Selecting a Minimum Diameter for Forest Biomass and Carbon Estimation: How Low Should You Go?

Forest inventories are traditionally designed to produce estimates of merchantable timber volume but are increasingly used to estimate stand-level biomass or carbon. Since inventory is volume-focused, it is common practice to tally stems 5 inches and greater in diameter (at breast height; d.b.h.). When estimating carbon or biomass, practitioners may be concerned about the effect of omitting smaller stems from the inventory. We present summaries to provide indicators of when smaller diameters may be less important for an accurate estimate to assist those foresters considering adding carbon estimates to traditional inventory objectives. Small stems (e.g., 2 or 3 inches d.b.h.) may contribute appreciable biomass within a few specific types of forest stands, such as for spruce-fir or lodgepole pine, but generally the contribution is minor. Similarly, the 5- or 6-inch stems, which are moderately large for some stands, will contribute very little to stand biomass for some forest types, particularly in western forests (e.g., Douglas-fir and ponderosa pine). These are well known patterns, and we suggest such information be used to inform the choice of minimum tally diameter. Pragmatic threshold values vary by region because the importance of smaller stems is not fixed at a single diameter over all U.S. forests.