LOGAN — Frosty temperatures Friday proved to be perfect weather for a hike in the woods with my five-year-old grandson, Mikey. Pausing a while in a sandstone recess cave, I sat on a rock watching him romp with my dog when I suddenly realized the cave was awash with light.
As the sun, on its downward trek to night fall, descended, it shot a beam of light from the rim of the hill directly into the rock formation, illuminating the leaves on the floor of the cave as well as a small American beech clinging to both its dried leaves and the ledge of the gorge below. The blast of light gave the scene the “look” of a warm autumn, color splashed day, despite the cold.
Sitting there amid the joyful sound of a boy, a dog and rustling leaves, I wondered…why do some trees hold on to their leaves through the winter and others go bare?
While most trees visible from the gapping mouth of the cave stood stark and leaf bare this glowing beech tree was clothed with near golden dried leaves.
There is actually a term used to describe this leaf retention, marcescence. But knowing its fancy name didn’t make me stop wondering why this happens.
Once back in front of my laptop, I found out…
I learned that marcescence, is most common in some oak species, American beech, witch hazel, hornbeam (musclewood), and hophornbeam (ironwood).
According to Jim Finley, PennState College of Agricultural Sciences, Department of Ecosystem Science and Management, under normal circumstances, “as deciduous trees prepare to shed their summer leaf coats, cells at the interface between the twig and the end of the leaf stem release enzymes and form an abscission layer that “unglues” the leaf — separating it from the vascular bundles, allowing it to fall free.”
At some time or the other, all trees, even conifers, shed leaves. This leaf drop benefits deciduous trees by reducing their water loss and allowing them to develop leaves that efficiently use available sunlight during warmer seasons.
“Sometimes, early cold weather or frosts may interrupt the abscission process or “kill” leaves quickly. In these cases, the occurrence of marcescent leaves may increase.” Finley explained in his article Winter Leaves that Hang On.
However, my question still was, why would a tree “decide,” during the absence of a killing frost, to retain its leaves?
I’ve noticed that frequently, you will see marcescent leaves on smaller trees or on the lower branches of larger trees, but why?
“In the case of smaller trees, which in forest conditions would be growing beneath taller trees, the reduced sunlight might slow the abscission process. By doing this, the understory tree leaves and the leaves on lower branches of larger trees would also have the opportunity to continue or even increase their photosynthetic process as upper leaves fall. Then, perhaps, leaves lower in the canopy are “caught” with cold temperatures and their leaves hang on,” Finley, observed.
It is also speculated that retained leaves may deter browsing animals. The dried leaves may actually conceal buds from browsers, like deer, or at best make them more difficult to nip from the twig.
Dried leaves, of course, are less nutritious. And according to Finley, a study in Denmark proved that point.
During the study researchers found out that deer offered hand-stripped twigs, preferred to marcescent twigs. In addition, nutrient analysis found the protein content of oak twigs was higher and the dead leaves had less lignin. The protein content of beech and hornbeam twigs was about equal to the leaves, however, the lignin content was nearly half again higher in the leaves. So the study proved that there might be something to those dried leaves hanging on to protect those tender nutritious little twigs.
If you could ask the trees, they might give you yet another reason for holding onto their leaves. This reason relates to their nutrient cycling. All of those leaves that tumble to the forest floor each fall joining leaves from other trees to lay there and decay. During the decaying process, their released nutrients could leach away and be unavailable to “feed” the trees the next growing season. By retaining their leaves, these smaller, understory trees, basically recycle their nutrients to themselves.
However, whatever the reason for marcescent leaves, when growth does begin in the spring the expanding buds actually push the dried leaves off and clothe the branches with new greenery.
Until that happens this spring, we stand on the chilly cusp of February with no other choice but to enjoy those rattling brown leaves and the comforting shelter they offer our winter birds.