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In State Forest, Autumn is Coming Later

Geography researchers at Indiana University have been studying forest productivity in Morgan-Monroe State Forest, just north of Bloomington. Scientists have found autumn now begins a month later than it did a decade ago.

The Morgan-Monroe State Forest is relatively young. Prior to the late 1920s, the land was filled with open fields and meadows. During the Great Depression, the Civilian Conservation Corps planned and planted the forest, which created jobs for local workers. The state Department of Natural Resources has managed the site ever since, and has allowed a portion of the forest to grow naturally, with minimal human interference.

This unmolested area was perfect for Dr. Danilo Dragoni and his team to study the growth and productivity of the deciduous forest. IU team began collecting air and soil samples from the forest, and began studying the tree canopy from a 150′ tall research tower in 1998. Using this research, Dragoni says his team determined that not only is the forest staying greener longer, but as a result, trees are growing faster, leaves are bigger and the amount of carbon absorbed has increased.

“The forest in 2008,” Dragoni explains, “was absorbing 30% more carbon than it was in 1998.”

Several factors might be responsible for a longer growing season and increased carbon absorption. Dragoni suggests temperature plays a pivotal role. “We knew the summer was coming later, and later,” he says, “but why is that? One of the things we found, though it’s not a smoking gun, is temperature in late summer is increasing as well.”

Though other research suggests a link between higher temperatures and increased carbon emissions, and Dragoni’s research says there is a link between higher temperatures and greater carbon absorption in Morgan Monroe State Forest, there is no way of knowing if the trees can take in more carbon because of the increase in CO2 in the atmosphere. Dragoni says more research is needed. The longer season though, does allow the trees to filter more carbon emissions from the atmosphere.

According to his research, Dragoni estimates 50% of the increased carbon absorption can be linked to the increased growing season.

Dragoni’s team is not the only group of scientists that has found an increase in deciduous forest carbon absorption. Dr. Sean McMahon, a post doctorate fellow at the Smithsonian Institute, found a similar trend.

McMahon has been noticing a longer growing season in trees outside of Washington DC. “The last frost of spring is coming earlier,” McMahon says, “and the first frost of fall is coming later, so the effective growing season for these trees has increased in the last century. So what we hypothesized is sometime in the recent past, the trees have been growing faster than they used to, they have more resources, and longer to use them.”

Dragoni first noticed the forest staying greener longer when his team began taking digital images of its canopy from the tower. Every year, they measured when the forest began to lose its leaves, and determined that process was beginning later each year.

“Fall was,” he explains, “on average, starting 3 days later. Starting in 1998-2008, we gained 1 month of vegetative season. From the end of September to the end of October.”

And although scientists are still not sure how much carbon trees absorb from the atmosphere, they do know that trees help filter CO2 created from the burning of fossil fuels the output of which is increasing, according to McMahon.

Besides measuring carbon absorption, meteorological data is also collected at the site in an attempt to determine what other factors affect forest growth.

One unanswered question though, is how much longer the growing season can increase. Though researchers agree the trend will continue, neither can say for how long.

“There is no way deciduous trees can maintain their leaves on and on,” says Dragoni. “There are limitations, nutrient limitations, light limitations. These effects will become more important than the increased temperatures in late summer. The process will slow down eventually and stop.”

Information Dragoni’s team gathered will be compared to data from other forests around the world, including the Amazon rainforest and forests in northern Michigan.

For more information on McMahon’s studies at the Smithsonian Institute, visit:

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