Ozone Effects on Tree Growth
What is Ozone?
Ozone naturally occurs in low concentrations in the earth's atmosphere. Ground level ozone, however, is considered a pollutant, and can be harmful to both plant and animal life, as well as overall ecosystem health. Ozone is formed when nitrogen oxides emissions from vehicles, powerplants, and other combustion sources react with volatile organic compounds from gasoline, solvents, and vegetation in the presence of sunlight. Ozone effects to plants include spotting, stippling, or bleaching of leaves and reduced growth and reproduction. Both crops and natural vegetation are affected. For its 2007 review of the ozone standard, EPA analyzed the effects of ozone on certain tree species. Seedlings of Quaking aspen, Black cherry, and Ponderosa pine were exposed to ozone in open-top chambers. Growth decreased as ozone increased. These results were used to predict growth loss to trees across the U.S., based on 2001 estimated ozone exposures (see maps). For more information, visit the EPA website.
What levels are harmful?
Two common ozone exposure metrics are the W126 and the SUM06. Both are cumulative, representing seasonal sums of ozone concentrations over 3 months during daylight hours from 8 am to 8 pm. The W126 is a weighted sum that preferentially weights higher ozone concentrations; the SUM06 is the sum of all one-hr average ozone concentrations greater than or equal to 0.06 ppm. A W126 of 13 ppm-hr is about a SUM06 of 15 ppm-hr.
Scientists have developed the following general guidelines on thresholds for injury to vegetation. For visible foliar injury to natural ecosystems, a W126 range of 5-9 ppm-hrs is likely to be protective; for growth effects to tree seedlings in natural forest stands, the range is 7-13 ppm-hrs; for growth effects to tree seedlings and samplings in plantations the range is 9-14 ppm-hrs.
Estimated Ozone Levels, 2001