Tree Growth Fifteen Years after Residual Biomass Removal, Soil Compaction, Tillage, and Competing Vegetation Control in a Highly-Productive Douglas-fir Plantation.

Collection of forest residual biomass is a potential concern in regions where this primarily non-merchantable wood, branch and needle material is harvested to provide a source of renewable energy.  Concern arises from the removal of nutrients contained in residual biomass, as well as from heavy equipment trafficking used to collect the material.  The Fall River Long-term Soil Productivity (LTSP) study in western Washington State U.S.A. 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 15 growing seasons.  Contrary to measurements at younger ages, at age 15-years, and over the age 10 to age 15 interval, the size and growth rate of Douglas-fir trees planted at the Fall River LTSP showed some statistically significant reductions (0.05 level) in total bole volume (-7.2%), bole volume growth (-6.4%) and total height (-3.3%) as a result of the total-tree removal.  The most severe removal treatment (total tree plus legacy wood removal) was significantly lower than the control (bole-only removal) treatment.   Soil compaction, without soil displacement, had no negative effects on tree growth and, if anything, compaction tended to increase tree growth and tree size over the 10 to 15 year growing period, although not significantly. In the years during and shortly after complete vegetation control, trees grew faster and were larger where non-tree vegetative competition was reduced.  However in the age 10 to 15 year growth interval, trees on non-vegetation control plots grew similarly to those where competing vegetation was controlled, despite the smaller initial size of the non-vegetation-controlled trees.   Treatments will continued to be monitored to determine how the treatments will respond as inter-tree competition and crown biomass increase water and nutrient demand.