Change in day length is the most important factor prompting the onset of fall foliage. As the days grow shorter and darkness increases, leaves stop producing chlorophyll, the green pigment that lets plants convert energy from sunlight. This breakdown allows other colors to take center stage: The yellow, orange, red and brown tones that we associate with fall are always present in leaves, but they are masked by an abundance of chlorophyll during summer’s long days.
Different tree species are associated with different colors. Hickory and sycamore leaves are golden-orange. Ash leaves tend to be yellow and purple. Oaks hold onto their leaves the longest and produce russet-brown foliage. Sugar maples take on an orange or red tone. There is evidence that red leaves are more prevalent when days are warm, dry and sunny, and the nights are cool (but not freezing). Red foliage has also been linked to fungus and drought.
During their lifetime, trees respond to a range of environmental influences, from airborne pollutants to insect pests, many of which can influence leaf color. One year might yield more red and another more gold. The Cary Institute is part of a National Phenology Network project exploring how environmental conditions affect the timing of when leaves change color and fall from trees. Careful monitoring can help answer bigger-picture questions, such as impacts of ozone pollution or climate change.
The Hudson Valley’s autumn leaf show is now on display; a walk on the Cary Institute of Ecosystem Studies’ campus is a great way to view autumn’s splendor. Armchair leaf-viewers can log onto our Tree Cam to see real-time aerial views of changing foliage near the main research building. The Cary Institute’s trails and grounds are open to visitors from 8:30 a.m. to 7 p.m. until October 31, when they close for the winter.
Learn more about phenology, including volunteer opportunities tracking nature’s clock, at www.caryinstitute.org/science/research/research-projects/phenology.
– Lori Quillen
Director of Communications
Cary Institute of Ecosystem Studies
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