The research, which was undertaken for 20 years in the northern hardwood forests of Michigan, was carried out by scientists at Michigan Technological University's School of Forest Resources and Environmental Science.
The scientists reached a surprising conclusion that moderate increases in temperature and nitrogen from atmospheric pollution actually improve forest productivity.
The team of researchers has been monitoring and measuring the temperature, moisture levels and nitrogen deposited by acid rain or varying levels of experimental nitrogen at four forest sites ranging from northwestern to southern Michigan since 1987.
According to Andrew Burton, an associate professor at Michigan Tech and head of the National Institute for Climatic Change Research's Midwestern Regional Center, the trees grow faster at higher temperatures and store more carbon at greater concentrations of nitrogen, a chemical constituent of acid rain, providing there is sufficient moisture.
"It may well be that increasing temperature and nitrogen deposition are good things, up to a point," Burton said.
The rise in temperature is extending the growing season, explained Burton.
So far, Burton and colleagues have measured 10 to 11-day longer growing seasons. "Our growing season isn't that long in the first place. So, 10 or 11 days is significant," he said.
A longer growing season could benefit the timber industry, enabling them to harvest more wood.
Now that woody biomass is being investigated as an alternative energy source by Michigan Tech and others, increased forest productivity could become a critical factor.
Burton and his fellow researchers, Don Zak at the University of Michigan and Kurt Pregitzer at the University of Nevada-Reno, want to discover if the increased annual growth of the forests is offset by an increase in tree mortality.
They also will examine whether the woody debris on the forest floor will decompose more slowly as nitrogen levels are increased, further increasing the ecosystem's ability to store carbon.
Burton calls the new work "a window into the future," an opportunity to see if there is a tipping point beyond which increased nitrogen harms rather than helps the forests.