AbstractThe effects of acidic precipitation and ozone on physiological processes in loblolly pine (Pinus taeda L.) were assessed over time to determine dose‐response relationships of cumulative exposure to varying levels of ozone and acid rain in combination. Seedlings of known provenance were grown in open‐top field chambers in a clear‐cut forest site in Duke Forest in the Piedmont of North Carolina and were exposed to pH 5.2 or 3.5 precipitation and several proportional ozone treatments (0.5‐3.0x ambient ozone). No direct effects of acid precipitation on gas exchange and carbon fixation were found. Elevated ozone exposures had strong negative effects on needle abscission and net photosynthesis. Exposure‐response relationships for photosynthesis indicated that a threshold level of approximately 100 ppm·h of cumulative ozone exposure (12‐h summation) had to be reached before significant inhibition occurred. Results suggest that chronic pollutant exposure over the growing season has direct negative effects on carbon fixation in loblolly pine that can significantly reduce long‐term growth potential. Current ambient ozone levels in the southeastern United States could be responsible for an approximate 19 reduction in instantaneous carbon fixation rates in loblolly pine by the end of one growing season. An 80 to 90 decrease in net photosynthesis was found at 2.25 and 3.0x ambient ozone concentrations, levels often found in major cities and surrounding area
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