Why do Trees Lose their Leaves in the Fall?

If you’re a Tree, you don’t get far in life unless you have a plan.

Trees have developed some pretty ingenious strategies to compete for resources, deal with threats, and adapt to change. One plan that many Trees in cold-winter climates have adopted is to drop their leaves in fall.

Deciduous Trees’ leaves lose a lot of water that can’t be easily replaced in winter, and photosynthesis slows to a crawl as the temperature dips towards freezing anyway, so deciduous Trees cut ’em loose.

It’s an event that’s planned for when the leaf is formed in the spring. A layer of cells called the “abscission layer” is created then at the base of the leaf-stalk, or petiole. In autumn, the shortening day-length triggers hormones that activate those abscission cells, cutting the apron strings that hold the leaf to the Tree.

Although the leaves are all eventually severed, they still serve the Tree by collecting at its feet and decomposing into a beneficial leaf-compost. Neat, huh?

Leaf-fall occurs over several days or a couple of weeks for most Trees, but for one notable—the Ginkgo—it can be quite sudden and dramatic. A Ginkgo can lose all of its leaves in a period of a few hours. Sometimes students or co-workers with a big Ginkgo in their midst will take bets on when its leaves will drop.

You may wonder why the leaves on some deciduous Trees, like Oaks, Beeches, and Hornbeams, often don’t fall until late winter or spring. We wonder that, too!

No one has been able to explain definitively why that happens. One theory is that the old leaves deter deer from browsing the tender buds. (You wouldn’t want to chomp through those dry, dead leaves either, would you?) Another theory is that this group of Trees (they are related) is not as evolved as other deciduous Trees. The evergreen strategy has been around a lot longer than the deciduous strategy. Maybe Oaks don’t have the hang of it yet.

As for Trees that truly do hold their leaves all winter, like Pines, Spruces, Firs, and Hemlocks, they are employing a different strategy altogether.

Instead of pushing out lots of lush growth all spring and summer and then going totally dormant, they are more even-keeled. They put out a modest flush of growth in spring and then go into maintenance mode. Their waxy, needle-like leaves help them retain moisture in the winter. They grow more slowly, but most of them get bigger and live longer than deciduous Trees.

So that strategy works, too! You just gotta have a plan – another great lesson we can learn from Trees!