Comparing field- and model-based standing dead tree carbon stock estimates across forests of the US

As signatories to the United Nation Framework Convention on Climate Change, the US has been estimating standing dead tree (SDT) carbon (C) stocks using a model based on live tree attributes. The USDA Forest Service began sampling SDTs nationwide in 1999. With comprehensive field data now available, the objective of this study was to compare field- and model-based estimates of SDT C stocks across the US to evaluate potential directions for improving National Greenhouse Gas Inventory (NGHGI) reporting and C dynamics research. Field inventory data indicated that most forests have relatively little SDT C stocks (< 1 Mg/ha), whereas large SDT C stocks (> 25 Mg/ha) are infrequent. Models used for past NGHGIs to predict SDT C stocks do not accurately reflect what was observed in inventory plots, resulting in an overestimation (similar to 100 per cent) of SDT C stocks at the national scale. These results indicate that the current estimate of the Nation's total forest C stock is overestimated by similar to 4.2 per cent due to overestimation of SDT C stocks that are a relatively small component of the total forest C stock. A field-based approach is suggested for use in future C reporting efforts to reduce estimation bias.