Winter Botany: Marcescent Leaf Mysteries

BlackOakMarcescentLeaves,KAOIf you happen to be wandering through Squam Swamp or Squam Farm as autumn gives way to winter, you may notice that while most of the trees have already shed their foliage, oaks (Quercus spp.) and American beech (Fagus grandifolia) may still retain old dried leaves rattling spookily in the wind. Some individuals seem to remain almost fully dressed, while others have only tattered remnants. This penchant for retaining leaves into the winter is called “marcescence,” and there are a few theories that have been proposed to explain why some trees display this trait and others don’t.

In pondering the “how and why” of this situation, it’s helpful to consider the reasons why our northern deciduous hardwoods drop their leaves in autumn in the first place. As day length shortens and temperatures drop, photosynthesis becomes less and less productive. Shorter days, and low sun angle mean that less light reaches the trees, and a certain temperature range is best for leaves to transform light into food efficiently (50-68°F). In the summer, trees have other adaptations to help cool their leaves to maximize their time spent in the optimum temperature range, but that’s a blog article for another season!

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Dried beech leaves turn a fawn color and many remain on the branch until spring when new leaves emerge.

To make matters worse, freezing temperatures rupture the cell walls of leaves as the water inside them expands, unless they are equipped with special protective mechanisms — also a blog article for another day.

Meanwhile, cold and drying winds make desiccation a real problem. Leaves become a losing proposition in a New England winter. Broadleaf trees would need to make a slew of adaptations to retain their leaves into the winter and make it worth their while. American holly (Ilex opaca) is one of the few trees that does this, with glossy leathery evergreen leaves

Retaining leaves is even more of a liability in areas prone to heavy snow and ice storms, because leaves keep snow from shedding effectively, and ice builds up on the larger surface area of leaves far more than on slender twigs or needles. Deciduous trees have evolved a dramatic solution to deal with this problem. Unlike evergreens such as pine and spruce, they create a “shutoff valve” in the stem of each leaf called an abscission layer when the days begin to shorten and temperatures drop. Oaks and beech do not have this shutoff valve, so the leaves may remain on the twigs until spring, unless they are torn off by wind and general wear-and-tear.

Something has to make it worthwhile for oaks and beech to keep their no-longer-photosynthesizing leaves long after other hardwood species cut their losses. One of the theories is that the leftover leaves—crunchy, dried out, and not very tasty — persuade browsers to browse elsewhere. Research has shown that the tough leaves are nutritionally poor compared with the young twigs and the buds, and deer and moose prefer leaf-free twigs when given a choice.

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What’s for lunch in winter? Fresh beech sprout twigs!

Considering the heavy browsing you will find on beech sprouts in Squam Swamp, maybe the trees will take any edge they can get! Perhaps it’s no big deal to occasionally lose some branches in your canopy from extra snow and ice loading, but being gnawed to the ground repeatedly means you will never get to rise above the forest floor to produce a seed crop of your own. That would be a pretty strong evolutionary pressure.

Whatever the benefits or drawbacks to marcescence, it becomes a handy shortcut to recognize groves of oak and beech as you move through the fall and winter forest. Long after the branches of tupelo and sassafras are stripped bare, oaks and beech are easy to spot from a distance. Since these species also provide valuable “mast” for wildlife (bumper crops of thousands of seeds or nuts every few seasons), hunters take notice of these hotspots. On the mainland, acorn and beech mast crops are a major food source for bears, turkeys, deer, squirrels, and other hungry animals. On Nantucket, deer and squirrels would be happy to take care of the abundance of nuts and seeds all by themselves, but get some help from birds such as blue jays and crows.

BeechCanopy Marcescence,KAO

This entry was posted in Botany, Forests, Natural History, Uncategorized, Winter and tagged , , , . Bookmark the permalink.

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