Changes in Soil Carbon Storage in Industrial Forests of Western Oregon and Washington Following Modern Timber Harvesting Practices

Carbon in forest soils is often overlooked because it is less conspicuous than the live trees, downed wood, and forest floor layer that are easily visible when walking through a forest. However, the amount of carbon in forest soils to one meter depth is generally one to two times the amount of carbon we see above ground in mature forests, making soils an important carbon storage pool in forest ecosystems. Given the large quantity of carbon stored in soil, there is some concern that disturbances to forest ecosystems could push some soils out of steady state and lead to a release of carbon from the soil, potentially contributing to the already large amount of greenhouse gas emissions from the burning of fossil fuels for energy. This has implications for the carbon neutrality of timberlands. Thus, careful investigation of the carbon cycle in forest soils is a key component in deciphering the gains and losses of carbon from forests, and ultimately understanding the effects of forest soils on the global carbon cycle. The study objective was to measure pre-harvest soil carbon stores to 1 m depth with enough precision to detect a small change upon resampling post-harvest. The 9 sites examined ranged from 100 to 400 Mg C / ha before harvest with minimum detectible differences around 5%. Three and a half years post-harvest the average of all 9 sites showed a very modest increase in mineral soil carbon as a result of modern timber harvest. Mineral soil carbon did not change significantly at 6 of the 9 sites, individually (range -2% to +5%), while two sites gained soil carbon (+6% and +11%) and soil carbon decreased at one site (-6%).