Organic Matter Removal and Soil Compaction Have Unexpected Results on Subsequent Tree Growth Early in a Douglas-fir Rotation in the Oregon Cascades.

Understanding the effects of woody biomass removals and any associated soil compaction is necessary to assess the sustainability (in an environmental and productivity sense) of harvesting woody biomass forest residuals as a source of biomass for bioenergy feedstock. Investigating the on-site environmental sustainability of collecting and utilizing forest harvest residuals from Pacific Northwest Douglas-fir forests for the production of biofuels is a key to the viability of the venture.  The long-term goal of the study is to contribute to our understanding of the amount of residual woody Douglas-fir biomass that can be removed during timber harvest without detrimental effects on soil quality, water quality, and wildlife. We will address these issues by installing a Long-Term Soil Productivity (LTSP) site in the southern Willamette Valley of Oregon on Weyerhaeuser ownership. This extends our inference space into warmer and drier parts of the Douglas-fir range.  Results of the second year tree growth indicate, contrary to expectations, that the more severe organic matter removal treatments did marginally better than the treatments that retained slash on site.  These early results appear to be correlated with an increase in soil temperature and may be fleeting as the stands ages and other factors, such as water and nutrients, influence tree growth more substantially.