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).
What’s interesting is that scientists haven’t figured out exactly why some trees retain their leaves.
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 Shed?
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.