Wildlife and Terrestrial Ecosystems
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  • Wildlife and Terrestrial Ecosystems
  • Southwest Forest Science Complex
  • 2500 South Pine Knoll
  • Flagstaff, AZ 86001-6381
  • (928) 556-2001
You are here: WTE Home / Invasive Species / Weed Impacts on Native Plant Communities

Weed Impacts on Native Plant Communities

Spotted Knapweed
Spotted Knapweed

Problem Statement

Exotic plant invasions pose a severe threat to the conservation of natural systems, and present a substantial challenge to land managers given that management activities often encourage the spread of weeds. Although native plants appear to be negatively affected by noxious weeds, previous research has rarely described the specific impacts of invaders on native plant communities. Such understandings are needed to allow evaluation of current weed management tools and improve implementation of management strategies.

Research Approach

We conducted a comprehensive study to quantify the specific impacts of spotted knapweed (Centaurea maculosa) invasion on native grassland communities located within coniferous forests of western Montana. By comparing pristine native and comparable spotted knapweed-invaded grasslands, we were able to document the native species and associated functional groups most affected by knapweed invasion. With this information, we can not only predict which native species and species groups are most at risk from knapweed invasion but also develop new hypotheses about how spotted knapweed achieves its invasive success.


Native grassland showing Arrowleaf Balsamroot in flower on the Lolo National Forest in western Montana
Native grassland showing Arrowleaf Balsamroot in flower on the Lolo National Forest in western Montana

Our results indicate that spotted knapweed, one of the most aggressive invaders of western North America, dramatically impacts native plant communities (Ortega and Pearson 2005), but that these impacts are not evenly felt by all native species. Perennial forbs, consisting of over 50 species, were abundant in areas without knapweed but showed steep declines as knapweed density increased. In contrast, bunchgrasses showed declines that were half as steep, and annual forbs did not appear to be impacted. The native species most impacted by knapweed was Arrowleaf Balsamroot (Balsamorhiza sagittata), a long-lived forb that is abundant in native systems and provides food for a host of animals. Spotted knapweed therefore appears to shift native plant communities away from perennial species, with the largest impacts on important forbs. These results are significant given that the primary tool for effective control of knapweed, broadleaf herbicide, also suppresses native forbs, thereby exacerbating the negative effects of knapweed on these species. For example, our research indicates that broadleaf herbicides drastically reduce abundance, seed production, and seedling densities of Arrowleaf Balsamroot (Crone_et_al._2009). While herbicide may be successful in reducing densities of knapweed and increasing productivity of native bunchgrasses, they may also lead to the further demise of native forbs, particularly when treatments are applied repeatedly (Ortega and Pearson in prep). This research provides valuable understandings to better inform managers about when and where to most effectively apply herbicides to reduce weeds such as spotted knapweed while also restoring native grassland species.


  • University of Montana
  • State of Montana
  • Montana Department of Natural Resources and Conservation
  • Lolo National Forest
  • Plum Creek Timber Company