PS 64-59
Net ecosystem productivity of US forests and carbon losses from the potential forest sink
International negotiations for a new global treaty to limit greenhouse gases are based in part on understanding the current and potential role of forest offsets. The US Forest Inventory and Analysis (FIA) data have been recognized as valuable data sources for estimating forest carbon stocks and changes. In this study, we developed spatially-explicit estimates of net ecosystem production (NEP) at 250m resolution for the continental US based on FIA data, empirical equations, forest types, and a forest age map.
Results/Conclusions
Our initial results indicate that the forests of the continental US are far from reaching productivity equilibrium and that most of them are robust carbon sinks with remarkable carbon accumulation rates. The total NEP of current US forests is about 544 Tg C/yr. However, this capacity is significantly larger than the C sink (net biome production, NBP) that is 234 TgC/yr. As a result, only 43% of NEP contributed to the sequestrated C in forests, while 57% was lost through different pathways, such as harvesting and natural disturbance, or laterally exported to rivers and estuaries. Regionally, forest NEP is influenced by different dominant factors, and is also vulnerable to various disturbances. Forests in the eastern US account for about 77% of the total NEP because of intensive industrial forestry practices in the southeast US and productive forests in the northeast and lake states that are still recovering from previous nonforest land-uses. In contrast, most forests in the western US are lower in NEP except some forests in the Pacific Northwest, and are highly vulnerable to disturbances such as fires and insect outbreaks. Although most current US forest ecosystems are still vigorously accumulating carbon, the ultimate size of the C sink is more controlled by abiotic factors. With observed and projected changes in climate and disturbance regimes recently and in the future, it is inevitable that the future C sink in US forests will be uncertain and indeed affected greatly by these changing causes.