Analysis of Yield in Silviculturally Treated Young Douglas-fir and Western Hemlock Plantations in the Pacific Northwest

Abstract

The objectives of this study were to evaluate the effects of treatment regimes including various levels of (i) pre-commercial thinning, (ii) commercial thinning, and (iii) initial planting density on the yield of pure Douglas-fir (Pseudotsuga menziesii mirb. Franco) and western hemlock (Tsuga heterophylla) stands as well as mixed stands of the two species. We utilized an optimal bucking algorithm to estimate log grade proportions and a linear mixed modeling approach to capture significant effects from each treatment regime at an age close to the common industrial rotation age. Pre-commercial thinning at higher intensities increased quadratic mean diameters (QMD) and higher quality log grade proportions, while decreasing cubic volume and pulp volume in Douglas-fir and western hemlock. Initial planting density was directly related with pulp volume and indirectly related with QMD for both Douglas-fir and western hemlock. Mixed stands had larger QMD in densities of 680 and 1210 trees per acre (TPA). These analyses expand the existing knowledge of silvicultural treatments and regimes effects on yield for Douglas-fir, western hemlock, and mixed species stands while utilizing a robust experimental design. Additionally, the study provides detailed estimates of log grade yields around rotation age for cubic- and board-foot volume to various top diameters, giving a foundation to improve forest modeling accuracy for yield response to silvicultural treatments in the future.