- Forest vegetation management (FVM) is an important component of reforestation in the Pacific Northwest (PNW). Several studies have demonstrated the benefits of vegetation management on planted conifer growth and survival. However, few reports have been published on the long-term effects of FVM treatments on total ecosystem biomass accumulation and aboveground net primary productivity (ANPP). In this study we assessed long-term effects of vegetation management on total tree and ecosystem biomass stock, and total tree and ecosystem ANPP 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). This study represents the first known attempt to quantify how FVM treatments impact long-term ecosystem biomass accumulation and ANPP of four different conifer species planted in the PNW.
This study contained two vegetation management treatments: control (C) and vegetation management (VM). Both the C and VM plots received a fall site preparation treatment. The VM plots then had sustained vegetation control using herbicides during the first 5 years after planting. Measurements were carried out during growing seasons 16 and 17 at the CR site and 15 and 16 at the CF site. Crop tree aboveground biomass was assessed using inventory data and species-specific allometric functions developed in this study. Ecosystem aboveground biomass was assessed by measuring, in addition to crop tree, midstory, understory and forest floor biomass. ANPP was calculated as the one-year increment in aboveground ecosystem biomass plus litterfall during the same period.
At age 16, at the CR site, average crop tree biomass stock of C plots was 95.3, 48.6, 19.0, and 38.2 Mg ha-1, for Douglas-fir, western hemlock, western redcedar, and grand fir, respectively. VM plots increased crop tree biomass stock by 26.5, 91.2, 44.7, and 96.1 Mg ha-1 for the same species. At the same age, at the CF site, average crop tree biomass stock of C plots was 76.6 and 18.7 Mg ha-1, for Douglas-fir and western redcedar, respectively. At this site, the gain of VM treatments over the control was 48.1 Mg ha-1 for Douglas-fir and 42.2 Mg ha-1 for western redcedar.
Ecosystem biomass stock was not affected by VM treatment on western hemlock, western redcedar and grand fir at the CR site, and only increased in treated plots of Douglas-fir at both sites and western redcedar at the CF site. Midstory 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 of C plots was 1.2 and 5.9 Mg ha-1, for Douglas-fir and western redcedar, respectively.
The average crop tree ANPP of C plots growing at the CR site was 14.5, 12.6, 2.6, and 10.2 Mg ha-1 yr-1, for Douglas-fir, western hemlock, western redcedar, and grand fir, respectively. At the CF site, crop tree ANPP of C plots was 11.0 and 3.2 Mg ha-1 yr-1, for Douglas-fir and western redcedar, respectively. Eleven years after vegetation management treatment ended, the crop tree ANPP of VM plots was greater than the C plots by 11.2, 7.9, and 14.4 Mg ha-1 yr-1 for western hemlock, western redcedar, and grand fir, respectively, at the CR site and 4.6 Mg ha-1 yr-1 for Douglas-fir and 6.5 Mg ha-1 yr-1 for western redcedar at the CF site. There was no effect of treatments on ecosystem ANPP (including production of understory and midstory biomass) at the CR site.
The results of this analysis demonstrate that sustained FVM treatments during the first 5 years of stand establishment increases both the biomass stock and ANPP. This suggests that FVM treatments can accelerate the long-term carbon sequestration rate of planted forests in the PNW. However, in analyzing other ecosystem components, there was no increase in ecosystem biomass stock and ANPP for western hemlock, western redcedar, and grand fir stands growing in the CR site at age 16. These results provide managers with options for FVM, depending on management objectives and site conditions. Differences observed between sites also serve to inform forest managers to know their site, i.e. temperature, annual rainfall, and competing vegetation species composition to develop an appropriate vegetation management plan.