Long-term effects of vegetation management on biomass stock of four coniferous species in the Pacific Northwest United States

Abstract

Silvicultural treatments applied during the early stages of stand development can have long-lasting impacts on forest ecosystem structure. Forest vegetation management (VM) is an important component of many reforestation programs and although several studies have demonstrated the benefits of VM on planted conifer growth and survival, few reports have been published on the long-term effects of VM treatments on total ecosystem biomass accumulation. In this study we assessed the effects of two contrasting VM treatments on total tree and ecosystem biomass stock for Douglas-fir, western hemlock, western redcedar, and grand fir growing in Oregon’s central Coast Range (CR) and Douglas-fir and western redcedar growing in Oregon’s Cascade foothills (CF). The assessments were made at age 16 years, 11 years after treatment application ended. The study contained two vegetation management treatments: control (C) and vegetation management (VM). Both the C and VM plots received a pre-planting herbicide site preparation treatment. The VM plots had, additionally, sustained vegetation control using herbicides during the first 5 years after planting. At age 16 years, at the CR site, the VM treatment increased the biomass stock of crop trees by 26.5, 91.2, 44.7, and 96.1 Mg ha−1 for Douglas-fir, western hemlock, western redcedar, and grand fir, respectively. At the same age, at the CF site, the VM treatment increased crop tree biomass stock by 48.1 Mg ha−1 for Douglas-fir and 42.2 Mg ha−1 for western redcedar. When other ecosystem components were considered, however, total ecosystem biomass did not differ between C and VM treated plots for western hemlock, western redcedar and grand fir at the CR site largely due to the development of an abundant hardwood midstory. On the other hand, VM treatments increased the ecosystem biomass stock of Douglas-fir and western redcedar at the site with a low abundance of hardwood midstory (CF site). Midstory biomass of C plots at the CR site averaged 52.9, 64.7, and 36.0 Mg ha−1, for western hemlock, western redcedar, and grand fir, respectively. At the CF site, midstory biomass of C plots averaged 1.2 and 5.9 Mg ha−1, for Douglas-fir and western redcedar, respectively. The results of this study demonstrate that sustained VM treatments during the first 5 years of stand establishment increases the biomass stock of crop trees, directing site resources towards planted crop trees.