Wait a second… Why do the Oak leaves remain…
Walking through the woods after a winter snow… Silence… Just the crunching of snow under your boots as you walk… In the wind you hear the sound of leaves rattling together… But all the leaves are covered in snow… A closer inspection of the sound will most likely reveal a tree that is covered in dead leaves… Leaves that are hanging on for dear life in the cold winter winds…
Long after all other trees lose their leaves in the winter, the dead leaves of an Oak Tree remain. This trait is extremely helpful in identification, often remaining until the buds break in the spring. This retention of dead plant matter is known as marcascence, and it is a genius evolutionary trait that I am going to try and explain.
In autumn, shortening day length tells the deciduous trees that it is time to stop growing. The tree then forms a layer of cells at the base of each leaf. This is called the abscission layer, it slows and finally stops the flow of sap to the leaf. Once the sap stops flowing the leaves lose chlorophyll and all the reds, yellows, and oranges that the green chlorophyll was hiding becomes visible.
Oak trees tend to be variable in this leaf retention, young trees will remain covered… while older trees may shed the top leaves, but retain much of the bottom… I’m not sure if many people pay close enough attention to the trees around them, but these leaves last until early spring. This is because the abscission layer forms much later on Oaks than on other trees. Though the leaves may look like they are nothing more than a fluke, there are actually many theories as to why this occurs.
One theory is the leaves act as a browsing shield from deer. In the deep winter when foraging is difficult and scarce, deer will target the young buds and branches of trees. The leaves that remain are very tough and prevent the deer from finding the tender oak buds. Another theory is that the tannin-rich dead oak leaves have a taste deer find unpleasant and therefore avoid the trees altogether.
Another is frost and wind protection. The new buds on a tree can be damaged by extreme cold and wind chills, leaves are retained to protect them. I don’t know if I am a fan of this theory, Oak trees have successfully flourished in extremely cold winters for a long time. This idea just seams like a stretch to me. Maybe possible on southerly oaks growing towards the northern end of their range… But still a stretch…
Lastly… And the theory I agree with… Spring nutrients and compost… In the early spring the forest floor is sprouting with life. Many spring plants are germinating through the partially composted leaves of last year. Around the same time the gardeners are putting down spring mulch, the oak tree drops its leaves. These leaves serve many purposes, but the obvious is to keep weeds to a minimum.
In nature, the leaves that fall to the forest floor in the autumn are slowly broken down. The freeze and thaw cycles of winter pulverize the leaves to a point where the previous years seed can germinate through them with ease. Normally these plants would finish the leaf decomposition as they grow, but the mighty oak has other plans with its leaves.
I find it interesting that many of those leaves remain right up to the spring weed push. These freshly fallen leaves help keep undergrowth to a minimum. As they break down over the summer they provide surface nutrients during a time when they would otherwise be scarce.
Regardless of the theory you choose, you have to admit marcascence is a cool trait. The idea that the leaves that stay on the tree through winter are their for a reason, is really something special. It is important to mention that you will not find this on every Oak tree, it is not uncommon to find one tree that has retained many of its leaves among twenty that did not. The variable nature of marcascence is what makes it such a mystery. A mystery that nature has yet to give up…
plant petunias and question everything – chriscondello
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And I thought I knew what was going on…brilliant.
Reblogged this on Some/Home and commented:
I have always been fascinated by the phenomenon of oak leaves keeping so many of their leaves through the winter. (Years ago, I wrote a song named “Oak Leaf Legend” that purported to explain it in Native American storytelling terms–this was for my nephew’s cub scout pack) So I was glad to see this post from Chris!
Interesting post. Cheers.
Really enjoyed this post … Thank you.
I am SO glad you explained this! I have often wondered why those leaves didn’t fall and if they somehow hindered the growth of new leaves in the Spring. I’ve told myself to try to remember the trees as winter comes to an end to see what would happen but suddenly new buds are everywhere and the brown leaves are just gone and I could never remember which one had the dead leaves on them.
I’ve asked out loud why this happens, but never got an answer so thanks for satisfying my curiosity! 🙂
Glad I could answer your question! And thank you for always reading!
You did a great job explaining the science of oak leaves — and much more poetically than any haiku.
Thank you so much… The haiku just got me thinking…
I tweeted a link to your post. I really enjoyed it.