A plot with heaving logging debris one year after the forest harvest in Matlock, Washington. USDA Forest Service photo by Dave Peter.
Harvesting methods such as clearcutting can disrupt the native plant community, leaving the site vulnerable to invasion by non-native plants like Scotch broom. Researchers Tim Harrington and Dave Peter were interested in what treatments are most effective for forest regeneration after harvest. In particular, how can forest managers enhance the growth of Douglas-fir and the native plant community on sites with drought-prone soils?
Harrington, Peter, and their colleague Rob Slesak (University of Minnesota) initiated research at a forested site on the Olympic Peninsula, Washington to better understand the potential benefits of leaving behind “slash” or logging debris—the cut limbs, tree tops, and needles leftover from the logging activity. They found that intensive forest harvesting practices that remove most of the logging debris increased abundance of Scotch broom.
In contrast, where heavy amounts of logging debris were retained, survival and growth of planted Douglas-fir increased. The debris acted as a trellis to facilitate increases in native understory species which kept Scotch broom from sprouting from the existing seedbed. The debris also acted as a mulch to reduce soil temperature and conserve soil water.
Areas treated with herbicides showed similar levels of Douglas-fir growth, but the native plant communities were negatively affected. The team’s results suggest that leaving logging debris after forest harvesting can provide sustained control of competing vegetation and promote conifer regeneration, while minimizing impacts to the native plant community. This study is providing the rationale for two forestry companies, Green Diamond Resource Company and Port Blakely, to retain logging debris as an aid to forest plantation establishment.