Belowground competition and the response of developing forest communities to atmospheric CO2 and O3

As human activity continues to increase CO₂ and O₃, broad expanses of north temperate forests will be simultaneously exposed to elevated concentrations of these trace gases. Although both CO₂ and O₃ are potent modifiers of plant growth, we do not understand the extent to which they alter competition for limiting soil nutrients, like nitrogen (N). We quantified the acquisition of soil N in two 8‐year‐old communities composed of trembling aspen genotypes (n= 5) and trembling aspen–paper birch which were exposed to factorial combinations of CO₂ (ambient and 560 μL L⁻¹) and O₃ (ambient = 30–40 vs. 50–60 nL L⁻¹). Tracer amount of ¹⁵NH₄⁺ were applied to soil to determine how these trace gases altered the competitive ability of genotypes and species to acquire soil N. One year after isotope addition, we assessed N acquisition by measuring the amount of ¹⁵N tracer contained in the plant canopy (i.e. recent N acquisition), as well as the total amount of canopy N (i.e. cumulative N acquisition). Exposure to elevated CO₂ differentially altered recent and cumulative N acquisition among aspen genotypes, changing the rank order in which they obtained soil N. Elevated O₃ also altered the rank order in which aspen genotypes obtained soil N by eliciting increases, decreases and no response among genotypes. If aspen genotypes respond similarly under field conditions, then rising concentrations of CO₂ and O₃ could alter the structure of aspen populations. In the aspen–birch community, elevated CO₂ increased recent N (i.e. ¹⁵N) acquisition in birch (68%) to a greater extent than aspen (19%), suggesting that, over the course of this experiment, birch had gained a competitive advantage over aspen. The response of genotypes and species to rising CO₂ and O₃ concentrations, and how these responses are modified by competitive interactions, has the potential to change the future composition and productivity of northern temperate forests.