Roads Impact the Distribution of Noxious Weeds More Than Restoration Treatments in a Lodgepole Pine Forest in Montana, U.S.A.

Abstract

A century of fire suppression has created unnaturally dense stands in many western North American forests, and silviculture treatments are being increasingly used to reduce fuels to mitigate wildfire hazards and manage insect infestations. Thinning prescriptions have the potential to restore forests to a more historically sustainable state, but land managers need to be aware of the potential impacts of such treatments on invasion by exotic plants. However, the effects of these activities on the introduction and spread of invasive plants are not well understood. We evaluated noxious weed occurrence over a 9‐year period (2001–2009) following thinning and burning treatments in a lodgepole pine forest in central Montana. Surveys were made in the treatment units and along roads for two shelterwood‐with‐reserve prescriptions, each with and without prescribed burning, burned only, and untreated controls. Five species listed as noxious weeds in Montana were recorded: spotted knapweed (Centaurea stoebe), oxeye daisy (Leucanthemum vulgare), Canada thistle (Cirsium arvense), common tansy (Tanacetum vulgare), and houndstongue (Cynoglossum officinale). With the exception of Canada thistle, noxious weeds were confined to roadsides and did not colonize silvicultural treatment areas. Roadside habitats contributed more to the distribution of noxious plant species than did silvicultural treatments in this relatively uninvaded forest, indicating the importance of weed control tactics along roads and underscoring the need to mitigate exotic plant dispersal by motorized vehicles. In addition, these findings suggest that roadways should be considered when evaluating the potential for invasion and spread of exotic plants following forest restoration treatments.