Is fall foliage a sign of seasonal change or response to heat waves?

As autumn settles in, we eagerly await the transformation of leaves into vibrant hues of red, orange and yellow. However, this year might tell a different story due to prolonged periods of hot weather. Ben McCallister, urban forestry specialist in Purdue University’s Department of Forestry and Natural Resources, sheds light on what to expect this fall.

The impact of weather on fall foliage timing

Right now, we’re seeing a mix of fall coming in and shifting temperatures, which is causing trees to respond. This back-and-forth hot weather is creating some color changes related to drought and stress. This can be expressed as brown color where leaves are starting to wilt and look a little crunchy from exposure to heat and drought stress," McCallister explains.

While the typical color change occurs from mid-September to early November, it’s not unusual for leaves to remain green at this point in the season. McCallister suggests that while the hot weather “most likely won’t affect anything visually, it could influence fall foliage over time as climate change progresses.”

He anticipates a more pronounced display of fall colors extending through October, with some species showing their colors into November. Ultimately, the vibrancy and timing of these changes will depend on daily temperature fluctuations and the amount of sunlight trees receive.

The environment also plays a significant role in how trees respond. For instance, urban areas with more concrete and buildings create a “urban heat island” effect, which can delay color changes compared to more rural settings.

The science behind color transformation

The science of fall foliage is both fascinating and complex. McCallister explains that different tree species respond to autumn at varying times, with chlorophyll being a key player in this process.

Changes in chlorophyll production leads to the color changes in leaves. Chlorophyll allows leaves to take up sunlight and convert it into energy, creating carbohydrates in the form of sugars – really anything that gives energy to the tree to help it grow, sustain itself and compartmentalize,” McCallister notes.

 As temperatures cool and the sunlight diminishes, chlorophyll production declines, allowing the green hues of leaves to fade. This decline reveals carotenoids and anthocyanins – two chemicals essential to fall foliage. Carotenoids contribute to the yellow and orange shades, while anthocyanins produce reds and purples. As chlorophyll wanes, these vibrant colors emerge.

“It’s a very cool process,” McCallister adds. “Each color absorbs different spectrums of light, altering energy uptake and preparing trees for leaf drop and dormancy. As leaves in deciduous trees transition, trees store energy in their roots and wood so they can brave the winter."

Viewing fall foliage

When it comes to enjoying fall foliage, McCallister emphasizes that Indiana offers many great viewing locations not far from Purdue’s campus.

Locally, Martell Forest, which is open to the public, is a great spot to check out the fall foliage and immerse yourself in the woods,” says McCallister.

What’s next?

Leaf drop is likely to occur anytime from mid-October into December, although this process may take longer than usual due to the milder winters we’ve been experiencing. Similar to color changing, the timing of leaf drop varies by tree species and their growth patterns, as well as the overall weather conditions.

For instance, ginkgo trees turn this brilliant yellow and have a cool reaction to leaf drop. Typically, all their leaves fall within a day or so, creating a carpet of yellow. In contrast, trees like maples and oaks, which have a mix of yellow, orange and red leaves, tend to hold onto their foliage a bit longer," McCallister recalls.

It's also important to note that while coniferous trees, like pines, spruces and furs, retain their needles year-round, some conifers like bald-cypress trees drop theirs. This sometimes causes confusion for homeowners who mistake these deciduous conifers for dying trees.

To provide the best care for your trees during these changing weather patterns, McCallister advises familiarizing yourself with different tree species. Resources like Google and dichotomous keys in forestry books can help with species identification and lead to tips for care. Regularly checking your trees’ health is essential year-round. Specialists in your local county Extension offices and ISA Certified Arborists can be valuable resources for proper tree management.