The Riddle of the Remaining Leaves
By Daniel Manget
As the excitement and energy of spring starts creeping up the mountain, we say goodbye to winter with its dull hues, long shadows, and distant leafless views that makes one feel the constant presence of the blue-grey giants. It is time to trade the quiet contemplative and meditative hike for the field guide driven adventure of an inquisitive naturalist. The tree buds that have been waiting patiently all winter are ready for their meteorological cue from the universe to burst forth to start the next cycle of flowers and food.
However, some of the more peculiar trees will have to use newly growing leaves to push to the ground dead leaves that have remained on the branch since last Fall. Have you noticed these leaves? In a behavior called marcescence, some deciduous trees do not shed their dead leaves in the winter. The reason why these leaves remain on the tree throughout the winter, while their neighbors shed them in autumn, is an ecological mystery that stimulates the minds of observant nature lovers who have wondered this strange phenomenon. These unassuming and stubborn brown leaves can’t help but stand out against a gray barren background, but it’s their rattling sound that really catches your attention as wind breaks the serene winter silence. Let’s explore this riddle of the winter forest before moving back into the season of the lush green canopy
Each fall, deciduous trees drop their leaves to conserve energy in a process called senescence. The energy to maintain the leaves outweighs the energy gained through a long tough mountain winter. Some of the hardy broad-leaved trees take on the challenge of keeping their leaves and do so with the evolutionary advantage of having small leaves and/or a waxy coating surrounding the leaves. This reduces the amount of water that escapes through the leaf during gas exchange. If water in the ground is frozen, the plant can’t absorb it to replace water lost through the leaves during photosynthesis. This is also why Rhododendron leaves curl up tighter the colder it is; to reduce the leaf’s surface area and therefore water loss. It’s a gambling lifestyle because when there is a warm day, there are few other green leaves to compete against for yummy sunlight.
Some trees, however, neither drop their leaves nor continue to photosynthesize throughout the winter. The leaves simply stay dead on the branches until spring. Have you noticed this before? This behavior is somewhat of an enigma to scientists and is a topic of much debate among naturalists. Why would a tree keep it’s brown dead leaves on it’s branches? The observant hiker would notice that this phenomenon occurs mainly on oaks and beeches (trees
in fagaceae or Beech family) but not solely. One might also notice that it occurs mainly on juvenile trees in this family or on the lower branches of older trees. This gives us an evolutionary clue to the purpose of this atypical and intriguing behavior called marcescence. Let’s discuss some of the speculated theories.
Delayed fertilizer due to lack of nutrients
One possible explanation for marcescence is that trees displaying this behavior are usually in areas with less nutritious soil. If trees hold onto these leaves until spring when new leaves push the old ones finally to the ground, they will get a boost of nutrients at a time when they are going to be needing a lot of energy. Energy to make leaves and flowers. Plus this way they can mooch off of the decomposing leaves of the neighboring trees all winter.
Deterrent to foragers of buds
So, what about the peculiar characteristic of marcescent leaves only being on juvenile trees and the lower branches of mature trees? What happens on those young trees and lower branches that would benefit the tree and help its survival? This is where the foraging happens. It’s a possibility that these brown, dead, non-nutritious leaves cover the yummy little flower buds that just grew in last fall making them undesirable to the deer passer by, ensuring their protection and therefore a higher chance of reproductive success come flower time.
Frost protection for buds
Brown leaves surrounding a growing dormant bud would probably make for a good windbreaker. This could prove essential on the most frigid windy nights and might mean the difference to save the bud from freezing and dying. A little protection from the elements can go a long way.
Getting in a few more bites
Since this phenomenon occurs on leaves that are on juvenile trees or lower branches of older trees, places that are usually shaded out from competition, holding onto these leaves for longer could give you a few more vital bites of energy from leaves that haven’t been damaged from sun exposure. These backstage lower leaves that don’t see much light or limelight during the summer months might be center stage when the suns rays finally break through the bare canopy. The tree therefore doesn’t form an abscission layer to drop the leaf in order to suck every ounce of energy from the last leaves before frost kills them on the branch.
There was a time on earth when all trees were evergreens because they all lived near the equator where there was no reason to drop leaves. As glaciers receded, trees evolved to escape the intense plant competition of the tropics through survival techniques to help their migration. Some became deciduous to conserve energy and some adapted leaves that lose less water due to a waxy coating (cuticle) and/or a small surface area. As trees transitioned, there had to have been a time when trees showed evidence of this transition by being neither deciduous nor evergreen. Perhaps the mysterious phenomenon of Marcescence isn’t as mysterious and romantic as naturalists think but is merely evidence of the tree’s separation from their evergreen cousins who chose a different route of survival. Or perhaps these trees found a successful niche being somewhere in between being deciduous and evergreen. Only the tree truly knows.