Rapid Biomass Measurement
As fuels reduction or biomass removal becomes more common in the forest, the need to rapidly measure that biomass increases. Whether that material is sold or exchanged for services, the contractor and the Forest Service should expect an accurate measurement of standing material to be removed. In many situations a measurement of diameter and/or height can be used to estimate weight. However, this process can be time consuming, especially in dense stands. An alternate means providing a rapid measurement may be possible.
A publication from the Rocky Mountain Research Station, Fires Sciences Lab, showed a high correlation between hemispherical, or fisheye, photography and weight per unit area of forest biomass (Keane et al 2005). Accurately measuring biomass with canopy images presents some excellent possibilities. If this method works well, then grabbing some quick images would provide a good estimate of onsite biomass (see Canopy Measurement). These images could be edited on site to reflect the various prescription options, which could assist an Inter-Disciplinary Team in assessing various thinning levels. Of course, this technique would be helpful in assessing carbon sequestration as well as canopy conditions. Still, this is only possible if the hemispherical photography is an effective method for measuring biomass.
To verify the ability to use hemispherical photography to measure biomass, we are working with Dr. Glen Murphy and his student Josh Clark at Oregon State University. Through destructive sampling in mixed conifer and douglas fir stands in the Pacific Northwest, canopy images will be compared with actual biomass removals. Canopy image collection precedes a basal area reduction; this is done repeatedly at each plot to provide multiple assessments per plot. Limbs and tops are chipped and weighed, while stems are measured and samples weighed.
This work has enabled both the analysis of hemispherical photography and validation of current biomass models. Preliminary results show that two commonly used biomass estimation models are quite accurate in estimating stem biomass, but are not good predictors of the biomass contained in the branches and tops. Alternately, hemispherical photography is proving to be an excellent means to estimate biomass from branches and tops. Incorporation of a height estimate further improves the accuracy of this estimate. While further analysis is needed, this is key information because biomass as a product is often secondary. Typically higher value products, such as saw timber or pulpwood, comprise a greater portion of the material removed. So, our estimate of our actual "biomass" product, which generally consists of limbs and tops, would be incredibly inaccurate when using traditional biomass models.
Over 100 metric tons of limbs and tops were chipped and weighed for this study. This data is currently being analyzed and all results will be available in December of 2009.