Leaves of a young Swamp White Oak (Quercus bicolor) may remain on the tree early into spring. Photo: Kathleen V. Salisbury
Just like attracting pollinators is the purpose for colorful wildflower petals there's a reason why some leaves keep on hanging on all winter. Marcescence (use it 3 times in a sentence today and you own this word!) is the term for this winter retention of leaves. Beech and oak are deciduous native trees, losing their leaves each fall. But young beech, as well as their cousins oaks, not to mention musclewood, witchhazels and parrotia, hang on to some of their leaves throughout the winter. They are marcescent.
How does this happen?
In the fall trees create a separation zone (abscission layer) between petiole (leaf stem) and branch. If the separation layer is complete, the leaves will drop to the ground, to add nutrients to the root zone as they decompose. Trees shed their leaves to prepare for harsh winter conditions by conserving valuable resources. They create this separation zone so the falling leaves do not damage the plant in the process of shedding. Marcescent trees do not form this abscission layer completely and so some of the leaves hang on through the winter.
Why does this happen?
As with many natural occurrences the jury is out on the definite reason, some think genetics and environmental factors are primarily responsible for the late season hangers-on, but some other hypotheses abound:
The fact that smaller, shorter, juvenile trees hang on to their leaves makes it likely the tree is protecting buds from hungry deer through the winter. The branches of young trees are at a perfect height for browsing nutritious buds during a snowy winter. Cloaking the pointy buds in dry leaves may keep deer from eating next year's growth.
Leaves that fall to the forest floor in autumn slowly decompose adding much needed nutrients and organic matter to the soil. The theory here is the young beech hang on to their leaves into the spring so they can release the leaves to decompose after the fall leaves have already become part of the soil, thus adding a much needed nutrient boost just in time for the spring growth spurt.
Another concept suggests these leaves act as a snow fence, slowing down snow and directing it to the base of the tree as it falls, ensuring moisture through the winter and into the spring (IF it snows).
"Why are these dead leaves still hanging on to my new tree?" a client asks you as you examine the wintery landscape. "Well…" you begin, starting a discussion, educating while encouraging your patron to examine, monitor and learn about the landscape you created and maintain.
Leaves and buds of a young American Beech (Fagus grandifolia) Photo: Kathleen V. Salisbury
Leaves of Persian Parrotia (Parrotia persica) surrounded round buds. Photo by Kathleen V. Salisbury