Recovery of Salix following Tamarix removal
Alexander Goetz1*, Ian Moffit1, Anna Sher1
1 University of Denver, Department of Biological Sciences, Denver, CO
Removal of invasive 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. Understanding the characteristics associated with Salix recovery can allow for more effective targeting of restoration efforts in the context of SWFL conservation. If we can identify conditions leading to more native vegetation cover as well as habitat protection for the SWFL, we can prioritize efforts more effectively and reduce conflict between conservation goals. Using a multi-state dataset of Tamarix removal sites in three different watersheds, we ask 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 riparian areas of the American Southwest. In total, there were 243 sites where Tamarix had been subject to active removal and/or biocontrol and 172 reference sites. We examined total cover of all Salix species and separately analyzed only S. exigua, the most dominant species. We used two measures of Salix and S. exigua response to restoration: final year of cover and cover change over time. We used linear mixed models with backward stepwise selection to predict response of Salix cover to multiple environmental and restoration factors. In addition, we tested effects of individual independent variables on Salix cover, both final-year outcomes and change over time. Finally, we constructed mixed models to compare Salix cover change over time with Tamarix cover change over time, both overall and by removal method. 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 S. exigua cover. 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.