Deciduous trees typically lose all of their leaves by late autumn. However, when snowshoeing or skiing through the woods this time of year, one is likely to find a scattering of deciduous trees that still have leaves clinging tightly to their branches. These plants are exhibiting marcescence, the trait of retaining plant parts after they are dead and dry.
Most deciduous trees form a layer of cells called the abscission layer at the base of each leaf’s stem, or petiole, where it attaches to the branch. This layer is composed of thin-walled cells that break easily, allowing the leaf to drop. A thin layer of corky cells seals the tree at the spot where the leaf was attached. Abcission layers are not formed on marcescent trees such as oaks and the American Beech, all members of the family Fagaceae. Therefore, their leaves do not fall off as readily, and many remain attached through the winter.
The evolutionary reasons for marcescence are not clear, though theories include defense against herbivory (e.g. browsing by deer), protection of leaf buds from winter desiccation, and as a delayed source of nutrients or moisture-conserving mulch when the leaves finally fall and decompose in the spring.
Leaf marcescence is most often seen on juvenile trees, and on the lower branches of older trees.
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