WWETAC Projects

Project Title: Tamarisk in the Pacific Northwest: current distribution, species-environment relationships, and threat assessment

Principal Investigator: Becky K. Kerns, Pacific Northwest Research Station, USDA Forest Service, Corvallis, OR


Collaborators: Catherine Parks and Bridgett Naylor, Pacific Northwest Research Station, USDA Forest Service, La Grande Forestry and Range Sciences Laboratory, La Grande; Michelle Buonopane, Pacific Northwest Research Station, USDA Forest Service, Corvallis, OR

Key Issues/Problems Addressed:
Tamarisk or salt cedar (Tamarix L., primarily Tamarix chinensis Lour., Tamarix ramosissima Ledeb., and their hybrids), a highly aggressive exotic species complex, is an increasingly dominant tree along rivers/streams in the interior Pacific Northwest. No comprehensive regional map of tamarisk invasions exists and little information is known about environmental influences on the ability of Tamarisk to invade sites.

Setting and Approach:
Pacific Northwest Tamarisk distribution data was obtained, a habitat suitability map was developed, and projected habitat changes due to climate shift were made using a smaller case study area and downscaled climate data. Information was acquired and combined from multiple sources and formats (herbarium records, state and local weed experts, online data) into a single data set. Data was visually examined using 1:2,000 black and white air photos to validate point locations and collect additional information. The species-environment relationship was modeled using an ecological niche process that does not require absence data and uses variables hypothesized as important for explaining the occurrence and abundance of Tamarisk: climate, hydrology, disturbance, and soils.

Key Findings:

  • Extensive populations of Tamarisk exist east of the Cascade Mountains.
  • Naturalized Tamarisk populations were present by the 1920s.
  • Major Tamarisk population centers are located in the central Snake River Plain, Columbia Plateau, and Northern Basin and Range, the warmest and driest environments in the region.
  • Habitat suitability model results indicate that 21% of the region supports suitable Tamarisk habitat. Less than 1% of suitable habitats are occupied by Tamarisk: the remainder are believed to be highly vulnerable to invasion.

Highly suitable Tamarisk habitat is predicted to increase 2 to 10 fold by the end of the century. Tamarisk habitat suitability maps of the Pacific Northwest region are available for the purpose of planning, detection, restoration, management, and eradication.

Kerns, B.K., Naylor,B.J., Buonopane,M. Parks, C.G., Rogers, B. 2009. Modeling tamarisk (Tamarix spp.) habitat and climate change effects in the northwestern United States. Invasive Plant Science and Management 2:200-215 (PDF, 9.8 MB)

WWETAC Project ID: FY07BK23

Figure 1. Habitat suitability (HS) for tamarisk as computed from a five-variable, four-factor ecological niche factor analysis.
From Kerns et al. 2009.Tamarisk Habitat Suitability