Wednesday, December 24, 2014

Winter Leaves that Hang On

Oak seedlings and saplings are know as marcescent.
As you spend the winter shoveling driveways, salting sidewalks, or just getting out and enjoying the wintery weather, have you noticed some trees hang on to their leaves after all the others have fallen to the ground?

Just like attracting pollinators is the purpose for colorful wildflower petals there’s a reason why some leaves hang on all winter. The winter retention of leaves is known as marcescence. Young beech, as well as their cousins, the oaks, not to mention musclewood, and witchhazel hang on to some of their leaves throughout much of the winter. They are known as "marcescent."

How does this happen?
In the fall trees create a separation zone, known as the abscission layer, between the leaf stem or petiole and the branch. If the separation layer is complete, the leaves will drop to the ground. 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. Marcescent trees do not form a complete abscission layer. So, some of the leaves hang on throughout the winter.

Why does this happen?
As with many natural occurrences the jury is out on the definite reason, most think genetics and environmental factors are primarily responsible for the late season hangers-on.

The fact that smaller, shorter, juvenile trees hang onto their leaves makes it likely the tree is protecting buds from deer throughout the winter. The branches of young trees are at a perfect height for browsing nutritious buds during 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 is the young beech and oak hang onto 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 abundant moisture into the spring.

Regardless the reason for marcescent leaves, when growth begins next spring the expanding buds will push them off and clothe the branches with new greenery. Until that happens, enjoy the waving brown leaves and the texture they add to forests and yards.

Kathleen Salisbury, Penn State Extension Horticulture Educator, Why Do You Just Keep Me Hanging On? December 12, 2014.

Jim Finley, Penn State Professor of Forest Resources, Winter Leaves that Hang On. December 17, 2012.

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