Why do trees reject their leaves in the autumn?
If you thought that "fall" was "fall" because the leaves, well, they descend from deciduous trees in the autumn, you are mistaken, according to Peter Raven of the Missouri Botanical Garden, for one. The leaves don't mind sticking around, but the trees kick them off! Yep, this isn't just about falling, it's more about shoving, evicting or dumping your steady date. I know that probably conflicts with your mental image of a picturesque fall season. If the leaves don't turn scarlet and gold where you live, you may even have worked hard to find signs of beauty in, for instance, poison ivy. (Eww.)
But what I am speaking of is that part of the ecosystem violently rebuffs another part of the same ecosystem. Until just a month ago the ecosystem members seemed completely interdependent. Now one party (the tree) is evicting the other parties (all of the leaves) with the help of the unindicted coconspiritors, the wind and the rain or snow. (I don't know whether I'm drafting a Restraining Order here, a Divorce Agreement, or a Declaration of Independence) .
I don't mean "why do they turn pretty colors," because most first graders in temperate zones can give you adorable little speeches about the green in the leaf (which was hiding all the other more interesting colors) going back into the tree, and the reds, yellows, oranges and browns which were there all along showing up. Older children may even talk about photosynthesis, pigmentation or chloroblasts in an attempt to obfuscate, and more sophisticated, artistic types may use words like "crimson," "scarlet," "ochre," "gold" or "emerald." But that is still not the question. Which is not how, it's why even bother? You're a tree, and everything's fine, why would you want to go and lose a bunch of perfectly good leaves?
Please don't forget that leaves, and other green plant material (I mean evergreens, of course, and ferns, but also sea weeds, mosses, lichens, algae, molds and other funky green stuff) are responsible for photosynthesis, and photosynthesis is responsible for enabling
Life on Earth.
Photosynthesis helps converts the sun's energy into sugar energy (or energy stored in chemical bonds) here on Earth instead of out in space. Whether you take the long view that photosynthesis → plants → [dinosaurs eating plants → dead dinosaurs] → fossil fuel reserves, or the more pragmatic view that photosynthesis enables wheat, corn, rice, soybeans and other crops to either be converted directly into human food or pass through animal farms on the way to becoming more food for humans, either way you can, I hope, see my point about photosynthesis being the driving chemistry for
Life on Earth.
But back to the leaves. There we were, millions of years ago, in fact about 360 million years ago, according to William C. Burger in Flowers: How They Changed the World. Evolution had come up with the dandy new idea of plants with stems and leaves, but they were all ferns, pines, or other gymnosperms. They all kept their leaves year-round, which wasn't so bad, because year-round was a balmy tropical climate.
But then it got colder, and consequently dryer (because cold air doesn't hold moisture), and as that happened the plants (or tree-like structures) needed to ditch their leaves. No problemo! Over the next few million years a new type of plant evolved: the angiosperms, or seed-bearing plants. There are many interesting things to learn about the angiosperms, but what I want to discuss today is the lovely, flat green leaf.
A broad flat surface like a leaf, suitable for absorbing the maximum amount of sunlight, left the plant open to heavy losses from transpiration (or evaporation) if the weather got cold and dry. The plant could lose raw materials if the wind or snow should happen to rip off this hypothetical leaf. What did our ever-adaptive plant do in response? It cleverly developed what Burger calls the "disposable leaf." Like a diposable brown paper lunch bag,
the disposable leaf was not costly in terms of raw materials for its plant. (Heck, let's call the plant a tree.) It could easily be reconstructed once sunshine was again plentiful (in other words, next spring). The disposable leaf could be abandoned, discarded, deserted, ignored by its dead-beat tree. Except you and I know, of course, that the trees which managed to trap these dead, disposable leaves among their roots ended up benefitting from the leaf mulch that formed naturally on the deciduous forest floor.
So when you see those beautiful leaves falling to the ground and asking to be raked, remember, even as they are being abandoned by their trees, they are performing one last function for them as they die, that of a formerly living mulch for the tree's roots. Don't be too quick to rake and dispose of the leaves; remember, the tree wasn't taking them to the dump, it was just relocating the leaves from the end of the branches (where they were no longer useful and would in fact be a liability) to just above the roots, for the coming cold and possibly dry season. Give these poor, rejected leaves a hand with their dying wishes; help them protect and fertilize tree roots. You will feel better about yourself; I promise.
PICTURES: Thanks to Sally G. Miller, Morguefiles, and David Goehring.
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