Leaves are beginning to change color across Central Alabama. The fall foliage will likely peak in the next couple of weeks, running a little bit behind the typical late October/ early November time frame. Scientists say predicting the timing and intensity of fall foliage is a complicated process that depends on a variety of environmental factors. Here are some of them.
The sun angle
The main reason for leaf change in general is the sun.
“Plants tend to use day length and the shortening days as a signal to begin to shut down their photosynthesis and prepare for the winter,” said Theresa Crimmins, director of the USA National Phenology Network.
The sun drops lower on the horizon as winter approaches in the Northern Hemisphere leaving areas farther north with fewer, and more rapidly changing daylight hours. That’s why fall foliage turns sooner in Huntsville versus Dothan.
Temperature
The ideal temperature setup for vibrant fall colors is mild sunny days and cool nights. Shorter days typically align with cooling temperature trends, but Crimmins said the trend toward warmer fall months in recent decades has disrupted the natural climate signals.
“With warmer conditions that we're experiencing globally on average, that's kind of messing things up,” Crimmins said. “We tend to see leaf color change and leaf fall drifting later in the season than it used to be.”
According to the nonprofit Climate Central, average temperatures have warmed by 3.7 degrees Fahrenheit with an additional 17 days of above average heat in the fall in Birmingham since the 1970s.
Temperature extremes earlier in the growing season can also impact the timing and intensity of fall foliage. For the month of October this year, temperatures ran about 5 degrees above average in Birmingham. Warmer weather means trees will delay changing color.
And it’s not just daytime temperatures that matter; nighttime temperatures do too.
“The nighttime temperatures are a signal to the plants to really start to shut stuff down,” Crimmins said. “Like kind of packing up as much of the carbohydrates and other nutrients that are in the leaf tissues and pulling them back into the branches and the bark of the tree, so that the leaves can eventually be shed.”
Crimmins added that once the tree absorbs the nutrients, the leaves will fall in preparation for the colder winter months.
But, a new analysis from Climate Central shows October nights have warmed by 5 degrees Fahrenheit in Birmingham over the past 50 years. Crimmins said even that small increase in warmer nighttime temperatures alone can also delay the leaf change and fall.
New research suggests cities like Birmingham might experience further delays in changing leaf color thanks to the urban heat island effect.
“Developed areas tend to be warmer than their surrounding areas,” Crimmins said. “And also have a lot more artificial light.”
On a global scale, Alabama’s state climatologist Lee Ellenburg suggested the strong El Niño may play its own role in the changing colors.
“If you look at the trends in general, El Niños do have a typically later freeze date on average, which can push back the timing of the foliage a little bit later,” Ellenburg said.
Moisture
More than a third of Alabama is under drought conditions. Though Birmingham has already received about an inch and a half of rainfall so far for November, the city ended October in a 2 inch rainfall deficit.
Ellenburg said trees prefer drier weather in the fall. But they need rainfall earlier in the growing season to fully thrive. Drier weather during the summer can put stress on trees – especially in a climate like Alabama that’s more accustomed to afternoon downpours. Mid-season drought conditions can prevent more saturated hues with leaves turning browner more quickly.
“Growing season conditions naturally would affect the health of the trees themselves,” Ellenburg said. “In dry times, some of the sycamores will start to drop their leaves really early – sometimes into August. That can make a big difference on what that foliage looks like.”
