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We investigated the potential impact of projected future temperature and CO2 concentrations in combination with tropospheric O3 on the annual biomass increment of Pinus ponderosa Doug. ex Laws. TREGRO, a process-based whole-tree growth model in which trees experienced a seasonal drought, was used to study the interactions of CO2, temperature, and O3 on tree growth along a latitudinal gradient in California, Oregon, and Washington, USA. The annual biomass increment increased in proportion to CO2 concen- tration, although the magnitude varied among sites. Increasing air temperature ( 11.38C) increased growth at most sites. Elevated CO2 increased the temperature optimum for growth at four sites and decreased it at two sites. The annual biomass increment decreased with increasing O3 exposure. The differences in O3 effects among sites were primarily controlled by differences in precipitation. Although increasing CO 2 can reduce the O3 impact, it does not eliminate the impact of O3. Elevated CO2 would enhance tree growth more if O3 exposures were reduced, especially in the more polluted sites. The greatest benefit for tree growth would come from reducing O3 exposures in the most polluted sites, but we must also consider locations that have high inherent O 3 sensitivity because of their mesic con- ditions. Limiting the increase of O 3 levels in those areas will also increase tree growth. |
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