Parched September may limit foliage display; trees need rain

University Park, Pa. -- Normally, a dry September in Pennsylvania means bright fall foliage, according to experts in Penn State's College of Agricultural Sciences. But this latest dry spell -- which followed persistent drought conditions across most of the state this summer -- may be bad news for leaf-lovers.

September was extremely dry, by almost any standard. "Most every county is at least 2 inches below normal in precipitation for the month," said Bryan Swistock, water resources specialist for Penn State Cooperative Extension. "It has been extraordinarily dry -- so much so that some counties are likely to set records. In some places, they received just two-tenths of an inch of rain."

Marc Abrams, professor of forest ecology and physiology, suggests that most years, dry weather in September contributes to the brilliance of Pennsylvania's famous foliage display. "I was worried about the drought that we had into early August, which was really quite severe," he said. "But then most of the state received significant rainfall, and the drought watch was called off in most counties. Even with that severe July drought, I didn't notice any early leaf drop or mortality. So we were expecting an impressive fall foliage display."

But then we had a month (September) with virtually no rain, and very little is expected anytime in the near future, Swistock reported. "Some of the state did get a minor storm late in the month, but the long-term weather forecasts for the next 30 days call for an extremely dry pattern to persist," he said. "And I trust the long-term 'Weather World' forecasts developed by Penn State's Department of Meteorology. They get it right.

"But of course the fly in the ointment with predicting fall weather here long term is tropical moisture," Swistock added. "The remnants from one hurricane could change everything. And there is a tropical disturbance brewing in the south Atlantic right now that could affect our weather. Let's hope it brings us plenty of rain without damaging wind and flooding."

From Abrams' perspective, that rain can't come soon enough to benefit the state's fall foliage. "Usually, dry conditions at this time of year promote fall colors, but if the current drought continues for another week or two, I won't be surprised to see early browning, premature leaf fall and less vibrant colors," he said. "This process has already started in Centre County. Trees across much of the state are being stressed by the lack of moisture. This is being exacerbated by the 80-degrees-plus temperatures we are having at the end of September."

Pennsylvania's extreme southwest -- Greene and Washington counties -- never received the early August rains that broke the drought's grip on the rest of the state. They are still under a drought watch issued by the state's Department of Environmental Protection. The state's southeastern corner is also extremely dry now, with the Philadelphia area getting virtually no rain for about a month. Abrams pointed out that some weather-related things must happen in coming days and nights, regardless of the drought conditions, for the foliage display to reach its potential. "First, it must cool off," he said. "Night temperatures dropping into the 30s and 40s are good for the foliage. The worst thing that could happen for the foliage would be if it stayed hot. We want to see bright, sunny-but-cool weather."

For two decades, Abrams has studied how seasonal precipitation and temperature influence timing and intensity of fall colors in central Pennsylvania. He believes that clear, bright days and low -- but not freezing -- temperatures and dry conditions promote the best fall colors. But not this dry.

"Cooler temperatures signal deciduous trees to stop producing chlorophyll, the gree n 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 noted. "The anthocyanins create the brilliant reds and purples seen in maple, sassafras, sumac, black gum and oak."