Marcescence: why young beeches and oaks keep their dead leaves all winter

Young beeches and oaks keep their dead leaves all winter because the layer of weak cells that cuts a leaf loose does not fully form in the fall. The tree finishes it in spring, when the buds swell, and the old leaves fall a few days before the new ones open. The trees are not dead and nothing is wrong with them. They are marcescent, and the habit is common enough in the East that a winter woods without it looks strange to anyone who grew up here.

What marcescence is

Walk through an oak woodland in January and the understory is not bare. Beech saplings stand in the snow holding every leaf they grew last summer, faded to the color of parchment and copper, and the young oaks above them rattle with dry brown leaves in the wind.

A deciduous leaf does not simply die and fall. In late summer the tree builds an abscission layer, a zone of weak cells across the base of the leaf stalk, and seals the twig side of it with a corky plate. When the layer is complete the leaf breaks away at the slightest pull, which is why leaves come down in a rush on a windy day in October.

In a marcescent tree that layer does not fully form in the fall. The leaf dies on schedule, loses its chlorophyll, dries and turns brown, but it stays fastened to the twig. It hangs on through winter, and only when the buds swell in spring does the tree finish the job and cut it loose. A marcescent woodland has a second leaf fall in April, a few days before the new leaves open.

“However, in species that retain their dead leaves throughout the winter, this abscission layer is delayed or only partially forms, allowing leaves to remain for a while longer.” (Illinois Extension, The stubborn beauty of oak leaves: marcescence explained)

Which trees do it, and where on the tree

American beech is the champion. Oaks come next: white oak, chestnut oak, swamp white oak, pin oak and scarlet oak are among the most reliable, though most oaks show some of it. American hornbeam and hop hornbeam hold leaves well into winter, and witch hazel often keeps its dead leaves around the flowers it opens in November.

Two patterns show up on every species. Young trees do it far more than old ones: a beech sapling keeps nearly every leaf, while a mature beech keeps a fringe on the lower and inner branches and drops the rest. And within one tree, the lower crown holds leaves longer than the top. A large oak in a park often carries a skirt of brown leaves on the branches within reach of the ground and is bare above. Whatever marcescence is for, it happens where the tree is young and low.

Blue Whale Trees grows oaks, both hornbeams and witch hazel from seed. Both patterns are plain in the beds each winter. The youngest seedlings hold nearly every leaf, while the same species a few years older keep them only on the branches nearest the ground.

The explanations

Nobody knows for certain why some trees do this, and it is possible that there is more than one reason, or none. These are the ideas with the most support:

• Browse deterrence. Dead, dry leaves loaded with tannin are poor food, and a twig wrapped in them is more work for a deer to reach and less rewarding to eat. This fits the pattern of young trees and low branches, which are exactly the parts within reach of a browser, and it fits the other ways trees defend themselves. In a winter feeding trial in a Danish deer park, published in 2001, Claus Svendsen found that deer browsed beech and hornbeam twigs more once their dead leaves were removed. Oak twigs were browsed the same either way, and beyond that one trial the evidence is thin.
• Bud protection. A cover of leaves may shelter the buds from drying wind, sun and the sharpest cold, a small benefit that would matter most to a sapling on an exposed site.
• Nutrient timing. Leaves that fall in October decompose through a season when roots are inactive, and some of what they release washes away. Leaves that fall in April land on a soil that is warming up, with roots ready to take up what the litter gives back. Oaks and beech are common on thin, dry, infertile ground, where that timing would matter most.
• Catching snow. Held leaves trap drifting snow, which melts at the base of the tree in spring. This is the least studied idea and may be an incidental effect.
• A leftover. The beech and oak family has many evergreen members, and marcescence may be nothing more than a halfway point between keeping leaves and shedding them, a trait that persists because it costs little.

None of these has been proven and none has been ruled out. The honest position is that marcescence is a real and consistent trait with a set of plausible benefits and no settled cause, which describes a good deal of tree biology.

How to see it

The best time is a still, bright morning after snow, when the copper of the beech leaves is lit against the white and the oaks above have turned the color of old leather. The best places are upland woods with a beech understory, ridge tops with chestnut oak, and the edges of swamps where young oaks stand in the open. Listen as much as look: a stand of marcescent trees in a January wind has a dry rustle that a bare woods never makes. The rustle is one of the pleasures of a winter walk once you know what it is.

How to tell it from a dead branch, and why it needs no action

A marcescent tree is healthy. It leafed out, grew, turned color and shut down on its normal schedule; it simply did not let go. There is nothing to prune, spray or fertilize, and pulling the leaves off would only expose the buds to whatever they are shielding them from.

“Leaves on such trees are either physically worn off by wind, rain, snow, sleet, etc., or are pushed off by developing buds in spring.” (UMass Extension, Landscape Message: November 6, 2015)

The thing to distinguish it from is a tree or branch that died in summer:

A hard freeze in early fall can also catch leaves before the abscission layer forms and freeze-dry them onto the tree, which looks alarming and is equally harmless. In April, as the new leaves push out, the old ones let go all at once and the tree carries on as if nothing had happened.

Common questions

Is a tree that holds brown leaves in winter dying?

No, if the leaves turned brown in autumn on schedule and the buds behind them are plump; leaves killed while green in summer shrivel in place on brittle twigs with shriveled or missing buds.

Why do only the young trees and the low branches do it?

Nobody knows, but that is the pattern on every species that shows marcescence, and it is the pattern the browse-deterrence idea predicts, since those are exactly the parts a deer can reach.

When do the leaves finally fall?

In April, a few days before the new leaves open, when the swelling buds finish the abscission layer the tree never completed in the fall.

Which oaks keep their dead leaves in winter?

White, chestnut, swamp white, pin and scarlet oak are among the most reliable, though most oaks show some of it. Young oaks hold the most, and older ones keep theirs mainly on the lower branches.