Competitive status influences tree-growth responses to elevated CO₂ and O₃ in aggrading aspen stands

1. Competition effects on growth of individual trees were examined for 4 years in aggrading, mixed-clone stands of trembling aspen (Populus tremuloides Michx.) at the Aspen-FACE free-air CO₂ and O₃ enrichment facility in northern Wisconsin, USA. During each growing season stands received one of four combinations of atmospheric [CO₂] (ambient vs ∼56 Pa) and [O₃] (ambient vs ∼1.5 x ambient). 2. Non-destructive measurements of annual tree growth were compared within and among clones and treatments in relation to an index of competitive status based on the difference between a tree's height and that of its four nearest neighbours. Competitive status strongly influenced tree growth, and the positive growth response to elevated [CO₂] was greater for competitively advantaged individuals than for disadvantaged individuals of most clones. 3. The magnitude of O₃ effects on growth depended on clone and competitive status: for some clones, negative O₃ effects were stronger with competitive advantage while others showed stronger O₃ effects with competitive disadvantage. The interactive effects of CO₂ and O₃ differed among clones, with negative effects of O₃ amplified or ameliorated by elevated CO₂, depending on clone and competitive status. 4. Treatments modified competitive interactions by affecting the magnitude of growth differences among clones. These modifications did not alter clone rankings of competitive performance, but when CO₂ and O₃ were both elevated, the differences in competitive performance among clones decreased.