2±1.1 C, ppt. 23.8±5.0 cm) sites. Our map and model will contribute to other tamarisk mapping efforts, serve as a basis for a comprehensive map, and provide information for early detection, rapid response, and tamarisk management and policy development.