Red Leaves in Fall

By Jacob Schor, ND, FABNO

Photo by Mike Gifford via Flickr, used under the Creative Commons License.

We traveled to the east coast recently. Ostensibly we were going to a wedding and for Parents Weekend the following weekend at our daughter’s college, with the days between spent at the family cabin in Maine. In truth, we were going to see the leaves.

The leaves of deciduous trees change colors in autumn before they fall off the trees. Obviously this process gives name to the season. Less obviously, the Greek term for this leaf falling phenomenon is the etymologic base for the modern term ‘apoptosis,’ which describes the process of cellular suicide, an honorable and desirable decision when initiated in cancer cells. Why trees exhibit such a wide range of colors has been one of those unanswered questions for both the questioning mind and modern biologists.

A series of papers now provide an interesting answer to this question, and lead to an even greater appreciation of fall colors while forcing us toward some interesting ruminations.

Your basic, generic leaf is green. This is because chlorophyll is green and leaves contain lots of chlorophyll. Plants use chlorophyll to absorb energy from sunlight and convert it into sugars and starch, your basic elements of what we call food.

When leaves begin to die in the fall, the chlorophyll green fades away and no longer blocks out the underlying yellow pigments. Leaves turn yellow when they die. Yet some leaves turn brilliant red.

Red leaves are a different story. Chemicals called anthocyanins are responsible for this red color; they weren’t present in the leaves during the summer. Some trees actively make anthocyanins as their leaves start to die. Why would a tree go to the effort (expense) of filling leaves with anthocyanins just before they become ground litter? An even better question is, why are red leaves common in New England and rare in Western Europe?

Anthocyanins serve a number of functions in plants. The list gets longer every few years.

In 2003, William Hoch of Montana State University, reported that anthocyamins helped send nutrients to the plants roots.  Blocking anthocyanin production resulted in the plant sending fewer nutrients to the roots for winter storage.ⁱ

In 2007 Habineck reported that trees that grown in nitrogen-poor soils produced anthocyanins because the pigments protected the leaves and kept them alive a bit longer giving them the chance to store  more nutrients in their roots before winter set in. Thus the red hues in the fall are the sign of a stressed tree trying to squeeze out a few more days of photosynthesis, to make every last bit of food to survive..ⁱⁱ

Anthocyanins also protect leaves from freezing as the temperature gets colder in the fall and actually help the leaves absorb more warmth from sunlight postponing the inevitable.

Anthocyanin pigments also protect against damage caused by insects. This ‘insecticide action’ explains why autumn leaves are redder in the U.S. than in Western Europe.  Oddly enough this explanation comes from Simcha Lev-Yadun of the University of Haifa in Israel, a country not known for fall colors. While earlier theories focused on the red leaves serving as a warning to insects that ‘we don’t taste good,’ Lev-Yadun’s explanation focuses on the insecticidal properties of anthocyanins.  A carpet of anthocyanin rich leaves surrounding a tree through the winter decreases insect breeding and subsequent attack from insects the following year.

North America and Europe have underwent repeated eras of ice ages during which trees have evolved to become deciduous, adapting to fluctuating seasonal and climatic conditions. In adapting to both seasonal cold and dry periods, trees also learned to repel seasonal insect attacks. Many trees learned the value of seasonal anthocyanin production. (We should note that in the tropics, many plants produce anthocyanins year round.)

There is a key difference between the pressures put on trees in North America versus Europe. In North America, the challenge to trees during the ice ages was different; plants and insects could gradually migrate north and south as the ice fields waxed and waned. In Europe the trees were trapped, along with the insects that wanted to feast on them, between ice sheets that advanced from the north and from the Alps. According to Lev-Yadun’s thesis, these European insects largely died out: "The anti-herbivore component in red leaf coloration was relaxed, and northern Europe became dominated by trees with yellow autumn leaves." The European trees didn’t need to make anthocyanins, so they lost the habit. Thus European trees turn yellow with little red.ⁱⁱⁱ

Here in Colorado, our thoughts quickly turn to Aspen leaves, which evolving high in the Rockies with reliable cold winters also had less need for insect defenses than trees in our eastern hardwood forests.

While solving this basic question of why leaves change colors just before they fall provides some relief, Lev-Yadun’s theory reminds us of a bigger question.

NDs often promote consumption of foods and nutritional supplements because of their high anthocyanin content.

Plants with high anthocyanin levels include the Vaccinium species (blueberry, cranberry and bilberry), the Rubus berries (black raspberry, red raspberry, blackberry, and blackcurrant), and a number of other plants including cherry, grape, red cabbage, violet petal, eggplant peel, black rice and black soybean.

Many nutritional proponents include these foods on their lists of the most beneficial things to eat.

Anthocyanins are but one example of chemicals that plants make that have insecticidal properties. When we start looking at other plant chemicals considered good for us, we eventually discover that the plant made them to be poisonous.

The yellow alkaloids in berberine and curcumin, the isoflavonoids in soybeans, the isocyanides in cruciferous vegetables and so on were all meant to be poisonous to bacteria, fungus, insects or animals.

Why is it desirable for us to swallow a botanical hodgepodge of poisons?

If we want to really understand this, we need to go back to basics. We need to view this question against an understanding of the basic laws of nature.

The first law of thermodynamics is often called the conservation of energy and says that, “Energy can be changed from one form to another, but it cannot be created or destroyed. The total amount of energy and matter in the Universe remains constant, merely changing from one form to another.”

The second law of thermodynamics tells us, “In all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state." This is also referred to as the law of entropy. The universe always moves towards states of greater entropy or disorder. (I also refer to this as the Law of Socks, as in “Fewer pairs of sock will emerge from the dryer than I wore last week.”)

Everything in the universe moves toward states of greater entropy, except of course for things that are alive. Life is the opposite of entropy, always striving toward levels of greater complexity and order.

Things that are alive have the capacity to use energy and resist entropy. Living things react and adapt to forces that would drive them toward entropy.

The second law of thermodynamics: “if no energy enters or leaves the system…”
Living things are able to resist entropy because they constantly bring more energy into the system: they eat! Food contains stored energy. Resisting entropy takes energy; creating states of greater order takes energy. To do either takes food. We eat in order to resist entropy.

Thus, in a primal sense, we might define life as the ability to utilize the energy in food in order adapt, maintain order and resist entropy.

The stored energy in food all comes from sun, which at some point is stored through photosynthesis in plants.

Without food, living things lose their ability to resist the forces of entropy; they lose the ability to adapt.

How do you know if something is alive? You poke it. If it’s alive it moves. Living things respond to the world, to stimuli.

‘Poke it, see if it is alive’
Living things adapt and respond in someway to external forces; they adapt to maintain order, either to get out of harm’s way or to move toward food or less entropic environments.

These basic definitions of the universe lead us to a biological term called “adaptive response.” In its simplest definition, adaptive response means “an appropriate reaction to an environmental demand” (Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier).

A fuller definition might read “The ability of a cell, tissue or organism to better resist stress damage because of prior exposure to a lesser amount of stress, observed in all organisms in response to a number of different cytotoxic agents.”

Adaptive response is now often applied to the ability to repair genetic damage and can be triggered by exposure to cytoxic exposure, to phytonutrients such as anthocyanins. Adaptive responses can also be triggered by whole organism stressors.

An example of whole body stressors is the Scandinavian habit of cold water swimming. Researchers there have done some interesting work on adaptive responses in individuals who enjoy this practice, which entails taking hot saunas and then leaping into holes cut into the winter ice of lakes for a swim.

One Finnish study tells us that this habit changes an individual’s response to cytokines: “These stresses appear to challenge both the neuro-endocrine and the immune systems and the results indicate that adaptive mechanisms occur in habitual winter swimmers.”^iv

As an aside, as we look at the many traditional practices of nature cure that were and are promoted by members of the naturopathic profession, we can see how many of them trigger adaptive responses.

Coming back to red leaves in autumn and anthocyanins, if we view anthocyanins as elements that will trigger various adaptive responses, such as triggering DNA repair and preparing our bodies to adapt to stressors, we can see how and why these chemicals can be both poisonous to insects and health-promoting to humans at the same time.

As we watch leaves turn red this autumn, perhaps we can also find some pleasure in the understanding of these complex relationships that give rise to these leaf colors that our minds simply perceive as a thing of beauty.

--
ⁱHoch W, Singsaas E, McCown B. Resorption Protection. Anthocyanins Facilitate Nutrient Recovery in Autumn by Shielding Leaves from Potentially Damaging Light Levels Plant Physiology 133:1296-1305 (2003)
ⁱⁱHabineck, EM. Correlation of soil development and landscape position with fall leaf colors. Paper No. 81-15 2007 GSA Denver Annual Meeting (28–31 October 2007)
ⁱⁱⁱLev-Yadun S. The shared and separate roles of aposematic (warning) coloration and the co-evolution hypothesis in defending autumn leaves. Plant Signal Behav. 2010 Aug;5(8):937-9.
^ivDugué B, Leppänen E. Adaptation related to cytokines in man: effects of regular swimming in ice-cold water. Clin Physiol. 2000 Mar;20(2):114-21.