Invasive tree cover covaries with environmental factors to explain the functional composition of riparian plant communities

Invasive species are a major cause of biodiversity loss worldwide, but their impact on communities and the mechanisms driving those impacts are varied and not well understood. This study employs functional diversity metrics and guilds - suites of species with similar traits - to assess the influence of an invasive tree (Tamarix spp.) on riparian plant communities in the southwestern United States. We asked: 1) What traits define riparian plant guilds in this system? 2) How do the abundances of guilds vary along gradients of Tamarix cover and abiotic conditions? 3) How does the functional diversity of the plant community respond to the gradients of Tamarix cover and abiotic conditions? We found 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 relying on sexual reproduction, in particular those 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. Additionally, we found a high degree of specialization (low functional diversity) in the wettest, most flood-prone, lowest Tamarix cover sites as well as in the driest, most stable, highest Tamarix cover sites. These guilds can be used to anticipate plant community response to restoration efforts and in selecting appropriate species for revegetation.