Leaves show their fall colors in Jericho in September 2020. File photo by Glenn Russell/VTDigger Josh Halman flew over Vermont’s woods late last summer and saw something that surprised him. Swaths of trees had already gone brown, scorched by an extreme drought that affected more than 64,000 acres.  This year, from the same vantage, things are looking up. “We haven’t seen anything even close to the level of damage we were seeing in the foliage last year,” said Halman, who leads a long-running tree-monitoring project at Vermont Forest Parks and Recreation. The rebound is good news for the leaf-peepers who pour into Vermont each fall. But for the foresters who study those same trees, the annual turning of the leaves is more than a spectacle. It’s a record. It shows, in real time, how a warming climate is quietly rewriting the rules by which Vermont’s forests live. After last year’s damage, experts say this season is largely back on track, though a few of the hardest-hit spots are turning early. And the reason the leaves turn at all is a process more precise, and more sensitive to weather, than most roadside admirers realize. “It is showing right in our backyard in Vermont that climate has an influence on how long trees, or sugar maples specifically, are able to be growing in a given year,” Halman said. His team has tracked individual sugar maples at two moments each year — when leaves emerge in spring, and when they turn in fall — since 1991. The stretch between those two dates is the tree’s growing season, and over three decades of data it has grown steadily longer. Last year, he said it ran 21 days longer than when the project began. A physical science course refresher That annual autumn process has a name. Senescence is the orderly shutdown that deciduous trees — the broad-leaved maples, oaks and birches that drop their leaves each year — go through every fall. Leaves are costly for a tree to grow, but they’re also how they turn sunlight into energy through photosynthesis. Chlorophyll in the leaves’ cells captures the light, and that chlorophyll is what makes summer leaves green. As days shorten and temperatures drop, the tree begins pulling its investment back inside. It draws nutrients — calcium, magnesium, potassium, nitrogen — out of the leaves and stores them in the roots and wood for next spring. As the green chlorophyll drains away, the oranges and yellows underneath are revealed.  They were there all along, just hidden. When the timing slips That much has held true for as long as there have been maples. What’s changing is the timing. A longer growing season might sound like a gift. But it comes with a catch, said Ali Kosiba, an assistant professor with the University of Vermont Extension Forestry Program. She studies how climate change is altering fall foliage. The amount of daylight, she said, declines on the same schedule every year, no matter the weather. Temperatures, on the other hand, are staying warmer later into the fall.  That mismatch makes for a difficult tradeoff. If they drop their leaves on schedule with the fading light, they could miss out on weeks of mild weather they could have spent making food. Hang on to them too long, and a sudden cold snap or violent storm can strip them before the tree has reclaimed those valuable nutrients. The Irasburg United Church framed by maple foliage in September 2020. File photo by Glenn Russell/VTDigger But, Kosiba said, trees are generally pretty good at making those tradeoffs. “Trees are really programmed to take advantage of conditions that benefit the amount of photosynthesis they can do and how much growth they can do in a year,” Kosiba said. “We are seeing that trees are responding to warmer fall conditions.” Fall foliage, she noted, is one of the less-studied corners of forest ecology, and researchers are still untangling what governs it. But the long-term drift is clear.  Weather also plays a part in the year-to-year variation. A drought one season can shape the colors the next. This year, Kosiba said, foliage might actually run a day or two early despite the broader slide toward later autumns.  Local conditions matter too. Trees in nutrient-poor wetlands are among the first to turn, which is why some are already showing color now. Even neighboring trees can turn at different times, depending on their genes, their soil and the storms they happen to weather. Halman, for his part, says he hasn’t seen the longer growing seasons harm the health of the forest. A healthy tree, he noted, can usually recover from a bad year. This year’s browning was minimal. What the foresters saw last year was not just senescence, but drought killing leaves outright, before they had the chance to turn. For anyone hoping to catch the show, Kosiba’s advice is to chase it. Vermont’s coldest regions, mapped by the state’s plant hardiness zones, which rank areas by how cold they get, peak first in late September. Each zone to the south follows about a week later, so the color rolls down the state over several weeks. “Travel around,” she said, “looking for different views.” Read the story on VTDigger here: Fall foliage varies from year to year, for leaf-peepers and trees. ...read more read less