Old-growth forests are often assumed to exhibit no net carbon assimilation over time periods of several years. This generalization has not been typically supported by the few whole-ecosystem, stand-scale eddy-covariance measurements of carbon dioxide exchange in old-growth forests. An eddy-flux tower installed in a >300-year-old hemlock-hardwood forest near the Sylvania Wilderness, Ottawa National Forest, MI, USA, observed a small annual carbon sink of CO 2 of -72 ± 36 g C m -2 year -1 in 2002 and -147 ± 42 g C m -2 year -1 in 2003. This carbon sink was much smaller than carbon sinks of -438 ± 49 g C m -2 year -1 in 2002 and -490 ± 48 g C m -2 year -1 in 2003 observed by a nearby flux tower in a 70-year-old mature hardwood forest (Willow Creek, WI). The mature forest had vegetation similar to the old-growth site prior to European settlement. Both sites had slightly larger carbon sinks in 2003, which was a drier and cooler year than 2002. However, the difference in sink strength between the two years was smaller than the uncertainty in the results arising from missing and screened data. Both sites also had significant systematic errors due to non-representative fluxes during certain micrometeorological conditions, which required careful screening. The difference in sink strength between the two sites was driven mainly by greater ER at the old-growth site (965 ± 35 g C m -2 year -1 in 2002 and 883 ± 69 g C m -2 year -1 in 2003) compared to the mature site (668 ± 21 g C m -2 year -1 in 2002 and 703 ± 17 g C m -2 year -1 in 2003). GEP was lower at the old-growth site (1037 ± 47 g C m -2 year -1 in 2002 and 1030 ± 41 g C m -2 year -1 in 2003) compared to the mature site (1106 ± 47 g C m -2 year -1 in 2002 and 1192 ± 51 g C m -2 year -1 in 2003), especially in 2003. Observations also suggested that growing season ER had greater interannual variability at the old-growth site. These results imply that old-growth forests in the region may be carbon sinks, though these sinks are smaller than mature forests, mostly likely due to greater ER.
Bibliographical noteFunding Information:
We thank Jianwu Tang, Leslie Kreller, Jon Martin, Deborah Hudleston, and other field crew members based at the University of Minnesota, Department of Forest Resources; John Gerlach, formerly of the University of Minnesota, Department of Forest Resources; Margaret Davis, University of Minnesota, Department of Ecology, Evolution and Behavior; Ron Teclaw, Dan Baumann, and Jud Isebrands, U.S. Forest Service North Central Experiment Station; Tom Steele, Karla Ortman, and Gary Kellner, University of Wisconsin Kemp Natural Resources Station; Bob Evans, Ottawa National Forest; Art Johnston, Chequamegon National Forest; Weiguo Wang, Pennsylvania State University; Larry Mahrt, Oregon State University; and two anonymous reviewers. This work was funded by the Office of Science (BER), U.S. Department of Energy Terrestrial Carbon Processes program, grant number DE-FG02-00ER63023, and by the Office of Science (BER), U.S. Department of Energy, through the Midwestern Regional Center of the National Institute for Global Environmental Change under Cooperative Agreement No. DE-FC03-90ER61010.
- Carbon balance
- Eddy covariance
- Old-growth forest
- Sylvania Wilderness