Oh, but isn’t the fall bittersweet. I can feel the warm glow of summer fading as the sun keeps setting earlier and earlier and the first frost curl back the dying Norway maple leaves. The plants are expressing their preparations for the impending end to the growing season and the pale yellows of striped maple and purplish reds of staghorn sumac are now the dominant colors. With each passing year I feel like I get a little more cued into the timing of fall foliage, the color palette of each species, and the pattern of how trees change colors.

Pattern in how color change occurs

Whether you’re looking at a leaf, a tree, or a forest, the pattern is generally the same in where fall colors first make their appearance and how they wash over the leaf/tree/forest. Color change tends to start on the fringes of a leaf and slowly taking over the leaf. Color change then works its way back along the branches towards the trunk , with the upper branches beginning this process first.

Clockwise from top left: American beech, northern hackberry, northern red oak, American elm, black locust, quaking aspen.

The general principal behind fall color change seems to be protect the most valuable first and the most expendable last. The top and tips of a tree are where the leaves have the most exposure to sunlight. As a result, these areas have the most capacity for future energy returns (in the form of photosynthesis). Rather than risking exposing these precious leaves to an early frost, fall comes earlier at the crown. On sugar maples (see image below), the tips are often bare while the mid-section is burnt orange and the bottom green. Similar for the edges of a leaf, the extremities are the areas most susceptible to cold temperatures and so the edge is also the first to change colors.

Left: Black locust leaf changing from tip towards the branch. Right: Cottonwood with bare tips at the top, yellow midsection, and green skirt (Centennial Woods, Burlington).
Left: Row of red x silver maple hybrids.
Center: Pair of sugar maples.
Right: Looking up at the red canopy of a red x silver maple, while the interior and lower branches are still red (Burlington)

It’s tough living in the shady understory of a forest, and there are only a couple brief periods of time when the forest floor gets full sunlight and it’s warm enough to photosynthesize: once in the spring before the canopy leaves emerge and once again in the fall after the canopy leaves have dropped. The shade tolerant trees and shrubs growing in the understory take advantage of this precious period of time by leafing out early and shedding their leaves later. This is less of a consistent pattern than what you see on a leaf, branch, or tree (striped maples, for example tend to be earlier), but is quite evident in forests with a thick shrub-dominated understory. Walking through a buckthorn filled forest in November is magical – a thick carpet of yellow and orange leaves on the forest and in the canopy frame the dark green understory of seemingly oblivious shrubs.

The challenge of being an understory tree is getting the timing right of keeping your leaves to photosynthesize after the canopy sheds their leaves but before the first hard frosts. Here, glossy buckthorn gets it wrong (Centennial Woods, Burlington)

Exceptions to the Rule

One of the basic Rules in biology is that there really aren’t any hard and fast rules (there are 3 laws – link – and a handful of Rules that serve as helpful guidelines). So of course there are exceptions to these patterns. Conifers don’t follow the rule at all. White pine, for example, keeps all of its current year’s needles but sheds all of the previous year’s needles, which are closer to the inside of the tree. Hemlock slowly loses its needles scattered around the branch from midsummer through late fall. Cottonwoods (and other aspens) tend to lose their leaves from the bottom up and keep a few stragglers up at the top. Oaksbeech, and a few others are marcescent, keeping their lowest leaves well into the spring (link). Other species don’t really seem to follow a really clear pattern (like Norway maple, boxelder, etc.), and their leaves all turn right around the same time.

A common pattern for cottonwoods to keep their top outer most leaves after the rest of the leaves have fallen (Rte 2, Richmond)

More on the topic

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