Plant community response to exotic tree removal in western watersheds

Background/Question/Methods

Removal of invasive species is often an important, if not central, component of many riparian restoration projects, however little is known about the ecological impacts of this action.  There is often a desire to increase native plant species, concurrent with concern that removal of one weed will invite a second invasion. In particular, control of the dominant tree Tamarix spp is increasingly conducted to improve ecosystem services, however previous research suggests mixed results from this activity.  Further, these studies results are confounded by projects of different ages (time since removal) and variability associated with different regions. To better understand what factors contribute to outcomes, we are monitoring vegetation response after concurrent Tamarix removal in 64 restoration sites in two watersheds of Colorado with the following specific questions in mind: 1) Does killing this tree increase native species cover or richness?,  2) Does removal technique affect plant community response? 3) to what degree does region predict vegetation response? 

Results/Conclusions

Tamarix control combined with a drought year significantly decreased native species vegetative cover in the first year after removal, but recovery in subsequent years was seen in most sites.  Application of herbicides negatively affected both exotic and native species, but these chemicals were also generally effective in shifting the dominance to native species.  Understory response to the deferent removal techniques show an increase of native species where Tamarix were removed by the “cut stump” method, but a decrease in sites when the Tamarix was treated by helicopter spray with herbicide, due to secondary invasion by Kochia scoparia.  There was no evidence of shifting dominance in those sites where there was only biological control by the tamarisk leaf beetle. Plant community response to restoration activities significantly differed between watersheds, reflecting the importance of water availability and differences between communities dominated by shrubs vs. herbaceous plants. Most interesting, relative native cover in the understory increased in sites with Tamarix removal but decreased in sites with no removal.  Taken together, these results suggest that “native” vs. “exotic” are meaningful ecological guilds, and that removal of a dominant exotic species in the overstory can have significant effects on relative dominance in the understory.