Fall River soil productivity study: ten-year results

The potential use of forest biomass and residues for energy has raised questions about the sustainability of intensive management practices, particularly the removal of currently unmerchantable biomass (foliage, branches, smaller trees) that are typically left on site following harvesting. To address sustainability concerns, states and other entities have developed guidelines for biomass harvesting which include provisions for leaving specific proportions of harvest residues on site. Many such provisions lack empirical support due to a dearth of long-term field studies.

The Fall River study in western Washington will help answer some important ecological questions about biomass utilization for Douglas-fir forests in the Pacific Northwest. The study was established on a highly productive site in 1998 by scientists at the University of Washington, Weyerhaeuser Company, and the U.S. Forest Service. Investigators are monitoring tree growth responses and other effects of replicated treatments representing different levels of biomass harvest intensity, soil compaction, and control of competing vegetation. Harvest treatments span a range from removing only merchantable boles to a treatment representing the theoretical limits of biomass residue utilization for energy (i.e., whole tree harvesting in combination with legacy wood removal).

Lead investigators include Dr. Robert Harrison of the University of Washington, Drs. Tom Terry (retired) and Scott Holub of Weyerhaeuser Company, and Dr. Constance Harrington of the U.S. Forest Service. NCASI has provided support for the study since its inception.

Consistent with results after five years, ten-year measurements in late 2009 and early 2010 have shown little effect of biomass harvest or soil compaction treatments on tree basal area or height. By contrast, control of competing vegetation continued to have a major, positive effect on Douglas-fir productivity (e.g., 24% increase in basal area). Work in 2010 will involve completing this biomass assessment and developing nutrient budgets for the different treatments. 

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