Modeling the effects of elevated tropospheric O-3 on two varieties of Pinus ponderosa

This study used TREGRO model simulations over a 3-year period to examine the responses of the western and eastern varieties (var. ponderosa and var. scopulorum, respectively) of Pinus ponderosa Dougl. ex P. & C. Laws. to multiple levels of tropospheric ozone (O-3) In the absence of O-3, TREGRO accurately modeled documented differences in midsummer photosynthesis, stomatal conductance, and total-tree biomass increase of var. ponderosa and var. scopulorum. Both varieties were exposed to five simulated O-3 exposures (0.5x, 1x, 1.5x, 2x, and 3x ambient levels). Simulated growth of var. ponderosa was reduced more than that in var. scopulorum at all O-3 exposures. In var. ponderosa 1xO(3) over 3 years caused reductions in biomass of fine-root (-117%), branch (-20%), and total nonstructural carbohydrate (60%) relative to growth in the absence of O-3. At 1xO(3) var. scopulorum displayed far less reductions in fine-root biomass (-10%) and total nonstructural carbohydrate (-5%). At 3 x O-3, total biomass increment in var. ponderosa and var. scopulorum was depressed 48% and 32%, respectively. The resistance to O-3 of var. scopulorum relative to var. ponderosa is attributed to lower stomatal conductance and greater carbon allocation to belowground tree compartments. These simulations indicated that assessments of the region-wide effects of O-3 on forest tree species and resources must address genetic variation in response to O-3.