If you live in an area with maple trees, you know it is spring when buckets are hanging from the trunks - sweet, watery sap will soon be maple syrup.
But how much? That has always been the puzzle for maple syrup producers. Eyeing the weather to understand spring sugar yields has been a rule of thumb but a new paper in Forest Ecology and Management provides a more scientific metric for predicting syrup production: how many seed helicopters rained down from the trees the year before.
The authors studied "mast" seeding events - years when trees collectively produce far more seeds than usual - at the Harvard Forest in Massachusetts. In sugar maples, mast seeding tends to occur every 2 to 5 years and recent mast seeding events occurred in Vermont in 2000, 2006, and 2011. They found that in Vermont, syrup production declined following every mast seed year.
"Weather affects how much sap will flow out of the tree, but sap volume is only one piece of the puzzle," says Dr. Josh Rapp of Harvard, first author of the paper, who analyzed the factors influencing 17 years of maple syrup production at 28 sites in Vermont. What really matters to maple syrup producers, Rapp explains, is the amount of sugar in the sap: "Sugar maple sap is 2 to 3 percent sugar. 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."
So, what predicts how much sugar is in the sap? "Not weather," says Rapp. "Weather alone was a surprisingly bad predictor of how much sugar came out of the taps over those 17 years," says Rapp. "That tells us there is something else at play."
"Both seeds and sugar are made from carbohydrates stored in trees," explains senior author Elizabeth Crone, associate professor of biology at Tufts. "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: "At the Harvard Forest, and likely throughout the northeast, the seed crop was small this year, suggesting the 2015 maple syrup harvest should be a good one."
Rapp explains weather's role in these predictions: "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% of the variation in syrup production in Vermont from 1998 to 2014."
Because seeds develop a full six months before 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."
"The idea of looking at the costs of seed production came from very abstract models developed by mathematicians in Japan," adds Crone. "One of the purposes of academic science is to come up with general insights that help us see applied problems in new ways. This is a good example of that kind of insight."