- The Washington Times - Thursday, November 14, 2002

Leaves can offer a source of cool comfort on a hot summer day or can be an aggravation as they fall on a homeowner's lawn each autumn. In either case, many people probably don't realize the leaf's overall importance to the survival of life on Earth.
Joseph Sullivan, an assistant professor at the University of Maryland's Department of Natural Resource Sciences and Landscape Architecture, who holds a doctorate in plant physiology from Clemson University, took a few minutes to discuss the magic behind and significance of the leaf.

Q: Why do trees need leaves?

A:
Leaves are the primary part of the plant responsible for photosynthesis, or the process by which green plants use light energy from the sun to turn carbon dioxide into chemical energy or carbohydrates for the plant. Without the leaves, the plant would die.
Leaves also are responsible for the process by which carbon dioxide, which is poisonous to humans, is removed from the air and replaced with oxygen, which we need to breathe.

Q: What is the importance of the leaf?

A:
Life as we know it would not have evolved without the cycle of the forested leaf canopy overhead, the leaves dropping to the ground and decomposing, creating nutrients for other life. Also, before there were plants capable of photosynthesis, the world was a very different place.
First and foremost, the role of plants and their leaves in the production of carbohydrates used throughout the food web is important because without them we would have nothing to eat. If you think about it, we either eat a plant that used leaves to produce its food or eat something that ate a plant with leaves, so on a very large scale, leaves are an important first step in our food chain.
Second is leaves' role in the removal of carbon dioxide from the air, replacing it with oxygen. Before the process of photosynthesis evolved, the world was a very anaerobic environment, or an environment absent of oxygen. In fact, oxygen was poisonous to the organisms present at that time.
As green plants evolved, the environment changed to an aerobic environment, leading to the development of the ozone layer that protects us from the sun's ultraviolet rays.

Q: What causes a leaf to change color, die and fall?

A:
A leading factor that causes leaves to change colors, in addition to the plant's genetics, is weather conditions, such as the amount of rainfall, sun and temperature. The biggest factor, however, is the longer nighttime periods that begin at the time of the autumn equinox, the time when our days become shorter and nights longer.
There are other factors, besides the change of the season that causes leaves to fall, often without normal fall coloration. During times of severe drought, such as we had this August, plants may lose some leaves as a protection mechanism.

Q: What determines a leaf's fall color?

A:
Genetics determines the color. When the leaf starts to grow, it maintains its green color due to chlorophyll, a green pigment responsible for absorbing the light energy used in the production of food in the plant.
In addition to chlorophyll, plants have other pigments that cannot be seen because of the abundance of chlorophyll. These extra pigments, called carotenoids, are yellow, orange or sometimes brown, and their role is to help the plant absorb the sunlight's energy.
When the nights become longer and cooler, the plant's balance of hormones changes, and the plant may enter into a dormant period for the winter. When this happens, a layer of cells is produced between the leaf stalk and the branch called the abscission layer. This layer of cells is corky, and it slows down the transportation of water and carbohydrates to the leaf, which slows the production of chlorophyll, and the dominance of the green color begins to fade. This allows the plant's other colors, the yellow, red and brown carotenoids, to shine through. This process also causes an increase in the production of anthocyanins, pigments that account for the purple and red colors of some leaves.

Q: What factors determine the size of a leaf?

A:
While the size of the tree has some bearing on leaf size, it really is dependant on genetics, just like the size of our feet. The environment in which a plant grows, however, also plays a role in a the size of the tree's leaves. A tree that is growing in a well-nurtured and watered environment will have larger leaves than a tree growing under stress, such as one planted in the middle of a parking lot where it is under a lot of environmental stress. Noticeably smaller leaves on a tree can be an indication that the plant is unhealthy, not receiving enough water, or has a nutrient deficiency or other problem.

Q: What is the best thing to do with the fallen leaves?

A:
What we don't want to do is put them in a plastic bag and throw them into the landfill. The best thing would be to try to compost them a mulching lawn mower works great for helping that process along.
Write to Joseph Szadkowski, The Washington Times, 3600 New York Ave. NE, Washington, DC 20002; or send e-mail ([email protected]).



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