Beeches are one of the few species that hold onto their leaves in the winter. Stripes on this beech leaf are evidence of beech leaf disease, detected in Sanford in 2024.
Photo: MDACF
Why Do Some Leaves Stay Put All Winter Long?
By Kevin McKeon, Maine Master Naturalist
In a previous column, we talked a bit about the colors of leaves and mentioned that some leaves remain affixed to their trees throughout winter. This trait, called marcescence, is specific to only a few trees – in this area, oak, beech, witch hazel, and hop-hornbeam. During our trail and woods walks, these are quite easy to spot, so now seems like a good time to investigate this phenomenon.
As the Northern Hemisphere tilts away from the sun, its light rays hit our area at a less-direct angle, so less heat is offered. This tilting also results in a shorter period of time when the sun’s rays flood our area, “shortening” our days. As we know, this brings on our winter. The shorter days are perceived by many organisms as a lowered photoperiod, which triggers various reactions by many plants and critters, including trees.
Trees that shed their leaves are called deciduous, as opposed to Maine’s famous evergreens. Not very well known is that evergreens lose about one-third of their leaves — needles — during leaf-fall time. But it’s the deciduous trees that have the remarkable reaction to lowered photoperiods by entering a dormant or hibernation phase. Part of this phase is the shedding of leaves that become less efficient at making food for the tree due to decreased sunlight and lowered photosynthesis. This is when the incredible evolutionary adaptation of leaf drop begins.
A layer of cells begins to form where the leaf stem attaches to the tree’s twig; this is called the abscission layer, and it performs two primary functions. First, it pulls chlorophyll and other nutrients back into the tree; the loss of the green chlorophyll from the leaf reveals its striking color we get to enjoy. The abscission layer also begins to stop the flow of food and water from the tree to the leaves. After the tree has sucked all the nutrients from the leaf, the cells complete formation of the abscission layer at this attachment point, effectively detaching the leaf from the tree, and it falls to the ground. There the leaf becomes one of many parts of the forest floor’s detritus layer — providing both homes and food for the tiny, microbiotic critters which, in turn, form the rich organic humus soil that feeds back to the tree and surrounding flora.
But not all trees release their leaves in the fall. The deciduous trees that hold onto these now-dead leaves have evolved a tactic to delay completion of the abscission layer, thus retaining their leaves. And it seems that this characteristic is more abundant on lower branches and on younger, shorter trees. So, scientists and naturalists are full of various speculations as to why some trees do this. Here are a few of those theories.
As noted above, dropped leaves decompose and add nutrients to the soil around the tree, effectively feeding its roots. Leaves that have fallen in the autumn often get wind-blown away from the tree’s root area, thus unable to enrich the immediate soil area. Also, late-winter/early-spring snowmelt could leach the leaves’ nutrients beyond the tree’s surface roots while still in dormancy, lessening nutrient availability for the tree. So maybe the tree holds onto its leaves in an effort to fertilize itself, dropping its leaves in the early, usually wet springtime, thereby holding leaves close to its surface root system and giving the tree a competitive, early season feeding advantage upon emerging from dormancy. The argument against this theory is, why would the tree, by holding its leaves, increase surface area for snow to accumulate, adding weight and increasing the possibility of branch breakage? The answer may be that more water is made available for hydration during its awakening from dormancy, because the accumulated snow on the leaves drops near the tree to melt in spring close to the surface root system.
Another theory: A 2001 Danish study had Claus Svendsen showing that marcescence has a protective effect for trees. Marcescent, dry leaves, which have lower nutritional value, were hand-picked, exposing the young leaf buds and twigs and making the tree more attractive for deer browsing compared to other trees, where the leaves were left alone. This bud/twig protective theory is further advanced by the seemingly proliferous marcescent leaves on lower branches and shorter trees.
(Interesting note: Deer get most of their wintertime nutrition from stored fat; their wintertime browsing mostly acts to activate their digestive system, which generates cold-weather warming.)
By holding its leaves until the food-scarce early springtime when browsers are very active and hungry, the now freshly shed leaves make the immediate area under the tree noisy when critter feet approach, making them more vulnerable for predation, discouraging browsers from entering the area; thus the tree’s fresh buds and twigs are protected.
Or maybe marcescent-leaved trees are behind the evolutionary scale and have simply not yet learned to drop their leaves and are maybe evolving other capabilities for survival — like acrid-tasting sap or thorn coverings.
So is marcescence an evolutionary adaptation for tree self-fertilization, increased spring hydration, protection from browsing, slow learning? Well, we trail walkers now have another of nature’s wonders to ponder while espying our area’s brown leaves.
So, get out in the woods this winter; there’s so much to see and appreciate!
The post Out in the Woods appeared first on Sanford Springvale News.
