Our forest-harvesting operations typically leave tons of organic material on the forest floor after harvest, including branches and treetops. Today, these harvest residuals have little economic value. A developing marketplace for woody biofuels, however, could transform these residuals into products with economic value.
But how much can we remove without diminishing the productivity of the next forest rotation or harming forest ecology? To find answers, we're collaborating with academic and government researchers on a long-term research study.
“We’re interested in understanding the effects, if any, of residual biomass removal on soil productivity and tree growth,” says Scott Holub, a Weyerhaeuser silvicultural research scientist based in Springfield, Oregon. “At the same time, our collaborators are examining broader ecosystem questions, from wildlife to water quality.”
Supported by a grant administered by the Northwest Advanced Renewables Alliance and funded by the U.S. Department of Agriculture, the research project started in 2011.
There are seven study groups, replicated four times, on 28 one-acre plots. Each group represents different levels of harvest intensity and biomass removal. Stumps were left on all 28 sites, and every plot contains several probes to continually monitor soil temperatures and moisture levels, two of the most critical variables related to tree growth. The sites were replanted with seedlings to study how different levels of harvest intensity might affect tree growth.
The questions we’re trying to answer aren’t just related to removing residual biomass for fuel. The knowledge we’ll gain from this research study will have practical applications related to our long-term timber harvest practices — for example, having better insight into how slash loads (branches and other organic matter left on a site following harvest) affect long-term soil quality.
While the trees on our test site are still young, making it too soon to draw conclusions, a similar Weyerhaeuser study offers some clues. After 15 years, researchers in southwest Washington state have observed no statistical difference in tree growth among plots with different levels of biomass removal.
This type of research is essential in understanding the sustainability impacts of harvest practices and exemplify our long-term practice of using science to inform decisions.