For more information about National Park Service air resources, please visit http://www.nature.nps.gov/air/.
Ozone Effects on Tree Growth
Controlled studies have found that ozone reduces tree growth significantly. The Environmental Protection Agency (EPA) analyzed the effects of ozone on seedlings of Populus tremuloides (quaking aspen), Prunus serotinablack (black cherry), and Pinus ponderosa (ponderosa pine) in open-top fumigation chambers. Growth decreased as ozone increased and the results were used to predict growth loss to trees across the U.S. (see maps). For more information, visit the EPA website.
Two common ozone exposure metrics are the W126 and the SUM06. Both are cumulative, representing seasonal sums of ozone concentrations over 3 months during the daylight hours (from 8 AM to 8 PM) of the growing season. The W126 is a weighted sum that preferentially weights higher ozone concentrations; the SUM06 is the sum of all one-hour average ozone concentrations greater than or equal to 0.06 parts per million (ppm). A W126 of 13 ppm-hr is about a SUM06 of 15 ppm-hr. For estimated ozone exposure levels in the U.S., visit Air Atlas Ozone Maps.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 protective range is 7–13 ppm-hrs; for growth effects to tree seedlings and samplings in plantations the protective range is 9–14 ppm-hrs (Heck and Cowling 1997).
Biomass Loss Estimates for Selected Tree Species and National Park Service units
Last Updated: August 21, 2015