Edited by Len Phillips

Most trees are genetically programmed to turn a certain color in the fall. Some turn yellow, some turn red, some turn orange and some don't turn any color at all. The reason that leaves produce the bright displays of red and purple foliage is because leaf sugars are converted to the red pigment, anthocyanin. Questions remain however as to what triggers the leaf sugars to change. Researchers speculate the leading triggers could be cold nights, rainfall, daylight length, a combination of these, or something else altogether.

Anthocyanin
For decades, forestry textbooks claimed that anthocyanin served no function as trees prepared to drop their leaves. It simply became visible as green chlorophyll molecules broke down in autumn.

But more recent research from around the world has proposed numerous ways that anthocyanin could benefit trees in autumn: as a sunscreen to protect leaves from excess light, as an antioxidant to help repair leaf damage, and to help resist cold and drought. Researchers also found that in sugar maples, the stems of red-colored leaves are more firmly attached than their yellow brethren. This observation means that anthocyanin may allow trees to keep absorbing sugars and nutrients from leaves later into the fall.

Cool Weather
Another hypothesis is that cool, but not freezing nights promote anthocyanin development. Cold at night may prevent the leaf from exporting all the sugars it made during the day and therefore they get trapped in the leaf.

Soils
Soils may also dictate the array of fall colors. According to a laboratory analysis of the autumn foliage on sweet gum and red maple trees, researchers found that in places where the soil was relatively low in nitrogen and other essential elements, trees produced more anthocyanins. When these researchers genetically blocked anthocyanin production in red-leafed plants, their leaves were unusually vulnerable to fall sunlight, and so sent less nutrients to the plant roots for winter storage. For trees living in nutrient-poor soils, it makes sense to produce more anthocyanins, which protect the leaves longer, so as much nutrient as possible can be recovered from leaves before winter sets in. Anthocyanins are an investment made by stressed trees in situations where they stand to gain from the extra recovery of nutrients from leaves. It's not about the showy color, but about survival.

Chlorophyll
The latest theory proposes taking a step 35 million years back to solve the color mystery. The green color of a tree's leaves is mainly due to the chlorophyll pigment. The change in color to red or yellow as autumn approaches is not the result of the leaves dying, but is rather the result of a series of processes which differ between the red and yellow autumn leaves. When the green chlorophyll in leaves diminishes, the yellow pigments that already exist become dominant and give their color to the leaves. Red autumn leaves result as the chlorophyll diminishes, anthocyanin, which was not previously present, is produced in the leaf.

Physiology
Some research suggests that the red pigment is produced as a result of physiological functions that make the translocation of amino acids to the woody parts of the tree more efficient in setting up its protection against the potential damage from light and cold.

Amino Acid
Other explanations suggest that anthocyanin is produced as part of the tree's strategy for protecting itself against insects that thrive on the flow of amino acids. Insects are not attracted to red leaves.

Evolution
But whatever the answer is, these explanations do not help us understand why the process of creating anthocyanin does not occur in Europe. An evolutionary ecology approach infers that the strong autumn colors result from the long evolutionary war between the trees and the insects that use them as hosts. Insects tend to suck the amino acids from the leaves in the fall season, and later lay their eggs, to the detriment of the trees. Aphids are attracted to yellow leaves more than red ones. Trees that expend the energy to color their leaves red may benefit from fewer aphids and fewer aphid eggs. In this case too, the protective logic of red pigmentation may be sound.

Mountain Ranges
According to another theory, until 35 million years ago, large areas of the globe were covered with evergreen jungles or forests composed of tropical trees. During this phase, a series of ice ages and dry spells transpired and many tree species evolved to become deciduous. Many of these trees also began an evolutionary process of producing red deciduous leaves in order to ward off insects. In North America, as in eastern Asia, north-to-south mountain ranges enabled plant and animal 'migration' to the south or north with the advance and retreat of the ice according to the climatic fluctuations. Along with them migrated their insect 'enemies'. Thus the war for survival continued there uninterrupted.

In Europe, on the other hand, the Alps and their lateral branches run from east to west, and therefore no protected areas were created. Many tree species that did not survive the severe cold died, and with them the insects that depended on them for survival. At the end of the repeated ice ages, most tree species that had survived in Europe had no need to cope with many of the insects that had become extinct, and therefore no longer had to expend efforts on producing red leaves.

According to the scientists, evidence supporting this theory can be found in the dwarf shrubs that grow in Scandinavia, which still color their leaves red in autumn. Unlike trees, dwarf shrubs have managed to survive the ice ages under a layer of snow that covered them and protected them from the extreme conditions above. Under the blanket of snow, the insects that fed off the shrubs were also protected so the battle with insects continued in these plants, making it necessary for them to color their leaves red.

Conclusion
The end result of all this research is that we have some good theories, but no firm reason why trees turn their beautiful shades of red, orange, yellow, and purple every autumn. We are just happy that they do.

Sources

• "When Good Maples Go Red: Why Leaves Change Color In The Fall", ScienceDaily, Nov. 26, 2008.
• "Why Do Autumn Leaves Turn Red? Soil May Dictate Fall Colors", ScienceDaily, Oct. 29, 2007.

• "Why More Autumn Leaves Are Red In America And Yellow In Europe: New Theory", ScienceDaily, Aug. 17, 2009.

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