Looking out my kitchen window, I noticed that I can see my neighbors’ houses through the trees. The leaves that hid them throughout the summer have now dropped from the maples, poplars, and sweetgums in my backyard. But there are a few trees still holding their leaves.
While most deciduous trees drop their leaves in the fall, several species exhibit marcescence, the condition of holding onto dead leaves through winter months. Oaks and beech trees are noted for this characteristic. Researchers aren’t sure why some trees keep their leaves, but they can explain how.
Life cycle of a leaf
Leaves are the food factories of plants. Cells containing chlorophyll align in leaves where the leaf surface meets the sunshine. For broadleaf plants, this is generally the upper surface. As long as the leaf receives enough water from the root system and light from the sun, chlorophyll cells produce carbohydrates to nourish the plant. Chlorophyll also gives leaves their green color.
During shortening days when summer slides into fall, trees receive fewer hours of daylight and experience longer, cooler nights. With less available light, chlorophyll cells begin shutting down. Leaves produce less food for the tree, and the fading chlorophyll pigment allows other colors to show. This process of changing from green to the red, orange, and gold colors of fall is called senescence.
All of these seasonal changes trigger certain responses from the tree. The tree slows growth and prepares for winter dormancy. This includes shedding leaves.
Leaf drop, or abscission, is also a process. Leaves have a zone of separation between the petiole (leaf stem) and the twigs that hold them. While the leaf is actively producing food, the tree keeps the separation cells open. When food production drops due to chlorophyll loss, however, the tree begins closing down the living tissue through which nutrients flow from the leaf to the tree. Water continues to flow to the leaf, but eventually, the tree closes these cells, too. When the leaf is completely sealed off from the tree, the weakened abscission zone allows the leaf to drop from the tree under the force of wind or rain. The sealed area protects the tree from insects and disease organisms much like a scab over a cut protects us from infection.
Marcescent leaves
Some trees don’t create an abscission zone in the fall. As a result, dry, brown leaves remain on the tree well into winter months. In the case of beech trees and some oaks, the marcescent leaves stick around until new spring growth shoves them off the tree.
Forestry experts have differing opinions about what, if any, advantage marcescence gives to trees. Since young trees often exhibit marcescence, one theory is that holding leaves until spring is a way for smaller trees to recapture nitrogen during active growth periods. If the leaves dropped in the fall, the nitrogen released through winter decomposition might get sucked up by larger trees, or it might leach into deep soil layers that are beyond the reach of tree roots. Another theory is that the dry leaves protect tender twigs and buds from browsing deer. Other researchers propose that leaves that continue to function later into the fall may simply die before the tree finishes creating the abscission zone. Because they retain a strong connection to the tree, these leaves don’t get blown off by wind and rain.
Whatever the reason they stick around, marcescent leaves serve at least two purposes. They provide protective cover for some birds, and they add sensory interest to winter landscapes.