The Mystery of Marcescence: What does Nature Know?
One of my favorite sights on a winter walk is a young beech tree with its pale creamsicle leaves
almost glowing in the largely monochrome forest.
Such trees are displaying marcescence, the somewhat mysterious phenomenon in which Autumn leaves don’t fall. In cultures across northern temperate zones both deciduous and evergreen trees are familiar and symbolic. With a few exceptions, deciduous trees drop their colorful leaves and give us the name of the season, while evergreens keep their green needles, signaling life through the cold and short days of winter. They drop their needles too, but gradually and continuously rather than all at once. The two distinct patterns are so familiar as to seem inevitable, but they point to the intelligence and adaptability of nature.
But a few deciduous trees, especially beeches and oaks, try a middle way, and hold onto their dried leaves through the winter. It’s especially common on young trees and sometimes on the lower branches of mature trees. Some research has shown that when they do drop, marcescent leaves do so in a short interval around the spring equinox. Deciduous trees shed their leaves to survive through the winter by conserving resources through the process of abscission (“cutting away”) that allows the leaves to drop without damaging the branch. First the tree reabsorbs and stores the nutrients contained in the chlorophyll that gives leaves their summer green color. This allows the reds, yellows, and oranges of Autumn to show. Then it creates two layers of specialized cells to seal the branch and separate the leaf stem from it, letting it drop.
Given the protective value of leaf drop and its fundamental place in the change of seasons, why don’t marcescent trees do this? What is the purpose of keeping those leaves? What does
Nature know here? There are several theories about why marcescence happens, but not much formal or conclusive
research, and it seems likely that it may depend on local conditions, environmental factors, and individual species traits.
One common explanation is that by holding onto unpalatable leaves, the trees protect new
buds from deer and other herbivores. Other explanations have to do with nutrients: keeping
the leaves may allow more time for nutrient reabsorption and photosynthesis; and dropping
them later, close to the tree, may positively impact decomposition and nutrient availability in the soil to support spring growth; retained leaves may catch snow and hold water like mulch; or they may provide habitat for birds and small animals that will fertilize the soil with their wastes.
All of these seem like good strategies for a young tree. And maybe they are true. All those possibilities certainly point to the creative power of nature to persist and adapt. But I sometimes like to think of that young beech as a just a spunky, contrarian spirit with a desire to
stand out in a winter crowd.
– Julie Burch, Founding member and future resident of RCE, January 13, 2024