Holding on Through Winter

As nights lengthen in the Autumn and food supply becomes more scarce, chlorophyll production slows, stops, and is eventually destroyed, causing deciduous trees in our temperate region to head into dormancy in truly spectacular fashion. The color change of leaves, leaves dropping from deciduous trees and shrubs, raking, and the smell of burning leaves are associations most midwesterners attach to Autumn, and many even look forward to. But why do some trees and shrubs still cling to them all through winter after most deciduous plants have dropped their leaves? This mystery is known as marcescence (pronounced ‘mar-CESS-enss’), which comes from the Latin word marcescere (“to fade”). In a temperate zone, deciduous trees go through a process called abscission in the fall. It is an active process that enables plants to shed their leaves in the winter to conserve energy and prevent moisture loss. An abscission layer forms at the base of the petiole, sealing off the tree from the leaf, causing cell death and self-pruning of the leaf. In marcescent plants, the abscission layer doesn’t form until spring, and the base of the petiole remains alive for this duration. 

Species of Oak (Quercus) still retain leaves in late January.

The most common woody plants in our area that typically display marcescence are oaks (Quercus spp.), American beech (Fagus grandifolia), American Hornbeam (Carpinus caroliniana), eastern hophornbeam (Ostrya virginiana), and witch hazels (Hamamelis). There are some species of Willow (Salix), that exhibit marcescence and other species that will infrequently hold their leaves depending on weather. 

What is the reason for marcescence? While this process is documented and studied, scientists are still speculating why. One theory states that dropping leaves later helps provide nutrient-rich mulch and moisture retention in dry, poor soil conditions in the spring when trees come out of dormancy and are actively growing. Another hypothesis is that the leaves provide more shelter to birds from winds throughout colder months. Additionally, retention of dry, wilted leaves prevents browsing by herbivores such as deer. Such leaves are an unappetizing food source for wildlife. Furthermore, this could explain why younger trees exhibit this trait, and often, retention only occurs on lower branches.

Although the reasons remain unclear, marcescence is an intriguing process that adds variety and interest to any wooded area throughout winter. Look for trees holding onto their leaves next time you’re at the Gardens. Observe the stark contrast against the majority of deciduous trees with bare branches. Before too long, we’ll be heading into spring. The marcescent trees will be pushing out new buds like the rest and, in the process, shedding those remaining leaves.

Kyle Strain

Manager of Public Horticulture