October marks the period when Chestnut Hill’s leafy landscape turns into a brilliant mix of red, orange, and yellow.

And according to local experts, that transition has started a little early in Pennsylvania. In its Oct. 2 fall foliage report, the state’s Department of Conservation and Natural Resources (DCNR) reported “early changes to fall color” at William Penn State Forest in the southeastern region, with red maple, black gum, and sycamore trees as the current frontrunners.

Leaves in Philadelphia County are starting to show their vibrant fall colors, according to the report, and some northern counties have already passed their peak.

Pennsylvania’s fall foliage season had a quick start due to the long period of dry conditions in the summer and cool nights in late August and early September, DCNR officials said in late September. But that transition has slowed down, as of Oct. 2, after the commonwealth got some rain at the end of September.

“The rainfall and warm nighttime temperatures likely stabilized the fall color timeline, which had been significantly advanced,” the agency’s report reads. “The expected cooldown over the weekend should stimulate noticeable color changes across Pennsylvania, especially in the northern half of the state and mountainous regions.”

The agency predicted an earlier yet shorter fall foliage season this year featuring plenty of vivid colors.

“We’re seeing some of the northern counties (have) already hit their peak color and (are) actually fading already,” Kayla Kehres, a service and urban forester with the Pennsylvania Bureau of Forestry, said last Wednesday. “It’s been very early and quick, probably also due to the heat that we’ve had and the drought stress.”

In late August and early September, some nights in Philadelphia cooled down to as low as the high 50s. And by the end of the month, many northern and eastern Pennsylvania counties were on drought watch. Kehres said the lack of rainfall caused some trees to turn earlier because they weren’t getting the water they needed to produce sugars.

Experts at Morris Arboretum & Gardens and Briar Bush Nature Center in Abington told the Local last week that they are seeing some early senescence, or premature fall color and leaf drop due to drought stress and other environmental factors.

“It’s just a little early. Usually leaves aren’t falling at the end of September. They usually wait a little bit, but there are some trees that do lose their leaves early,” Vince Morocco, the director of horticulture at the Morris Arboretum, said. “It’s not uncommon to have plants starting to lose their leaves a little earlier than normal.”

Apart from drought stress, the brief cooldown in early September could have also signaled some trees to start preparing for winter, he said.

What mainly causes the leaf color change, though, is the shorter days in the fall. The lack of daylight leads chlorophyll to break down, revealing pigments that give leaves their fall colors like xanthophylls and carotenoids, Morocco said. Weather affects the intensity of color. Anthocyanin pigments, which creates reds and purples, need moist soil conditions, bright and sunny fall days, as well as cool but not freezing nights.

“If we get some more cool nights, I think we should be primed for some really nice fall color,” he said. “It depends on what happens in the next couple of weeks.”

Last week featured sunny skies and warm days, with temperatures staying in the 50s and 60s at night.

Kehres said fall foliage in Philadelphia usually peaks two to three weeks after the northern parts of the state, so residents should make leaf-peeping plans for mid-October.

Mark Fallon, the director of Briar Bush, said the height of Abington’s fall foliage season usually occurs around the first week of October to the first week of November, when the center’s maples and oaks produce vibrant shades of red.

“There is a pretty wide diversity of trees, so the fall color there is, I would say, pretty spectacular,” he said. “Coming around sunset is always a great way to enjoy those colors. They kind of pop a little more.”

Morocco said the season is already looking better than last year’s, when the severe summer drought caused lots of early leaf fall and muted color. Some trees still produce vibrant colors every year no matter what the weather conditons are, he said. These include black tupelos and sumac trees.

It’s unclear how climate change impacts leaf color, but it does have lasting effects on the trees themselves. Droughts cause heat stress and Morocco said longer autumn seasons can encourage plants to grow into the winter, causing injuries to the trees during the first winter cold snap.

Apart from Morris Arboretum & Gardens and Briar Bush Nature Center, here are some other nearby leaf-peeping destinations:

•Wissahickon Valley Park. Friends of the Wissahickon has a list of ideal leaf-peeping spots on its website.

•Pastorius Park

•Pennypack Park

•Huntington Park

•John James Audubon Center

•Fort Washington State Park

•Awbury Arboretum

•Laurel Hill East and West cemeteries

•The Schuykill River Trail, especially along Kelly Drive

•Blue Bell Park

•Evansburg State Park

•Norristown Farm State Park

Abby Weiss is an environmental reporter for the Local and a Report for America corps member. She can be reached at Abby@chestnuthilllocal.com.