Riparian Plant Communities Remain Stable in Response to a Second Cycle of Tamarix Biocontrol Defoliation
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PHYSICAL AND BIOTIC DRIVERS OF CHANGE IN RIPARIAN ECOSYSTEMS
Riparian Plant Communities Remain Stable in Response to a Second Cycle of Tamarix Biocontrol Defoliation Eduardo González 1
&
Patrick B Shafroth 2 & Steven R Lee 3 & Sasha C Reed 4 & Jayne Belnap 4
Received: 14 January 2020 / Accepted: 9 September 2020 # US Government 2020
Abstract Reduced abundance of non-native Tamarix shrubs in western U.S. riparian systems following biological control by a defoliating beetle 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 was accompanied by small increases in native and non-native herbaceous species, with variable responses 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 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 but supported common management goals, including a 47% average increase in cover of a native shrub (Salix exigua), and no secondary invasions by other non-native plants. We suggest that the effectiveness of biocontrol programs must be assessed case-by-case, and on a long-term basis. Keywords Biocontrol . Defoliation . Riparian plant community . Salix . Tamarix . Upper Colorado River
Introduction Riparian ecosystems provide important ecological functions and services to society, but these are jeopardized by human activities such as hydrological regulation, overexploitation of water resources, and floodplain occupation by agriculture, and urbanization (González et al. 2017a). One of the Electronic supplementary material The online version of this article (https://doi.org/10.1007/s13157-020-01381-7) contains supplementary material, which is available to authorized users. * Patrick B Shafroth [email protected] 1
Department of Biology, Colorado State University, 251 W Pitkin St., Fort Collins, CO 80521, USA
2
U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., Building C, Fort Collins, CO 80526, USA
3
U.S. Geological Survey, Western Ecological Research Center, 40298 Junction Dr., Suite A, Oakhurst, CA 93644, USA
4
U.S. Geological Survey, Southwest Biological Science Center|, 2290 S.W. Resource Blvd., Moab, UT 84532, USA
primary impacts of human activities in riparian zones has been the spread of invasive plants (Richardson et al. 2007), and controlling plant invasions is a central focus of riparian zone management worldwide (González et al. 2015). Along North American rivers, for example,
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