REGIONAL— It’s not just your imagination. Leaves are changing color later than in the past here in the North Country, and we’re not alone. All across North America, the dramatic colors of fall are coming later than in decades past, driven by the general warming of autumn temperatures as a result of climate change.
Temperature isn’t the only factor that governs the timing and brilliance of the fall color change, but it is one of three factors that most influence the transition. According to University of Minnesota forest ecologist Lee Frelich, day length and soil moisture are two other primary factors that influence fall color.
All three factors clearly played a role in this year’s unusual fall season, notes Frelich. The late summer drought and a cool start to September seemed to spark a more typical maple change here in the North Country, with a peak around the end of the third week of the month. But significant rains in late September and record heat in early October seemed to put autumn on hold for at least ten days, as many maples held their showy leaves and later-turning trees, like aspen and birch, stayed green.
Frelich said some trees in the Twin Cities that had changed in late September actually turned green again as the return of moisture and summer-like heat prompted the trees to restart the production of chlorophyll, which is what makes leaves appear green.
Aspen, which used to peak right around Oct. 2 or 3 on average here in the North Country, peaked this year on Oct. 14, or almost two weeks later than used to be the rule. That’s according to well-known phenologist John Latimer, who is based in the Grand Rapids area. Latimer has documented the aspen peak in the region since 1984 and his data, which he shared with the Timberjay, shows that the aspen peak now comes about eight days later than it did forty years ago. Back in the 1980s, peak aspen color arrived on Oct. 1 on average. Today, it doesn’t arrive until Oct. 9 on average.
His data shows a remarkable amount of variation within that trend, with the earliest peak reported on Sept. 23 in 1986, coming less than a year after the latest peak in his records, reported on Oct. 21, 1985. The late change in 1985 was a distinct outlier at the time, but aspen peaks in the middle of October are increasingly common in the region, while September peaks have become increasingly rare. In the first ten years of Latimer’s record-keeping, aspen peaks arrived in September (average: Sept. 26) six years out of ten. In the last ten years, however, only 2016 saw a September aspen peak, and that came on Sept. 30, the latest day possible.
The impact of climate change is being seen in the trend toward the later arrival of fall colors, according to Frelich, primarily as a result of warming temperatures, particularly overnight lows. The autumn months of September and October are among the fastest-warming months in Minnesota based on recent trends.
Precipitation is a factor in leaf change but there’s no clear signal on whether climate change is making Minnesota drier, or wetter, or simply prone to more extremes. Frelich noted how an exceptionally dry year, like 1976, accompanied by an early late-summer chill, contributed to one of the earliest fall color seasons on record. “Fall color peaked that year around Labor Day,” he recalls.
Climate change is making conditions like experienced in 1976 increasingly unlikely, and that’s not just because of milder temperatures in the fall. He said the increasing level of CO2 in the atmosphere is also likely to keep plants producing chlorophyll later into the season. CO2 levels in the atmosphere have increased from roughly 280 parts per million in the pre-industrial age to approximately 425 ppm currently.
One thing that won’t be affected by climate change is day length and Frelich said that sets an absolute date by which trees will begin to shut down. “At a certain point day length becomes controlling,” he said. He figures by the end of October in most of Minnesota, the daylight hours are too limited to support the production of chlorophyll, no matter how warm temperatures might be. That means even though fall color may come later in the future, it will still be part of the Minnesota experience for years to come.