Invasive tree covaries with environmental factors to explain riparian plant guilds

Annie L. Henry and Anna A. Sher, Department of Biological Sciences, University of Denver, Denver, CO, Eduardo González, Biology, Colorado State University, Fort Collins, CO, Bérenger Bourgeois, Département de Phytologie, Université Laval, Québec, QC, Canada

Background/Question/Methods
Invasive species have become an inextricable part of the landscape, particularly in riparian plant communities, and removal is often a key component of restoration programs. Species-based assessments used to monitor the consequences of removal do not typically provide insight into the mechanisms underlying plant community response. This study employs functional diversity metrics as well as guilds - suites of species with similar traits - to assess the influence of Tamarix (a dominant invasive tree in the southwestern United States) cover on the functional composition of riparian plant communities in the presence of a defoliating biocontrol agent. We ask: 1) What traits define riparian plant guilds and how does guild abundance vary along a gradient of Tamarix cover and abiotic conditions? 2) How does the functional diversity of the plant community respond to the combined gradient of Tamarix cover and abiotic conditions?
Results/Conclusions
We found lower diversity in high Tamarix cover sites. We identified nine distinct guilds primarily defined by reproductive strategy, as well as height, seed weight, specific leaf area, drought and anaerobic tolerance. Guild abundance varied along a covarying gradient of local and regional environmental factors and Tamarix cover. Guilds focused on sexual reproduction, i.e., producing many light seeds over a long period of time, were more strongly associated with drier sites and higher Tamarix cover. Tamarix itself appeared to facilitate more shade tolerant species with higher specific leaf areas than would be expected in resource poor environments. Understanding the mechanisms underlying plant community responses to invasive species establishment and removal provides a crucial first step in anticipating changes in ecosystem function and resilience in invasive dominated systems.