Ten-year Douglas-fir regeneration and stand productivity differ among contrasting silvicultural regimes in western Washington, USA

Abstract

In the Pacific Northwestern USA, concerns regarding impacts of forest harvesting on visual quality, wildlife habitat, and carbon management have prompted evaluations of alternative silvicultural regimes for coast Douglas-fir (Pseudotsuga menziesii var. menziesii). Research was initiated in 1998 near Olympia WA USA to conduct long-term comparisons among six silvicultural regimes: clearcut (harvest all trees), two age (harvest all except 38 mature trees ha−1), patches (harvest 20% of the area in 0.6–2.0 ha tracts), groups (harvest 20% of the area in 0.1–0.5 ha tracts), thinning (reduce stand density to 45% of the biological maximum for Douglas-fir), and a non-treated control. Harvested areas in the first four regimes were planted with Douglas-fir seedlings. This report focuses on tree regeneration and stand productivity during the first decade of the study. Fifth year height of planted Douglas-fir was greater in the clearcut regime (1.8 m) than in the patches and groups regimes (1.1–1.2 m). Fifth year tree regeneration in the clearcut and two-age regimes was dominated by Douglas-fir (80–86% of seedlings), but regeneration in the patches and groups regimes was composed of a mixture of conifer and hardwood species. Ten-year periodic annual increment (PAI) in Douglas-fir ingrowth volume was greater in the clearcut regime (1.4 m3 ha−1 yr−1) than in the control (0.1 m3 ha−1 yr−1), whereas ingrowth volume PAI of other conifer species was greater in the two-age regime (0.4 m3 ha−1 yr−1) than in the control (0.0 m3 ha−1 yr−1). Tree regeneration responses indicated increasing abundance of shade tolerant species in the non-clearcut regimes, especially western hemlock (Tsuga heterophylla). Fifth year cover of the vine, California blackberry (Rubus ursinus), in the clearcut regime (21%) was over four times that in the control (5%). Ground disturbance in the clearcut regime reduced 5th-year height of the shrubs, salal (Gaultheria shallon) and red huckleberry (Vaccinium parvifolium), by 40–50% compared to the control. Ten-year volume PAI of Douglas-fir was linearly related to post-harvest stand basal area (r2 = 0.94), and the relationship did not vary significantly among the three replicate sites. During the first decade of the study, Douglas-fir regeneration and stand productivity differed among silvicultural regimes because of post-harvest variation in stand edge competition, species composition of tree seedlings and ingrowth, and residual stand density.