Sam Levinson of Squirrel Hill and a senior Carnegie Mellon University student, heads off after taking photos of the fall foliage on Flagstaff Hill in Schenley Park.
By David Templeton / Pittsburgh Post-Gazette
Unusually warm October weather and less September rain explain why leaves failed to produce brilliant splashes of gold, orange, red and purple, with many remaining green into the first week of November.
It also raises the spectre of climate change.
Every year has seasonal variations, but some scientists say this year may be a harbinger of a more likely occurrence in coming years -- warmer temperatures pushing back the peak foliage season from the third week of October to later in the month or even early November. Such a trend also forebodes duller leaf coloration.
Warmer fall temperatures and resulting duller leaves also signal that local tree species, including sugar maples, will begin migrating northward with other plant and animal species, in search of ideal climate. More extreme temperatures, storms and droughts are anticipated.
"This is precisely the sort of thing we expect to happen," said Penn State University climatologist Michael E. Mann. "Fall comes later, spring gets earlier and summer gets hotter. NASA just reported that the globe just saw the warmest September ever."
In coming decades, he said, extreme weather conditions and warmer autumns "will become the new normal."
"What we are seeing is the loading of the dice," he said. "How late any particular fall is, or how warm the early spring is, is random. But climate change is loading the dice so there are more sixes. We've erased the one and replaced it with a six. Now we have two sixes, which means you roll a six on average once every three rolls rather than once every six. That's twice as often."
The Appalachian State University biology department has published a position paper that duller autumn colors portend problems.
"Although less brilliant fall foliage displays may not rank high on the list of concerns about global change, those muted colors could be the canary in the mine shaft telling us that these shifts could be markers for more subtle and potentially more consequential changes in our world," states its essay, "Will Global Climate Change Affect Fall Colors?"
"A tree stressed is a symptom that something larger is wrong with our world. Our civilizations depend on healthy, functioning ecosystems for services such as clean water, erosion control and clean air, not to mention food, fiber and shelter."
In October, the National Weather Service reports, temperatures were well above normal, with an average high of 65.1 degrees (2.5 degrees above the normal October high of 62.6) and average low of 47.5 (4.6 degrees above the normal low of 42.9). The monthly average of 56.3 degree was 3.6 degrees above the normal of 52.7 degrees.
September had normal temperatures but only 2.31 inches of precipitation, or nearly an inch below normal.
"Where it gets interesting is the 4-degree anomaly in October. Only one degree can be a factor," Mr. Mann said. "Whether it was the extreme heat in the summer of 2012, which was the warmest year that the United States has ever seen, it's an indication that we are seeing the loading of the dice. We still will roll ones and twos, but this fall we rolled a six."
Wilkins native John R. Seiler, a forest biologist at Virginia Tech University, said seasonal variation explains the late leaf color change and falloff this year and was less inclined to blame climate change. But he said climate change is a potential factor. "If it stays warm, that's going to drag the process later into the season," he said.
Length of daylight is key to color change and leaf fall. The underlying color of leaves is yellow but photosynthesis produces chlorophyll in warm months that results in the dominant green. When photosynthesis declines in fall, many leaves return to the base yellow before browning and falling. Red, though, is sunscreen for certain leaves, explaining why bright autumn sun creates bold red colors.
"The current theory is that red acts like sunscreen while the tree is pulling nutrients from the leaves for winter," Mr. Seiler said. Cold but sunny weather reduces chlorophyll production, leaving the leaf vulnerable to sun damage. So the leaf produces sugar compounds known as anthocyanins, which are the protective red and purple leaf pigments. "When the tree is wasting sugar on something [like leaf color] it better have a purpose."
Botanist Cynthia Morton of the Carnegie Museum of Natural History said leaves change color because less daylight and cooler temperatures signal leaves to shut down. Warm, sunny days and cold but not freezing nights produce the boldest yellows, oranges and reds, she said. Veins carrying fluids in the leaves close off and form a scab of cork cells that causes the leaf to fall from the branch.
"The best colors are when we have a spring that is warm and wet, a summer that is not too hot or dry and a fall with warm sunny days and cool nights," Ms. Morton said. "This is why the colors are OK and not great this year."
David Templeton: email@example.com or 412-263-1578.
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