Why do some trees not lose their leaves?

As the seasons change, we expect certain things to happen in nature. As we look forward to spring, we see buds on trees and shrubs. As spring starts to creep along, we look for wildflowers on the forest floor. Then suddenly the trees have leaves and the world is green as far as the eye can see. By the time fall begins, we are ready for the leaves to drop so the cycle can start over.

But not all deciduous trees seem know they are supposed to lose their leaves. Most trees will shed them all by the time winter sets in, but a few trees hang onto them. For these trees, their leaves can survive even the fiercest winter storms.

These leaves that just can't seem to let go get the first part of the equation right. The green color fades, but that's where the process stops. Those leaves never seem to fall.

Trees that do not lose their leaves are said to be marcescent. It is most common among certain kinds of trees. In Illinois, oak trees and beech trees are among those that most often are marcescent.

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Words to know

Deciduous: Shedding its leaves annually.

Theory: A system of ideas intended to explain something.

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It is most commonly seen in younger trees rather than fully mature trees. These withered brown leaves don't last forever, though. When spring sets in, the leaves will finally fall.

Normally, trees shed their leaves when the structure that connects the leaf to its plant, called the petiole, forms a cluster of cells that allow that plant and leaf to separate from one another. This spot is called the abscission zone. For most trees, the abscission zone is activated in fall. But for marcescent trees, the activation is delayed until spring, just before the tree's new set of leaves begin to emerge.

The reason for the delayed timing is a mystery, but there are a few theories. One is that keeping their leaves may prevent wildlife from nibbling on the tree. This could give the young trees a greater likelihood of surviving to maturity. This also explains why trees outgrow it as they mature.

Retaining their leaves through winter may also be a way for the deciduous trees to compete with evergreen trees in areas where the soil lacks nutrients. Evergreen trees typically grow better in these environments, so marcescence may be a survival strategy in less-than-ideal conditions.

Speaking of evergreen trees, do they lose their leaves? They do, but it's quite different than how deciduous trees lose their leaves. The leaves on conifer, or evergreen, trees are needles. And evergreen trees do shed needles every year, but they don't shed all their needles at once like deciduous trees.

Evergreen trees add new green needles each year, and those new needles replace older needles. Just like with leaves, the old needles will turn yellow or brown before dropping.

When evergreen trees will drop their leaves/needles varies. Most pine trees shed needles in the fall, but junipers tend to lose theirs in late summer or early fall. And some evergreen trees may lose needles in spring or early summer.

How long needles last can vary too. While leaves on deciduous trees last just one year, many evergreen trees keep their leaves for three years. Some junipers can keep theirs for a decade or even longer!