We sampled trees grown with and without competing vegetation control in an 11-year-old Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) plantation on a highly productive site in southwestern Washington to create diameter-based allometric equations for estimating individual-tree bole, branch, foliar, and total aboveground biomass. We used these equations to estimate per-hectare aboveground biomass, nitrogen (N), and carbon (C) content, and compared these results to (1) estimates based on biomass equations published in other studies, and (2) estimates made using the mean-tree method rather than allometric equations. Component and total-tree biomass equations were not influenced by the presence of vegetation control, although per-hectare biomass, C, and N estimates were greater where vegetation control was applied. Our biomass estimates differed from estimates using previously published biomass equations by as much as 23 percent. When using the mean-tree biomass estimation approach, we found that incorporating a previously published biomass equation improved accuracy of the mean-tree diameter calculation.Keywords: Douglas-fir, plantation, biomass, allometry, carbon, nitrogen.ii
SummaryThere are few published biomass equations for young Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) plantations. Equations developed from regional-scale data, which include older trees, may not produce accurate biomass estimates for young, fast-growing trees. We sampled trees grown with and without competing vegetation control in an 11-year-old Douglas-fir plantation on a highly productive site (Class II+) in southwestern Washington to create diameter-based allometric equations for estimating individual-tree bole, branch, foliar, and total aboveground biomass. We used these equations to estimate per-hectare aboveground biomass, nitrogen (N), and carbon (C) content, and compared these results to (1) estimates based on biomass equations published in other studies, and (2) estimates made using the mean-tree method rather than allometric equations. Our component and total-tree biomass equations did not differ between treatments with and without 5 years of intensive vegetation control. Estimated total aboveground tree biomass at year 11 was 89.7 and 73. , with and without vegetation control, respectively. Per-hectare aboveground tree biomass estimates using previously published Douglas-fir biomass equations differed from those made with our equations by -8 to +23 percent; the published equations producing biomass estimates most different (≥20 percent) from our estimates included those developed from large, diverse samples. When using the mean-tree biomass estimation approach for our site, we found that incorporating a previously published relationship between diameter and biomass for young Douglas-fir improved accuracy of the mean-tree diameter calculation.iii