August drought, cool September bring out early fall colors

University Park, Pa. -- A severe August drought followed by cool temperatures in early September resulted in many pockets of early fall color in central and northeastern Pennsylvania and southeastern New York, according to a forest expert in Penn State's College of Agricultural Sciences.

Peak colors normally occur in mid-October, but by the middle of September this year a significant number of trees already were turning red, orange and yellow. These include red maple, sugar maple, ash, black gum and sumac, thanks to the unusual weather conditions over the last six weeks.

"Luckily, this has occurred in a relatively small, but noticeable, number of trees in the region. Therefore, we should still be set up for awesome foliage this fall -- but it is not a sure bet yet," said Marc Abrams, professor of forest ecology and physiology. "Trees enjoyed extremely favorable growing conditions this year and seem to be very healthy in general. But the most important thing for foliage colors is what's going to happen in the next few weeks."

According to Abrams, rainfall for the first three weeks in August was nearly 2 inches, or 23 percent, below normal and temperatures during the first half of September were 5.8 degrees below normal. Those are significant variants, he pointed out, that had a major influence on some trees.

Although many wish for Indian summer this time of year -- with hot autumn days and mild nights -- Abrams warned that those who want to see really brilliant foliage in October should hope for cooler weather. "Fall weather is very important," he explained. "We need to have cool temperatures falling into the high 30s or 40s at night, as well as bright sunny days with little rain."

If temperatures cool in coming weeks with little rain falling, fall foliage watchers in Pennsylvania should be treated to a superior display of color this year, Abrams predicted. "If we cross our fingers and Mother Nature cooperates, we should have great color by the middle of October," he said.

For two decades, Abrams has studied how seasonal precipitation and temperature influence the timing and intensity of fall colors in central Pennsylvania. "We believe that clear, bright days, low -- but not freezing -- temperatures and dry 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 anthocyanin, Abrams explained. 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. Much of Pennsylvania experienced a significant drought this summer, but Abrams doesn't believe most trees were stressed enough to significantly impact fall colors.

This year's drought, however, may cause some highly stressed trees to change color earlier than normal or to display rather dull coloration. Overall, there is still a high potential for most trees to show their true, vibrant colors this year, Abrams predicted.