Drivers of change in FIA inventories: effects of land-use change, disturbance, and management on forest carbon flux

Background/Question/Methods

Forest ecosystems are of primary importance in understanding fluxes in terrestrial carbon because of their potential to accumulate large amounts of carbon. Carbon flux from forests are determined by the balance of establishment, growth, mortality, and logging across millions of hectares of land. Ground-based forest inventory measurements are crucial for assessing carbon stocks and paramaterizing remote-sensing based estimates. Remeasurement of permanent field plots allows calculation of change in live and dead wood carbon pools, and in characterizing the causes of those changes, including land use change, natural disturbance, or management. The goal of this study was to determine the importance and magnitude of these drivers on carbon flux across the forest lands of Oregon and Washington, USA. We used repeated measurements on 12,000 Forest Inventory and Analysis (FIA) plots and allometric equations of biomass and carbon density to quantify tree carbon flux due to growth, land use change, disturbance, and logging.

Results/Conclusions

Land use change resulted in a net increase of 1.1x105 ha of forest land in Oregon between and a net loss of 1.5x105 ha of forest land in Washington between 1986-2001. However, there was a net loss of 6 Tg of live tree C because most of the losses were on productive forests and most of the gains were on low-productivity juniper woodlands. Live woody C decreased significantly in the drier eastern part of the region, primarily as a result of insect outbreaks and wildfires. Although potentially undesirable from a sequestration standpoint, the reduction in C density in eastern forests follows decades of fire suppression and may be desirable from a forest health perspective. On non-federal lands, a majority of this mortality was converted to wood products through salvage logging. C stores increased significantly in the more productive western part of the region, due to large accumulations from growth and low levels of logging on public lands during this period. Patterns of C accumulation varied with stand age and site productivity. While carbon sequestration in undisturbed stands can be prodigious in the Pacific Northwest, land use change and disturbance have been equally important.