Ozone Impacts on Forests

Prediction of Ozone Impacts in Forest Ecosystems

Background

The nation’s clean air “yardsticks”—referred to as the National Ambient Air Quality Standards (NAAQS)—are established by EPA as part of the Clean Air Act mandate. EPA has set national air quality standards for six principal pollutants (referred to as “criteria” pollutants): particulate matter, sulfur dioxide, carbon monoxide, nitrogen dioxide, lead, and ground-level ozone. “Ambient” refers to surrounding or outside air quality.

Areas of the country that are unable to meet clean air standards for these pollutants are referred to as “nonattainment” areas. As such, these areas must develop and implement strategies to bring themselves into “attainment” with the standard. Failure to do so could result in the imposition of significant economic development restrictions. In order to keep the NAAQS current, the EPA reviews them every five years and, if appropriate, recommends revisions.

There can be two types of NAAQS. “Primary” standards establish air quality limits for the protection of public health, including the health of sensitive populations, such as asthmatics, children, and the elderly. “Secondary” standards set limits to protect public welfare, including protection against decreased visibility and injury to animals, crops, vegetation, and buildings.

Secondary Ozone Standards

Recommendations to date for EPA’s secondary ozone standards have been based largely on short-term chamber studies of annual crops and seedlings of a few tree species. Future reviews of the ozone standards probably will place greater emphasis on impacts on vegetation instead of just concentrating on human health issues. Ground-level ozone can reduce the ability of plants to photosynthesize, thereby reducing crop and forest productivity.

To realistically assess the impact of ozone on forest ecosystems, it is necessary to have a reasonable way to project ozone exposure across the landscape. This exposure—or the amount of ozone surrounding the plants over time—needs to be converted to a dose experienced by the plants making up the forest. Dose refers to the amount of ozone actually taken up by plants over time. The EPA’s approach to this process in the past was to use complex statistical modeling that suffered from many simplifying assumptions that ignored regional and local ozone differences. EPA also has not taken advantage of recent improvements in the modeling of air pollutant transport, photochemistry, and plant exposure. Because of the difficulty in conducting large-scale experimental studies, future assessments may have to rely on models that use results from studies on individual trees and forest stands to project ozone impacts at the landscape level.

Modeling the Impact of Ozone on Forests

To accurately model the impact of ozone on forested regions, it is necessary to identify the key environmental parameters that control ozone uptake by foliage at the tree level, such as air temperature, soil moisture, light, and wind speed. Linkage of ozone transport and climatology models with estimates of ozone uptake by trees will allow the evaluation of the potential ozone response of the forest.

In order to develop the necessary linkages, the Tennessee Valley Authority, working with the Electric Power Research Institute (EPRI), EPA, and a number of universities, established the ROVE project in 1991. Over the past nine years, ROVE has combined a series of lab, field, and modeling studies. The first step in this process, ROVE I, was a series of controlled experiments. During these chamber studies, scientists compared responses of mature trees and seedlings to the impact of a range of ozone levels. These studies found that, even within the same species, ozone impacts on key physiological processes, such as photosynthesis, stomatal conductance, and respiration, vary across plants of differing age and size. These results brought into question, at a national level, the use of seedling studies as the basis for ozone standards designed for entire forests.

The next step of the ROVE project involved ozone uptake and response measurements collected in natural forest stands. ROVE II, a long-term field study across an elevational gradient, was designed as a test-bed for evaluation of findings from previous seedling and mature tree chamber studies, as well as findings from tree and stand growth modeling efforts. Four sets of scaffolding were constructed in the Great Smoky Mountains National Park to allow investigators to climb to the top of the forest canopy to measure ozone uptake in mature stands. These studies found ozone uptake to vary significantly, depending upon tree species, the tree's size and position in the canopy, and the soil and climate of the site. Ozone stress was found to cause decreased root growth, thus impacting on a tree's overall growth rate and ability to compete for water and nutrients. A similarly designed U.S. Forest Service study in the western United States also found that seedlings of a particular species were not good predictors of ozone response for overstory trees of that same species.at same species.

The last step of the ROVE project involved using outputs from tree and stand models to assess how realistic environmental conditions might affect the sensitivity of representative tree species to variations in ozone exposure. Modeling assessments using data on the variability in response to ozone exposure among tree species and across site conditions gave an indication of the range of responses that might be expected under natural conditions over a large region.

Modeled overall growth rates, in the absence of ozone, showed wide variation between sites and years, and this variation also was seen in responses to ozone exposure. Annual decreases in net photosynthesis in response to recent ambient ozone conditions in the Great Smoky Mountains were on the order of 1 to 6 percent. In general, trees growing on moist sites, and in wet years, had high growth rates, but they also had high actual ozone doses at any given exposure level. As a consequence, models predicted large decreases in the net photosynthesis of the trees in response to increasing ozone levels. On the other hand, modeling results suggested that trees grown on drier sites, and in years with low precipitation rates, responded less to ozone exposure in absolute terms. This reduced response was due to a lower ozone actual dose. Under dry conditions, tree responses were also similar across a wide range of ozone levels.

Conclusions

The results of the ROVE studies interject caution in relying upon the current models, especially when they rely upon short-term observations and/or seedling responses only. ROVE studies found ozone uptake to vary significantly, depending upon tree species, the tree’s size and position in the canopy, and the soil and climate of the site. Ozone stress was found to cause decreased root growth, which reduced overall growth rate and ability to compete for water and nutrients. The ROVE results indicate large differences in leaf gas exchange and ozone uptake between the forest canopy and the understory, depending on the particular species being studied. These results underscore the importance of tree size and canopy position in evaluating forest tree response to ozone using process-based models. Therefore, regulatory decisionmaking based on short-term chamber studies using seedlings should be performed with extreme caution.

Information Contacts

L. Suzanne Fisher, 865-632-6935, lsfisher@tva.gov
P. Alan Mays, (865) 632-1634, pamays@tva.gov

Inquiries and comments should be sent to wjparkhurst@tva.gov.