Marcescence — an ecological mystery

Young beech trees retain their leaves throughout the winter months.
(Photo provided by Richard Gast)

Young beech trees retain their leaves throughout the winter months. (Photo provided by Richard Gast)

We’re blessed to live in an area that offers some of the most beautiful fall foliage found anywhere in the world. And this fall proved to be one of the most remarkably enduring that I’ve ever experienced: The maples, birches, poplars, oaks and beeches created a landscape literally exploding in shades of gold, crimson, and orange, which lasted for several weeks.

As cold weather approaches, many species of trees shed their leaves as a strategy to reduce water loss and frost damage. Triggered by hormone change (the balance of auxin levels between leaves and branches), it’s all part of an important and complicated process known as abscission; in which trees seal off the point where the leaf petiole connects to the twig (the abscission layer).

With winter upon us, the deciduous trees in our yards, along the roads and in fields and forests have all lost their leaves. Or have they? Look around. There are often juvenile, deciduous trees that somewhat defiantly hold onto their brown, bleached, withered leaves long into and even throughout the winter — until wind or snow breaks them off or they are finally forced off by expanding buds, come spring.

There’s a name for this winter retention of leaves: marcescence (mahr-CESS-ents), from a Latin root meaning “to shrivel.” And while scientists understand how tree genetics and environmental factors contribute to the occurrence of marcescent (withering without falling off) foliage, no one can say with absolute certainty why it happens.

The simple explanation is that sexual maturity determines whether the leaves fall off or not. But speculation and debate abound. One commonly accepted belief is that by concealing next year’s growth — the tasty, nutritious new-twig growth and buds that lie beneath them — the desiccated, bitter-tasting, difficult-to-digest leaves, which have little food value, may act as a deterrent to browsers such as deer and moose.

The retained leaves may also help protect the young trees from exposure to damaging winds. And by trapping falling and wind-blown snow and directing it to the base of the tree, which results in more moisture at the base of the tree throughout the winter and into the coming spring, the withered-but-steadfast leaves may help alleviate potential water-stress issues, too.

A growing number of botanists, biologists and ecologists believe that marcescence is tied to nutrient cycling and availability, the theory being that young understory trees with smaller root systems, especially those on marginal soils, may benefit by waiting until the spring to release their leaves to decompose into the soil. In doing so, they are essentially recycling their own leaves, providing additional organic matter to the soil in the root zone just in time to make those nutrients available for spring growth.

Whatever the reason (if, in fact, there’s any reason at all), the sight of a small stand of young beech alongside the road — their dried, bleached marcescent leaves highlighted by the midday sunshine — can add interest and texture to bleak, white and grey winter landscapes. And then there’s the wonderful sound of marcescent leaves trembling and rustling, rattling in the breeze, almost magically breaking the stillness of the woods along the trail in the silent, early hours of morning, and the peaceful feeling that brings. And the fact that they may also provide protection from the elements and predators for birds visiting backyard feeders.

Nature can be remarkably beautiful, even in the dead of winter. And even though the Old Farmer’s Almanac is calling for a cold winter with a lot of snow across the Northeast, winter can still be one of the most fascinating and appealing times of year. You just have to get into it.

Did you know?

¯ The word “autumn” originated from the French word “automne,” which comes from the Latin word “autumnus.”

¯ The word “harvest” comes from the Norse “haust,” which means “to gather or pluck.” According to Dictionary.com, the term harvest was used to describe autumn until sometime in the 17th century, when poets coined the phrase “the fall of leaves,” which was eventually shortened to “fall.”

¯ During the fall, in response to colder temperatures and less light, leaves stop producing chlorophyll, the green pigment that helps capture sunlight to power photosynthesis. As the green fades, the leave’s other pigments shine through, such as orange and yellow carotenoids and vibrant red anthocyanin.

¯ Scientists believe global warming may eventually affect autumn colors. As the world warms, leaves may delay changing their colors. Some trees may not use their sugars to create red pigments at all. They may, instead, use that fuel to grow new twigs. Global warming may also alter the habitats of trees, such as the sugar maples, which produce some of the most vibrant fall colors.

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