Logging doesn't immediately jettison carbon stored in a forest's mineral soils into the atmosphere but triggers a gradual release that may contribute to climate change over decades, a Dartmouth College study finds. The results are the first evidence of a regional trend of lower carbon pools in soils of harvested hardwood forests compared to mature or pristine hardwood forests. The findings appear in the journal Global Change Biology Bioenergy. A PDF of the study is available on request.

Despite scientists' growing appreciation for soil's role in the global carbon cycle, mineral soil carbon pools are largely understudied and previous studies have produced differing results about logging's impact. For example, the U.S. Forest Service assumes that all soil carbon pools do not change after timber harvesting.

The Dartmouth researchers looked at how timber harvesting affects mineral soil carbon over 100 years following harvest in the northeastern United States, where soils account for at least 50 percent of total ecosystem carbon storage. Mineral soils, which underlie the carbon-rich organic layer of the soil, make up the majority of that storage, but are sometimes not included in carbon studies due to the difficulty in collecting samples from the rocky, difficult terrain. The researchers hypothesized that the mineral soil carbon would be lower in forests that had been harvested in the last century than in forests that were more than 100 years old. They collected mineral soil cores from 20 forests in seven areas across the northeastern United States and compared the relative amounts of carbon in the soil from forests that were logged five years ago, 25 years ago, 50 years ago, 75 years ago and 100 years ago.

The results showed no significant differences between mineral soil carbon in the older versus harvested forests. But there was a significant relationship between the time since forest harvest and the size of the carbon pools, which suggested a gradual decline in carbon across the region that may last for decades after harvesting and result in increased atmospheric carbon dioxide.