Current projections indicate steady increases in both trophospheric ozone and carbon dioxide well into the next century with concurrent increases in plant stress. Because information about effects of these interacting stresses on forest trees is limited, we have conducted ozone and carbon dioxide experiments using ozone-tolerant and ozone-sensitive trembling aspen (Populustremuloides Michx.) clones (clones 216 and 259, respectively). Aspen plants were grown either in pots (square-wave study) or in the ground (episodic study) in open-top chambers. Plants in the square-wave study were exposed for a single growing season to charcoal-filtered air (CF) or to CF plus elevated carbon dioxide (CO₂), ozone (O₃), or O₃ plus CO₂ (O₃ + CO₂). Plants in the episodic study were exposed for three growing seasons to CF, twice simulated ambient (2x) O₃ (2x O₃), or 2x O₃ plus CO, (2x O₃ + CO₂). Photosynthetic measurements were made either in the open-top chambers at treatment CO₂ concentrations or in controlled-environment cuvettes with various CO₂ concentrations, producing assimilation versus intercellular CO₂ concentration (A/C_i) curves. Ozone decreased photosynthetic rate and stomatal conductance and accelerated leaf senescence. Elevated CO₂ increased photosynthetic rate and decreased stomatal conductance when measured at treatment CO₂ concentrations, and exacerbated the negative effect of O₃ on photosynthesis. For example, for clone 259, photosynthesis decreased 9% for the O₃ treatment compared with the CF treatment, but decreased 24% for the O₃ + CO₂ treatment compared with the CF treatment. Similar decreases for clone 216 of 2% and 6% for O₃ and O₃ + CO₂, respectively, were not significant. A/C_i curves showed that O₃ decreased carboxylation efficiency and maximum photosynthetic rate and that photosynthetic inhibition in response to O₃ was greater with elevated CO₂. The simultaneous declines in all factors of photosynthetic gas exchange measurements suggest that the equilibrium between stomatal conductance, carboxylation, and light harvesting systems was not disrupted by O₃ and O₃× CO₂ interactions. Carbon dioxide did not ameliorate the detrimental effects of O₃ on the leaf photosynthetic apparatus. In fact, the O₃-tolerant clone appeared more sensitive to O₃ with elevated CO₂.

Increased forest productivity has been obtained by improving resource availability through water and nutrient amendments. However, more stress-tolerant species that have robust site requirements do not respond consistently to irrigation. An important factor contributing to robust site requirements may be the distribution of biomass belowground, yet available information is limited. We examined the accumulation and distribution of above- and below-ground biomass in sweetgum ( Liquidambar styraciflua L.) and loblolly pine ( Pinus taeda L.) stands receiving irrigation and fertilization. Mean annual aboveground production after 4 years ranged from 2.4 to 5.1 Mg·ha^–1·year^–1for sweetgum and from 5.0 to 6.9 Mg·ha^–1·year^–1for pine. Sweetgum responded positively to irrigation and fertilization with an additive response to irrigation + fertilization. Pine only responded to fertilization. Sweetgum root mass fraction (RMF) increased with fertilization at 2 years and decreased with fertilization at 4 years. There were no detectable treatment differences in loblolly pine RMF. Development explained from 67% to 98% of variation in shoot versus root allometry for ephemeral and perennial tissues, fertilization explained no more than 5% of the variation in for either species, and irrigation did not explain any. We conclude that shifts in allocation from roots to shoots do not explain nutrient-induced growth stimulations.