Stem and Branch Diameter Response in Pruned Douglas-fir Plantations (Pseudotsuga menziesii var. menziesii): Implications for Volume and Clear Wood Production in the U.S. Pacific Northwest

Thesis (Master's)--University of Washington, 2016-06

Enhancement of timber quality from self-pruning in Douglas-fir stands typically begins to occur around 60-80 years of age. Due to 40-year rotations in production settings, this species has the potential to benefit from the silvicultural treatment of pruning. Previous literature indicates that pruning may increase overall height and diameter growth rates, reduce log taper and produce more valuable clear wood. This study measured 426 trees located in Stand Management Cooperative installations throughout Western Washington and Oregon to determine stem and branch diameter response to pruning and implications to volume and clear wood production. Pruned trees received a 20, 40, 50, or 60% crown reduction with either an “all pruned” or “pruned with followers” treatment. Stems were measured for diameter at breast height (4.5’) and top the 1st, 2nd and 3rd logs (17.5’, 34’, 42’) with the largest branch diameter recorded in each quadrant of the logs. Results indicate that, compared to the controls, removing any portion of the lower crown begins to decrease DBH and height growth rates as well as reducing log taper. The reduction in taper does not appear to be sufficient to account for the loss in overall tree volume. Branch sizes above the pruning treatment showed a slight increase in vigor but not enough to degrade the value of the upper log. Pruning produced clear logs with little epicormic branching in the 40-60% crown removal treatments. Recommendations for land managers desiring to prune is to (1) remove as little of the live crown as possible to minimize growth reductions while still meeting clear wood goals, (2) remove the “followers” in a subsequent entry to prevent the overtopping of the pruned trees, and (3) pruning should not be hindered by concerns of degrading upper logs.