Climate Change & Foliage

I spent a few days in the Berkshires of Western Massachusetts last week, where fall was just starting to show up. I hadn’t really thought too much of the colors I saw on the surrounding ridgelines, which seemed like the usual mixture of greens with the occasional red or yellow upstart shining through, until I overheard a woman describe this fall as being “kind of meh”. Let me acknowledge up front that my former environmental educator, nature-is-all-that’s-good-in-the-world, reaction was to be somewhat affronted by the use of a single rather derogatory syllable to describe one of the greatest shows on Earth. So I borrowed her perspective for a moment, took another look around, and began to wonder if what she saw as “meh” was really just a forest that hadn’t quite realized it was fall yet. This fall isn’t boring…it’s late.

The fact that the leaves of deciduous New England trees (and one conifer—here’s looking at you, tamarack) change color and fall to the ground in autumn at all is worth more than a meh. Changing day length is the primary driver for this process but there are other factors at play, such as temperature and precipitation. And although we humans haven’t managed to change the amount of daylight the Earth receives in a single day (yet), we are having an impact on those other two. Anthropogenic climate change is altering both average temperatures and the amount and intensity of rainfall across New England. According to the National Oceanographic and Atmospheric Association, New England has seen an increase of 0.7˚F to our average temperatures over the last 100 years. Increases of anywhere between six and 10 degrees F are predicted in the next 100. Rainfall is predicted to arrive in intense, periodic bursts rather than the slower, steadier rainfalls we see now. Those bursts will punctuate longer periods of drought, meaning longer periods of stress to our forests. The amount of temperature change plus the degree of drought will lead to differences in the timing and colors we experience, and which people pay millions of dollars annually to view.

A University of Connecticut study examined these factors and noted distinct differences in how Northern New England would experience rising temperatures versus Southern New England. In Northern New England, the maple and birch-dominated forests showed later leaf color changes. These trees are sensitive to changes in moisture and climate, and respond to drought and heat by delaying the dormancy that leads to fall colors. Rising temperatures will also mean periods of drought between intense rain events, rather than the more evenly-spaced rain storms we see now. The moderate levels of drought further north could also delay foliage changes. In some cases, moderate drought means more vibrant reds as red maples show stress from lack of water, but this only happens when the nights are cool enough to signal to these trees that it’s time to switch from chlorophyll to anthocyanins, the source of those reds. Warmer nights don’t provide that cue, so maples delay color change until nights are cooler. Sugar maples are also sensitive to temperature changes, but the moderate warming in Northern New England should allow this species to stay in the mix, especially at higher elevations.

Southern New England will have faster-rising temperatures, which could lead to earlier color changes and leaf drop. Oaks are a significant part of these forests and are more drought-tolerant than the birches and maples further north. Because temperatures will rise faster here, drought is also predicted to be more extreme. This level of stress in trees means early color in a short burst, followed by rapid leaf drop. Certain years of extreme drought resulted in trees skipping that vibrant wave of yellows and oranges, going straight to dry brown leaves that fall off almost immediately upon color change. Sugar maples may disappear from Southern New England forests entirely, or perhaps persist only in cold pockets and higher elevations.

An October in New England full of shades of brown and bare branches is a sad future to contemplate. The end of chlorophyll production would no longer mean the beginning of leaf peeping season, no longer bring visitors to see the spectacle of hillsides and valleys adorned in fiery shades as trees prepare for the harsh winter ahead. But beyond the economic value of tourism, the loss of fall color signals a New England forest out of step. Our deciduous trees evolved this strategy over millions of years as a way to maximize resources while heading into a lean and challenging time. I’ve heard hardened Mainers joke about the relief climate change might bring, as if the only result would be less plowing and no need for multiple jackets just to survive a trip to the grocery store (truly, spoken like a species that evolved in a much, much warmer environment). But our tree species need those cool falls followed by cold winters. They need steady rain through the spring and fall. Take those factors away and we risk losing more than a few extra layers of clothing. We risk losing that which characterizes the very place where we live, that last gasp of color before we fade into the grays and whites of winter, that glory spread equally along highways and rural roads, along meadows and deep into the woods. And without those trees, we lose insects, birds, on and on. We lose the songs that break winter’s silence, we lose the trills that signal warmer days. We lose the flashes of color and movement that reveal long, strenuous migrations. We lose beauty and resign ourselves to “meh”.