Most winter trees fall into one of two categories we readily recognize. There are the deciduous trees, whose leaves fall off in winter, and there are the evergreen trees, who keep their leaves throughout the winter. But there are trees that don’t squarely fit into these categories.
In the photo above, you are looking at an American Beech, which is a deciduous tree. And although its leaves have wrinkled and faded, they hang on tenaciously to the tree. Take a walk in the Sheldrake woods and you’ll notice them right away, like a ballroom of debutantes frozen in time, not ready to hang up their petticoats for the season.
There is a name for what these trees are doing: marcescence, or the retention of dead plant organs. In deciduous trees autumn triggers a chemical change that makes the tree grow a layer where the leaf stems connect to the branch. This layer creates a separation between the leaf and the branch, and prevents water and nutrients from reaching the leaf. That separation, also known as abscission, is the reason leaves change color, shrivel, and fall off. But in marcescent trees such as oaks, beeches, and witch hazel, even though the abscission layer forms, and the leaves shrivel and fade, they do not fall off.
Why? Why would a tree want to hold on to dead leaves that can no longer produce food?
This is one of those delicious questions that we actually do not know the answer to. There are theories, but in the absence of sufficient testing, we cannot say which is correct.
Hypothesis #1: Protection from browsers
It is possible that marcescent leaves may provide cover for tree buds from large browsing herbivores such as deer or moose. Crumbly zero-nutrition leaves may provide a layer of deterrence, since animals would have to eat through the leaves to get to the nutritious buds and twigs they like to eat, if they can even see them through the leaves!
In support of this theory is that marcescent leaves are primarily found on lower branches and young trees, whose buds are within reach of browsers.
Hypothesis #2: Strategic decomposition
A second theory posits that there is an advantage to dropping the dead leaves in the spring, as new growth finally pushes off the old leaves. Marcescent trees such as beech and oak traditionally grow on infertile soil. It’s possible that by dropping the leaves in the spring, they will decompose at the time when the tree most needs the organic compost to spur new growth.
Personally I find this explanation less convincing, for it does not explain why only the younger trees or lower branches sport the dead leaves. Also, decomposition in winter is very slow due to the shorter hours of sunlight and the dormancy of many decomposers, so I am not certain there is much to be gained by waiting until spring to drop to the ground.
Hypothesis #3: Evolutionary vestiges
The first trees were all evergreens, keeping their waxy leaves through winter. As they succeeded and spread, they evolved to suit the different climates in which they found themselves. Some found it advantageous to shed their leaves over winter, to avoid freezing or depleting their nutrient store, and deciduous trees were born. It is possible that marcescent trees such as beeches and oaks are an evolutionary halfway point–deciduous trees that retain their (dead) leaves through the winter. Perhaps these clinging ghosts are an evolutionary vestige, like the human appendix or tailbone, which serves no function but has persisted over time. Lending support to this explanation is the fact that some beech and oak species are evergreens. The Southern Beeches are 43 species of evergreen trees and shrubs that until recently were classified in the same genus as beeches (They now have their own genus, Nothofagaceae, which translates to “bastard beeches.”). Their fossil record dates back to the Cretaceous period. Among oaks, the Quercus ilex is an evergreen.
What do you think? Which do you find most convincing? Whatever the explanation, take this time to walk in the woods and enjoy the rustle of the wind rattling these stubborn stragglers.