Updated: Aug 10, 2020
Years ago when I was studying the lyric essay at university, I wrote an essay titled The Journey, in which I detailed a trip taken from Kelowna, BC, to Vancouver by car. On this journey, I noticed that the deciduous leaves had started to change colours on the Coquihalla Highway even though they had not yet begun to change at home. Was it altitude, I wondered in this piece of writing, that caused the leaves to change? Was it temperature? Was it something else? I noted that I’d often wondered about the reasons behind the phenomena and debated the value of researching the answer versus having the pleasure of asking myself this question every year. I never did discover the answer.
According to Wikipedia (and elementary school science class), leaves are green because of chlorophyll, which is a green pigment. In the growing season chlorophyll is abundant in the cells of leaves, which is why leaves are predominantly green in colour.
Leaves need chlorophyll to capture the sun’s rays and use the energy produced to make the simple sugars which are the plant’s food. While making the food, the chlorophyll is broken down and used up, but it is also replenished in the growing season. Once daylight hours shorten and temperatures drop, the veins of the leaves (which carry the chlorophyll) gradually close off and a cork layer forms at the base of each leaf. The cork layer reduces water and mineral intake for leaves. This begins slowly but progresses more rapidly as the season changes. Although chlorophyll may still be in the veins of the leaf (keeping the veins green), the tissues between the veins dry out and change colour.
According to David Bradley of SciScoop, “the change in color to reds and yellows in autumn is not caused by the leaves dying, but by a series of controlled biochemical processes.” With less chlorophyll in the leaves as a result of the formation of the cork, less green pigment exists. The yellow pigments which already exist in the leaves can then become dominant. The leaves ‘turn’ yellow.
The process is different with red leaves. As the quantities of chlorophyll in the leaf get smaller, the leaf actually produces a new pigment called anthocyanin. This pigment, which is red, did not exist inside the leaf previously. For anyone with a rudimentary knowledge of colour theory, this makes some sense, considering red and yellow would make an orange hue which, when added to the green of the chlorophyll pigment, would simply turn all leaves a muddy brown.
Since the information about anthocyanin was only recently discovered by scientists, there has been an effort to explain why this process occurs. Anthocyanin has a toxic effect, and the red foliage appears to be a warning to insects that these leaves are toxic. The red colour is actually how the trees ward off insects. Red leaves, as beautiful as they might appear, are how certain tree species defend themselves in autumn.