More Maple Seeds, Less Maple Syrup

A study finds a way to predict sap quality both more accurately and further in advance

bucket attached to maple tree for sap tapping

For decades, maple syrup producers have eyed the weather to help gauge spring sugar yields. But new research by Tufts scientists reveals that the quantity of seed helicopters that rain down from the trees the year before is a far better predictor of the quality of the sap.

“Weather affects how much sap will flow out of the tree, but sap volume is only one piece of the puzzle,” says Joshua Rapp, a postdoctoral fellow working with Elizabeth Crone, an associate professor of biology in the School of Arts and Sciences.

“Both seeds and sugar are made from carbohydrates stored in trees,” says Elizabeth Crone. Photo: Ingimage“Both seeds and sugar are made from carbohydrates stored in trees,” says Elizabeth Crone. Photo: Ingimage
What really matters to maple syrup producers is the amount of sugar in the sap. “Sugar maple sap is 2 to 3 percent sugar,” Rapp says. “The rest is just water to boil off. Sweeter sap is more profitable. If you start with sap that’s 3 percent sugar, it takes a third less sap to make a gallon of syrup than sap that’s 2 percent sugar.”

So what foretells how much sugar is in the sap? Not weather, apparently. “Weather alone was a surprisingly bad predictor of how much sugar came out of the taps,” says Rapp. “That tells us there is something else at play.”

For several years, Rapp and Crone have been studying what are called “mast” seeding events—years when trees collectively produce far more seeds than usual—at the Harvard Forest, a 3,750-acre reserve in Petersham, Massachusetts. They have also analyzed the factors influencing 17 years of maple syrup production at 28 sites in Vermont. Their research was published online in late October in the journal Forest Ecology and Management.

In sugar maples, mast seeding tends to occur every two to five years. Recent mast seeding events occurred in Vermont in 2000, 2006 and 2011. Rapp and Crone’s research shows that in Vermont, syrup production declined following every mast seed year.

“Both seeds and sugar are made from carbohydrates stored in trees,” Crone says. “When a tree produces a lot of seeds one summer, then the next spring the carbohydrate bank account is low for making sugar. It’s a matter of budgeting resources.”

Looking ahead to next year’s harvest, Rapp notes that at the Harvard Forest—and likely throughout the Northeast—the seed crop was small this year, suggesting the spring 2015 maple syrup harvest should be a good one.

That’s not the only factor influencing the volume and quality of the sap, of course. Rapp says the best way to predict syrup production “is actually a combination of factors: proportion of trees with seeds, minimum and maximum March temperatures and maximum April temperature. Those factors together explained 79 percent of the variation in syrup production in Vermont from 1998 to 2014.”

Because seeds develop a full six months before the syrup harvest, Rapp hopes this study can help syrup producers plan ahead. “Maple syrup is a complicated natural resource,” he says. “Hopefully this research can give producers a window into the upcoming season.”

Clarisse Hart is the outreach manager at the Harvard Forest. She can be reached at

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