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To cause tree damage, O₃ must be absorbed by the plant through the stomatal openings found on the surface of leaves in a process known as stomatal conductance. Stomates open during daylight hours to permit the exchange of gaseous compounds (CO₂, O_2, and water vapor) necessary for photosynthesis. At night, stomates close, preventing the transpiration of water. Because stomates are open during the day, daytime O₃ concentrations are most likely to damage trees. Rates of stomatal conductance vary by species and age, and these rates directly determine both the quantity of O₃ uptake and the plant’s response to a given concentration of O₃ (Kelly and others 1995). In general, ozone-sensitive tree species under high O₃ stress experience reduced leaf area, slower growth during drought conditions, and lower vertical growth rates (Southern Appalachian Mountains Initiative 2001).

It appears that O₃ affects growth and vitality indirectly by predisposing trees to injury from other biotic and abiotic stressors (Chappelka and Freer-Smith 1995). For example, ponderosa pine exhibits increased sensitivity to bark beetle attack in the San Bernardino Mountains following O₃ damage (Cobb and others 1968). In the South, pines typically do not show visible symptoms of O₃ injury under ambient O₃ conditions (Berrang and others 1996) except during extended periods of high O₃ levels when injury is readily visible.

The amount and way that O₃ affects trees depend on the age of the trees and the species. Given similar amounts of O₃ exposure, immature hardwoods generally exhibit more growth loss than softwoods (table 18.2) (McLaughlin and Percy 1999). Based on the limited number of studies available, mature hardwood growth rates appear to be more sensitive to O₃ exposure than mature softwood growth rates (table 18.2). According to Dougherty and others (1992), an average mature loblolly pine tree growing in a plantation experiences a 3-percent annual loss of gross primary production under ambient O₃ conditions in the South. In a review of 19 studies measuring the influence of O₃ exposure on growth of slash pine, shortleaf pine, and loblolly pine seedlings and saplings, Teskey (1996) concluded that annual growth reductions for pine seedlings in the South were between 2 and 5 percent. For mature loblolly pines, Dougherty and others (1992) used a process model to estimate annual growth reductions of about 3 percent under ambient O₃ concentrations.

Hogsett and others (1997) found that black cherry has strong O₃ sensitivity, and tulip poplar has moderate O₃ sensitivity. Southern yellow pine species showed little response to changes in SUM06 O₃ concentrations, and sugar maple exhibited a threshold response in which annual biomass increased dramatically between 26 and 38 ppm-hour per year SUM06.

Overall, it appears that the growth of mature southern yellow pines is being reduced by current typical ambient O₃ levels at annual rates that vary from 0 to 10 percent per year. Annual growth reductions for pine seedlings in the South are probably between 2 and 5 percent (Teskey 1996). However, at present there are no indications of community level changes (competition dynamics, community structure, and function, etc.) attributable to O₃ (McLaughlin and Percy 1999). Although O₃ may be reducing annual growth of trees in the South, other air-borne chemicals such as CO₂ and nitrogen and sulfur compounds may be simultaneously increasing growth (Teskey 1996), thereby effectively masking the negative effects of O₃ on overall forest health.