What Autumn Leaf Fall Reveals About Global Warming

New England without colorful fall foliage? It’s hard to imagine, but it’s a real possibility if global warming isn’t checked. That’s because sugar maples, whose vibrant hues bring billions of tourism dollars into the region’s economy, don’t thrive in very warm conditions.

To help scientists predict the effects of global warming, Pamela Templer, a Boston University College of Arts & Sciences (CAS) associate professor of biology, and her PhD students in the CAS Biogeoscience Program, conducted a two-year study that provides a clearer understanding of how trees respond to temperature change. Forests “are so important, both naturally, in terms of their plant and animal life, and economically to the people of New England,” she says, “so I think it’s important to understand how they’re changing.” She and the students published their study results in the Annals of Botany in 2015.

Templer, who directs BU’s Biogeoscience Program and received a Metcalf Award for Excellence in Teaching from the University in 2015, has been studying how trees respond to warmer spring and summer temperatures and to soil freezing in winter. Until now, she hasn’t spent much time thinking about how they respond to temperatures in fall, which she calls an “understudied period of the year.” To help fill the gap, Templer and the students in her biogeoscience course examined the link between temperature and leaf senescence—a phenomenon that’s often defined by 50 percent leaf fall and that signals the end of the growing season—in deciduous trees in the northern hemisphere. They found that warmer temperatures are postponing the end of trees’ growing season by roughly 0.33 days each year and that October temperatures are the key predictor of the date when trees lose half their leaves. The warmer an October, the longer trees hold on to their leaves.

They also discovered that where trees are located determines how strongly they respond to changes in temperature and to the amount of sunlight they receive. Northern hemisphere trees that are closer to the equator are more likely to postpone senescence because of warmer temperatures than trees farther north. Trees in higher latitudes are also more prone to shed their leaves because of the lessening amount of sunlight in fall.

Understanding the mechanisms behind trees’ growing season is important for predicting the impact of climate change on New England and the rest of the country, including “how forest carbon storage will change in a warmer future,” says study coauthor Allison L. Gill (GRS’18). Currently, forests in the Northeast take in more carbon than they release, but warmer temperatures could change that, says Templer. A longer growing season allows trees to take in more carbon, she says, but it also means they—along with the soil and any critters, from bacteria to insects, inside—are metabolizing at a higher rate than in winter, which causes them to release carbon. There’s a possibility that this could cause northeastern forests to start releasing more carbon than they take in. “One of the major drivers of climate change is the concentration of CO2 in the atmosphere,” says Templer.

Warming isn’t just a menace to the environment; it also poses an economic threat. In the maple syrup industry, for example, which Templer says requires “below-freezing nights followed by warm days” for tree sap to circulate, “the tapping season is getting earlier and earlier.” She points to data that Michael Farrell of Cornell University has collected in Uihlein Forest in Lake Placid, New York. His work shows that the tapping season has shifted about a week earlier over the last 30 years, leaving three to four fewer days for maple sap production. While the industry can adapt for now, Templer wonders “if one day that tapping period is going to get so early that it just disappears.”

Templer hopes her study’s findings will help produce more effective terrestrial biosphere models, which scientists and the government use to predict the impact of climate change. The more information is known about how the Earth responds to rising temperatures, the more precise these models will be—and the bigger role they can play in shaping policy on issues like carbon trading and carbon taxes.

In their paper, Templer and her students also recommend that scientists adopt a more standard definition for the date of leaf senescence. “We found huge variation in the methods different researchers used,” Gill says. Templer recommends scientists use the date of 50 percent leaf fall, the most common measurement in the studies she and her students examined, and leaf color change—which she says is an even stronger predictor of senescence, despite the subjectivity involved. If money were no object, she’d suggest widespread use of PhenoCams, digital cameras placed in forests that record time-lapse images of foliage, and Eddy Flux towers, which measure the carbon dioxide exchange between the forest and the atmosphere.

As it is, scientists often rely on a more mundane, and inexpensive, tool that they place under trees to collect fallen leaves to be dried, weighed, and used in their calculations: a laundry basket.