Why Leaves On Some Deciduous Trees Stay On Through Winter
We all learn in school that deciduous trees turn brilliant shades of yellow, orange, red or purple and then drop their leaves in fall. So why do your deciduous tree’s leaves sometimes hang on long after fall is over? Should you worry about tree leaves that stay on your tree throughout the winter?
In this article, we look at why deciduous leaves normally drop in fall, why some don’t, and what you should (or shouldn’t) do when tree leaves stay put until spring.
What Normally Happens To Tree Leaves In Fall
When the days shorten and weather turns cold or a sudden freeze occurs, a tree signals to its leaves that dormancy is arriving and their annual cycle is finished.
This is when the yellow, orange, red and purple underlying colors of leaves show up. The green, photosynthesizing leaf cells that have masked the more vivid colors during the growing season are turned off and the spectacular fall color show begins. We share more details about this autumn display here.
The signal to shut down for dormancy and drop this year’s leaves is common to all deciduous tree and shrub species. Most deciduous leaves have a special type of cell structure where they attach to the branch that makes a tidy cut-off, or abscission, point. When dormancy starts, the cells stop working and the leaf’s connection to the tree is cut. With the connection cut, the abscised leaves naturally fall off with wind and rain.
You can find traces, or scars, of this occurrence on twigs where leaves were once attached. Different species have recognizable patterns of leaf scars that help tree care professionals to identify trees in winter.
But Some Leaves Won’t Leave…
Not all tree leaves get the signal to fall off. Instead, they turn brown, shrivel up, and then hang on throughout most of the winter.
Marcescence is the term used to describe the situation in which leaves continue to hang on after they stop photosynthesizing. Sometimes leaves that have finished their job but just won’t fall off are also called “persistent”.
Tree SPECIES WHOSE LEAVES HANG ON
Tree species that may be familiar to you often have persistent leaves into winter. The most common ones in the Dayton, Ohio area include:
- American beech (Fagus),
- oak species (Quercus, which is a member of the beech family),
- witch hazel (Hamamelis), and
- hornbeam (Carpinus).
Why Leaves Don’t Fall Off
The reasons why their leaves hang on may have to do with evolution and the benefits leaves provide to the dormant, but living, trees.
Deciduous trees have evolved to respond to sunlight and temperature cycles, as well as available water and nutrients. Each spring, leaf buds on bare branches open to begin making energy and food through photosynthesis. It’s a gamble on the tree’s part that there will be no more freezing temperatures that could kill the new leaves but, thankfully, the trees usually win that bet.
After a leafy summer, when the growing season is over and the temperature drops, the tree shuts off its leaf connections and awaits dormancy.
Benefits of Persistent Leaves
The depleted, annual leaves still benefit the tree, but in different ways. Normally, we think about using fallen leaves to benefit our garden by making leaf mold or compost. But what about leaves that don’t fall off?
Early spring “fertilizer” – One theory is that leaves hang on through winter until new growth pushes them out of the way in early spring. In spring, when there is warmth and rain, these already-decayed leaves fall to the ground and quickly enrich the soil, providing immediate nutrients to tree roots. This is beneficial to trees that grow in areas with poor soils, where nutrients are scarce or are quickly washed through sandy soils by rainwater, or for shrubs and trees with limited root spread.
Winter protection – Another theory is that the persistent leaves protect young twigs and shoots from damage, either from freezing and drying, or from predation. Dead leaves can hide the tasty shoots and twigs from view so they don’t get eaten. And browsing animals don’t like branches with persistent leaves – the dead leaves are unappetizing and nutrient poor, and they’re frightening as they move and rattle with wind gusts or when shaken by hungry browsers.
Shelter for songbirds – One important beneficiary of persistent leaves is songbirds. These little insect-eating machines need protection from wind, rain, and snow in winter, and persistent leaves can help insulate them and shield them from predators. When spring begins, new leaves will help nest builders by screening eggs and hatchlings from predation. Maintaining songbird populations is more important than ever, as habitat areas shrink and climate change alters the once-predictable seasons.
What To Do With Leaves On Trees During Winter
The short answer is to do nothing. Your trees are following their evolutionary habits. Regardless of whether dead leaves stay on over winter, you can expect your trees to produce crowns of new, green leaves in spring that will eventually push last year’s stragglers out of the way.
If you have destructive browsing animals, leave the leaves and help your trees by making them unappealing.
For smaller shrubs such as witch hazel, however, removing dead leaves is not difficult and will enhance their early bloom display. Just be careful not to damage the shrub as you do this.
If the appearance of the dead leaves is an aesthetic concern, they can be removed. But if your trees are large, it will be harder for you to reach the height of the remaining leaves from the ground and improvised tools will likely damage twigs and branches (please don’t climb into your tree to remove leaves!). Tree care professionals can remove the remaining leaves without damaging the dormant tree or compromising next spring’s growth, although there are no tree health benefits to doing so.
Then again, if your tree is pruned for structure or health while it’s dormant, any remaining leaves can be removed at the same time and all debris can be removed at once.