Estimating the quadratic mean diameter of fine woody debris for forest type groups of the United States.

The Forest Inventory and Analysis program of the Forest Service, U.S. Department of Agriculture conducts a national inventory of fine woody debris (FWD); however, the sampling protocols involve tallying only the number of FWD pieces by size class that intersect a sampling transect with no measure of actual size. The line intersect estimator used with those samples requires a measurement of the quadratic mean diameter (QMD). Published information regarding appropriate QMDs by FWD diameter class by species or forest types across the United States is lacking. Thus, the objective of this study is to employ a technique known as the graphical estimation (GE) method for estimating FWD QMDs for major forest types across the United States. Results indicate that the GE method, along with adaptations proposed in this study, allows for rapid estimation of FWD QMDs without additional fieldwork. The value (-1.81) of the scaling power function between the number of woody pieces by size class closely matches published scaling values (-2.00) between bole and fine branch diameters. Validation results indicate that estimates of QMD from the GE method differ substantially from published QMDs; however, published QMD results differ from one another to an even greater extent. Given the mixing of FWD from a diversity of tree and shrub species in the Nation's forests, this study concludes that a set of general QMDs for FWD across the United States should be used in the absence of empirically derived or power function-based QMD estimates.