Progress reported in study of soil resilience to intensive harvesting in Douglas-fir

Biomass harvesting removes higher quantities of organic matter and nutrients from forested sites than traditional harvesting and has the potential to impact soil productivity. Uncertainties about the effects of biomass harvesting on soil and other ecological values has led to the development of biomass harvesting guidelines by states and other entities with provisions to retain specific quantities or proportions of harvest residues on site. Field studies across North America have shown many sites are resilient to harvest residue removals, with minimal effects on productivity of regenerating stands, whereas others sites are more susceptible to impacts. 

Factors accounting for the resilience or sensitivity of forested sites to biomass harvesting are not well understood. One hypothesis is that increased soil organic matter decomposition and nutrient mineralization resulting from higher soil temperature and moisture conditions following harvesting increases site resilience. Alternatively, higher inputs of decomposing roots following harvesting may lead to increased nutrient availability and resilience.

To test these hypotheses, a study was initiated with NCASI support as part of a larger project supported by Weyerhaeuser Company and a grant from the Northwest Advanced Renewable Alliance – USDA Agriculture and Food Research Initiative.  Principal Investigator of the project is Dr. Jeff Hatten of the Oregon State University College of Forestry. As part of the study, investigators are using soil chemicals derived from roots, needles, wood, and soil microbes along with other techniques (stable isotopes and soil density fractions) to elucidate factors contributing to soil resilience following harvesting of Douglas-fir at a site outside of Springfield, Oregon. Treatments included bole-only, whole-tree, and whole-tree + forest floor removal with and without moderate compaction. Preliminary results show minimal immediate post-harvest effects of the treatments on soil carbon (C) and nitrogen (N) content but an increase in soil C concentrations and C:N ratios, suggesting fresh organic matter additions to soil from dead roots or harvest residues.

The initial contribution of roots to soil carbon may buffer soil C loss caused by increased decomposition rates.  Further analysis of other soil chemical and physical property data should strengthen conclusions and implications of the study.

The investigators anticipate completing the study by mid-2017.

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