Elevated CO2 and temperature alter the response of Pinus ponderosa to ozone: A simulation analysis

We investigated the potential impact of projected future temperature and CO 2 concentrations in combination with tropospheric O-3 on the annual biomass increment oi Pinus ponderosa Doug. ex Laws. TREGRO, a process-based whole- tree growth model in which trees experienced a seasonal drought, was used t o study the interactions of CO2, temperature, and O-3 on tree growth along a latitudinal gradient in California, Oregon, and Washington, USA, the annu al biomass increment increased in proportion to CO2 concentration, although the magnitude varied among sites. Increasing air temperature (+ 1.3 degree sC) increased growth at most sites. Elevated CO, increased the temperature optimum for growth at four sites and decreased it at two sites. The annual biomass increment decreased with increasing O-3 exposure. The differences i n O-3 effects among sites were primarily controlled by differences in preci pitation. Although increasing CO2 can reduce the O-3 impact, it does not el iminate the impact of O-3. Elevated CO2 would enhance tree growth more if O -3 exposures were reduced. especially in the more polluted sites. The great est benefit for tree growth would come from reducing O-3 exposures in the m ost polluted sites, but we must also consider locations that have high inhe rent O-3 sensitivity because of their mesic conditions. Limiting the increa se of O-3 levels in those areas will also increase tree growth.