Soil carbon fails to recover in cleared forests

Hubbard Forest

Hubbard Brook Experimental / Forest credit Science History Institute

If a forest is clear-cut, then allowed to grow back, the new forest may look identical to the naked eye – but science produced by Woodwell Climate Research Center has shed new light on damage that’s invisible, large, and lasting.

In forests, carbon is stored not just in the plant biomass but in the soil. While previous work on forest carbon stocks took place over short time spans, Woodwell Climate scientist Dr. Jonathan Sanderman and his colleagues spent 16 years tracking a single forest in New Hampshire. The research team sampled soils prior to its clear-cutting, and then sampled again in years 3, 8, and 15 following the harvest.

The scientists found carbon losses in the soil can be large and lasting, publishing their study in the journal Biogeochemistry. “Clearcut harvesting can have sustained negative impacts on soil resources,” Sanderman said.

Research area

It took 15 years of biomass accumulation to simply offset the carbon losses incurred in the mineral soil, leading to a near-zero net carbon gain over the study period. In other words, the soil loses significant carbon while treeless, and even when the tree canopy returns, the soil’s carbon storage remains well below its original carbon level even after 15 years.

Dr. Sanderman is a biogeochemist who specializes in understanding how soil carbon and nutrient cycles have been altered by land-use and climate change. He began working on the study shortly after completing his undergraduate degree at Brown University.

“While this is only one location, this study used massive soil sampling campaigns over time to do an excellent job tracking soil carbon pools, so we are very confident in the findings. Most studies of soil carbon changes following forestry use what’s known as a chronosequence – studying several distinct sites of varying ages and assuming they’re representative over time,” Sanderman said.

It’s unclear exactly what happens to the carbon in the soil. The two most probable fates are decomposition and release to the atmosphere as CO2 or runoff into waterways, but the researchers say future studies are needed.

Left: Collected soil samples

Right: Soil carbon test

The study has implications for forest policy, especially in light of increasing demand for woody biomass electricity production. If forest harvesting is expanded, then it will take substantially longer than previously assumed to offset harvest- or bioenergy-related carbon dioxide emissions with carbon uptake during forest regrowth.

Previous Woodwell Climate research has shown that historical land use and land cover change has resulted in substantial losses of carbon from soils globally. Dr. Sanderman’s research has shown that agricultural land uses have resulted in the loss of 133 petagrams of carbon from the world’s soil. Sanderman also worked with Woodwell Climate senior geospatial analyst Greg Fiske to produce maps indicating hotspots of soil carbon loss, often associated with major cropping regions and degraded grazing lands, suggesting that there are identifiable regions that should be targets for soil carbon restoration efforts.