Contrary to the conclusions of dozens of other studies, a new paper appearing in  Global Change Biology argues that as the climate warms and growing seasons lengthen, subalpine forests are likely to soak up less carbon dioxide. The authors, scientists at the University of Colorado, say that more of the greenhouse gas will be left to concentrate in the atmosphere as a result.

The researchers found that while smaller spring snowpack tended to advance the onset of spring and extend the growing season, it also reduced the amount of water available to forests later in the summer and fall. The water-stressed trees were then less effective in converting CO2 into biomass. Summer rains were unable to make up the difference.

Drier trees also are more susceptible to beetle infestations and wildfires, said co-author Russell K. Monson said.

Even as late in the season as September and October, 60 percent of the water in stems and needles collected from subalpine trees along Colorado's Front Range could be traced back to spring snowmelt. The team was able to distinguish between spring snow and summer rain in plant matter by analyzing slight variations in hydrogen and oxygen atoms in the water molecules.

The results suggest subalpine trees like lodgepole pine, subalpine fir and Englemann spruce depend largely on snowmelt, not just at the beginning of the summer, but throughout the growing season, according to the researchers.

"As snowmelt in these high-elevation forests is predicted to decline, the rate of carbon uptake will likely follow suit," said co-author Jia Hu.

Subalpine forests currently make up an estimated 70 percent of the western United States' carbon sink, or storage area. Their geographic range includes much of the Rocky Mountains, Sierra Nevada and high-elevation areas of the Pacific Northwest.

Citation: JIA  HU, DAVID J. P. MOORE, SEAN P. BURNS, RUSSELL K.  MONSON, 'Longer growing seasons lead to less carbon sequestration by a subalpine forest', Global Change Biology, February 2010, 16(2), 771 - 783; doi: 10.1111/j.1365-2486.2009.01967.x