As you spend the winter shoveling driveways, salting sidewalks, or just getting out and enjoying the winter weather, have you noticed that some deciduous trees hang on to old brown leaves while most are bare?
Marcescence
The term for trees retaining their leaves in winter is known as marcescence. The strict definition of marcescence is, “the withering and persistence of plant organs that normally are shed.” It is most commonly applied to plant leaves. And of course, is most obvious in deciduous trees that retain leaves through winter. They are known as “marcescent.”
Leaf marcescence is most often seen in young trees and may no longer occur as the tree matures. Several species of trees commonly have marcescent leaves. These include certain species of oak (Quercus), beech (Fagus) and hornbeam (Carpinus). We sometimes even see the trait in witch-hazels, which tend to hang on to some of their leaves throughout much of winter.
In the fall trees create a separation zone between the leaf stem, called the petiole, and the branch. This zone is known as the abscission layer. If the abscission layer is complete, the leaf will drop to the ground. Trees that are marcescent do not form a complete abscission layer. So, the leaves hang on throughout the winter.
Image Shows Marcescent Oak Seedlings in the Forest Understory
Why Does Marcescence Happens?
The evolutionary reasons for marcescence are not totally clear. Theories include protecting leaf buds from winter desiccation and browsing deer, providing a delayed source of nutrients in spring, as well as water conservation.
Since young trees are more commonly marcescent, leaf retention is a possible evolutionary strategy to avoid being browsed by deer. Retaining dry leaves in winter may protect the buds and keep deer from eating next year’s growth.
Leaves on the forest floor slowly decompose over time adding nutrients to the soil. Marcescent trees, such as beech and oak, may get an added nutrient boost during shoot elongation by dropping their leaves in spring, after the fall leaves have already decomposed and become part of the soil.
Some also suggest that marcescent leaves may act as a “snow fence,” trapping snow and accumulating it at the base of the tree, thus ensuring abundant moisture for spring growth.
Regardless of the reason for marcescent leaves, when growth begins next spring the expanding buds will push the old leaves off. Until that happens, enjoy the crinkled up brown leaves and the texture they add to our yards and forests.
Written by: David R. Jackson, ISA Certified Arborist, Cutting Edge Tree Professionals