Ozone

Ozone is potentially the most significant pollutant affecting forests in North America (Barnard and others 1991). A chemical composed of three linked oxygen atoms, ozone is highly beneficial in the upper atmosphere. At ground level, however, it is a powerful oxidizing agent that is capable of killing tissues (SAMAB 1996). Small amounts of ozone occur naturally, but the large quantities measured at ground level are formed primarily through chemical reactions between nitrogen oxides, volatile organic compounds, and sunlight. Ozone exposures above 0.05 ppm are of greatest concern to scientists, resource managers, and the public; these concentrations are frequently found in the eastern United States (Lefohn and Jones 1986, SAMAB 1996)

Ozone and Potential Vegetation Damage

Ozone is highly damaging to tissues inside of plant leaves, which it enters through small pores called stomates. Once inside the leaf, the ozone is either destroyed by biochemical processes, or the ozone kills the cells found just below the upper leaf surface and turns these cells reddish or black -a process called stippling (SAMAB 1996).

Several factors affect the uptake of ozone by a plant. Different plant species have differing sensitivity to ozone. Also, sensitivity to ozone may vary within a species. Other factors, such as light, temperature, relative humidity, soil nutrients, and soil moisture also influence uptake of ozone. Damage from drought and ozone exposure are believed to be inversely related. Drought is thought to minimize ozone effects because it causes stomates to close, preventing ozone from entering the leaves. On the other hand, ozone-sensitive species growing at high elevations may be more sensitive to ozone exposure than those growing at lower elevations. Sufficient moisture to prevent stomatal closure at high elevations may be obtained from cloudwater, thus allowing more ozone to be absorbed even in relatively dry periods (SAMAB 1996).

Symptoms of ozone injury in leaves are well known, and these symptoms have been observed on leaves of sensitive species throughout the southern Appalachians. Ozone exposures, when soil moisture is adequate, may be sufficient to cause growth losses to the most sensitive species in the southern Appalachians, such as black cherry. However, little to no growth losses are likely in moderately sensitive species, such as yellow-poplar; or in resistant species, such as red oak. Highly sensitive vegetation in the northern and southern portions of the southern Appalachians may have experienced the greatest frequency of ozone damage (SAMAB 1996).

Current efforts by state, local, and federal air pollution agencies provide evidence that ozone exposures in rural forests could possibly be reduced in the future. For example, there could be a lowering of ozone exposures in the southern Appalachians as soon as ozone nonattainment areas outside the study area implement control strategies that bring the region back into compliance with federal law. Currently, Whitetop Mountain in Virginia is the only area designated as nonattainment for ozone because the 0.12 ppm standard was exceeded. Also, regional strategies that reduce emissions of nitrogen oxides, possibly the limiting factor in ozone formation, may result in lower ozone exposures for the southern Appalachians (SAMAB 1996).

What are the implications of ozone exposures for the health of forests in the southern Appalachians? The forest products industry will be adversely affected if ozone reduces tree growth and impacts future timber supplies. Ozone exposures could also reduce the genetic diversity of some species, such as white pine. Furthermore, little is known about the effect ozone exposures may have on rare and endangered plant species (SAMAB 1996).