Why Do Some Autumn Leaves Turn Red?

THE ABSENTEE GARDENERS

By Kit Flynn and Lise Jenkins

Many of us look forward to the arrival of autumn with its cooler temperatures and brilliant colors. We know that there is a breakdown of the chlorophyll compound, so necessary for the photosynthesis process, in preparation for the winter slumber. However, that still doesn’t explain the reason why some leaves turn such brilliant colors before
turning brown and falling.

In grammar school we learned that leaves stop producing the chlorophyll so essential for photosynthesis. Consequently, leaves revert to their original colors, red, orange and yellow. We assumed that the colors had no function.

If only it were that simple and uncomplicated.

There are so many unanswered questions for the non-scientist: Why are the undersides of a leaf comparatively more drab in color than the leaf’s topside in autumn? Why do the preponderance of European trees turn yellow whereas trees in the US, Canada and East Asia turn a variety of colors?

Why are leaves using their sugar storage to produce the reds and the purples? After all, these leaves will ultimately die and fall off — and the sugar reserves could help to sustain the tree during its winter slumber.

What seems to be accepted in the scientific community is that the red pigments, the anthocyanins, offer some sort of protection to the tree, but what kind of protection? On this there is little agreement.

Today, scientists are delving into the mysteries of autumn leaf colors, especially the reds and the purples produced by the anthocyanins. The pH determines whether the color will be red or purple, with an acidic pH producing the reds and less acidic pH responsible for the purples.

One theory, known as photoprotection, holds that the changing colors act as a sunscreen, thereby allowing the tree to absorb more nitrogen before the leaf drop. Another theory holds that it is a reaction to drought resistance — the anthocyanins reduce water loss and even might reduce the leaf’s freezing point.

Could this be an attempt to protect against cold temperatures? This theory holds that red leaves convert the light to heat. However, this theory doesn’t relate to yellow leaves. Other scientists have proposed that perhaps the colors attract birds to eat the fruits, thereby spreading their seeds. However, this only refers to trees bearing fall
fruit.

Trees actually zap up production of anthocyanins at the same time the chlorophyll production is waning. Scientists are now wondering if this serves as a warning to insects. Red leaves are of poor nutritional quality so migrating insects might take this as a sign to stop by those trees that still have green or yellow leaves.

Yet there is the unanswered question about why more European tree leaves turn yellow whereas in the U.S. and East Asia, more trees bear red leaves. Are there fewer insects in Europe to protect against?

Another theory is the camouflage one, citing that insects and their eggs are more visible on green leaves than red ones. Yet the majority of insects such as aphids will lay their eggs on yellow leaves. A theory that amounts to an anticamouflage one holds that autumn colors actually accentuate the presence of insects on the leaves, thereby leaving them more vulnerable.

None of these theories adequately explain why some leaves turn red — and they don’t account for those leaves that turn orange or yellow.

What we do know is that fall color is most pronounced when there has been adequate rainfall throughout the year. Dry periods reduce photosynthesis production, thereby triggering leaf fall before the colors develop. The best environment for wonderful fall color includes warm, sunny days and cool nights.

Another mitigating factor is the first frost that causes leaf drop. If it’s later in the season, the better the chances for greater fall color.

As for us, we’re going to head back to our grammar school explanation while we lie back, enjoying autumn’s amazing colors.

Absent from their gardens, Kit and Lise enjoy roaming our region exploring the intersection of horticulture and suburban living. More on Instagram @AbsenteeGardener or email: info@absentee-gardener.com