Disturbance and climate effects on carbon stocks and fluxes across Western Oregon USA

Abstract

We used a spatially nested hierarchy of field and remote‐sensing observations and a process model, Biome‐BGC, to produce a carbon budget for the forested region of Oregon, and to determine the relative influence of differences in climate and disturbance among the ecoregions on carbon stocks and fluxes. The simulations suggest that annual net uptake (net ecosystem production (NEP)) for the whole forested region (8.2 million hectares) was 13.8 Tg C (168 g C m−2 yr−1), with the highest mean uptake in the Coast Range ecoregion (226 g C m−2 yr−1), and the lowest mean NEP in the East Cascades (EC) ecoregion (88 g C m−2 yr−1). Carbon stocks totaled 2765 Tg C (33 700 g C m−2), with wide variability among ecoregions in the mean stock and in the partitioning above‐ and belowground. The flux of carbon from the land to the atmosphere that is driven by wildfire was relatively low during the late 1990s (∼0.1 Tg C yr−1), however, wildfires in 2002 generated a much larger C source (∼4.1 Tg C). Annual harvest removals from the study area over the period 1995–2000 were ∼5.5 Tg C yr−1. The removals were disproportionately from the Coast Range, which is heavily managed for timber production (approximately 50% of all of Oregon's forest land has been managed for timber in the past 5 years). The estimate for the annual increase in C stored in long‐lived forest products and land fills was 1.4 Tg C yr−1. Net biome production (NBP) on the land, the net effect of NEP, harvest removals, and wildfire emissions indicates that the study area was a sink (8.2 Tg C yr−1). NBP of the study area, which is the more heavily forested half of the state, compensated for ∼52% of Oregon's fossil carbon dioxide emissions of 15.6 Tg C yr−1 in 2000. The Biscuit Fire in 2002 reduced NBP dramatically, exacerbating net emissions that year. The regional total reflects the strong east–west gradient in potential productivity associated with the climatic gradient, and a disturbance regime that has been dominated in recent decades by commercial forestry.