Plant-plant interactions mediate the plastic and genotypic response of Plantago asiatica to CO₂: An experiment with plant populations from naturally high CO₂ areas

Background and Aims The rising atmospheric CO₂ concentration ([CO₂]) is a ubiquitous selective force that may strongly impact species distribution and vegetation functioning. Plant-plant interactions could mediate the trajectory of vegetation responses to elevated [CO₂], because some plants may benefit more from [CO₂] elevation than others. The relative contribution of plastic (within the plant's lifetime) and genotypic (over several generations) responses to elevated [CO₂] on plant performance was investigated and how these patterns are modified by plant-plant interactions was analysed. Methods Plantago asiatica seeds originating from natural CO₂ springs and from ambient [CO₂] sites were grown in mono stands of each one of the two origins as well as mixtures of both origins. In total, 1944 plants were grown in [CO₂]-controlled walk-in climate rooms, under a [CO₂] of 270, 450 and 750 ppm. A model was used for upscaling from leaf to whole-plant photosynthesis and for quantifying the influence of plastic and genotypic responses. Key Results It was shown that changes in canopy photosynthesis, specific leaf area (SLA) and stomatal conductance in response to changes in growth [CO₂] were mainly determined by plastic and not by genotypic responses. We further found that plants originating from high [CO₂] habitats performed better in terms of whole-plant photosynthesis, biomass and leaf area, than those from ambient [CO₂] habitats at elevated [CO₂] only when both genotypes competed. Similarly, plants from ambient [CO₂] habitats performed better at low [CO₂], also only when both genotypes competed. No difference in performance was found in mono stands. Conclusion The results indicate that natural selection under increasing [CO₂] will be mainly driven by competitive interactions. This supports the notion that plant-plant interactions have an important influence on future vegetation functioning and species distribution. Furthermore, plant performance was mainly driven by plastic and not by genotypic responses to changes in atmospheric [CO₂].