Maple tree in brilliant fall color. Photo by Donna L. Long.

Why Trees Shed Leaves in the Fall

Sugar Maple turning brilliant scarlet red in the September sun. Photo by Donna L. Long.
Sugar Maples (Acer saccharum) turning brilliant scarlet red in the September sun. Photo by Donna L. Long.

We humans think we understand why trees shed leaves in the fall. As the leaves stops making chlorophyll, the tree seals off the old leaf and then essentially “cuts” it loose from the tree as it no longer needs it.  Not only is the production of chlorophyll stopping and leaves dropping, but another process is happening that we are beginning to understand.

In the late summer, the leaves of the maple trees in my city neighborhood have lost their healthy, deep green color of summer.  The leaves change to a dull green, dry, and shut down their chlorophyll making factories. The leaves look old and spent.  The trees have started to break down some of their nutrient-rich proteins and chlorophyll and store the substances in their roots over the winter.  The substances are converted to either carbohydrates or fats. In spring the nutrients travel in water up from the roots, into the branches to feed the fast-growing leaves. This is the sap that rises in trees in the spring.  But in September, the maples, particularly the Sugar Maples (Acer saccharum) begin to turn a brilliant red. A deep, mind-blowing scarlet with touches of gold and orange.

 

Maple tree in brilliant fall color. Photo by Donna L. Long.
Maple tree in brilliant fall color. Photo by Donna L. Long.

 

For deciduous trees, the winter temperatures are too low to carry out photosynthesis. The respiration of the tree would release water through the leaves which could not be replaced by taking in water from the frozen ground. In the Philadelphia area, the ground freezes around the time of the winter solstice.

Another chemical process is happening as the chlorophyll-making shuts down. The leaves begin to make a red pigment called anthocyanin. Anthocyanin’s function is not understood. There are several theories about what the anthocyanin is doing.

Some scientists suggested the anthocyanin in fall leaves is a warning or signal to aphids to avoid the tree for egg-laying because of harmful substances.  This idea is not supported by scientific research and evidence.

The more scientifically accepted idea is that the production of anthocyanin protects compounds in the leaf from being broken down by sunlight.

And there is another theory that I find fascinating. As a gardener, it makes tremendous sense to me.

Red leaf shed by tree in autumn Photo by Donna L. Long.
Red leaf shed by tree in autumn Photo by Donna L. Long.

Brian J. Ford a British biology writer has a theory that the shedding of leaves is a mechanism by which plants excrete waste. Brian J. Ford created a controversy when he stated the idea that the majority of the dinosaurs were aquatic. This idea was met with wide-spread disagreement from the paleontology community. Ford is not a paleontologist. But, I think his idea on plant excretion may have some merit. Here is his theory.

Ford was observing palms and Philodendron plants while sitting on a log at the edge of a tropical rainforest near Cairns, Queensland, Australia. He noticed leaves were shed both by plants that were healthy and had plenty of nutrients and those that were struggling to survive. Both the healthy plants and the struggling plants had leaves that turned yellow and dropped off the plant. Why would a healthy plant receiving nutrients and water need to shed leaves during summer?

Trees are living beings. Living beings carry out certain functions to be considered “living” in western science: cellular organization, homeostasis, excretion, movement, respiration, reproduction, irritability, nutrition, and growth. We can understand how trees can fulfill all these criteria except one, excretion. How do trees excrete waste? When leaves release oxygen into the atmosphere, they are transpiring or giving off water vapor containing water and waste products. The waste products the plant is releasing is dissolvable in water. But plants take in substances like heavy metals and or toxins that can’t be dissolved in water. How does it rid itself of those toxins? I have wondered about this for years.

As we have understood the process so far, deciduous plants lose their leaves because the plant is shutting down for the winter. Evergreens (conifers) plants do not shed all of their leaves at the onset of autumn but keep them and stay green through winter. Evergreens shed their leaves, too, just not all at once at the same time each year. When you walk under or around a large evergreen tree such as an Eastern White Pine (Pinus strobus), the ground beneath the tree is covered with brown dead pine needles. Why does the pine tree shed its needles?  Not because the leaves can’t survive the harsh weather of winter. Evergreen leaves can and do survive the winter.

Sugar Maple leaf shutting down chlorophyll and filling cells with anthocyanin. Photo by Donna L. Long.
Sugar Maple leaf shutting down chlorophyll and filling cells with anthocyanin. Photo by Donna L. Long.

According to Ford, plants use the dropping of leaves as the primary way of excretion. Trees send toxic substances such as tannins, oxalates, and heavy metals into the leaf shortly before it drops from the branches. That is pretty convenient. The leaf is shed from the tree and toxic substances that can’t be dissolved in water and evaporated through the leaves, is carried along with it. According, to an article published by Ford (Biologist, December 2006) “individual cells acts as repositories and each is filled with pigments before another takes on the task of storage. As the micrographs make clear, deeply pigmented palisade (columns of) [sic.] cells increase in number one by one, as the leaf changes colour. ” Here he is referring to anthocyanin, the red pigment found in fall leaves.

Not only is the plant using the soon-to-be shed leaf to get rid of waste, but it is systematically filling up cells with the pigment anthocyanin.  I haven’t come across research that tests Ford’s theory. I will be keeping my eye out for any studies which do test it.

What if it were true that trees use the shedding of autumn leaves to eliminate wastes from its tree body before shutting down for winter? Would it change in some small measure, how humans think about trees? It would certainly change how I understood the lifeways of plants in my gardens.

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