On a brisk late February day, some bulge with maple sap while others sport little sap icicles. The blue bags are a little less romantic than the metal pails that Rapp's forbearers hung off trees, but they have the benefit of being easily weighed and transported.
Measuring those blue bags is crucial for Rapp, a scientist at Harvard Forest in Petersham, Mass., who is studying the impact climate change is having on syrup availability and quality. Sugar maples generally have to be about 40 years old before they're ready to be tapped, which means the knowledge scientists are building now incredibly valuable for the future of the industry.
"We've been having huge flows early this season," Rapp said, standing next to a maple syrup boiler as a sweet steam rose from the bubbling vat of syrup through the vent in the roof of the sugar shack.
In what has been the warmest winter on record for New England, producers tapped trees two to three weeks earlier than usual. A little to the east of his sugar shack, Rapp said it was the "first time in memory" producers tapped their trees in January, an annual ritual usual relegated to the second or third week of February.
Maple syrup production is intimately tied to the weather. Sap only flows when temperatures rise above freezing during the day and drop below it during the night. That temperature difference creates enough pressure to push sap out of the tree - one of nature's amazing feats.
But warming driven by human-caused climate change is having an impact on these natural processes. The easiest one to spot is the shift in maple syrup season as winters have warmed by about 1°F (0.6°C) per decade across much of the Northeast since 1970.
"In general over New York and New England, the season is now beginning about seven days earlier than it did 40-50 years ago and ending 10 days earlier," Timothy Perkins, the director of the University of Vermont's Proctor Maple Research Center, said on a phone call.
New York and the New England states account for about three quarters all the maple syrup produced in the U.S., an industry worth $117 million in 2014. While a change in the dates of the season is notable, it's not exactly going to bankrupt the industry.
Warming on the southern fringe, however, is a major concern. Virginia syrup producers are already tapping at the coldest time of the year and future warming could make even that time unsuitable for maple syrup production.