Amateur Naturalist: The Autumn Leaves
Los Alamos
Autumn now is arriving. The most obvious sign is with leaves. The leaves of trees and bushes are turning yellow or red. Another sign is with birds. Hummingbirds drink flower nectar not only to meet their needs from day to day, but also to store nutrients for their migration flight south. As an example hummingbirds drank a pint of sugar water in a feeder on a daily basis at the start of September. This suddenly came to a stop toward the end of September when the first cool front of autumn arrived. The feeder now remains full, day after day. The hummingbirds have flown south. They may fly 1,000 miles south into central Mexico or up to 1,800 miles to reach Guatemala.
Turkey vultures here in New Mexico also migrate. They may fly far south into central or South America. A hummingbird may beat its wings 70 to 80 times per second and fly by itself while migrating. A turkey vulture may glide much of the time and be part of a flock. Consider a hummingbird beating its wings 4,200 in a minute while a vulture may flap its wings six times.
But what is occurring within plants? Chlorophyll gives the green color to a leaf. The underlying red or yellow colors in a leaf become visible when chlorophyll is disbursed. Chlorophyll typically weighs in the range of 2 milligrams to 10 milligrams for each gram of a fresh leaf. The average is about 5 milligrams. A leaf may weigh about five grams. Chlorophyll therefore may weigh 1/1,000 of a leaf's weight. A very slight change in the weight of leaf results in a visually dramatic change in color when chlorophyll departs. (Note that there can be considerable variation about these numbers.)
A tree may have thousands of leaves. A tree with 1,000,000 leaves therefore may have 100 grams of chlorophyll. This is equivalent to about 1⁄4 pound. What does this mean for oxygen production? Projections vary, but one projection for a mature tree is 100 kilograms per year. A human breathes about 740 kilograms per year in comparison. This suggests that seven or eight trees are needed to create the oxygen used by a person annually.
It is interesting to think about what is happening to a forest when its leaves are gone. It would seem that the oxygen level would fall since oxygen is no longer being produced. It also would seem that the carbon dioxide gas level would increase since it is no longer being absorbed as part of the photosynthesis process. However oxygen is produced elsewhere such as by plankton in the oceans. It is blown by the wind around the world. Air also rises as it is warmed by the sun throughout the day. It becomes mixed with other air and blown elsewhere. The changing color of a leaf is a sign of something happening on both a small and large scale.
Aspen groves become fields of yellow as the trees prepare to become dormant for winter. Photo by Robert Dryja
Consider that each five gram leaf had about five milligrams of green chlorophyll in it. Photo by Robert Dryja
