Larry Lyons: Why do oaks keep leaves all winter?

Published 1:20 pm Wednesday, March 17, 2010

lyonsA while ago Bill, a regular follower of the column, wrote wondering why some oak trees keep their leaves all winter instead of dropping them in the fall like other deciduous trees.

Great question. To explain it we must first review Tree Function 101.  During the warmer months a tree produces its own food in the form of sugar. There are three ingredients in a tree’s sugar recipe, water, carbon dioxide and sunlight. Winter weather conditions cause problems in gathering these ingredients. Short, cloudy days don’t offer much sunlight and freezing temperatures hinder the absorption and transport of water. Thus most deciduous trees find it more efficient to just put life on hold during the winter and go dormant.

In the fall as days grow shorter (it’s actually the longer nights that trigger the event), the veins that transport sap in and out of the leaves slowly begin to close off. The tree absorbs the nutrients from the leaves and stores them, making sort of a winter food cache if you will. That’s when the leaves turn colors in the fall.  Since the leaves will no longer be making food they’re just useless, excess baggage so why not get rid of them? The tree begins forming a layer of cells at the base of each leaf called the separation or, technically, the abscission, layer.

The walls of the cells forming this layer are weak allowing easy detachment of the leaf. When this fragile separation layer is complete, the leaf breaks off.

You’ve surely noticed that as fall progresses some species and even individual trees drop their leaves earlier than others. It’s all a matter of timing when this separation layer is fully formed.  In the case of some species of oaks, notably pin oaks but other oaks, too, as well as beech trees, the formation of the separation layer on many of the leaves isn’t completed until spring. That’s when those brown leaves that hung around all winter finally drop off.

That’s all pretty straightforward but if you pay attention you’ll occasionally see dead leaves all winter on other tree species that aren’t supposed to do that. Right now in my yard are several small sugar maples with many of the lower leaves still clinging obstinately to the branches. What’s up with that? If a leaf is killed before the separation layer is complete the layer never forms and the leaf remains on the tree. This is most often caused by an early freeze or a branch partially breaking, causing premature death of the leaves. Eventually they rot off.

Then there’s the question of why most trees evolved to drop leaves in the fall but not the oaks? Here it gets to pure speculation. The theory explaining fall leaf drop is to get rid of those bulky leaves before the snow, ice and winter winds arrive to minimize branch breakage. As for the spring drop of the oaks, you’ll notice it’s only the younger trees that retain a large portion of their leaves. The larger oaks either drop all of the leaves in the fall or only retain those on the lower limbs throughout the winter.

The most common theory is that the unpalatable dried leaves protect the young trees and lower limbs of larger trees from browsing by ungulates such as deer and moose (back when moose had a wider range). One must question, though, why just the oaks and beeches? Many, many tree species are heavily browsed, why didn’t at least some adopt the same defensive tactic?

Another theory, at least for the more mature oaks, is they retain their lower leaves so as not to cover up the acorns that fall to the ground underneath. The upper leaves are shed and dispersed by the wind while the lower leaves that receive less wind and would drop straight down are retained to leave the acorns exposed. Then the squirrels, birds, mice and other acorn gourmands can find and disperse them. Of course, that doesn’t address the non-acorn bearing young trees retaining all their leaves. Maybe they’re just practicing for when they grow up. More likely, we humans are just clueless about the whole thing.
Carpe diem.

Larry Lyons writes a weekly outdoor column for Leader Publications.
He can be reached at