Tree growth ten years after residual biomass removal, soil compaction, tillage, and competing vegetation control in a highly-productive Douglas-fir plantation

• Residual biomass removals did not significantly affect tree size or growth.
• Soil compaction at this site did not significantly affect tree size or growth.
• Tillage following compaction did not significantly affect tree size or growth.
• Not eliminating competing vegetation decreased plot-level tree volume by 30%.
• Water availability is potentially most limiting to tree productivity at this site.

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.