In response to warming, Eastern forests inhaling more carbon dioxide than they're exhaling

Scientists have known for many years that rising temperatures are lengthening the growing season in many northern and mid-latitude forests. New research indicates that in the eastern United States, increased carbon uptake has outpaced a simultaneous increase in carbon dioxide “exhaled” into the atmosphere through respiration. Overall, it seems that eastern forests are acting as increasing “sinks” for carbon dioxide.

One reason for the increased carbon uptake is that trees up and down the eastern half of the country are leafing out earlier in the spring. The map shows a trend toward earlier spring green-up over twelve years (2001-2012), based on both satellite vegetation data and ground observations from long-term research sites in forests across the Northeast. Shades of green indicate locations where the onset of spring is occurring earlier. White indicates very little change, while pink shows locations where the onset of spring is occurring later in the year. On average, spring is arriving about 10 days earlier than it used to only two decades ago.

The shift toward earlier spring leaf out is due to warming in the U.S. East, and has been mirrored by a delay in when trees drop their leaves in autumn. In a recent study published in Nature Climate Change, researchers found that enhanced “greening activity” during this extended growing season increased the amount of carbon that forests removed from the atmosphere through photosynthesis—the process by which plants convert carbon dioxide in the air into sugar molecules to use for food and to grow.

But just like animals do, plants and soil organisms burn sugars for energy and “exhale” carbon dioxide as a byproduct, a process known as respiration; warming also increases ecosystem respiration. While respiration increased in eastern forests during the extended growing season, the forests absorbed more carbon dioxide than they released, leading to a total net increase in carbon storage.

It’s not a given that the increased carbon uptake would dominate increased respiration. In northern (boreal) forests, which are primarily dominated by evergreen species, previous research found an overall decrease in carbon uptake despite an extended growing season. That’s a sign that that these two ecosystems have responded very differently to a warming climate, although additional research is needed to know exactly why.  

Increased carbon uptake in forests may help reduce the growth rate of atmospheric carbon dioxide and slow future warming. But at the same time, climate change is increasing the vulnerability of many U.S. forests to fire, insect infestations, drought, and disease outbreaks. These disturbances raise the potential for large releases of carbon dioxide back into the atmosphere. Despite recent increases in forest growth due to elevated carbon dioxide and temperature changes, it remains unclear how future net forest carbon storage in the United States will respond to accelerating mortality and trends in land use and forest management.

References:

Keenan, T. F., Gray, J., Friedl, M. A., Toomey, M., Bohrer, G., Hollinger, D. Y., Munger, J.M., O'Keefe, J., Schmid, H.P., Wing, I.S., Yang, B., and Richardson, A. D. (2014). Net carbon uptake has increased through warming-induced changes in temperate forest phenology. Nature Climate Change, advance online publication. doi:10.1038/nclimate2253

Joyce, L. A., S. W. Running, D. D. Breshears, V. H. Dale, R. W. Malmsheimer, R. N. Sampson, B. Sohngen, and C. W. Wood­all, 2014: Ch. 7: Forests. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 175-194. doi:10.7930/J0Z60KZC.