Have you noticed a tree around town that holds its brown leaves all winter instead of dropping them?
There's a term for this curious leaf-retention phenomenon. It's called marcescence. And if it's a conical-shaped understory tree with bleached, light tan leaves, it's probably an American beech (Fagus grandiflora).
"Basically, that means that things hold onto stuff," said Jim Finley, a Pennsylvania Extension Service forester who is also a professor of forest resources and director of the Center for Private Forests at Penn State. Marcescence occurs in other trees beyond beech trees. Leaf retention also occurs in many oak species, witch hazel, hornbeam (musclewood) and hophornbeam (ironwood), said Finley, who added that it's more common with smaller trees, or more apparent on the lower branches of larger trees.
What's interesting is that scientists haven't figured out exactly why some trees retain their leaves. "It’s all speculation," sad Finley, who said there appears to be little new literature about the topic in recent years.
"I did a scholar search and went through about 200 publications," he said. "Many of the dates, at least in North American publications, were somewhere between about 1936 and 1975 or 1980." The only recent article on marcescence he found was a deeply scientific piece published in 2013. Interestingly, he added, there seems to be more interest in botanical literature about marcescence in palm trees in Mediterranean and tropical climates than in hardwood trees in North America.
Theories about leaf retention
Though there's a lack of scientific conclusions about why marcescence occurs and its possible benefits, there's no shortage of speculation. That speculation, Finley said, essentially involves nutrition recycling and water conservation and protection against browsing animals. Here are his thoughts on the matter:
Nutrition cycling and water conservation. If leaves of marcescent trees fell off in the fall, two things could happen that could deprive the tree of nutrients in the spring when it begins a new growth cycle. One is that winter winds would scatter the leaves here and there and the tree would lose the nutrients it would otherwise get from decaying leaves. The second is that even if the winds didn't blow the fallen leaves away during winter, the nutrients from leaves that fell in the autumn and joined others on the forest floor would be leached away before they could become available to "feed" the trees the next growing season. This might be especially important to small understory trees with smaller root systems. Perhaps, therefore, beech and other marcescent trees retain their leaves through the winter so that when they fall in the spring there’s some likelihood the leaves are going to remain near the tree. In doing so, they would create a mulch layer that’s going to stay there a little while. So that possibility involves not only nutrient cycling but conservation of water resources.
Protection against browsing animals. It's possible that the dried leaves may conceal buds from browsers or make them difficult to nip from the twig. Researchers have found that dried tan and brown leaves are less nutritious than green leaves. At least one study out of Denmark found that deer offered hand-stripped twigs preferred those to marcescent twigs, especially of beech and hornbeam, but not so for oak. Nutrient analysis found the protein content of oak twigs was higher and the dead leaves had less lignin, complex organic polymers that form the main part of woody tissue in vascular plants. The protein content of beech and hornbeam twigs was about equal to the leaves; however, the lignin content was nearly half again higher in the leaves.
What causes marcescent leaves to fall off as spring approaches?
All trees shed leaves, even conifers, though conifers generally retain their needles for more than one year, Finley pointed out. What happens, he explained, is that as deciduous trees 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 of weak cells that "unglues" the leaf and allows it to fall free. Leaf drop benefits deciduous trees by reducing water loss through leaf respiration and allows the trees to create new leaves that efficiently use available sunlight during warmer seasons.
Sometimes, early cold weather or frosts may interrupt the abscission process or "kill" leaves quickly, Finley continued. In these cases, the occurrence of marcescent leaves may increase. But, lacking killing frosts, why would trees "decide" to retain their leaves? Well, it's impossible to know since botanists can't ask the trees!
Another factor that might affect and slow the abscission process in the case of smaller trees, which in forest conditions would be growing beneath taller trees, is reduced sunlight. In this instance, the understory tree leaves and the leaves on lower branches of larger trees would also have the opportunity to continue or even increase their photosynthetic process as upper leaves fall. Then, Finley, observed, perhaps, leaves lower in the canopy are "caught" with cold temperatures and their leaves hang on.
Regardless of the reason for marcescence, when growth begins in the spring, new leaf buds will expand, push the old leaves off and clothe the branches with new greenery. Until that happens, Finley suggests we should just enjoy the waving brown leaves rattling in winter winds and the texture they add to forest and yards. But, he admits, marcescence does raise a question.
Why should we care?
Marcescent trees can provide shelter for birds against the elements and predators. (Photo: K M/flickr)
It's natural for people to care about something as obscure as marcescence, Finley said. "I'm as much a social scientist as I am a botanist, and I did a study for the U.S. Forest Service about people’s love and concern for forests. People have some amazing connections to trees and forests. There's just a natural linkage there."
There are also some practical reasons for people to know about marcescence, Finley added. "Having a tree that retains its leaves all winter long is a good place to put a bird feeder. It is kind of nice because it provides some protection from the elements and predators."
In addition, "it's just a fun thing to know as you drive around and you see these things," he said, adding that it helps people understand what's happening in the natural world around them. And, for those who happen to have a mountain or lake getaway, planting an understory of beech trees can provide another layer of cover to evergreens such as laurel, rhododendrons and hemlock. They can also create bedding and feeding areas for wildlife such as turkey and deer.
Finley said his study showed that even people who might not routinely think and care about trees and forests and things associated with them, such as marscence, do care about the natural world and can be deeply affected by what they see.
Christopher Martin is such a person. Martin teaches English at Kennesaw State University in Kennesaw, Georgia, and creative nonfiction at the Appalachian Young Writers Workshop. He is also a prize-winning writer and author of the poetry collection "Marcescence: Poems from Gahneesah." Gahneesah is the anglicized form of the Cherokee name for Kennesaw Mountain, an outcrop north of Atlanta that was the site of the Battle of Kennesaw Mountain during the Civil War. In the fighting, Confederate forces under the command of Gen. Joseph E. Johnson tried but failed to stop Maj. Gen. William T. Sherman's Union army as it advanced on Atlanta.
"Gahneesah" means "burial ground" or "place of the dead," which adds a layer of myth and richness to the literal, botanical process of marcescence — essentially, dead leaves clinging to living trees until they're replaced by new growth," said Martin. At the time of his winter visit to the battlefield, now a national park, he was unfamiliar with the marcescence habit of the beech leaves that inspired him to write poem. "The moment described in the poem led me to do some casual research on beech trees, and that led me to the word," he said. "So the poem itself was a process of discovery, which was cool."
To keep the circle of art and science going, here's the poem, published with the author's permission.
I hike a horse trail, tread moss and mire west of Kennesaw Mountain,
cross Noses Creek’s crumbling banks. I stop, rest, sit on a rotting log
where stone piles of Confederate earthworks cover the ground,
testaments to what this place has seen, remnants of what it has been.
Here the woods are white, brittle with leaves still clinging to beech trees.
From a fallen beech, a hermit thrush murmurs, flutters farther into brush
when it sees me. Three whitetail does stand vigilant, in an instant vanish
through dusk, tails flared, one with trembling leaves these branches
will bear until spring, will bear as my own limbs hold whispers stirring,
these stories of what it means to die, yet remain bound to a living thing.