Estimation of Aboveground Forest Carbon Flux in Oregon: Adding Components of Change to Stock-Difference Assessments

Abstract

A substantial portion of the carbon (C) emitted by human activity is apparently being stored in forest ecosystems in the Northern Hemisphere, but the magnitude and cause are not precisely understood. Current official estimates of forest C flux are based on a combination of field measurements and other methods. The goal of this study was to improve on existing methods by directly tracking components of change in tree C across a large region using field measurements. We used repeated Forest Inventory and Analysis (FIA) measurements on permanent plots to quantify aboveground live tree C flux over an 11-year period due to land-use change, disturbance, and harvest, on 1,073 plots across 5.96 million ha of nonfederal forestland in Oregon. Land-use change resulted in a 110,000 ha (1.9%) net increase of forestland between 1986 and 1997. However, there was a net loss of 3.4 Tg of live tree C due to land-use change because the forestland lost was more productive than that gained. Live woody C decreased significantly in eastern Oregon (−14.4 Tg), with mortality and harvest exceeding growth, primarily due to severe defoliation by western spruce budworm. However, C stores increased significantly in western Oregon (19.2 Tg) due primarily to large accumulations from growth on nonfederal public lands. We demonstrate that C accounting that uses remeasured probabilistic field sample data can produce detailed estimates of C flux that identify causes and components of change and produce more consistent estimates than alternative approaches.