Trees and shrubs are often categorized into evergreen or deciduous depending on how the transition from summer to winter impacts their leaves. Coniferous species, such as pine, spruce, cedar and fir are green throughout the year, including the winter.
The waxy-coated needles of these “evergreens” are able to retain green chlorophyll throughout the cold months. It’s important to note that evergreen needles don’t actually last forever, but rather are shed every 2-5 years depending on the species. This needle drop often goes unnoticed as it overlaps with newer needles that stay green.
Deciduous species, on the other hand, generally drop all their leaves in autumn, which is why we also call it fall. Cooling temperatures and shortening daylength triggers deciduous species to develop an abscission layer at the base of the leaf petiole (stalk), leaving the leaf dangling until wind or gravity pull it off. However on a few species, this abscission layer does not develop completely and the withered leaves can be retained well into winter and even into the following spring. This retention of dead leaves is called marcescence.
In our region, the most prominent marcescent species are oaks. Although most oaks, with the exception of bur and chinkapin oak, show some level of marcescence, shingle, white, scarlet, black, hills and pin oak are especially capable of holding on to their spent leaves well into winter. Other species that have varying degrees of marcescence include beech, ironwood, musclewood, horsechestnut, smoketree, boxelder, Japanese maples and some sugar maples. Marcescent shrubs include witchhazels and some viburnums.
Marcescence is not just about leaves as it can also apply to flowers, seeds and fruits. Many trees retain fruit parts or seeds through the winter—coffeetree, ash, catalpa, sycamore, pagodatree, hawthorns and crabapples. And the marcescent male flowers (deformed by gall insects) can be very prominent on male white ash trees, providing a very useful winter identification aid.
Marcescence can be quite variable depending on genetics, growing conditions and the age of trees. Oaks often lose much of their leaf retention as they mature. And leaves protected from wind and snow can also last longer on some species. Scientists aren’t sure why marcescence evolved, but one theory is that it may provide some protection from deer and animal browsing during winter. Another theory is that winter leaf retention can help capture more snow and increase spring soil moisture.
Depending on one’s perspective, marcescence has both benefits and drawbacks. For wildlife enthusiasts, marcescent trees and shrubs provide winter protection for several types of birds, greatly increasing backyard diversity. Also, for someone like me who doesn’t like to rake leaves, I appreciate that oak leaves are shed gradually throughout the fall and winter and thus allowing more time for leaf cleanup (or lack thereof) to occur. People who like to get their raking done promptly in the fall, however, often despise this spread out chore. Another potential problem is that marcescent species can be more prone to damage from ice storms and heavy snow loads, although oaks seem to weather these occurrences fairly well.
One benefit of marcescent species growing in importance is that they help expand the palette of trees suitable for windbreaks and visual screens. As we lose many of our evergreens to a growing list of diseases, insects and weather extremes, we’d be smart to diversity our shelterbelts by adding more marcescent species like shingle, scarlet and black oak. And across a community, marcescent species are helping to soften the harsh winds, thus making our winter existence much more tolerable.
Justin Evertson, Nebraska Statewide Arboretum, plantnebraska.org