Improving forest carbon flux estimates with gain-loss measurements from regional inventories in Oregon

Background/Question/Methods

The increasing concentration of carbon dioxide in the atmosphere is implicated as one of the primary causes of recent increases in global mean temperatures and sea levels. A substantial portion of the carbon (C) being released through human activities is apparently being stored in terrestrial ecosystems in the Northern Hemisphere, but the magnitude and cause are not precisely understood. Forest ecosystems accumulate much larger amounts of carbon than other terrestrial ecosystems, so they are a focus of efforts to understand changes in carbon storage (i.e., fluxes) over time. Current “stock-change” estimates of forest carbon flux are based on an amalgam of field measurements, surveys, remote sensing classifications, and models. The goal of this study was to improve on existing carbon accounting methods by using the “gain-loss” method and directly tracking components of change in tree carbon across a large region with strong environmental gradients using field measurements. We used repeated Forest Inventory and Analysis (FIA) measurements on permanent plots to quantify tree carbon flux due to land use change, disturbance, and harvest on 1,067 plots across Oregon.

Results/Conclusions

Land use change resulted in a net increase of 1.1x10⁵ ha of forest land in Oregon between 1986-1997. However, there was a net loss of 3.4 Tg of live tree C because most of the losses were on productive west-side forests and most of the gains were on east-side juniper woodlands. Live woody C decreased significantly in eastern Oregon (-14.4 Tg), with mortality and harvest exceeding growth. Much of the mortality and subsequent harvest was associated with severe defoliation by western spruce budworm. However, C stores increased significantly in western Oregon (19.2 Tg) due to large accumulations from growth on public lands, and harvest being balanced by growth on private lands. Patterns of C density associated with stand age differed by site productivity class and forest type. The increase in C density with stand age was greater on more productive sites due to higher growth rates in young stands, but contribution of those sites to growth decreased as harvest rates increased with stand age. We demonstrate that gain-loss accounting from a probabilistic field sample can produce detailed estimates of carbon flux that identify causes and components of change, and produce more consistent estimates than combining alternative approaches.