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4.5 Ozone Discussion and Conclusions

The growth impacts of ambient O₃ levels on southern pines appear to be statistically significant at this time (see reviews by McLaughlin and Percy 1999, Teskey 1996). Additional increases in tropospheric O₃ will almost certainly have negative impacts on growth of pine species in the South (Teskey 1996).

Another important consideration for future forest health is the frequency and intensity of forest fires. Forest fires produce carbon monoxide, NO_x, and gaseous hydrocarbons that are the precursors of atmospheric O₃ (Bohm 1992). Therefore, forest fires may contribute to O₃ production in wilderness and rural areas (Bohm 1992). Bohm (1992) observed that O₃ has been found to accumulate near the location of a burn and substantial increases in O₃ concentrations (>50 percent above ambient) have been detected downwind of burned areas and at the top of burn plumes.

The important relationship between soil moisture, stomatal conductance and tree sensitivity to O₃ levels highlights the importance of climate in predicting future impacts of O₃ on forest health. Under future climate scenarios, trees in areas of the South characterized by periods of persistent drought and poor soil water storage capacity will be more sensitive to O₃ pollution and will likely incur substantial visible foliar damage (Maier-Maercker 1999).