Study shows resilience of Douglas-fir to intensive harvesting and soil compaction over 10 years

There is considerable interest in the sustainability of biomass harvesting and other intensive management practices designed to increase forest productivity and the availability of feedstocks for traditional products and energy markets. Questions and concerns about these practices have led states and other agencies and non-governmental organizations to develop biomass harvesting guidelines that include provisions for retaining specific proportions of harvest residues and restricting biomass harvesting on some sites.

Scientific justification for such provisions based on long-term field studies is limited. The Fall River study, led by investigators from the Weyerhaeuser Company, the University of Washington, and the U.S. Forest Service, is the longest-running study of its kind for Pacific Northwest Douglas-fir. NCASI has provided support for the project over the last 16 years.

A recent paper published from the study shows little impact of harvesting intensities beyond those used in practice and of associated compaction on Douglas-fir growth over 10 years.

Consistent with other studies throughout North America, control of competing vegetation had a much greater influence on subsequent forest productivity than did intensive biomass removals and compaction, presumably due to the alleviation of water limitations at Fall River. The study demonstrates considerable resilience of the Fall River site to management practices. Similar studies have been established on contrasting sites in the region to determine responses across a wider range of soil types.

The paper, “Tree growth ten years after residual biomass removal, soil compaction, tillage, and competing vegetation control in a highly-productive Douglas-fir plantation,” was written by S.M. Holub, T.A. Terry, C.A. Harrington, R.B. Harrison, and R. Meade and published in Forest Ecology and Management (2013) 305:60-66. The abstract follows.

Forest residual biomass harvesting is a potential concern in regions where this primarily branch and needle material is removed to provide a source of renewable energy or where total-tree yarding takes place.  Concern arises from the removal of nutrients present in residual biomass, as well as from heavy equipment trafficking used to collect the material. The Fall River Long-term Soil Productivity (LTSP) Trial in western Washington State, USA was designed to study potential impacts on Douglas-fir tree growth of residual biomass removal on a productive soil in the Pacific Northwest region. The objective of this investigation was to examine the effects of residual biomass removal, soil compaction, tillage following compaction, and competing vegetation control on subsequent stand productivity through 10 growing seasons. The size and growth rate of Douglas-fir trees planted in the Fall River LTSP Trial were not significantly affected by any residual biomass removal level, indicating substantial resilience to high levels of residual biomass harvest at this productive site with deep, nutrient-rich soil. Soil compaction, without other soil disturbance or displacement, had no negative effects on tree growth or tree size over the 10-year growing period. Tillage following soil compaction also led to a modest, but not significant, increase in tree growth and size. In the years during and shortly after complete vegetation control, trees grew faster and were larger where non-tree vegetative competition was reduced. Water availability during the growing season appears to be a growth-limiting factor at this site where annual precipitation is high but very little falls during the summer growing season. Tree growth will continued to be monitored to see how the trees will respond to the treatments as inter-tree competition and crown biomass increase water and nutrient demand.  

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