Technical Bulletin No. 0970: Forest Herbicide Effects on Pacific Northwest Ecosystems: A Literature Review

The use of silvicultural herbicides to control competing vegetation has evolved over the past 75 years and has become an integral component of today’s forestry management practices. Although the direct effects of herbicides on non-target biota are generally well understood and documented, comparatively little information is available on indirect and long-term (more than five years) effects. This review summarizes extant scientific literature by providing brief synopses of the direct effects of forest herbicides on plant and animal communities, but primarily emphasizes the indirect effects to wildlife in the Pacific Northwest. Herbicides increase survival and growth of crop trees and are effective at controlling target vegetation. Many studies have shown that herbicides have low toxicity to wildlife, tend to dissipate quickly, have limited mobility (rapidly fixed in environments), and do not bioaccumulate. Forest herbicides can enter aquatic ecosystems via accidental runoff or drift, but this riparian contact is minimized through the use of vegetation buffers, drift prediction models, application timing, and droplet size. When herbicides are applied at recommended rates in managed forests, direct toxic effects on wildlife and fish are not expected. Based on general measures of vegetation composition and structure, plant communities found in intensively managed forests recover quickly following chemical treatment (typically within two to three years). Wildlife responses to vegetation changes induced by herbicides at a stand level tend to be species- and site-specific, with timing of wildlife community recovery linked directly to the pace of vegetation recovery.