An ambitious ecosystem-level experiment in Minnesota is evaluating the response of northern peatland ecosystems to warmer temperatures and elevated carbon dioxide.
“Results so far suggest that although surface peat releases carbon in response to increasing temperature, the large reservoir of carbon in deep peat layers below ground is stable,” said Randy Kolka, research soil scientist at the USDA Forest Service’s Northern Research Station. “That’s good, and we did not expect it. The old carbon is recalcitrant.”
Peatlands cover just 3 percent of Earth’s land surface but store about 30 percent of its soil carbon, roughly the same amount stored in all land plants. The layers of peat build up over centuries because the plant material decomposes very slowly in such a wet environment. By inducing whole-ecosystem warming, the researchers seek to understand environmental change processes affecting both carbon and organisms, which will improve data used in global circulation models and allow scientists to better predict future climate trends. In spite of their importance, peatland ecosystems currently are not well-represented in global models.
The Spruce and Peatland Responses Under Changing Environments (SPRUCE) Experiment on the Marcell Experimental Forest north of Grand Rapids, Minn., uses 10 testing chambers, five with temperatures from 0 to 16.2 degrees Fahrenheit), and five with similar boosts in temperature that include carbon dioxide levels approaching 900 parts per million, which is more than twice current levels but consistent with end-of-century projections for atmospheric carbon dioxide.
On the Marcell Experimental Forest, populations of black spruce and tamarack trees, shrubs, and moss grow on the peat. In the above ground test chambers, early data indicate growth rate of black spruce trees and lichens slowed dramatically in response to artificially elevated temperatures; in contrast, some low shrub species thrived under similar conditions.
Another early finding from SPRUCE is that plants growing in the warmest test chambers, 16.2 degrees Fahrenheit above ambient temperatures, are greener and still growing up to three weeks longer in both spring and fall than plants growing at ambient temperatures. It appears the plants, especially some low shrub species, are rather quickly adjusting to the longer growing season. The trees however are having more difficulty adjusting to the longer growing season partially because of earlier springs where they began to put out foliage only to have another big cold snap where they lose their needles again before actual spring occurs. The trees use a lot of resources when they have to put out foliage multiple times a year.
The experimental warming also dramatically increases methane gas emissions and to a lesser degree, carbon dioxide emissions.
Organic material also decomposes faster under elevated ground temperatures. The scientists pushed cotton strips into the peat and regularly tested the material’s tensile strength as a measure of its decomposition and early results indicate that the cotton strips were weaker in the higher temperature chambers.
“The SPRUCE experiment took over four years to get up and running because after we built it, we needed to test it thoroughly to make sure all of the equipment functioned properly in such a wet environment that can dip to minus 40 degrees Fahrenheit during the winter,” Kolka said. “These early research results indicate just how beneficial an ecosystem-wide field experiment can be for understanding and predicting environmental change. We probably have some more surprises in store for us. It’s exciting to be part of something this big and this significant.”
SPRUCE is a cooperative venture between the Forest Service and the U.S. Department of Energy’s Oak Ridge National Laboratory, along with 50 other collaborators. All data from the SPRUCE Experiment will be archived for sharing with scientists, educators, and the public. In addition to generating crucial data for understanding Earth’s carbon cycle, DOE wants more accurate global models to better predict future energy demands.