Expert: Drought conditions to dampen foliage display

University Park, Pa. -- If it seems to you, as you drive around Pennsylvania, that the leaves on many trees began turning colors early this year, it's because they have, according to a forest expert in Penn State's College of Agricultural Sciences.

On the heels of one of the hottest, driest summers on record for Pennsylvania, hardwoods across the state began going from green to gold, orange, red and purple -- and to dull brown -- in mid-September. "It has been so dry, and trees in some areas are so challenged by drought conditions, that their leaves just went straight to brown and are falling off the branches already," said Marc Abrams, professor of forest ecology and physiology.

"I think we have to lower our expectations on what we hope to see this year because the climate has put tremendous stress on some tree species," he said.

All of Pennsylvania is under a drought watch, and state officials recently declared a drought warning for 24 counties. "Throughout most of the state, counties are 4 to 10 inches below normal rainfall levels," said Bryan Swistock, water resources extension specialist in the college's School of Forest Resources. "The driest areas are the two ends of the state -- counties along the New Jersey and Ohio borders, and a few counties in the southwest around Somerset."

Swistock pointed out that for Pennsylvania to break out of the summer-fall drought, the remnants of a tropical storm or hurricane must track over the state. "There is a lot of tropical moisture around, but so far none of it has gotten to us," he said. "But there will be plenty more opportunities in the next couple months to get prolonged rains as tropical storms form in the south Atlantic and travel north."

Those rains, however -- if they come -- will be too late to do much for the foliage. Still, Abrams noted, Pennsylvania's forests are amazingly resilient, so there will be some colorful foliage in mid-October. "There remains the possibility for decent fall color despite the difficult climate," he said. "But clearly this won't be one of our better years."

For more than two decades, Abrams has studied how seasonal precipitation and temperature influence timing and intensity of fall colors in central Pennsylvania. "We believe that clear, bright days, low but not freezing temperatures, and dry but not drought conditions promote the best fall colors," he said.

Cooler temperatures signal deciduous trees to stop producing chlorophyll, the green pigment responsible for photosynthesis, he explained. The chlorophyll breaks down and disappears, unmasking other leaf pigments. It's these other pigments -- called xanthophylls and carotenes -- that create the 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 santhocyanin, according to Abrams. The anthocyanins create the brilliant reds and purples seen in maple, sassafras, sumac, black gum and purple oak.

The amount of anthocyanin produced each year is related to starch levels in the tree. Trees often produce less starch during droughts, such as the one the state is experiencing.

"One thing that I have been impressed with in my 20 years of gauging foliage is the resiliency of the display," Abrams said. "Year after year, despite the conditions, there are places where the trees show good color, but perhaps not great color.

"People should go out and search for those pockets of bright color, because they will be there. They just may be a little harder to find this year than usual. If we do get some rainfall in the next couple weeks, that should help brighten the colors."
 

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Last Updated November 18, 2010