If you’ve seen trees hold onto some brown leaves during the winter, this adaptation is called marcescence.
We all know it’s normal for trees to loose their leaves in the fall. Cold weather signals the tree to do so. When this happens, the cells around the vein of the leaf stem, which carries the water and nutrients to the leaf, slowly closes off. These cells form the abscission layer at the base of the leaf petiole (leaf stalk). When the leaf falls off, it exposes a leaf scar. Above the scar is a leaf bud, which is where the new leaf will emerge in the spring. You can actually ID trees in winter by the shape of the leaf scar.
In trees that have marcescence, the cells in the veins carrying water to the leaf closes over winter instead of so quickly on the fall. The leaf doesn’t form an abscission layer at the base of the leaf petiole (leaf stalk) until later winter. Some think this may be due to the lack of an enzyme the tree has. Wind of course can cause the leaf to go ahead and break off.
Some tree species are considered everciduous, and show marcescence no matter their age like oak, beech, hornbeam, and witchhazel. Sometimes juvenile trees exhibit marcescence, but looses it as it matures. Some trees may hold onto its leaves just on lower branches, or they may do it one year but not the next. Abnormal weather can cause marcescence on trees, such as maples, when there’s a long warm autumn followed by quick cold.
There are several hypothesis for marcescence. One is that it may protect the leaf bud during the winter weather.
Another reason is the leaves are beneficial to the tree and it saves some for spring. Releasing them in the spring provides a mulched layer that holds moisture and increases the soil’s organic matter, which improves soil texture and ability to hold and release moisture. The leaves also provide nutrients as they break down into the soil in spring. I think this hypothesis makes the most sense.
Another theory is that the leaves on the trees hold onto snow and water. When this melts in warmer temperatures, it provides more moisture to the tree.
The leaves may also deter animals like deer from feeding on the trees since they aren’t palatable or maybe because they’re noisy and scare the browsing animals. This seems possible on young trees that hold onto their leaves on lower branches.
All of these hypothesis are beneficial factors to the tree, so it could be an adaptation that serves multiple purposes. Next time you see a winter tree hanging onto some leaves, you can remember that’s called marcescence and know that it’s there to help the tree survive better.