Methodologies for Estimating Forest Carbon Stocks From Annual Forest Inventory Data

The USDA Forest Service Forest Inventory and Analysis (FIA) program performs an annual forest inventory that includes measurements of down woody material and soil quality. These measurements are taken on a systematic nation-wide grid of approximately 125,000 plots where each one may represent up to 38,850 ha of forest. Between ten to twenty percent of these plots are measured every year. Tree measurements include species, height, and diameter at breast height. Carbon content of live tree biomass is estimated with carbon conversion constants. Down woody material carbon stocks (coarse and fine woody debris) are estimated using line intercept transects and carbon conversion constants. The soil quality indicator includes volumetric sampling of the forest floor and the collection of mineral soil cores representing depth increments of 0 - 10 cm and 10 - 20 cm. Carbon content of the soil samples is determined by dry combustion. We combined tree, down woody material, and soil measurements collected over three years (2001 - 2003) to evaluate current methodologies for estimating the total carbon sequestered in forests of the United States. Carbon storage by pool is roughly ranked as follows: standing tree biomass > 0 - 10 cm mineral soil > 10 - 20 cm mineral soil > coarse woody debris > the forest floor ~ fine woody debris. The tree measurements actually occur on a denser sampling network than either the down woody material or soil measurements. Addition research is underway to determine effective methodologies for estimating the carbon estimates from these two stocks at all tree measurement locations.