Atmospheric CO₂ and O₃ alter competition for soil nitrogen in developing forests.

摘要

Plant growth responses to rising atmospheric CO₂ and O₃ vary among genotypes and between species, which could plausibly influence the strength of competitive interactions for soil N. Ascribable to the size-symmetric nature of belowground competition, we reasoned that differential growth responses to CO₂ and O₃ should shift as juvenile individuals mature, thereby altering competitive hierarchies and forest composition. In a 12-year-long forest FACE experiment, we used tracer 15N and whole-plant N content to assess belowground competitive interactions among five Populus tremuloides genotypes, between a single P. tremuloides genotype and Betula papryrifera, as well as between the same single P. tremuloides genotype and Acer saccharum. Under elevated CO₂, the amount of soil N and 15N obtained by the P. tremuloides genotype common to each community was contingent on the nature of belowground competition. When this genotype competed with its congeners, it obtained equivalent amounts of soil N and tracer 15N under ambient and elevated CO₂; however, its acquisition of soil N under elevated CO₂ increased by a significant margin when grown in competition with B. papyrifera (+30%) and A. saccharum (+60%). In contrast, elevated O₃ had no effect on soil N and 15N acquisition by the P. tremuloides genotype common in each community, regardless of competitive interactions. Under elevated CO₂, the rank order of N acquisition among P. tremuloides genotypes shifted over time, indicating that growth responses to CO₂ change during ontogeny; this was not the case under elevated O₃. In the aspen-birch community, the competitive advantage elevated CO₂ initially conveyed on birch diminished over time, whereas maple was a poor competitor for soil N in all regards. The extent to which elevated CO₂ and O₃ will shape the genetic structure and composition of future forests is, in part, contingent on the time-dependent effects of belowground competition on plant growth response.