Subdued hues: Bad news for autumn leaf-lovers


UNIVERSITY PARK, Pa. – Normally, a dry September in Pennsylvania means bright fall foliage. 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.”
Just teasing. 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.
“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 reports. The long-term weather forecasts for the next 30 days call for an extremely dry pattern to persist.
Tropical unpredictability. “But of course the fly in the ointment with predicting fall weather here long term is tropical moisture,” Swistock said. “The remnants from one hurricane could change everything.”
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.
Trees across much of the state are being stressed by the lack of moisture. This is being exacerbated by the 80-degrees-plus temperatures at the end of September.
Still in drought. 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.
What to hope for. Abrams points 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.
The science. “Cooler temperatures signal deciduous trees to stop producing chlorophyll, the green pigment responsible for photosynthesis,” he explains.
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 notes. “The anthocyanins create the brilliant reds and purples seen in maple, sassafras, sumac, black gum and oak.”

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