Thirteen-Year Height and Diameter Growth of Douglas-Fir Seedlings under Alternative Regeneration Cuts in Pacific Northwest

Abstract

Interest in managing Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) forests in the Pacific Northwest under silvicultural systems other than traditional clearcutting has prompted research on the efficacy of alternative systems for successful regeneration and sustained timber productivity of Douglas-fir. The College of Forestry Integrated Research Project, implemented by Oregon State University, was established to compare various ecosystem responses and public perceptions among treatments implemented under clearcutting, shelterwood-with-reserves, and group selection silvicultural systems. The objective of this analysis was to quantify the following three responses of planted Douglas-fir seedlings to initial regeneration cuts: cumulative 13-year height growth (H_13yr; 1992–2004), cumulative 13-year diameter growth (D_13yr; 1992–2004), and most recent 5-year height growth (ΔH_5yr; 2000–2004). Differences in variability of overstory density at the treatment level led to significant differences in the variance of understory growth responses. After accounting for heterogeneous variance, analysis of variance indicated significant treatment effects for all three responses. Treatment effects were explained by the decline in H_13yr, D_13yr, and ΔH_5yr with increasing overstory competition as represented by basal area of residual trees immediately after harvesting (initial basal area). Predicted height:diameter ratio of Douglas-fir seedlings increased as IBA increased. Under regeneration methods that retain a portion of the overstory, a residual overstory with basal area <80 ft²/ac allows establishment, growth, and continued survival of Douglas-fir regeneration during the 13 years following harvest.

shelterwood with reserves, group selection, patch cut, clearcut with reserves, light availability, residual density, uneven-aged management, even-aged management