Atmosphere and Plants

Plant distribution and health is controlled by properties of the atmosphere such as climate, hurricanes, lightning, and pollution. Plants also play a large role in controlling the atmosphere. In fact, the atmosphere at any one location is not only the result of global atmospheric weather patterns, it is also the end result of the type and amount of vegetation.

Effect of Plants on Climate

Over the course of a day, the bulk of energy from the Sun is used in one of two processes: raising temperatures or changing water from a liquid or solid state to vapor form. (Photosynthesis uses only about 2 percent of the Sun's energy.) If a large amount of vegetation is present and is actively conducting photosynthesis, most solar energy is used to convert liquid water in leaves to water vapor. This release of water vapor from leaves is called transpiration. If there is no vegetation, as after a logging clear-cut, evaporation can take place from the soil, but much more energy is used to increase temperatures. Therefore, clear-cutting large forests can significantly raise temperatures. In the Amazon forest in South America, the process of transpiration produces high humidity in the atmosphere, which in turn is then returned to Earth as rain. As much as half of the precipitation in the western Amazon is from this recycled moisture. In these and other ways, plants influence the climate in their region. The type of vegetation also affects the radiation budget, that is, the percent of solar radiation that is reflected back into space; for instance, sparsely vegetated areas reflect more light than densely vegetated areas.

Plants and Atmospheric Oxygen

Forests—in particular, the tropical forest—are often called the lungs of the planet. It is true that plants produce oxygen during photosynthesis, but they and the organisms living in their ecosystems also consume about the same amount of oxygen during respiration. Oxygen makes up 21 percent of Earth's atmosphere but annual production of oxygen by plants is only about.05 percent of the atmospheric amount. There is so much atmospheric oxygen that completely destroying all vegetation would have only a minor effect on atmospheric oxygen levels. Doubling the amount of vegetation would increase atmospheric oxygen by only.5 percent. So it is not true that plants are responsible for our global oxygen supply, at least in the short term. Other processes relating to the weathering of rocks and oceanic circulation operating at the timescale of tens of thousands of years are principally responsible for regulating oxygen levels. Plants are, however, very important in the cycling of carbon dioxide.

Plants and Volatile Organic Compounds

Easily evaporated compounds containing hydrogen and carbon are known as volatile organic compounds (VOCs). There are thousands of VOCs in the atmosphere, and plants produce many of them. Monoterpenes and isoprenes are the best-known plant-produced VOCs. Most monoterpenes are produced by conifers in leaves, wood, and bark. Once produced, monoterpenes stay in the plant tissue and are used by plants for defense against herbivores . Isoprene, produced by deciduous trees, spruces, and mosses, does not stay in the plant. The production of isoprene helps plants conduct photosynthesis at high temperatures that would otherwise be very damaging. Production of other VOCs gives many plants their characteristic aroma.

VOCs also react quickly in the atmosphere. In the presence of nitrogen oxides and sunlight, VOCs react to form ozone, a major component of smog. In urban areas, industrial activity and the use of cars can produce very high levels of nitrogen oxides. This, combined with the production of VOCs by plants, can be a major contributor to urban pollution. In cities with large forest populations, such as Atlanta, Georgia, plant-produced VOCs can account for a large portion of the urban smog problem. Human activities, however, are still responsible for the bulk of urban smog as well as the production of the large amounts of nitrogen oxides reacting with the VOCs. In Switzerland, a highly industrialized country, plant-produced VOCs made up only 23 percent of total VOCs. Trees should therefore not be blamed for most smog.

Air Pollution and Pollution-Tolerant Plants

Our industrial society produces large amounts of pollution. Sulfur dioxide is produced by the combustion of a variety of high sulfur fuels, especially coal. Acid rain is produced from sulfur dioxide. Aluminum and glass factories produce fluoride, a pollutant that can accumulate in plants. Ozone and peroxyacetyl nitrate, both produced in the presence of sunlight, nitrogen oxides, and VOCs, are major components of smog and together are the most serious air pollution problem faced by plants.

Pollution enters the plants through stomata, tiny pores used by leaves for gas exchange. Yellow or brown coloration along leaf edges and veins are signs of pollution damage. Cell membranes are destroyed and the biochemical reactions of photosynthesis are slowed or stopped. Air pollution itself does not usually kill plants, but it can severely reduce crop yields and makes plants more susceptible to diseases and insects. The damage created by pollution depends on the concentration of the pollutant as well as on the duration of the pollution event. For example, long-term exposure to low pollution levels may be less damaging than short, intense pollution events. Long-term processes such as acid rain, though, can damage forests by changing soil acidity over many years.

Plants vary greatly in their ability to resist pollution. In some cases, plants are resistant to sulfur dioxide, but not to ozone. In Australia, radiata pines are usually more resistant to sulfur dioxide than broadleaf eucalyptus trees. Yet in Sweden, broadleaf trees resist ozone better than the conifers. There is also tremendous variation in ozone resistance within the Eucalyptus genus. Sweeping generalizations about individual species are virtually impossible, but plants do seem to follow several patterns:

• thick leaves are pollution-resistant
• species or varieties that have high rates of stomatal conductance (the process that brings carbon dioxide and pollution into the leaf) experience more pollution damage
• plants can often adapt to high pollution over time
• older plants are more resistant than younger plants.

For most species that have been studied, botanists can develop plant varieties that are resistant to a certain pollutant or combination of pollutants. Therefore it is usually better to assess local pollution problems and to select or breed plant varieties for that situation than it is to identify universally resistant species.

see also Acid Rain; Carbon Cycle; Defenses, Chemical; Global Warming; Human Impacts; Photosynthesis, Carbon Fixation and; Terpenes; Water Movement.

Michael A. White

Bibliography

Cozic, Charles P., ed. Pollution. San Diego: Greenhaven Press, 1992.

Gay, Kathlyn. Ozone. New York: Franklin Watts, 1989.

Jones, Hamlyn G. Plants and Microclimate, 2nd ed. New York: Cambridge University Press, 1992.

Miller, Christina G., and Louise A. Berry. Air Alert. New York: Atheneum Books for Young Readers, 1996.

Sharkey, Thomas D., Elizabeth A. Holland, and Harold A. Mooney. Trace Gas Emissions by Plants. San Diego: Academic Press, 1991.

Tolbert, N. Edward, and Jack Preiss, eds. Regulation of Atmospheric CO ₂ and O ₂ by Photosynthetic Carbon Metabolism. New York: Oxford University Press, 1994.