Is Climate Change Ruining Fall?
New England displays some of the most dramatic fall color in the world. The crowds of leaf-peepers who come here to see color in the mountains also show up to take in Harvard’s yellowwoods, maples, oaks, and honey locusts. But foliage hunters may have noticed that lately the Yard has been looking greener than it ought to at the end of September. Temperatures reached the high eighties this past weekend and earlier this week—and the National Weather Service forecasts above-average temperatures for most of the country for the rest of the year. Trees’ leaves have barely turned. “This is a year in which we’re having a later fall than usual because we have this combination of moderate temperatures and moderate rainfall,” says Richard Primack ’72, a professor of ecology at Boston University (who held a Putnam Fellowship at the Arnold Arboretum in 2002, and conducts research at the Harvard Forest). “This is a very typical year for the effects of climate change.”
People may associate global warming with scorching high temperatures, like the 100-plus-degree heat wave that hit parts of Europe this July. But daily lows—the temperatures reached as the air cools overnight—are warming up much faster than highs. And these nighttime temperatures, which humans may not notice as much as daytime highs, are particularly important for signaling to deciduous trees that it is time to drop their leaves in the fall, as they sense that cold weather is coming.
“As the days get shorter and nighttime temperatures get cooler, that usually signals to plants to go into what we call dormancy,” says Elizabeth Wolkovich, an associate professor at the University of British Columbia and visiting scholar at Harvard’s department of organismic and evolutionary biology, where she was an assistant professor from 2014 through 2017. “That allows them to start protecting their tissues, so when it gets really cold they don’t get hurt....They’re trying to pull back a lot of the resources in [their] leaves and bring them back into the wood structure and protect them.” This process also breaks down green-colored chlorophyll, revealing the yellows, oranges, and reds of fall.
Nighttime fall temperatures in Boston have been markedly higher in the last decade than they were 70 years ago. Between 1940 and 1948, the mean daily low for the period spanning September 1 - October 31 was about 51.5 degrees Fahrenheit. Between 2010 and 2018, the mean daily low for the same time of year was nearly three degrees higher: 54.4. The difference in daily highs between these two periods is less pronounced: 68.5 degrees in the 1940s, and 69 in the 2010s. (The data used in these calculations are downloadable from the National Centers for Environmental Information.)
Even those years that had higher daily highs in the 1940s than in the current decade still tended to have lower lows: climate change has significantly narrowed the gap between daily highs and lows. That comports with what is already known about greenhouse gases: they act like a blanket to prevent nighttime cooling, as heat radiates back into space. Nationwide, falls are getting warmer, and it’s happening more quickly in the Northeast, according to an analysis by Climate Central. Between 1940 and 1948, using the same Boston data cited above, there was an average of seven nights in October when the temperature dropped to 40 degrees or lower. Between 2010 and 2018, there was an average of four.
“Certainly we know fall is getting later with climate change,” Wolkovich explains. During periods when “it’s super hot and you’re not getting cold nighttime temperatures, we expect that that makes the chlorophyll not break down as well. And if the chlorophyll is not breaking down, you’re not going to see the other amazing colors...and then the horror is that if the leaves are still brown or green, and you start to have wind or rain, you’ll lose the leaves before you even have fall color, or you just won’t have good color because the chlorophyll will never fully break down.”
Day Length and Other Factors
Research into climate change’s complex effects on fall has increased significantly in the last few years, Primack says, just as its impact has become more apparent in daily life. In addition to temperature changes, trees use day length to sense when they should begin this process of senescing. But a 2015 meta-analysis showed that at lower latitudes in the Northern Hemisphere, which includes Boston and all of the continental United States, signals from temperature seem more important than day length—and senescence has been delayed more at these latitudes than at those farther north.
Rainfall is also a critical factor. “The ideal conditions for fall colors usually are not too much rain, a slight drought. That’ll often help plants pull resources back into their core,” Wolkovich explains. The Northeast has become wetter overall with climate change, especially in the fall and spring, which contributes to trees’ delayed dormancy. But in years when precipitation is unusually low, the opposite can occur: in 2016, for example, drought conditions led to a premature fall. “We had much higher than average temperatures combined with low amounts of rainfall, or no rainfall, in August and September,” says Primack. “The trees started to undergo senescence in August. We started having pretty dramatic color change in September….If you went out into the forest in August in 2016, the ground was covered with yellow leaves….That was because of very high degrees of heat stress and drought stress.”
Dulling Distinctive Colors
New England falls are special in part because this region’s native tree species naturally display brighter colors than trees in similar deciduous climates around the world, Primack says: “Eastern North American maples show more red color than maples from Europe and Asia.” Why might that be? The theory is that “New England has one of the most variable temperate climates of anywhere in the world.” Seasonal weather varies year to year here more than it does on the other side of the Atlantic, he explains. “In the fall, it’s unpredictable when the first frost is coming….Plants react to that by being cautious.” They’ve evolved to break down chlorophyll early, undergo senescence gradually, and produce more pigments.
These trees’ ecosystems are increasingly being changed by the proliferation of insects not native to the region, which often thrive in warmer temperatures. Among the preferred foods of Asian long-horned beetles, for example, are maple trees, and this could amplify the devastating effects of climate on fall foliage. The emerald ash borer, another beetle species, is killing ash trees, “one of the most important trees for fall foliage,” Primack says. “They often have very bright yellow or orange or sometimes kind of a purply foliage.”
These insects’ populations are checked by cold weather. But as temperatures rise, Primack adds, they’ll become more abundant and spread farther north: “That’s the really scary scenario of a warming climate.”