Improving Restoration Outcomes and Forest Resilience: Testing Pretreatment Weed Control Prior to Management Actions

Photo Credit

USDA Forest Service Photo by Becky Kerns.

Crews apply herbicide prior to forest management treatments to target invasive plants. 

Thinning is a common forest management practice in federal forests throughout the Pacific Northwest. It is often used to mitigate wildfire risk and enhance forest resilience or to promote the development of stand characteristics found in older forests. However, many invasive plants thrive in sunny locations and newly turned soil. The reduction in overstory canopy cover and soil disturbance after thinning and harvest operations can create ideal conditions for the establishment of invasive plants. Some managers have noted that it is potentially more effective to treat weeds before conducting other vegetation management than to try and control infestations afterward.

Testing the timing of herbicide treatments 

To test this idea, we collaborated with the Siuslaw and Colville National Forests to implemented field-based studies in productive Douglas-fir forests. We designed the experiment to address the following questions:
1)    Does herbicide pretreatment of invasive plants before harvest reduce postharvest invasive plant populations and promote native plant recovery? 
2)    What are the impacts on nontarget species?
3)    Does seeding native plants reduce postharvest populations of invasive species and promote postdisturbance native plant recovery? 
4)    Is native plant seeding more effective when combined with pre-treating invasive plants? (This question is specific to the study site in the Colville National Forest.) 
5)    How does thinning alter understory abundance and does it promote invasive plant species? 
6)    How do outcomes vary by pretreatment invasion levels, distance from roads, existing plant community, and disturbance intensity associated with thinning?

Testing seed mixes used after fire

We also designed a third study in collaboration with the Willamette National Forest focused on testing the effects of two different postfire seed mixes. The seed mixes included a standard mix composed of yarrow, blue wildrye, and California brome seeds, and a dry seed mix composed of yarrow, Roemer’s fescue, and California oat grass. 
This study addresses the following questions: 
1)    How does establishment vary between the two seed mixes?
2)    Do understory communities vary with and without seeding?
3)    Does seeding impact the expansion of invasive plant species?
4)    Do outcomes vary by native seed mixes, pretreatment invasion levels, distance from road, plant community composition, and disturbance intensity associated with fire severity? 
5)    Does the heterogenous wildfire history influence postfire recovery or recolonization of certain species, plant groups, or seed mixes? 
 

Keywords: Invasive plants, herbicide, seeding, roadsides, post-fire invasion, thinning, logging, Douglas-fir

Although herbicides are commonly used to control invasive plants in forests on public lands or in restoration settings, little is known about the primary herbicides being used, their efficacy and costs, and their effect on native species in these settings. This project is part of ongoing work to fill this knowledge gap.
This research need was identified as part of a Regional Invasive Species Research Needs Assessment that the Western Wildland Environmental Threat Assessment Center (WWETAC) conducted in 2021. During follow-up webinars, we identified further study sites and target species. 

Project Deliverables

Deliverables will include:

• Three peer-reviewed papers
• Webinars to share study results
• Field-site visits with land managers to discuss study results and management implications 

In April 2025, we hosted a project update webinar for 25 Forest Service managers and other staff.