Refining volume estimates of down woody debris

Down woody debris (DWD) plays a vital role in forest ecosystem structure and function. Although Volume is likely the most common metric used to characterize DWD. an evaluation of the formulae used for volume estimation on individual DWD pieces has received little attention. We determined actual volume of 155 diverse DWD pieces (types, species, lengths, and diameters) by detailed field measurements. By comparing the actual and calculated Volumes from six commonly used formulae, we assessed their bias, precision, and accuracy. Based on observed DWD forms, we developed a new formula. namely the "conic-paraboloid", which was included in the assessment. Among the formulae that require length and two end diameter measurements, the conic-paraboloid had the lowest bias, highest precision, and hence greatest accuracy. Newton's and the centroid formulae had higher accuracy yet require more field measurements. Smalian's, conical frustum. and average-of-ends formulae had poor performance relative to the others. Accuracy of all formulae decreased with increasing piece length. Thus, partitioning pieces into two, three, and four sections for additional measurement improved accuracy. As decay advances, pieces become progressively more elliptical in cross section. Using the cross-sectional area derived from only the long axis of the ellipse leads to substantial volume overestimates for well-decayed DWD.