By most predictions, the Northern forests that cover much of North America, Europe, and Asia, should be getting greener. Scientists have always thought that plant growth in the world’s Northern forests was limited by temperature. Arctic summer provides a brief period in which plants can develop before the cold of winter ends the growing season. Over the past century, however, temperatures have gone up and the length of the growing season has increased, nearly doubling in sections of Alaska. With carbon dioxide, one of the key ingredients in photosynthesis, also on the rise, plants should be thriving. But they are not.
A greening forest was what Scott Goetz, an ecologist at Woods Hole Research Center in Woods Hole, Massachusetts, expected to see when he began to track the forest’s health using satellite data. As he tracked changes between 1982 and 2003, he noticed something strange: the forest was getting browner, not greener as he expected. His results are shown in this image, which depicts how photosynthetic activity, an indicator of growth, has changed over the past two decades. For parts of the region, growth has not changed (gray), but in interior Alaska and a wide swath of Canada, growth has declined (brown). Only in the far north, regions of tundra, has growth increased (green).
Goetz’s observations were complemented by the work of another scientist, Alon Angert, at the University of California, Berkeley. Angert was also tracking the health of the forest from 1982 to 2002, but he wanted to know how much carbon dioxide the forest was taking out of the atmosphere. As trees grow, they absorb carbon dioxide, so a faster-growing forest should soak up more carbon dioxide. He noticed that after 1994, the carbon dioxide uptake declined during the growing season, hinting that forest growth had slowed during the past decade.
These two studies and several field studies hint that the world’s northern boreal forests may be in decline. Field studies have shown that cold-loving trees like the Northern White Spruce actually grow more slowly as temperatures rise. A related reason for the forest’s decline may be a lack of water. Warmer, longer summers dry the trees. While theories of global warming predicted a greener forest, they also predicted that eventually the forest would run out of water and begin to decline. Only time will tell if the decline Goetz and Angert independently observed is the predicted long-term effect of climate change or a short-term drought, but the downward trend may have already begun.
To read more about the impact of global warming on Northern forests, please see Forest on the Threshold on the Earth Observatory.
Angert, A., Biraud, S., Bonfils, C., Henning, C.C., Buermann, W., Pinzon, J., Tucker, C.J., and Fung, I. (2005) Drier summers cancel out the CO2 uptake enhancement induced by warmer springs. Proceedings of the National Academy of Sciences, 102(31), 10823-10827.
Arctic Climate Impact Assessment. (2004). Cambridge University Press. http://www.acia.uaf.edu/
Goetz, S.J., Bunn, A.G., Fiske, G.J., and Houghton, R.A. (2005) Satellite-observed photosynthetic trends across boreal North America associated with climate and fire disturbance. Proceedings of the National Academy of Sciences, 102(38), 13521-13525.