by Ryan Reed
You probably noticed the lack of rainfall over the past six weeks or so. It’s likely you also have been seeing an accelerated transition to fall color in your neck of the woods. That’s because dryness is a well-known fall foliage driver. It’s no accident that most early color can be seen along the drier forest edges and south/west facing slopes, where more sunlight (and more intense sunlight) reaches the ground.
Shearer Dug Trail, Tuscarora State Forest, 2024.
Abundant chlorophyll (the green pigment) in a leaf is basically a gamble that the tree is willing to make in the name of energy production. Recall that chlorophyll is the biochemical in the leaf that functions like a solar panel, harvesting the sun’s energy, which in turn is used to assemble carbon dioxide molecules into longer chains known as complex sugars (these take the form of plant fibers like starch, cellulose, and lignin). A critical part of that process is water, which is exactly why trees won’t grow without it. Put simply, if there is no water, there is no need for chlorophyll because photosynthesis can’t occur. Persistent dry weather stimulates the tree to cease chlorophyll production, revealing the fall colors underneath. Colder air, as we typically see with the onset of fall, also happens to hold less moisture. Couple that with less available sunlight with the shortening day length and you have a perfect recipe for fall foliage.
In the lead-up to fall foliage season, dry periods will hasten the color changes and can lead to particularly vivid displays. This sometimes comes with a cost though, as the duration of the period of peak color is often shorter. Some callers and reporters have recently asked if the premature leaf shedding they have noticed is a concern for the overall prospects of this fall foliage season in Penn’s Woods. I don’t think so. Keep in mind that some shedding of leaves is normal throughout the growing season as a tree attempts to maintain a proper internal water balance with the varying precipitation patterns that are typical in summer.
With the fall foliage season upon us, please don’t forget about our annual fall foliage reports, the first of which for 2025 will appear this coming Thursday, September 25. And yes, that is one week earlier than the usual start date as we try to capture the earlier changes this year. Earlier, later, longer, shorter, it really doesn’t matter. Fall foliage season is always awesome in Pennsylvania!
https://www.pa.gov/agencies/dcnr/conservation/forests-and-tree/fall-foliage-reports
https://www.explorefall.com/states/pennsylvania
The Remarkable Science of Leaves Changing Color
Fall color on Cole Run Road, Forbes State Forest, 2024.
