While the leaves usually start to turn in the northeastern United States within the next few weeks, much of the country won't see the first signs of autumn until mid-October. With cooler weather in the long-term forecast and a little less rain, this year's display has the potential to be perfect.
This week's hurricane Isabel would probably not hurt the autumn leaves' colors, said Bill Hoch, a plant physiologist at the University of Wisconsin at Madison. Strong winds could break off the leaves and heavy rain would remove the drought stress that gives the leaves their color. But since the storm is expected to hit the East coast before the leaves begin changing color, trees probably won't feel the storm's effects very much.
"The weather during senescence is the most important factor in how brilliant the colors will be," Hoch said. The leaves' changing colors are a stress response in trees, as they respond to the normally cooler temperatures and slight drought of approaching winter. "Sunny, cool days are best, but other stresses during this time such as drought also induce more vibrant colors."
Senescence is a seasonal aging period when leaves stop producing chlorophyll and start storing valuable nutrients in the tree for next year before the leaves drop off. Biochemical triggers in the leaves cause trees to stop producing green-colored chlorophyll, their source of energy, and allow molecules that look red or yellow to show through. "The yellow and orange hues are produced by xanthophylls and carotenoids," said Marc Abrams, a forest physiologist at the Pennsylvania State University. "It is the same pigment that makes carrots orange."
Red leaves, on the other hand, are produced by anthocyanins. "Anthocyanins filter light that could otherwise damage the way the plant makes food, which is becoming increasingly vulnerable at this time," said Hoch.
"Anthocyanins act like a sunscreen, an antifreeze, and an antioxidant, qualities that could help leaves deal with an array of environmental stresses," said Paul Schaberg, a plant physiologist with the United States Forest Service. "We are studying stresses on tress such as drought, pollution, acid rain, and cold and we think that the red anthocyanins could be produced by the leaves to help them deal with stress."
The color displayed by the leaf has a lot to do with the species of tree it is, which determines the specific chemicals produced by the leaves during senescence. According to Schaberg, red leaves also seem to have a stronger attachment to the tree than yellow leaves at the same stage of senescence. "The longer the leaves stay on the tree, the more time there is for their nutrients to be sent back into stems for storage for next year," says Schaberg.
William (Bill) Hoch
University of Wisconsin-Madison
Pennsylvania State University
United States Forest Service
American Institute of Physics