The Evercidious: Understanding Marcescence
Marcescence in American beech (Fagus grandifolia) observed at Audubon Acres by Mark McKnight.
The plant world spares no expense when it comes to being strange and peculiar. Half a billion years of evolutionary persuasion has been a mighty force on the direction and complexity of plant life and habit we see today. Carnivorous plants, towering trees, fall foliage and the other bizarre phenomena of nature are all the product of a geological time scale that is truly unfathomable! Like some of us, the wild and weird flora are borne out of a bit of unconvention. One such tale of quirky plant evolution may be found on a walk through the woods on a cold January day.
Ornately dangling from the branches of a smooth, gray-trunked juvenile American beech (Fagus grandifolia) are vestiges of yesteryear’s foliage. Dried and sandpaper-brown, these dead leaves have clung to their branches since the last leaves of fall have dropped. Until spring, or the forced eviction from mother nature (such as wind), these leaves will stay in place to decorate an otherwise dormant and ho-hum forest landscape. This retention of leaves, normally discarded seasonally, is known as marcescence.
Marcescence only occurs in deciduous trees and shrubs - and is due to the lack of particular enzymes that create what is known as an “abscission zone” found at the base of the leave’s petioles (stems), where they attach to the branch. Abscission, latin for “to cut away,” aptly describes the function of this cellular layer, allowing a plant to remove its own foliage. Normally, most deciduous trees will begin to create this abscission zone during early autumn, cued into action by declining daylight hours and cooler temperatures. When fully formed, the abscission zone’s layer closest to the leaf snaps free from a weak cellular wall, while the bottom layer’s cork-cell structure insulates and protects the twig from water loss and the harsh realities of winter. Without a fully-formed abscission layer, a tree will struggle to independently rid itself of dead leaves until next year’s budding. Trees that exhibit marcescens can still abscise some of its leaves, though typically in higher branching, such as the crown. Even more perplexing, some trees will retain all of their dead foliage throughout winter. This trait is highly variable, affecting certain species more than others, as well as age and even particular branches.
For our temperate deciduous forests, Oak species (Quercus spp.), American beech (Fagus grandifolia), Witch-hazels (Hamamelis spp.), Maples (Acer spp.), Hornbeam (Carpinus caroliniana), and Eastern Hophornbeam (Ostrya virginiana) are the usual species that exhibit marcescence. However, a sudden cold snap preceded by a long, warm autumn can inhibit the full formation of an abscission layer in some species that might not normally display marcescence.
Oaks are the species that exhibit this trait most frequently. All oak species can display foliar marcescence in their lifetime. Perhaps the most iconic example of this trait is the American beech. Its marcesced foliage, bronzed-orange to light brown is striking against the muted colors of the dormant forest.
While observed to be primarily a juvenile trait, most of these angsty teenage trees will grow out of this leaf-retention rebellion to become seasonally-sensible, bald adults - there is a candid parallel to be found in there somewhere. That’s not to say mature trees don’t exhibit marcescens, though. Immature zones, especially in the lower branches of these older trees, can exhibit this trait throughout most, if not all of its life. It has also been observed that those limbs that retain leaves rarely produce fruit and seed during the productive season.
Why marcescence occurs is still somewhat of a mystery, and it has opened many intriguing lines of inquiry. Like most things in nature, there usually isn’t just one clear answer, and it always helps to know the evolutionary history.
The evolution of marcescence should be told via the broader story of deciduous and evergreen plant traits. To better understand the evolution of these traits, it might help to first dive into the arrival of trees on Earth. This event occurred approximately 380 million years ago, about 100 million years after plants migrated from the vast oceans of Earth to colonize and conquer the terrestrial landscape. During this period of warm and relatively stable climate, plants began to rapidly evolve and spread, eventually causing resource competition for light. This instigated a need that was answered by the first of our mighty trees. These trees were a bit of an oddball character when they arrived on the plant scene, and with a name such as cladoxylopsids, how could it not be? Very different from the trees we see today, these ancestral species underwent many evolutions, extinction events, and geological epochs to get to their form they have today.
Somewhere in this evolutionary story, Earth’s plants split into two camps: those that retain their leaves year round (evergreen), and those that go dormant for the colder months of the year (deciduous). For millions of years, this has been a stable ontology for the plant world that has remained virtually unchanged. Earth’s temperate zones eventually became the dominion of deciduous flora, while the tropics and polar zones were incorporated by evergreens. These two vegetative habits would end up being the defining advantage for the habituation and domination of their respective climate zones. Which strategy came first is still being debated by evolutionary biologists. Some evidence suggests that deciduous habits evolved first alongside a gradually cooling climate.
Ultimately, the diverging of these life history traits would create the impetus for the emergence of marcescence, as something of an intermediate between the two. This “everciduous” trait’s true origin is still not well understood. Some have speculated on the trait being a vestige of an ancient, evergreen common ancestor left over from the long grind of time. Exhibited in palms somewhat extensively as well, marcescence in these plants is considered a primitive trait by evolutionary biologists, meaning that the trait is more or less unchanged from that of a common, ancient ancestor. Does this trait still serve purpose, and if so, what is it? The answer is still unfolding through research.
Perhaps the most accepted and practical theory for the advantage for a marcescence habit is the deterrence of herbivory. New, fresh buds on branches and twigs are always tastier and more nutritious to the local forest grazer, when compared to the gritty, dead, crunch of old leaves. When observed in more mature trees, most marcescence is exhibited in lower, more browsable areas of the tree. It makes sense to put emphasis on the protection of the vulnerable areas of new growth, especially adolescent trees that are almost all fresh growth. Adding to this dining drama, some have speculated this adaptation could have been selected for by mammoths and giant ground sloth feeding habits! The protection of new growth by its old foliage could also mitigate some of the biting conditions of winter and wind, offering another possible answer.
A different theory to explain this phenomenon set forth by ecologists is a fascinating one. By retaining its dead foliage until spring, winter’s snow can be trapped and subsequently released during melt to provide moisture to areas, especially in drier sites. While on the stem, leaves of these trees will also decay slower, and upon removal in spring, will act as compost and mulch to help aid in early season nutrient sequestration and soil moisture retention. It turns out that nature, can in fact, tend to itself…and be its own gardener.
There are very few subjects that allow one the space to wax poetic on giant ground sloths, self mulching plants, and the virtues of being bald all in one breath, like the world of plants do. Whatever the reason might be for the emergence and retention of this quirky plant habit known as marcescence, it still stands as one of the most intriguing and fantastic botanical bounties to be found on a walk through the woods on a cold January day.
- By Dylan Hackett, Nursery and Greenhouse Manager
In 2019, Dylan started work at Reflection Riding as an educator, teaching the community aspects of ecology and the flora and fauna around them. Dylan now serves as Nursery Manager, facilitating the propagation of hundreds of species of native plants endemic to the region. Dylan’s work brings him great joy, especially working with community members to learn about environmental stewardship and sending them home with the power of a little bit of good in their hands.