Information

Why do some trees hold their leaves through fall and winter?


Why do some species of oak (Quercus spp.) retain their leaves through fall and winter? I've found that these leaves are called marcescent leaves. There must be some benefit that the tree gains from this because it seems like it would increase the possibility of limb damage due to snow accumulation on the leaves.


The trait might not necessarily allow for an advantage, but a few possibilities have been proposed:

  1. Nutrient return to the soil when needed in the spring
  2. Less palatability to grazing animals

Source:

http://northernwoodlands.org/articles/article/why-do-some-leaves-persist-on-beech-and-oak-trees-well-into-winter


Why trees shed their leaves

November 2017 on Maple Street in Johnson City, Tennessee. Image via Teri Butler Dosher.

In temperate forests across the Northern Hemisphere, trees shed their leaves during autumn as cold weather approaches. In tropical and subtropical forests, trees shed their leaves at the onset of the dry season. Many types of trees shed their leaves as a strategy to survive harsh weather conditions. Trees that lose all of their leaves for part of the year are known as deciduous trees. Those that don’t are called evergreen trees.

Common deciduous trees in the Northern Hemisphere include several species of ash, aspen, beech, birch, cherry, elm, hickory, hornbeam, maple, oak, poplar and willow. In tropical and subtropical regions, deciduous trees include several species of acacia, baobab, roble, ceiba, chaca and guanacaste.

Image via Tosca Yemoh Zanon in London Photo via Daniel de Leeuw Photography

Most deciduous trees have broad leaves that are susceptible to being damaged during cold or dry weather. In contrast, most evergreen trees either live in warm, wet climates or they have weather-resistant needles for leaves. However, there are exceptions in nature, such as tamarack trees that shed their needles every autumn and live oaks that retain their broad leaves for the entire year even in relatively cool climates.

Shedding leaves helps trees to conserve water and energy. As unfavorable weather approaches, hormones in the trees trigger the process of abscission whereby the leaves are actively cut-off of the tree by specialized cells. The word abscission shares the same Latin root word as that in scissors, scindere, which means “to cut.” At the start of the abscission process, trees reabsorb valuable nutrients from their leaves and store them for later use in their roots. Chlorophyll, the pigment that gives leaves their green color, is one of the first molecules to be broken down for its nutrients. This is one of the reasons why trees turn red, orange, and gold colors during the fall. At the end of the abscission process, when the leaves have been shed, a protective layer of cells grows over the exposed area.

Layer of abscission cells separating a leaf from its stem. Image Credit: U.S. Forest Service.

The shedding of leaves may also help trees to pollinate come springtime. Without leaves to get in the way, wind-blown pollen can travel longer distances and reach more trees.

Autumn leaves. Image Credit: Tracy Ducasse.

Bottom line: Many types of trees shed their leaves as a strategy to survive cold or dry weather.


Why Do Trees Lose Their Leaves at All?

As with most everything else in the natural world, tree behavior revolves around the sun. Once trees detect a reduction in the amount of daylight, they start to reduce the amount of chlorophyll they produce.

Chlorophyll, of course, is the pigment that makes leaves green and is the primary producer of energy for almost all plants. Once chlorophyll production stops, it gets broken down and taken back into the tree. This reduction in chlorophyll allows other pigments that have always been present in leaves, but not visible due to the overwhelming amount of chlorophyll, to be seen (like yellows, oranges and reds).

Read more about why leaves change color in a previous blog post.

Image from the Public Library of Science.

To go along with this change in color, trees prepare to shed leaves by growing a layer of cells between the leaf stem and the tree branch known as an abscission layer. This layer stops the transport of nutrients and water to the leaf and becomes the main physical reason trees lose their leaves. The abscission layer also helps protect this sensitive area of the plant from winter cold and dryness.

The main reason for leaf drop on most trees is that, come winter, it gets pretty cold and dry in our part of the world. Rather than expend energy to protect these fragile organs, trees shed leaves to conserve resources.

The main problem is that water is essentially unavailable to most plants in the winter (because it’s ice), so there is no way for them to replenish what they lose. And, as far as I know, there is no revitalize and replenish lotion available for trees.


Benvie, Sam. Encyclopedia of North American Trees. Buffalo, NY: Firefly, 2000.

Brockman, C. Frank. Trees of North America: A Guide to Field Identification. Ed. Herbert S. Zim. New York: Golden, 1986.

Cliburn, Jerry, and Ginny Clomps. A Key to Missouri Trees in Winter: An Identification Guide. Conservation Commission of the State of Missouri, 1980.

Collingwood, G. H., Warren David Brush, and Devereux Butcher. Knowing Your Trees. Washington: American Forestry Association, 1978.

Dirr, Michael. Dirr's Hardy Trees and Shrubs: an Illustrated Encyclopedia. Portland, Or.: Timber, 1997.

Elias, Thomas S. The Complete Trees of North America Field Guide and Natural History. New York: Book Division, Times Mirror Magazines, 1980.

Grimm, William Carey. The Book of Trees. Harrisburg, PA: Stackpole, 1962.

Hightshoe, Gary L. Native Trees, Shrubs, and Vines for Urban and Rural America: a Planting Design Manual for Environmental Designers. New York: Van Nostrand Reinhold, 1988.

Little, Elbert L. National Audubon Society Field Guide to North American Trees. New York: Chanticleer, 1996.

Martin, Alexander C., Herbert S. Zim, and Arnold L. Nelson. American Wildlife and Plants. New York: McGraw Hill, 1951.

Mitchell, Alan F., and David More. The Trees of North America. New York, NY: Facts On File Publications, 1987.

Randall, Charles E. Enjoying Our Trees. Washington: American Forestry Association, 1969.

Settergren, Carl D., and R. E. McDermott. Trees of Missouri. Columbia: University Extension, 1995.

Sternberg, Guy, and James W. Wilson. Native Trees for North American Landscapes: from the Atlantic to the Rockies. Portland: Timber, 2004.

Wharton, Mary E., and Roger W. Barbour. Trees and Shrubs of Kentucky. Lexington: University of Kentucky, 1973.

Wyman, Donald. Trees for American Gardens. New York: Macmillan, 1965.


Some trees hang tight to their leaves

It’s that time of year when the leaves are changing color and dropping onto our lawns. Even though it may seem like all the trees are losing their leaves, there are a few deciduous trees that hold onto their dried leaves throughout the winter.

Marcescent (mahr-CESS-ent) is the term used to describe this type of leaf retention which is common among many species of oaks (Quercus), American beech (Fagus grandifolia), witch hazel (Hamamelis virginiana), musclewood (Carpinus caroliniana), and ironwood (Ostrya Virginiana). These leaves are dry, but yet springy — they don’t crumble into pieces. Rumor has it some early settlers used marcescent leaves, instead of straw, to stuff their mattresses. The leaves will stay on the tree until wind or snow break them off, spring’s expanding buds push the leaves off the branch, or there may be some leaves that stay attached.

Tree genetics is the main reason for leaf retention, but early cold weather or frosts can increase the occurrence of marcescent. It may also be a sign of problems if it occurs before seasonal changes or if it is on isolated stems. Marcescent is common with young trees and may disappear as the tree matures, small trees, or more apparent on lower branches of larger trees.

Nancy Rose, a horticulturist with the University of Minnesota Extension explains why some trees drop their leaves and other retain them. “In autumn, the leaves of most deciduous trees develop an abscission layer where the petiole (leaf stalk) meets the branch. This allows the leaves to fall off without leaving an open wound on the stem. Dry leaves stay on marcescent trees because the leaves didn’t develop the normal abscission layer in autumn.”

There is much speculation on why a tree holds onto its leaves. One idea is that the dried leaves serve as protection for the trees, either from being eaten by deer as the leaves tend to be low in nutrients and difficult to digest, and/or protecting the buds and twigs from frost throughout the winter.

Another thought is the trees hold onto the leaves through the winter so that when the leaves are dropped in the spring there is more organic material as the leaves decay, available to the tree when it is needed the most. This might be important to small understory trees with smaller root systems, by holding onto their leaves until spring they retain and recycle the nutrients for themselves.

There is also the theory that the dried leaves are an effective way to trap snow, which leads to more moisture at the base of the tree come spring.

Whatever the reason these trees hold onto their leaves, the dried leaves can add winter interest to the landscape and may serve as a source of winter protection for birds.

Chris Jacobs is a certified master gardener in Dodge County.

Get to the root of the problem

The Reporter periodically publishes responses to questions submitted to the Dodge County Master Gardener Association on topics of general gardening interest. If you have a question, send an email to [email protected] A certified Master Gardener will send a reply directly to you in a timely manner.

Dodge County Master Gardeners Association is an all-volunteer organization that assists UW-Extension by helping people in the community better understand horticulture.

It is made up of individuals who have taken 36 hours of training in topics that include soils, botany, entomology, plant pathology, houseplants, landscaping, turf, vegetable, fruit and ornamental plants.

Each member completes a minimum of 10 hours of continuing education and 26 hours of volunteer work every year. Master Gardeners are available as speakers for civic, church, school and social organizations.


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!

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.


Why Do Leaves Fall in Autumn?

In temperate regions of the world, autumn is marked by the brightly colored foliage that slowly drops from trees and shrubs to carpet the ground. But why do some plants shed their leaves before winter? It turns out autumnal leaf drop is a form of self-protection. While evergreen plants in cold climates have thick waxes and resins to protect their leaves from freezing and fracturing, deciduous species generally have thin leaves that are susceptible to cold temperatures. Since water expands when frozen, the tender leaf cells would rupture during the winter, making them useless for photosynthesis. Without dropping these leaves, such a tree would be stuck with thousands of unproductive appendages and no way to make food! As if that weren't reason enough, the surface area of all those leaves would also pose a threat to the plant’s physical integrity. Winter months are often windier than other seasons, and the wind against the broad leaves on a cold, brittle tree could cause major breakage. The same goes for the weight of snow collecting on all those leaves. Finally, by the end of summer, many leaves are insect-eaten, diseased, or otherwise damaged. Dropping them gives the plant a fresh start in the spring, and the nutrients from the decaying leaves are recycled to help grow the next leafy generation.

Interestingly, autumn leaves are not simply blown off trees but are separated from the plants in a highly controlled process. As day length shortens and temperatures cool, hormones within the plant are activated to begin the abscission process. Chlorophyll production stops and the pigment starts to degrade, often revealing showy reds and yellows that were masked by green. The vessels that carry water to the leaf and sugars to the rest of the plant are closed off, and a layer of cells, known as the abscission layer, starts to grow between the leaf stalk and the twig holding it. These cells serve to slowly cut the leaf from the plant without leaving an open wound. As the leaves fall, the plant enters dormancy, saving its energy for the great bud burst of spring.


Autumn Leaf Colors

During summer days, leaves make more glucose than the plant needs for energy and growth. The excess is turned into starch and stored until needed. As the daylight gets shorter in the autumn, plants begin to shut down their food production.

Many changes occur in the leaves of deciduous trees before they finally fall from the branch. The leaf has actually been preparing for autumn since it started to grow in the spring. At the base of each leaf is a special layer of cells called the "abscission" or separation layer. All summer, small tubes which pass through this layer carry water into the leaf, and food back to the tree. In the fall, the cells of the abscission layer begin to swell and form a cork-like material, reducing and finally cutting off flow between leaf and tree. Glucose and waste products are trapped in the leaf. Without fresh water to renew it, chlorophyll begins to disappear.

The bright red and purple fall foliage colors come from anthocyanin (an-thuh-'si-uh-nuhn) pigments. These are potent antioxidants common in many plants for example, beets, red apples, purple grapes (and red wine), and flowers like violets and hyacinths. In some leaves, like maple leaves, these pigments are formed in the autumn from trapped glucose.

Why would a plant use energy to make these red pigments, when the leaves will soon fall off? Some scientists think that the anthocyanins help the trees keep their leaves a bit longer. The pigments protect the leaves from the sun, and lower their freezing point, giving some frost protection. The leaves remain on the tree longer, and more of the sugars, nitrogen and other valuable substances can be removed before the leaves fall. Another possible reason has been proposed: when the leaves decay, the anthocyanins seep into the ground and prevent other plant species from growing in the spring.

Brown fall foliage colors come from tannin, a bitter waste product. Other colors, which have been there all along, become visible when the chlorophyll disappears. The orange colors come from carotene ('kar-uh-teen) and the yellows from xanthophyll ('zan-thuh-fil). They are common pigments, also found in flowers, and foods like carrots, bananas and egg yolks. We do not know their exact role in leaves, but scientists think they may be involved somehow in photosynthesis. Different combinations of these pigments give us a wide range of colors each fall.

As the bottom cells in the separation layer form a seal between leaf and tree, the cells in the top of the separation layer begin to disintegrate. They form a tear-line, and eventually the leaf is blown away or simply falls from the tree.

One more important question remains. What causes the most spectacular display? The best place in the world for viewing fall colors is probably the Northeastern United States. This is because of the climate there, and the wide variety of deciduous trees.

The brightest colors are seen when late summer is dry, and autumn has bright sunny days and cool (low 40's Fahrenheit) nights. Then trees make a lot of anthocyanin pigments. A fall with cloudy days and warm nights brings drab colors. And an early frost quickly ends the beautiful fall foliage color display.


Some trees tend to hang on to a portion of their leaves through the winter, making spring leaf drop perfectly normal. We usually think of fall as the season for shedding, but there are a few tree species that go against the grain.

But if you don’t have a tree that naturally loses its leaves in spring, your tree could have an infection. First, see what type of tree you have . Then, examine its fallen leaves to see if they’re curled and brown instead of smooth and green.

What are the trees that naturally lose their leaves in spring?

If your tree is dropping leaves that look green and healthy, all is probably good! You likely just have a tree that naturally sheds in spring. Below are the most common trees that do this.

Common Trees That Lose Their leaves in Spring

  • Hackberry
  • Hickory
  • Holly
  • Live oak
  • Southern magnolia

I don’t have one of those trees, so why are my tree’s leaves falling in spring?

If your fallen tree leaves appear curled, spotted, or brown, anthracnose could be the issue. Anthracnose is the catch-all name for different fungal diseases that attack all kinds of trees. Plus, it’s most common in damp, cool springtime weather.

What if my ash tree is losing leaves in spring? Is it likely anthracnose or something else?

Ash trees, particularly white and green ash, are often affected by anthracnose. You’ll see the same signs as listed above.

What treatment is there for anthracnose?

Fortunately, most tree types–including ash– can easily shake off anthracnose. While the fungus can cause some leaves to fall, a flush of fresh leaves should come in within a few months.

While you wait, the best thing to do is get rid of the branches seriously affected and reboot your tree’s health.


Why do my trees still have leaves in November?

Why do some of my trees still have leaves, even though it’s November? This is a question I receive every nearly every year. Although the question is simple, the answer is a little more complicated. Quite simply, it depends …

The simplest answer is that some tree species simply hold onto their leaves longer into the fall or winter than other species do. Our native ironwood tree is a classic example. The leaves first change to a yellow/orange color in the fall, and then to a tan hue later on.

Ironwood leaves, November 13, 2012.

And they stay on the tree in that color for a long time. I often see ironwood trees in the forest understory in both North Dakota and Minnesota. Red oak, which also grows in Minnesota but not in North Dakota, also tends to hold its leaves well into winter. The fruit of American linden (a.k.a. basswood) stays on the tree late in the fall, and it has a bract that looks like a leaf.

Leaf-like bract from an American linden fruit, November 13, 2012.

European buckthorn (a.k.a. common buckthorn), an exotic invasive species, holds its leaves much longer than other species and they often remain green late into the fall.

European buckthorn leaves remain green late into the fall, November 13, 2012.

The second possibility for trees holding onto their leaves relates to the origin of those trees. Quite simply, is this tree from an area south of here? To some extent, trees that originate further south tend to hold onto their leaves a little longer in the fall. Another way to look at this is that they don’t begin the process for becoming dormant (or winter hardy) as early as the native trees do. This is a bit dangerous as an early fall frost can kill twigs or even the whole tree if it hasn’t hardened off sufficiently. Planting ornamentals in a hardiness zone that they’re not adapted to can result in problems like this.

Similarly, we need to review the management of the tree during the growing season. If the tree receives excess nitrogen too late in the growing season, it tends to focus its energy on tender new growth. In the fall, it won’t be able to harden up as quickly as it normally would, so it remains green and tender, holding onto its leaves late in the fall. For this reason, we recommend avoiding fertilizer applications during July, August and the first half of September. For watering, we recommend cutting back during August. This “mini” drought stress should kick-start the dormancy processes, resulting in a tree that is fully hardy once winter arrives. Obviously, withholding too much water can cause problems as well, so a balance is needed.

Lastly, we need to look at the health of the branch/tree that’s still holding onto its leaves. If a branch dies suddenly during the growing season, it will retain its leaves and will not drop them in the fall. Certain diseases can also result in trees holding onto their leaves late in the fall. In many cases, this is an indication of a broader health problem.

In summary, there are several possible reasons that trees might hold onto their leaves. Usually, it’s not a good sign. It may indicate damage that has already been done, or it might be an indicator of damage yet-to-come. But in some cases, it might be nothing at all!