Cameron Douglass, Impacts of Chemical and Mechanical Tamarisk Control Methods on Plant Communities

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Tamarisk infestations often have strongly negative effects on the biotic communities at invaded sites, especially on the plants that persist underneath the tamarisk canopy and nearby. Tamarisk removal may eliminate the physical impacts of infestations, but may also cause unintended consequences that can define how well understory and surrounding plant communities can re-establish. Our ongoing research studies how common tamarisk removal and control methods (chemical control -- aerial and backpack applications; mechanical control -- Hydro-axe mulching and tree excavation; and biological control -- Diorhabda carinulata releases) affect patterns of subsequent site re-vegetation.

Imazapyr and triclopyr are the two most frequently used herbicides for controlling tamarisk. Therefore, our research has focused on the effects from initial herbicide applications and also on the impact of soil residues. Helicopter applications of imazapyr resulted in widespread plant community decline, with kochia being the most common and abundant species that establishes following application. Individual plant applications of imazapyr minimized the area of non-target impacts, but still killed many plant species. Triclopyr basal bark applications were much safer on the understory plant community, and in particular for grasses. The soil residues of imazapyr in particular were persistent, and twelve months after application negatively affected sensitive native plant species.

Mechanical removal methods tended to have a short-term positive effect on plant communities, increasing species diversity and numbers. Interestingly, the small-sized tamarisk debris created by Hydro-axing and other mulching equipment was found to encourage initial plant establishment. However, by the second year after treatment the disturbances caused by mechanical tamarisk removal favored recruitment by perennial, invasive species. Species that were found to be especially common and problematic included perennial pepperweed and Russian knapweed.

Overall, while tamarisk removal by any means can at some sites facilitate the establishment of other noxious plants, there is generally an inherent ecological capacity for desirable and functionally important plants to establish. The specific removal method that is chosen can affect this capacity, and the broad use of non-selective herbicides in particular can diminish desirable re-vegetation. Secondary establishment by other invasive plant species can often be managed efficiently and successfully if identified early on when infestations are small and easy to treat. Controlling tamarisk is only one component of effectively managing infested natural area sites, and the selection and implementation of appropriate removal methods can help to preserve a site's capacity for passive restoration.

Cameron H. Douglass and Scott J. Nissen, Colorado State University, Fort Collins

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