Recovery of a native tree after invasive tree removal in riparian systems of the western US

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Alexander R. B. Goetz , Ian Moffit, Anna A. Sher

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Removal of invasive species does not completely restore an ecosystem to its pre-invasion state, instead creating a novel ecosystem that can have variable recovery trajectories leading to a range of functional outcomes. Removal of Tamarix spp. in the American Southwest has had deleterious impacts on habitat availability for the endangered Southwestern willow flycatcher (Empidonax extimus trailii, abbr. SWFL), which nests readily in Tamarix when native Salix canopy is not present. If we can identify conditions leading to more native vegetation cover as well as habitat protection for the SWFL, we can prioritize restoration efforts more effectively and reduce conflict between conservation goals. To determine whether reduction in the invasive tree led to more habitat, as measured by increased cover of Salix and specifically Salix exigua (the most common species), we compiled data on vegetation, soils, and geographic conditions in 243 sites where Tamarix had been subject to active removal and/or biocontrol and 172 reference sites. We found that (1) while decreased Tamarix cover is associated with an increase in Salix, the increase does not compensate for the overall losses in canopy cover. (2) We did not find a significant difference in Salix cover among Tamarix removal methods or relative to negative reference sites; however, sites where herbicide was applied at any point had higher Salix cover. (3) We found significant impacts of several environmental characteristics including soil properties, distance to water, and initial Salix cover on Salix and specifically S. exigua cover across sites. Our data reflect the fact that Salix and Tamarix occupy distinct environmental niches. Our findings demonstrate that removal of an invasive species does not necessarily lead to reestablishment of the native species it appeared to displace, and that this may be due to the invader occupying a different niche than the native species. We suggest that in the case of promoting habitat for SWFL and other birds, outcomes of restoration activity can be improved by focusing Tamarix removal on sites more likely to promote Salix growth based on environmental characteristics.

Resource Type:
Conference Presentation, SER2021

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