Driving around town recently, I noticed many trees still full of leaves. Typically, we think about leaves changing colors and falling off the tree in October or November but often, several species of deciduous trees hang onto their leaves for much of the winter.
The term used in the world of arboriculture to describe leaf retention is “marcescence.” Normally, deciduous trees release enzymes and form an abscission layer that releases the leaves, allowing it to separate from vascular bundles that keep it attached to the stem.
Even conifers drop their needles but most often they hold them for more than one year and release them after new needles are present. Needles are thicker and have a thick waxy coating that helps prevent water loss.
Leaf drop help deciduous trees by reducing water loss during the winter. During the winter, water is often scarce or frozen. Sometimes early winters or frosts interrupt the process and kill leaves quickly and the occurrence of marcescent leaves may increase.
So why would trees decide to keep their leaves if early cold snaps are not the case? We can only speculate.
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Plants in general have built in defenses. Blackberries for example have thorns to deter birds and other animals from eating its fruit. Some plants produce toxins or noxious odors like the mint plant.
Foxglove produces several deadly chemicals such as steroidal glycosides that cause nausea, hallucinations, or convulsions.
Trees may keep their leaves on through the winter to help hide and protect their new buds from herbivores. Dried leaves are less nutritious than green leaves and they are noisy when disturbed. It makes sense that browsers such as deer would be deterred.
There are some nutrients in dried leaves. As they decay, they release nutrients and organic matter into the soil. Some science speculate that trees hold on to their leaves until spring in order to take full advantage of these nutrients instead of dropping them in the fall to benefit other understory plants. Dropping their leaves at the right time can provide a competitive advantage to the tree. Of course, marcescent leaves also provide good cover for birds and squirrels throughout the winter.
Some ecologists suggest that marcescence is an adaptive trait that some species have developed over time for trees growing on dry, infertile sites. Beech and oak trees often are found in areas that are dry and infertile and both species hang onto their leaves most of the winter. Other scientists speculate that beech and oak are just a juvenile trait and these species have not evolved as other species have.
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Michael Snyer, Vermont State Forester, agrees that marcescence may indeed be helpful to trees in dry, cold, deer-infested areas but it may also just be that these species are evolutionarily delayed, still on their way to becoming fully deciduous trees.
In the end, we really are not sure why some trees hold their leaves while others do not. What I do know is that those trees that hold their leaves sure make it difficult to clean up the lawn.
P. Andrew Rideout is the University of Kentucky Extension Agent for Horticulture at the Henderson County Extension Office. You can reach him by email at firstname.lastname@example.org