SciTech

How Things Work: Fall Leaves

My affection for autumn leaves is like a CS nerd?s love for Linux. Have you even noticed those gorgeous red maples next to Warner, the stunning orange foliage near the tennis courts, or the yellow glory of the stately oaks along Baker?
And if you think a few oak trees are nice, try a walk in Schenley Park, where a trail is transformed into a stairway to heaven. Lest this piece descend any further into effusive adjectives, I now present the science of dead leaves: that is, why leaves turn those pretty colors that they do.
So suppose that you?re God, and you?ve decided to give Earth?s weather a little variety. You?ve already managed to get the sun in place, with the Earth and its moon in a fairly circular orbit around it. The question is: Do you stretch earth?s orbit to create seasons, or do you tilt the earth? For reasons of good astrophysics, the answer turns out to be the latter.
In fact, the earth?s orbit is really quite circular: Our distance from the sun differs by about three percent over the course of a year. The reason for our seasons is the tilt ? 23.45 degrees of it, to be precise. During the colder months, one side of the earth is tilted away from the sun, making the days shorter and reducing the heating efficiency of sunlight.
Of course, the only thing trees care about is the fact that in autumn it?s colder and the nights are longer. When the length of the night reaches a certain threshold, cells near the juncture of the leaf and the stem begin to undergo a form of cell division called meiosis, blocking the flow of nutrients to the leaf.
Sunlight is then free to break down the chlorophyll molecules in the aging leaf. However, the nutrients in these chlorophyll fragments ? magnesium ions and nitrogen-rich proteins ? are still useful to the tree. These compounds are reabsorbed by the tree and recycled. In effect, the greenness of the leaves is drained back into the tree.
But a leaf does not contain only chlorophyll. There are also red and purple pigments known as anthocyanins which are manufactured from the sugars trapped in the leaf. Also present are pigment molecules that aid chlorophyll in capturing light: yellow xanthophylls and orange carotenoids. When the chlorophyll is gone, these pigments are not recycled, but left behind as waste. And what waste it is! If only other kinds of garbage were so attractive.
After the last of the minerals have been mined from the leaf, cells near the leaf stem produce the enzyme pectinase. Pectinase causes the majority of the cells at the base of the stem to separate from each other. (Think seppuku.) The weight of the leaf then rips the remaining cells apart, and the leaf falls quietly to the ground. Some trees retain their leaves through much of the winter, but the leaves do not remain colorful for long. Just as chlorophyll breaks down quickly, so do the other color pigments, once they are exposed to sunlight and the autumn chill. The only remaining pigments are tannins, which are brown. All good things must come to an end.
A beautiful fall is a rare thing, requiring a delicate set of weather conditions. The most colorful autumns require warm days, lots of sunlight, and cold nights. Brisk temperatures let chlorophyll be broken down faster, and promote the formation of anthocyanins. However, an early frost will hinder the process responsible for manufacturing anthocyanins, as will too little rain. Too much rain may cause the leaves to be lost before they become fully colored.
So be grateful for what you get. You may wish you were living in sunny California, but there?s no place like Pennsylvania for a gorgeous fall season. Take a minute and spend some time enjoying these fall colors. If you want to get really crazy, you could be drinking cider, eating caramel apples, or roasting chestnuts. It doesn?t get much better than that.