Trophospheric ozone is the most significant regional air pollutant impacting forest health in the South. Ozone is formed by the reaction of nitric oxides and volatile organic compounds in sunlight. These gases are primarily the by-products of anthropogenic sources such as industrial emissions and motor vehicles. High concentrations of ozone are found near or downwind from industrial facilities when geographic or meteorological conditions are conducive to stagnant or low air flow.

Plants that have been identified as reliable indicators of phytotoxic levels of pollutants are known as bioindicators. Examples of bioindicators for ozone include black cherry, blackberry, common milkweed and yellow-poplar.

The most common visible symptom of ozone injury on broad-leaved bioindicator species is uniform interveinal leaf stippling (Figure 29). As a gaseous pollutant, ozone enters the stomata of plant leaves through the normal process of gas exchange, damaging the palisade tissue. Ozone also reduces growth and reproductive capabilities.


Figure 29. Stippling of the leaf surface is typical of foliage affected by ozone.

Monitoring and Reporting Ozone Incidence

Ozone monitoring and reporting is a cooperative effort, primarily involving the Environmental Protection Agency, the Electric Power Research Institute, the Tennessee Valley Authority, the National Park Service, and the Digital Arborist.

The primary sources of information concern hourly mean concentrations of ozone. Similar, although less abundant, data also exist for sulfur dioxide and nitrogen dioxide. These data have been collected from EPA monitoring stations across the South since 1973.

A discussion of procedures can help in understanding the limits and utility of the available data. Monthly ozone air-quality data were characterized as follows: (1) 7-hour average (0900 to 1559) (2) number of hourly occurrences greater than or equal to 80, 100, 120, 140, and 150 parts per billion (ppb).

Earlier reports indicated that ozone concentrations in large city centers were lower than those detected in more rural areas. To determine the relevancy of each monitoring site for the Southern Forest Atlas, large-scale aerial photographs that show land use around each site were obtained. These photos were interpreted by the Aerial Survey Team of the Forest Health Unit in Atlanta, Georgia. Land use was estimated by a quadrant in a 2.5 mile radius around each site. Based on these values, a peer-review panel of scientists decided to use the monitoring sites with forest area of five percent or more and urban area of ninety percent or less. The sites also had to have 70 percent ozone data capture.

These selection criteria resulted in the rejection of approximately 100 site/years of data. The loss of data from these sites created concern as to whether ozone could be characterized throughout the South.

To determine whether or not sites needed to be dropped from the complete data set, kriging interpolation was performed on two subsets of the original data. Kriging is an interpolation technique which uses intersite correlation in the data to determine the weights that are applied to measured values in estimating unknown values. It also quantifies interpolation error, thereby providing a measure of the uncertainty that may arise. Kriging has been widely used to interpolate regional air quality.

In the first test, kriging was used to predict monthly means of the daily 7-hour ozone values from all monitoring sites for April, July, and August 1984. Results showed that the error of krige estimates compared to the actual values on the selected sites was less than 0.5 ppb. Thus, there was no indication that kriged estimates deviated significantly from actual measures. Figure 30 shows the average daytime concentrations in the South from June through September, 1978, based on data from EPA monitoring stations that were entered into the GIS system.

Ozone Surveys in Wilderness Areas

In addition to surveying for actual incidence of ozone concentrations as described above, the Digital Arborist also surveys for ozone impact as evidenced by ozone indicator plants in Forest Service wilderness areas. Surveys of ozone injury in the wilderness areas of Southern Region national forests have been conducted periodically according to recommended procedures. One purpose of the surveys is to document visible symptoms of ozone injury as prescribed by the Clean Air Act and other regulatory guidelines. Table 1 shows where surveys of ozone injury have been carried out in class 1 wilderness areas across the South.

Recently published literature predicts an increase in ambient ozone concentrations across the South. This should mean an increase in visible injury to sensitive plant species. However, the best evidence of trends in ozone injury would be obtained over a relatively long time frame, with foreknowledge of ambient ozone concentrations and site conditions. This approach would require co-locating plots with stationary ozone monitors, and, preferably with plants of known genetic sensitivity.

Our understanding of the mechanisms of ozone-plant interaction continues to evolve rapidly. Consequently, bioindicator plants will doubtless take on increased utility in the years ahead. Additionally, the Forest Health Monitoring program has established a commitment to support ozone bioindicator plants as a meaningful measure of ozone severity. Finally, stations for monitoring ambient air continue to improve, providing data that are more reliable and cost effective.