Scientists: Climate Change Not Sweet For Sugar Maples

By Andrea Smardon

On the left, central New England is seen with colored foliage in Oct. 2003; on the right, central New England showed little color in October 2005.

BOSTON — As New England travelers take to the roads this holiday season, they’ll be seeing the last of the leaves on the trees. No two autumns alike -- every year, there’s some variation in the color and timing of the fall foliage. But scientists at the University of New Hampshire have observed some significant changes in the leaves of the sugar maple over the last few years. The scientists say the maple leaves are less brilliant in color, and the sap is less sweet.

They think these things are related -- and that climate change is the cause.

Martha Carlson is a graduate student in natural resources at the University of New Hampshire, but she’s not following the typical academic path. Her hair is mostly silver and she’s already reached the age when many people retire.

There’s something personal motivating Carlson to drive down to Durham from her home an hour and a half away in Sandwich, NH. She owns a sugar bush – a “farm” of sugar maples used to harvest syrup – and she’s concerned about the sugar maple trees on her farm.

Vials of maple syrup line a shelf in Martha Carlson's "Maple Watch Lab." Carlson's research suggests the sap used to make maple syrup is less sweet than it used to be. (Andrea Smardon/WGBH

“I had read that climate change was supposed to kill off all the sugar maples in the U.S.,” Carlson says. “I didn’t want that to happen to my maples. So I came down here to find out more about that, and to study biology in a more formal way.”

Carlson studies symptoms of stress in maple trees, collecting leaves and sap from her farm and 12 other sites across New Hampshire.

On a shelf in what Carlson calls her “maple watch lab,” dozens of small vials display syrup samples ranging in color from light amber to dark brown. “Last year in 2009 all over the state of NH, the sap came out dark and it all made dark syrup like this one right away,” she says, holding up a vial of dark brown syrup. “Usually you get very light syrup and it gets browner and browner until the end of the season. So the season was short. That was when I started saying well there’s got to be something different.” If a tree is stressed, it won’t hold on to its leaves as long, and that means less time to make sugar.

Not only is the season shorter, but Carlson has found that maples across the region are producing sap with less sugar. She says sap containing 2.5-3% sugar was the norm fifty or sixty years ago. “Today the sap this year was 2.5-2%. It got down at the end of the season to 1 percent. If you’re a tree and you’re used to having 3 percent sugar in your body, and now you have 1 percent, I think that’s going to affect the health of trees.

It also affects the syrup producers.  Carlson says it takes many more gallons of sap today to produce the same amount of syrup.

Carlson’s findings are backed up by her advisor Barrett Rock, a professor of forestry and botany at UNH.  Rock has been studying New England’s woodlands for more than three decades, using NASA’s satellite images of forests to detect changes in leaf color from year to year. 

Every year, at the onset of fall, tree leaves naturally stop producing the green pigment chlorophyll and turn yellow. Sugar maples also make a red pigment – anthocyanin – and the combination of red and yellow is what gives sugar maples their signature burnt orange fall foliage. But low sugar levels cause a decline in anthocyanin that Rock said has been visible in recent years. “For the last number of sugar seasons that’s what we’ve been seeing. We don’t get the really, intense brilliant orange,” Rock said.

Rock has also observed a change in the timing of when the leaves reach their peak color: It’s getting later in the season.  He says the changes in color display and sugar seasons correlate with predicted climate change patterns, including a longer growing season in the fall and an earlier onset of warmer weather in the spring.

Rock says Martha Carlson’s work bolsters the evidence that the sugar maple is responding at least in part to climate change -- but what’s surprising is how quickly it’s happening. “At 68, I didn’t think I would live long enough to see the impact of a changing climate.  But I’m beginning to think living another 10-15 years, I will see some significant changes, and as Martha’s work has suggested, we’re already seeing significant changes,” Rock said.

Average temperatures in New England are 2-4ºF warmer than they were a hundred years ago.  Rock says if the average temperature gains another 6ºF, then New England will no longer have sugar maples at all. Under worst-case scenarios, that sort of warming might be possible late this century.

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