Combined effects of biological control of an invasive shrub and fluvial processes on riparian vegetation dynamics

Authors: GONZALEZ, EDUARDO - Colorado State University; SHAFROTH, PATRICK - Us Geological Survey (USGS); LEE, STEVEN - Us Geological Survey (USGS); Ostoja, Steven; BROOKS, MATTHEW - Us Geological Survey (USGS)

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2020
Publication Date: 4/9/2020
Citation: Gonzalez, E., Shafroth, P., Lee, S., Ostoja, S.M., Brooks, M. 2020. Combined effects of biological control of an invasive shrub and fluvial processes on riparian vegetation dynamics. Biological Invasions. 22:2339-2356. https://doi.org/10.1007/s10530-020-02259-9.
DOI: https://doi.org/10.1007/s10530-020-02259-9

Interpretive Summary: Invasive plant species have the ability to reshape how natural plant communities look and function. A plant called salt cedar or tamarisk is one such species which is more or less ubiquitous with along rivers and other riparian areas of the southwestern United States. Since introduction this plant, which can be either a small tree or large shrub has effectively taken over in the areas where it occurs to the cost of native flora. In an effort to control tamarisk the USDA-APHIS released a biocontrol beetle as a defoliating agent to reduce the dominance of the weedy plant with the hope that native plants could re populate the areas where it was effective. This study considered the effectiveness of the beetle by looking at the post defoliation vegetation in context of landscape features. It was found that native shrubs and trees were able to regain dominance, but the identity and abundance depending on local site-specific characteristics.

Technical Abstract: Plant community responses to biocontrol of invasive plants are understudied, despite the strong influence of the composition of replacement vegetation on ecosystem functions and services. We studied the vegetation response to a folivore beetle (Diorhabda genus, Coleoptera) that has been introduced along southwestern US river valleys to control the invasion of non-native shrubs in the genus Tamarix (Tamaricaceae). We collected detailed plant compositional and environmental data during four different surveys over 7 years (2010–2017), including two surveys prior to when substantial beetle-induced dieback occurred in summer 2012, along the lower Virgin River, Nevada. The study river was of special interest because it is one of only a few largely unregulated rivers in the region, and a large flood of 40-year return period occurred between the first and second surveys, allowing us to study the combined effects of fluvial processes, which typically drive riparian plant community assembly, and biocontrol. Vegetation trajectories differed as a function of the dominant geomorphological process. Tamarix cover declined an average of 75% and was replaced by the native shrub Pluchea sericea as the new dominant species in the floodplain, especially where sediment deposition predominated. Following deposition, and especially erosion, opportunistic native herbs, Tamarix seedlings, and noxious weeds colonized the understory layer but did not increase in cover over time. Stands of the native shrub Salix exigua, a desirable replacement species following Tamarix control, only increased slightly and remained subordinate in the floodplain. Overall, our results showed that, by successfully controlling the target non-native plant, a biocontrol agent can substantially modify the replacement plant communities in a riparian system, but that fluvial processes also strongly influence the resulting communities.