Oak Leaf on the Move
As the snow blankets the earth, there are patches of orange, rust and brown painting the mostly naked forest. These leaves cling to the tree, only being freed by harrowing winter winds or warming days of spring pushing out the new leaves.
I am embraced by the quiet serenity of this place, contemplating a mystery afoot which is perking my curiosity throughout my winter walk in the park. Only the sound of the leaves rattling above my head keeps my inquisitive focus. Stillness is not a part…
Look at it go….
As my attention is drawn to the lone oak leaves dancing across the new fallen snow, pirouetted on pointed end, making their way to the ice capped banks blown free of the glistening white blanket.
Above, the brown, supple leaves, still clinging to the tree in February, rustling as the wind whips through are a testament to an ecological development since the very first evergreens graced our planet. In fact, try to crumple leaves of oak and beech. Dry, they don’t even break into pieces. This springiness was why they were used in stuffing mattresses rather than using straw which was easier to collect, yet compressed easily. Early settlers were very industrious and inventive.
There are the evergreens such as pine, hemlock, spruce and fir which retain needles all winter, yet not all conifers are evergreen to lose needles in winter, such as larch and bald cypress.
There are the deciduous trees such as maple, birch and cherry that shed their leaves to expose their very stark nakedness.
Japanese Maple Leaf on the Move
But not all trees in this forest are naked in their winter glory.
There is this odd group in the middle, composed of beech and oak. And, I am assuming larch and bald cypress as well. The question then becomes, is retaining dead leaves an evolutionary and ecological advantage for a tree that is in between? Why do these trees delay shedding when there is apparent benefit for doing so for others? Most tree species have picked a side. Either evergreen or not.
Being evergreen is believed to be an advantage to a tree by increasing the time available for its leaves to remain photosynthetic. They also reduce nutrient loss associated with dropped leaves and seal themselves to protect precious water.
Weighted Down with Ice
A deciduous tree has developed a redesign to allow trees in changing environs to reduce water loss and avoid frost damage during most unfriendly seasons. They then are increasing their photosynthetic efficiency during favorable seasons. So what is it called being in the middle…
The term is called marcescent. Which in laymen terms, is when a plant part dies and is not shed. The leaves may curl in a chrysalis-like fashion, but they hang on like a scared child gripping his mother hysterically on the first day of school.
A botanist may say or agree that marcescence is a juvenile trait for a tree and it is usually observed on a young tree, but it also occurs on the lower branches of older trees.
Older Oak with Marcescent Leaves on Lower Branches
There is an idea that perhaps this acts as a means of trapping snow at the base of the tree, allowing for more moisture come springtime. But also consider that snow-covered branches weigh down more heavily on a tree and limbs may break as a consequence, so this can be a double-edged sword for an oak if you follow this logic.
Following the two leaves bouncing across the ice and snow has yielded little damage.
Other theories suggest that the retention of leaves may provide frost protection for new buds and twigs to weather out the winter, shielding them from biting, brutal winds.
Browser with a tasty belly full. Yummy on those yews.
Yet, others have suggested that the dead leaves act as a deterrent to browsing deer preventing the buds from being eaten. All plausible theories giving credence to marcescent leaves being helpful to trees living in dry, cold, deer ravaged forests.
Many Young Oak at the Base of the Mother Oak, a lot of busy squirrels.
But it is also possible the oak is an example of a tree that is evolutionarily delayed in development, not quite being a full-fledged deciduous tree.
Above you can see how successful the oak is in perpetuating itself. Oaks outcompete other species in areas that are dry and less fertile. And another theory suggests a reason why. It is proposed that by retaining the leaves until spring, the tree delays the decomposition process, enabling it to provide mulch and compost for itself and its offspring, thereby warming and fertilizing the less than optimum soil. This is when the saplings and the mother tree need the most nutrients and then get a quick jumpstart to the season.
Leaf Buds and Leaf Loss
So it appears to be a matter of great timing, shifting the competitive balance of one species over another. Very cool stuff.
Get some air…
Beech and oak are delayed in some respect to being full-fledged deciduois because, they are decendant from their more evolutionary, evergreen past. Beech, genus, (Fagus), and oak, genus, (Quercus), are in the same family , Fagaceae, which includes evergreens such as live oak and tanoaks.
Big Oak with Younger Oak on Either Side
Does anyone really know the reason why marcescence occurs? It is understood how it occurs. The abscission layer, separating a leaf and its twig, forms a seal in autumn on a tree.
Abscission at Dark Area, (I am Guessing) Andrea will Know
This switches off the leaf from its water and nutrient supplies like a gate valve, causing the leaf to usually, and in most cases, disengage. The base of the leaf stem produces an enzyme which forms this separation layer or abscission zone to help prevent infection and the tree starts the process of leaf drop.
Two Leaf Stems Attached, See New Buds?
The abscission layer is not fully formed with marcescent leaves. Simple, sort of until you consider the hormonal changes necessary, then it becomes all kinds of complex.
The hormone that stimulates abscission is called ethylene. It is believed that the hormone auxin inhibits abscission by preventing the abscission layer from forming.
For some science in less than sciencey terms, see my popular post Why Do Trees Change Different Colors on the Same Tree?.
Papery, Yet Still Springy
I hope you enjoyed the papery textured leaves in there whithery best. They bring a lot of winter interest to the landscape and you have marcescence to thank.
And if you missed my winter interest piece, here is the link. And for those that really like winter photography, I have a post sure to please. My coldest shots yet, plus more very red birds. This time the boys. Please stop back for my monthly GBBD Magazine for the 15th. Cold Niagara here too.