3 min read

While satellite models show New England’s foliage arriving a week later than it did 70 years ago, Maine’s weekly forest ranger maps reveal a climate system in flux, with peak dates swung wildly between early drought-induced leaf drops and warm, delayed autumns.

This shifting schedule is watched closely by Maine residents, for whom the vibrant crimson and gold canopy is cherished, and by leaf peepers who pump nearly a half-billion dollars in economic activity into Maine’s tourism economy each fall.

How leaves change color every autumn is dictated by plant biology.

As the days grow shorter and cool temperatures arrive, deciduous trees stop pumping out green chlorophyll, allowing underlying yellow, gold and red pigments to finally be revealed. However, chlorophyll breakdown is slowed by rising fall temperatures, keeping leaves green into October.

But ecologists say temperature is only one piece of the puzzle. Seasonal foliage timing and color brilliance are also heavily influenced by rainfall, soil moisture and pest outbreaks, which is why year-to-year ranger reports can mask the long-term trends in Maine’s changing foliage.

Comparing weather and climate is akin to mood versus personality. Daily moods can be easily swayed by immediate events, much like short-term weather anomalies. In contrast, a person’s underlying personality is formed gradually over decades, much like long-term climate baselines.

A review of the state forest rangers’ weekly foliage progression reports underscores the year-to-year variability. In 2020, after a severe summer drought, more than half the state’s seven foliage zones peaked in late September. It would be another six years before any zone peaked so early.

Tree stress caused by severe summer droughts often triggers premature browning and leaf drop, creating the appearance of an early peak. Conversely, fungal blights promoted by heavy fall rain and sudden windstorms can denude entire canopies before peak vibrancy.

How can the long-term delay in fall color be confirmed when the ranger reports look so chaotic?

The underlying shift was uncovered by geographer Stephanie Spera through research at Acadia National Park. She used decades of satellite imagery alongside historical diaries and park photos to strip away annual weather noise.

She identified a distinct pattern: Peak foliage at Acadia, which would fall into Maine’s Zone 2 in the statewide fall foliage map, was arriving nine to 10 days later than it did in the 1950s, or about one day later with each passing decade.

Acadia has warmed by 3.4 degrees Fahrenheit over the last century. One out of five plant species documented in the park a century ago are gone. Seven of the park’s 10 most common tree species are expected to decline or possibly even disappear by 2100.

Climate change is driving not just rising temperatures, but several of the other variables that play a role in Maine’s annual foliage show. The future climate will likely see more more droughts and deluges play out in the same calendar year, according to the Maine Climate Council.

Because fall nights are warming across Maine, trees are staying active later in the season. Thus, while year-to-year peak foliage dates remain at the mercy of the fickle weather moods, Maine’s climate personality is predicted to push peak colors deeper into the calendar over time.

Maine’s modern foliage tracking traces back to 1959, when then Forestry Commissioner Austin Wilkins ordered weekly foliage updates from state forest rangers. These early assessments were radioed to state headquarters and manually distributed to local reporters by phone.

In 1996, Maine became the first state to post weekly leaf conditions on the web. Over time, these reports have established a baseline for seasonal environmental tracking and created a marketing tool for the state’s growing leaf-peeping economy.