Last week I had the pleasure of picking apples with my son. Fruit covered the trees and colorful leaves were underfoot, but the weather was far from crisp. Here in the Hudson Valley, October has been mild. According to data collected at the IES Weather Station, our mean temperature has been above the fifty year average, and we've broken maximum temperature records three times.
Northeasterners cherish fall. It's a season of harvest, a time for pumpkins and pie. Our forests turn a blaze of color and, for a few weeks, we are dazzled by nature's march into winter. This reverence for fall has led a number of people to contact IES, concerned that the warm weather trend might be having a negative impact on fall foliage.
The color palette that emerges from the dying leaves of deciduous trees is a complicated process. A short warm spell is unlikely to result in a suppression of color. As a matter of fact, warm days and cool nights can actually increase the amount of red that we see.
Leaves change color in the fall when trees stop producing chlorophyll, the pigment that keeps them green. Chlorophyll is abundant during the spring and summer, when leaves turn water and carbon dioxide into glucose by harnessing energy from the sun. This process, called photosynthesis, is essential to tree growth.
Change in day length is the most important factor regulating fall foliage. As the days grow shorter and darkness increases, trees begin to shut down their glucose factories. Leaves stop producing chlorophyll and the chlorophyll remaining in leaves breaks down. As this happens, the green begins to fade and other pigments take center stage.
Yellow, orange, and brown tones are produced by carotenoids. These pigments, which play a role in photosynthesis, are always present in leaves. Their color was just masked by an abundance of chlorophyll. Red and purple hues, called anthocyanins, are produced in the fall. They form when sugars and proteins in leaves react with sunlight. You might notice that vibrant crimson canopies occur in sunny spots.
Different species develop different characteristic 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.
The ecology of red leaves is cloaked in mystery. There is some evidence that they are more prevalent when days are warm, dry, and sunny, and nights are cool (but not freezing). Research suggests that anthocyanin pigments protect leaves from sun damage, allowing trees to conduct a last gasp of photosynthesis. Red foliage has also been linked to fungus and drought.
It is important to remember that when we observe the foliage coloring on a hillside or valley, we are seeing a complex mix of individual trees. During their lifetime, trees respond to a myriad of environmental influences, from airborne pollutants to insect pests, many of which can influence subsequent leaf color. One year might yield more red, and another more gold.
The brightness of the day, the presence of clouds, or the position of the sun can influence our perception of fall's vibrancy. Memory is also a fickle; our moods and life experiences can shape how we recall the colors of past autumns. My advice: go out and enjoy nature's display while it lasts. And, if you are lucky, you might not need to pack a jacket.
The next EcoFocus will explore what happens to leaves once they hit the ground! Decomposing leaves supply vital nutrients to forest ecosystems.
Lori Quillen is the Communications Director at the Institute of Ecosystem Studies, Millbrook, NY. |
