The Tamarisk Takeover: Tamarisk Invasion Along the Canyon Corridors of the San Juan River, SE Utah.
 Kennedy A. Perry1*, Cynthia E. Dott2

1Fort Lewis College, Durango, Colorado, USA
2Fort Lewis College, Durango, Colorado, USA

Tamarisk is an invasive species that has taken over in the Southwest. Tamarisk establishes on recently disturbed open and moist bars, and its establishment is strongly linked to patterns of discharge. (Birken and Cooper, 2009). It is important to understand conditions that led to the historic establishment of tamarisk to better understand the future of riparian habitats, as well as manage and limit future potential invasions. This study investigated which hydrologic conditions led to tamarisk invasion in the riparian canyon corridors along the San Juan River in southeastern Utah (River Miles 10-44). Tamarisk establishment patterns have not been previously studied on the San Juan River. Sites were chosen within canyon constraints on BLM land and were roughly 1 mile apart. In total, there were 66 viable samples of varying age classes at 17 sites. Samples were cut as close to the base as possible, and when there were multiple stems, the largest stem was taken. GPS, distance from river, and stem diameters were recorded per sample. In the lab, rings were counted with a microscope, and establishment year was calculated. We used a GLM to test which predictor variable was the most parsimonious in predicting tamarisk establishment. We found that tamarisk establishment (from 1959 to 2018) was linked to high peak flows the year of establishment, followed by a year with a low peak flow. Major peak flows the year of establishment cleared riparian habitats, providing the optimal site for tamarisk seedlings. Seedlings were not washed away when there were low peak flows the year after establishment, allowing them to become more solidly anchored and harder to dislodge. Tamarisk showed distinct zonation along the narrow canyon corridors. Tamarisk of similar ages grew in similar bands throughout the study sites. This shows that tamarisk establishes in response to hydrologic conditions- waves of establishment occurred in distinct zones after large floods. Future climate and hydrologic conditions are expected to become more drought-like, and it is unknown how tamarisk will respond, but the better we understand past conditions for its establishment the more tools we will have for its future management in the arid West.