Forest vegetation management (FVM) is an important component of reforestation projects in the Pacific Northwest (PNW). Long-term effects of vegetation management on total tree and ecosystem biomass stock, and total tree and ecosystem aboveground net primary productivity (ANPP) were assessed 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 fully replicated study included two treatments of vegetation management: control and treated. The control plots consisted on no vegetation control. The treated plots consisted on sustained vegetation control using herbicides during first 5 years after planting. Measurements were carried out during growing seasons 15 and 16. Crop tree aboveground biomass was assessed by measuring tree inventory and determining species-specific allometric functions. Ecosystem aboveground biomass was assessed by measuring, in addition to crop tree biomass, forest floor, understory and midstory biomass. Net primary productivity was assessed by adding to the yearly increments in biomass, the yearly litterfall measured monthly during the two years of evaluation. This study represents the first known attempt to quantify how FVM treatments impact ANPP and ecosystem biomass accumulation of conifer plantations in the PNW.
Results/Conclusions
Treated plots showed larger biomass stock than control plots, averaging, across both sites, an increment of 24.7, 84.6, 41.5, and 98.5 Mg ha-1 for Douglas-fir, western hemlock, western redcedar, and grand fir, respectively. Ecosystem biomass stock was also increased on treated plots, averaging an increment of 30.8, 28.6, and 50.6 Mg ha-1 for Douglas-fir, western hemlock, and grand fir, respectively. For western redcedar at the CR site, control plots showed 27.2 Mg ha-1 more ecosystem biomass than treated plots.
Treated plots showed increased crop tree ANPP, averaging, across both sites, a gain of 2.5, 10.8, 9.2, and 11.6 Mg ha-1 yr-1 for Douglas-fir, western hemlock, western redcedar, and grand fir respectively. There was no effect of treatments in ecosystem ANPP at the CR site. At the CF site, treated plots showed an average gain of 4.6 and 6.9 Mg ha-1 yr-1 for Douglas-fir and western redcedar respectively.
Our results demonstrate that sustained FVM during the first five years of stand establishment have no detrimental effects on ANPP and biomass stock of planted conifer forests in the PNW, suggesting that FVM treatments can be used as a tool to accelerate the long-term carbon sequestration rate of planted forests in the region.