Developing improved tools and methods used to estimate the amount of residual biomass potential contained in a softwood plantation or forest is a key task for the NARA project. The amount of forest residues available affects how much biojet fuel and co-products are produced, the location sites for depots and conversion plants and the overall sustainability of the residual wood to biojet fuel industry.
A report, partially funded by NARA and entitled “Estimating Tree Biomass, Carbon, and Nitrogen in Two Vegetation Control Treatments”, was recently published and advances knowledge on how softwood biomass growth can best be estimated and how vegetative treatments affect growth. Data in the report was obtained from an 11-year old Douglas-fir tree plantation situated on the Fall River Long-term study site. The report takes on two objectives:
Objective 1: Sample Douglas-fir trees on the site to develop estimates for total-tree aboveground biomass, carbon and nitrogen content with and without competing vegetative control.
Objective 2: Compare biomass estimates obtained in objective 1 with estimates obtained using previously published equations and protocols.
Sampling trees to determine biomass, carbon and nitrogen
All of the trees on this site were planted in the year 2000, In 2011, total height, height to live crown (HLC), diameter at breast height (BDH) and the diameter at 15 cm above ground (D15) measurements were either directly taken or extrapolated from 2010 measurements.
In March following the year-11 growing season, 26 trees (24 randomly selected and two chosen with mean dimensions) were destructively sampled to create equations for estimating tree biomass. Destructive sampling involved falling the tree, drying and weighing the bole (trunk), branch and foliar segments and determining the carbon and nitrogen content of each segment.
By correlating individual tree measurements with their biomass weight, carbon and nitrogen content, equations were generated that allow total aboveground biomass, carbon and nitrogen estimates on a per-hectare basis for the site.
The effects of vegetation control
Of the 26 trees sampled, half were in an area that received five years of competing vegetative control and half received no vegetative control. Per-hectare biomass was over 20% higher where vegetative control took place. Vegetation control had no significant effect on the relationship between DBH and biomass, however, indicating the robustness of the equation for use on stands with or without vegetative control. Nitrogen and carbon amounts were proportional to total biomass and were not apparently affected by vegetative control treatments.
Comparing published equations with sampling results
Eight published equations that related DBH to total-tree biomass were applied to the study site and the resulting biomass calculations were compared to the biomass results generated in this study. Total-tree estimates generated from the previous reports varied from 8 to 23 percent from the site-specific biomass equation estimates. These results show the error range possible of applying previous equations developed from off-site forests to determine biomass estimates.
Comparing the “Mean Tree Method” with sampling results
The authors then compared their destructive sampling biomass estimates to an estimate obtained using the mean tree method. This comparison relied on destructively sampling a single tree on site that had a mean DBH value to the surrounding trees. The remaining biomass on the site was then calculated using the slope established from a previously published equation. This method over estimated the site biomass by 19.2 %. The authors suggest that the mean tree selected in this study deviated significantly from the DBH-biomass regression line in the published equation and that more than one mean tree should be sampled for more accurate results.
Additional biomass calculation models
This study is part of a coordinated effort to provide superior biomass estimate equations. Doug Maquire’s team at Oregon State University is conducting a more expansive sampling project covering a wide range of trees varying in age and size. The sampling for this study is over 80% complete and results should be communicated soon.