Recent Technical Changes to the United States Forest Carbon Inventory

A national system of field inventory plots is the primary data source for the annual assessment of US forest carbon (C) stocks and stock-change to meet reporting requirements under the United Nations Framework Convention on Climate Change (UNFCCC). The inventory data and their role in carbon reporting continue to evolve. The Forest Inventory and Analysis (FIA) program of the USDA Forest Service is charged with conducting the field inventory of US forest C. The FIA program employs remotely sensed imagery to define forest and nonforest plots which are systematically distributed approximately every 2,428 ha across the conterminous US. More than 125,000 plots in the current field inventory have at least one forested condition where field crews measure tree- and site-level attributes (e.g., diameter and tree height) at regular temporal intervals. A subset of forested plots is measured for additional variables related to forest non-tree C pools (e.g., downed woody materials, understory vegetation, and soils). The FIA program does not directly measure forest C stocks. Instead, a combination of empirically derived C estimates (e.g., standing live and dead trees) and models (e.g., forest floor C stocks related to stand age and forest type) are used to estimate forest C stocks. A series of recent refinements in FIA estimation procedures have replaced some of the purely modeled forest C stock estimates (e.g., downed dead wood) with estimates based on direct measurements from the national field inventory. Results indicated that models of non-live tree C pools differ significantly from field-based estimates at the plot-level but demonstrate only slight divergences of total C estimates at the national scale. The results of these studies in the context of forest carbon accounting and future refinements are discussed.