Why Do Some Restoration Projects Fail and Others Succeed? A Quantitative Look at 243 Sites for Environmental, Management, and Social Factors
Anna Sher1*, Annie L. Henry2, Lisa B. Clark2, Alex Goetz2, and Eduardo González2,3
1University of Denver, Dept. of Biological Sciences, Denver, CO, USA; anna.sher@du.edu 
2University of Denver, Dept. of Biological Sciences, Denver, CO, USA
3Colorado State University Dept. of Biology, Fort Collins, CO
Tamarix control projects in riparian systems vary widely in their success at meeting project goals. Researchers have investigated the role of removal methods and the environment to explain this variability, but the human component has rarely been explored. Our previous research had found that in this system, land managers mostly follow scientific recommendations regardless of background or attitudes, however, the question remained how much these choices, or even aspects of background or experience, actually explained restoration outcomes. This research quantifies the relative roles of environmental factors and both manager decisions and traits for explaining the impact of Tamarix removal projects throughout the southwestern U.S. To do this, we have created 243 pairs of sites where Tamarix has been removed with controls to quantify impact. Our response measure was a PCA of those metrics that mattered most to managers as a measure of success, that is, a change in 1) Tamarix cover, 2) total native species cover, 3) relative understory native cover, and 4) understory noxious species cover. We then determined how much of the variability in this dependent variable could be explained by commonly used environmental factors such as soil texture and chemistry, geography, measures of water availability, and removal method. We then determined to what degree human factors explained the remaining, unexplained variance (i.e., residuals). These human data were collected from 45 corresponding managers of these sites who completed questionnaires about their practices and backgrounds. We found that decisions made by managers beyond removal method mattered for the degree to which Tamarix removal changed plant communities, including what priorities had been established for the site and how many collaborators were involved with the project. This is the first study to quantify the direct relationship between human traits and vegetation in this ecosystem type, and with implications for improving restoration outcomes in the future.