Monday, October 15, 2007

Why Do Leaves Fall From Trees?

Fall is the season that we get to enjoy the colors of many different types of leaves changing from green to brilliant red, yellow, orange, and/or brown colors. However, the leaves are also undergoing other physical changes besides the changing colors - that is, the leaves begin to fall from different trees and plants. But why do the leaves fall, and why do some plants have leaves that do not fall? We will first talk about the different types of trees that lose or do not lose their leaves. Then we will discuss the reasoning behind how or why the leaves fall off.

There are two different types of trees and plants: deciduous and evergreens. Deciduous trees (like elm or maple trees) grow in temperate climates and usually lose all of their leaves for part of the year. Evergreen trees, like pine or spruce trees, keep their leaves in the fall because they are resistant to water loss and cold temperatures. Deciduous trees generally have broad leaves, while evergreen trees have long, thin needles for leaves. The evergreen needles are coated with wax to keep the water in all year long.

Trees are naturally tough plants - a tree's roots, branches and twigs can tolerate freezing temperatures. However, most trees' leaves cannot withstand really cold temperatures so the tree must shed the leaves at some point during the year in order to survive during the winter. Leaves are made up of cells that are filled with water sap. At the base of each leaf stem on a tree, there is a layer of cells called the abscission or separation layer. During the summer, small tubes in the separation layer carry water into the leaf and food back to the tree. In the fall, this layer swells and makes a cork-like substance that stops the flow of water and food between the leaf and the tree. This cork-like layer is formed when the veins that carry sap into and out of a leaf gradually close, which destroys the tissues that nourish the leaf. These veins close as a result of the days getting shorter during the fall months (i.e., there is less sunlight per day). The separation layer then forms a tear-line, and soon the leaf blows away due to the wind or it falls from its own weight. The tree then seals itself where the leaf detached, which is similar to how our bodies can seal small cuts with scabs, and it is now ready for the winter. An exception to all deciduous trees losing their leaves is the oak tree. Even though oak trees are considered deciduous, some oak leaves remain on the tree through winter because the separation layer never fully detaches the dead oak leaves.

So the next time you are asked to rake the leaves of your front yard, you will understand why they have fallen in the first place!

Little Lion Experiment:

This experiment is more of an observation. It is simple and involves spending time outside looking at trees trying to identify whether the trees you see are deciduous or evergreen trees. The trees can be in your neighborhood, in a nearby forest or park, and/or at your school. You will need an adult to come with you as you look at the different trees. You may also want to take a small bag to collect any leaves that you might want to keep. Take notice of the trees that have already begun to lose some of their leaves compared to those that have not lost many of their leaves yet. Try to visit the same trees a few more times this month to observe the changing leaf colors and the amounts of leaves falling. Did you find any trees that did not lose any leaves? Did you find any trees that did not change colors?

Even though we did not talk about leaves changing color much, here's an online scrapbook that shows what some leaves look like when they change color: