Plant Community Response to a Second Cycle of Tamarix Biocontrol Defoliation in the Upper Colorado River Near Moab, UT
Eduardo González1*, Patrick B Shafroth2, Steven R Lee3, Sasha C Reed4, Jayne Belnap4
1 Colorado State University, Department of Biology, Fort Collins, CO, USA; edusargas@hotmail.com
2 U.S. Geological Survey, Fort Collins Science Center, Fort Collins, CO, USA; shafrothp@usgs.gov
3 U.S. Geological Survey, Western Ecological Research Center, Oakhurst, CA, USA; stevenrandalllee@gmail.com
4 U.S. Geological Survey, Southwest Biological Science Center, Moab, UT, USA; screed@usgs.gov, jayne_belnap@usgs.gov
Reduced abundance of non-native Tamarix shrubs following biological control in western U.S. riparian systems has led to concerns that replacement plant communities could be dominated by other invasive species and/or not provide some of the ecosystem services that Tamarix was providing. In previous studies, Tamarix decline following biocontrol has been accompanied by small increases in native and non-native herbaceous species, with uncertain response of woody vegetation. However, none of these studies spanned periods longer than a decade since beetle release. This is an important caveat, given the cyclical nature of plant-herbivore interactions and potential lags in vegetation recovery. We report plant community response to an eight-year-long second cycle of Tamarix defoliation-refoliation in two reaches of the upper Colorado River in eastern Utah, 11-13 years after since beetle arrival. Tamarix cover across sites initially declined an average of ca. 50% in response to the beetle, but then recovered. Changes in the associated plant community were small and slightly favoured more desirable plant species: a native shrub (Salix exigua) increased 47% on average and no secondary invasions by other exotic plants were detected. We suggest that the effectiveness of biocontrol programs must be assessed case-by-case, and on a long-term basis.