UNIVERSITY PARK, Pa. -- Don't be discouraged by gloom and doom stories, says an expert in Penn State's College of Agricultural Sciences. Although the drought has affected some sensitive trees, we still have the potential to see good fall colors.

"The bad news is that some of our very important color-producing species -- red maple, black gum and the birches -- are starting to turn early," says Marc Abrams, professor of forest ecology and physiology. "This is a classic drought response. So we may see a less defined peak in the second week of October. But the good news is that only 10 to 20 percent of the trees have been affected. The real test will be whether we get the normal cool-down period this fall."

For 13 years, Abrams has studied how precipitation and temperature influence timing and intensity of fall colors. He's found that the critical period for coloration runs from the middle of September to the second week of October. "At that point, we need nice cool nights in the high 30s and low 40s -- but not a hard frost -- and bright sunny days," he says.

Cooler temperatures signal deciduous trees to stop producing chlorophyll, the green pigment responsible for photosynthesis, he explains. Photosynthesis is the way plants trap light energy and convert it to sugars and starches, the food and building materials for plants.

As the chlorophyll breaks down and disappears, it unmasks other leaf pigments. It's these other pigments -- called xanthophylls and carotenes -- that create the glowing yellows and oranges seen in the leaves of yellow poplar, hickory, sycamore, honey locust, birch, beech and certain maples.

After chlorophyll production stops, trees also produce another pigment in their leaves called anthocyanin. The anthocyanins create the brilliant reds seen in red maple, sassafras, sumac and black gum.

"Lab and greenhouse research indicates that more anthocyanin is produced when starch levels in the leaves are high," Abrams says. "Because drought reduces photosynthetic rate -- which in turn decreases starch levels -- some people believe that drought will affect fall coloration.

"But I'm not convinced," he says. "We've had a number of droughts in the past 13 years, and as long as we get that nice cool-down period, starch levels have been adequate to produce good color."

One thing Abrams has found to be very detrimental to fall colors is a very warm late summer and early fall. "The trees get confused," he says. "They try to stay green longer, to capture the longer growing season. This may be good for the tree, but it's not good for peak coloration."

So it's really too early to say what will happen this year. "But keep in mind that fall coloration is very resilient," Abrams says. "Over the range of climatic conditions and challenges we've seen, we've still had good fall colors."

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EDITORS: For more information, contact Marc Abrams at 814-865-4901.

Contacts: Kim Dionis KDionis@psu.edu 814-863-2703 814-865-1068 fax