Recovery of a native riparian tree following removal of an invasive competitor

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Alexander Goetz, Ian Moffit and Anna A. Sher, Department of Biological Sciences, University of Denver, Denver, CO

Background/Question/Methods
Invasive plants, by definition, displace native species, however this does not necessarily mean that their removal leads to competitive release and thus recovery. The lack of such recovery can have cascading effects through ecosystems. In riparian areas of the American Southwest, removal of invasive Tamarix spp. trees 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 spp. (willow) canopy is not present. However, previous research has found conflicting results regarding recovery of native plants when Tamarix is removed, and none have specifically focused on Salix. Using a multi-state dataset of Tamarix removal sites in three different watersheds, we asked the following questions: (1) Does removal of Tamarix lead to the establishment of Salix? (2) Which Tamarix removal methods have the best outcomes in terms of Salix cover? (3) What environmental conditions are required to implement a successful Salix restoration effort? We compiled data on vegetation response to Tamarix removal consisting of plant cover, soils, and geographic conditions in 243 sites where Tamarix had been subject to active removal and/or biocontrol and 172 reference sites. We examined the response variables of total cover of all Salix species as well as S. exigua (narrowleaf willow) specifically, which is the most dominant Salix species in the study extent. We used linear mixed models with backward stepwise selection to predict response of Salix cover, both final-year outcomes and change over time.

Results/Conclusions
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 that Salix cover was greater when soils were sandier and less saline, and under cooler and wetter climate conditions. Our data reflect the fact that Salix and Tamarix occupy distinct environmental niches. Our findings suggest that Tamarix removal does not necessarily lead to favorable outcomes for SWFL conservation but that outcomes can be improved by focusing on sites more likely to promote Salix growth based on environmental characteristics.