Cornellians trudging up Libe Slope or walking along Tower Road can now hear the crunch of fallen leaves beneath their feet and look up at fiery treetops, marking the peak of autumn in Upstate New York.
But this season has seen peak fall foliage well past October, which is two weeks later than the peak season of last year. Prof. Taryn Bauerle, plant biology, explained that while warmer temperatures and increased rainfall contributed to the delay in the leaves changing color, minor variations in weather can be expected year-to-year.
According to Bauerle, the chlorophyll within the cells of plants photosynthesize to provide nutrients for the plants. This molecule appears as the green color while another class of compounds, carotenoids, make the yellow and orange colors that start to become more visible in the fall. While carotenoids are always present in the leaves, they are typically masked by the abundant green of the chlorophyll, and this process occurs every year, Bauerle explained.
“Keeping leaves green is an expensive process for trees,” Bauerle said. “Because our days become shorter in winter, where there is much less light, it becomes less economically feasible to hold onto those leaves.”
Once the days get shorter, the trees stop producing chlorophyll, and over time, the carotenoids within the leaves start to shine through, bringing bursts of fall colors to the forefront, Bauerle said.
But this year, the spectacular fall colors remained longer than usual — caused by an unseasonably warm early fall in Ithaca, according to Bauerle. She added that higher levels of precipitation this past summer and fall could also have caused this delay.
“When you have warmer temps and plenty of rainfall, which [are] optimal growing conditions for plants, the plants might as well continue to make sugars and food,” Bauerle said. “That is probably why we are seeing this delay this year.”
Prof. Timothy Fahey, natural resources and the environment, agreed that unseasonably warm fall temperatures were responsible for the leaves changing color so late, peaking in the Finger Lakes Region last week. Fahey added that leaf abscission, or the falling of leaves, is caused by two main environmental factors — the amount of sunlight during the day, and the dip in temperature at night.
However, early predictions for when fall foliage will reach peak colorization may not always hold true. According to Bauerle, large amounts of rainfall, significant freezing events or drastically lower sunlight could trigger leaves falling sooner than expected.
Even if leaves changed later than normal this season, yearly variation is to be expected as with any weather-related event. Optimal fall weather of moderately warm, sunny days, paired with cool, crisp nights, make for the brightest and boldest fall foliage, according to Bauerle.
Color intensity is also specific to the species of trees themselves. Bauerle says maple trees are known for having vibrant fall foliage with a variety of colors, including spectacular reds and purples, while oak trees never make great fall foliage, transitioning from yellow to brown.
While weather conditions caused minor delays in the onset of colorful foliage this year, Bauerle said major changes to this hallmark of fall are unlikely. However, drastic changes to the timing of fall foliage could be downstream effects from other climate change-driven environmental stresses to forests, such as if droughts or strong storms cause leaves to fall quicker.
“The upward trend of rainfall and warmer temperatures can be expected, but the primary driver of leaves turning color are the shortening of our days in the winter, and that’s not going to change,” Bauerle said.