Marcescence: Why Some Trees Keep Their Leaves in Winter

January 22, 2020, 5:49 PM EST

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Above: This photo tells an entire story. Taken at Roan Mountain State Park in Tennessee in January, it shows most of the deciduous trees are bare. However, you can easily see the light tan colors of the beech trees, which have retained their leaves. Finally, to round out the little eco-system notice the evergreen rhododendrons near the forest floor and the conifers with their green needles higher up in the canopy. All images in this post are by Tom Niziol unless otherwise noted.

I love to walk through the woods at all times of the year. Fall is so wonderful because of the change in the colors of the leaves, and as we head toward winter, the deciduous trees pretty much shed their leaves and become bare—well, most of those trees. I often wondered why some of these trees seem to keep their leaves into early winter and some keep their leaves right through until the next spring.

That process is known as marcescence, and it’s defined as the retention of dead plant organs that normally shed. In this case, it’s those leaves that are normally shed by deciduous trees in at the end of the growing season, in contrast to trees that are “evergreen” and do not shed their leaves (as shown in the image at top).

Beech trees at Knox State Park in East Aurora, New York
Figure 1. It may be winter, but these beech trees at Knox State Park in East Aurora, New York still have leaves on them. This is called marcescence.

The process of shedding leaves is really interesting and shows the intricate evolution of nature as a way to survive through all seasons. When the days grow shorter and the amount of sunshine available to leaves decreases, the process that makes food for the trees ends. Chlorophyll begins to break down, the green color disappears, and we get those splendid colors of the fall before most trees drop their leaves.

The process of leaf drop is also a neat little trick of nature. At the base of their stem (referred to as the petiole), leaves have a zone called the abscission layer, located near the branch to which they are attached.

basic parts of the leaf connection to the twig
Figure 2. This photo shows the basic parts of the leaf connection to the twig, including the stem of the leaf, or petiole, the abscission layer, and the new leaf bud for next year's growth.

The word abscission (sounds like scissors) comes from the Latin “to cut away”. The abscission zone has special cells that act like scissors, cutting the leaf off from the main part of the tree in autumn. The part of the leaf stem, or petiole, nearer the leaf contains a separation layer of thin-walled cells that break readily, allowing the leaf to drop. On the branch or twig side of the petiole, there’s another special layer of cells that have a corky structure, which forms a protective layer on the tree, neatly closing up the break to prevent injury or disease. So, the cold of winter gets sealed out, while precious water that the tree continues to use through the winter is sealed in. When spring finally arrives, the return to rapid growth from the trees limbs makes leaf buds expand and swell, and the old leaves finally break off if they haven’t already. Nature is so cool!

Most, but not all, deciduous trees go through the abscission process. But there are a number of species that exhibit marcescence, or the retention of their leaves, to some degree through the winter months. That’s what you may see when you walk through the forest in the winter. marcescence is most common by far in the beech, followed by many species of oak as well as hornbeam.

Figure 3. Examples of beech and oak trees that have kept their leaves on a crisp winter day at Roan Mountain State Park, Tennessee, while the rest of the deciduous trees are bare.

What causes marcescence?

Scientists have not established the exact reason why certain trees exhibit marcescence—you just can’t go and ask a tree. However, there are some common theories. A few of those theories are based on the observation that marcescent leaves are found most often on younger or smaller trees or on the lower limbs of bigger trees.

One theory suggests trees may keep their leaves to deter deer and other browsing animals from eating the nutrient-rich twigs. The leaves may conceal sumptuous new buds. In fact, researchers have found that the dried leaves are less nutritious than the twigs, and that characteristic might keep the animal from trying to munch on the lower twigs of trees.

Researchers suggest another possibility for trees holding their leaves through the winter. It relates to the availability of nutrients for trees as they head into the growing season in the spring. When leaves drop in the fall, the nutrients from those leaves that accumulate on the forest floor are pretty much gone by the next spring when the tree needs food to kick off the growing season. This mulch layer would also hold in precious moisture for the trees. If the tree holds its leaves until spring, then releases them to the ground below, they may act as quick-start nutrients as the growing season begins, and this is most important for the smaller trees under much of the canopy from larger trees.

On a related note, in some years, rapid onset of early frosts or freezes may halt the abscission process and cause many other deciduous trees to hold their leaves into part of the winter season. This would include varieties of maples and other species, but as the winter wears on most of these trees finally do lose their leaves.

There’s no debate that the muted browns and yellows of marcescent leaves provide a beautiful backdrop in the bare forests of the winter. In addition, one benefit of trees like the beech, which keep most of their leaf canopy during the winter, is for birds who can seek shelter from the cold winter temperatures and winds among those clumps of leaves.

For those who choose to take that wonderful saunter through a forest path during the winter, you now know why there are trees who choose “not to go naked” during the season, but wait to complete a quick change as nature’s spring fashion season swings into full gear. Enjoy your time outdoors; nature is truly amazing!

Tom Niziol (left) and Dave Whitman (right) skiing
Figure 4. Tom Niziol (left) and Dave Whitman (right) enjoying a day out in the woods with beech trees, fully adorned with those tan leaves, in the background at Knox State Park, East Aurora, New York. Image courtesy Tom Niziol.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Tom Niziol

Tom Niziol recently retired as winter weather expert for the Weather Channel after a 32-year career as a forecaster, science and operations officer, and meteorologist-in-charge at the National Weather Service office in Buffalo, NY. Tom has published several papers and taught forecasters around the world through the COMET Program. His keenest winter weather interest is lake-effect snow.

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