Welcome back to Ask a Naturalist: your own personal “Google” for information on all things natural on PEI! In October, I posted about how and why PEI’s broad-leaved trees lose their leaves in fall (an orderly biological process called ‘senescence’, about which you can read more here: https://www.pei-untamed.com/post/autumn-leaf-colour). Recently, I’ve had questions from several people about why some of these trees hang on to their dead leaves all winter. Let’s take a look!
Red Oak (Querus rubra, shown here) is one species that commonly shows this phenomenon, but you can often see it in America Beech (Fagus grandifolia) and Ironwood (Ostrya virginiana) as well. In all cases, tan leaves stand out against the white winter landscape, and young trees are much more likely to retain leaves than their older kin. The biological term for this leaf retention is ‘marcescence’.
Once a hardwood tree has recovered everything it needs from the leaf, it usually seals off the connection with special cells that are a bit like Lego – designed to pull apart cleanly. This creates an ‘abscission zone’: a spot where the leaf can break easily from the tree. Most trees form this abscission zone in fall, but marcescent species wait until spring.
While botanists have known of this phenomenon for centuries, have a name for it, and understand how it happens, we still don’t know for sure why. There are some theories, though.
We do know that - given a choice - browsers such as deer prefer bare twigs over leafy twigs as winter food. One theory is trees such as Oak retain their leaves over winter as protection against being eaten. (While we don’t have deer on PEI, our Oaks don’t know that!). Smaller, younger trees are more susceptible to browse damage, which could explain why marcescence is more commonly seen at that stage.
Another theory is that trees hang onto the dead leaves to give them a head start in spring. Rather than losing leaves in fall and having them blow away or rot, it’s possible that keeping leaves all winter allows them to create a fresh layer of mulch at the base of the tree when they drop in spring. By contributing nutrients, holding moisture, and reducing competition, this could give young trees an advantage when they start to grow again.
My own theory is that overwintering leaves protect young, delicate bark from too much sunlight. Once taller trees lose their leaves, those in the understory are exposed to much more direct light. Sunny winter days can warm tree bark enough that cells become active, only to be killed once the sun goes down and temperatures plummet at night. It’s possible marcescence has a role in preventing this damage (called sunscald) on more delicate young trees. It’s also possible the reason is a combination of these explanations, or something different entirely!
Everything in nature is a balance between cost and benefit. Trees that hold on to their wind- and snow-catching leaves have a higher risk of losing branches in winter storms. So, while we may not know the reason(s), we can be sure they are good enough to outweigh that liability.
No matter the reasons, the subtle colours and rustling sounds of marcescent leaves add to the beauty of the Island’s winter woodlands. Those dead leaves will drop once the tree awakens for spring, so now is the time to enjoy the display.
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