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Ozone Effects on Vegetation

Photo of healthy (top) and ozone-injured (bottom) tulip tree (yellow poplar) foliage. Ozone-injured leaf has interveinal dark brown/black discolorations on the upper leaf surface.
Photo of healthy (top) and ozone-injured (bottom) tulip tree (yellow poplar) foliage.

One of the most widespread air pollutants is ground-level ozone. Naturally-occurring ozone in the upper atmosphere forms a layer that absorbs the sun's harmful ultraviolet rays and protects all life on earth. However, ground-level ozone can harm vegetation as well as human health. It is not emitted directly from smokestacks or vehicles but is formed when other pollutants, primarily nitrogen oxides and volatile organic compounds, react in the atmosphere in the presence of sunlight. Ozone causes considerable damage to vegetation throughout the world, including agricultural crops and native plants in natural ecosystems. The Environmental Protection Agency (EPA) has established an ozone standard to protect human health and vegetation from ozone effects. However, there is evidence to suggest that this standard, based on human health studies, is not protective of very sensitive plant species. These plants may be harmed at ozone levels below the standard.

Ozone enters plants through leaf openings called stomata and oxidizes plant tissue, causing changes in biochemical and physiological processes. Both visible foliar injury (e.g., stipple and chlorosis) and growth effects (e.g., premature leaf loss, reduced photosynthesis, and reduced leaf, root, and total dry weights) can occur in sensitive plant species. Many other factors can magnify the extent of ozone injury such as soil moisture, presence of other air pollutants, insects or diseases, and other environmental stresses. Ozone effects on natural vegetation have been documented throughout the country, particularly in many areas of the eastern U.S. and in California. To learn how ozone affects tree growth, read more »

Featured Content

Impacts icon

Bioindicator plant species can reveal ozone stress in NPS ecosystems. These species are known to be sensitive to ozone and ozone exposure produces distinct visible and identifiable injuries to plant leaves. Learn more...

Studies and Monitoring icon

Ozone effects studies help the NPS understand environmental impacts of ozone. Access ozone risk assessment project results for 270 NPS areas to learn about the impacts of ozone at specific parks. Learn more...

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Last Updated: January 10, 2013