Why do some trees’ dead leaves not fall off the tree?

marcescence in trees

Each autumn, the leaves of trees throughout the neighborhood transition from shades of green to a tapestry of bold oranges, reds and yellows. But as quickly as the leaves change color, they tumble to the ground.

Normally, as deciduous trees (which include hardwoods and some conifers) prepare to shed their leafy summer coats, cells at the interface between the twig and the end of the leaf stem release enzymes and form an abscission layer that “unglues” the leaf – separating it from the vascular bundles, allowing it to fall free. All trees shed leaves, even conifers; however, they generally retain their needles for more than one year. Leaf drop benefits deciduous trees by reducing water loss and allows them to develop leaves that efficiently use available sunlight during warmer seasons.

There are a few species of trees that hold their dead leaves, Oaks, hornbeams, and Beech trees. This is a phenomenon called marcescence. Younger oaks may keep what looks like a full complement of dead brown leaves, while more mature oaks may keep them on just their lower branches.

Sometimes, early cold weather or frosts may interrupt the abscission process or “kill” leaves quickly. In these cases, the occurrence of marcescent leaves may increase.

Some people speculate that retained leaves may deter browsing animals, such as deer. The dried leaves may conceal buds from browsers or make them difficult to nip from the twig. Researchers have found that the dried leaves are less nutritious. At least one study, conducted in Denmark, found that deer offered hand-stripped twigs preferred those to marcescent twigs, especially of beech and hornbeam, but not so for Oaks.

The other reason trees might give for holding onto their leaves relates to nutrient cycling. Leaves that fall in the autumn would join others on the forest floor and begin to decay. As they decay, released nutrients could leach away and be unavailable to “feed” trees the next growing season. This might be especially important to small understory trees with smaller root systems. By holding onto their leaves, they retain and recycle their nutrients to themselves.

The marcesent leaves provide a bit of shelter for wintering birds as they perch among the rattling leaves, away from winter’s wind.

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 forest and yards.

If you are uncertain about the health of the trees at your home or business, be sure to consult certified arborists when you hire a tree service in Columbus.

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