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Abscission and Marcescence in the Woods

November 9, 2009

While on a recent hike in the hills of Vermont, I noticed that some trees had lost their leaves while others still had golden brown leaves all over them. It seemed that all of the Red Oak and the American Beech trees still had a nearly full canopy, while the Sugar Maple along the old woods road were completely naked.

I snapped a photo of it so I could do a bit of research at home. It turns out that Oak and Beech trees have marcescent leaves. Marcescence is when a plant part dies but is not shed. In the photograph you can see that the Sugar Maple trees in the foreground along the old carriage road have completely shed their leaves, while the Beech trees on the hillside are still fully loaded.

A tree is full of vascular cells that transport water and sap from root to leaf. As the amount of sunlight decreases in autumn, the veins that transport sap into and out of the leaves slowly close. A layer of cells, called the abscission layer, develops at the stem base. The leaf falls off when this layer is completely formed.

Oak (Quercus), Beech (Fagus) and Hornbeam (Carpinus) are an exception. The separation layer doesn’t fully allow the leaves to detach. That’s why most of their dead leaves remain on the tree through winter until the wind rips them away. But one has to wonder what advantage this would have for the trees. It may be that it deters foraging of young buds and branches by deer. If there are a lot of dead leaves on the ends, it may not be as palatable. It may aid in protecting the tree from water or temperature stress during the winter. Or, perhaps it is a left over ghost from the past that is now neutral, neither hindering nor helping the species prosper. Whatever the reason, in late fall you can easily see the forest for the oak and beech trees.

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