Intraspecific variation in juvenile tree growth under elevated CO₂ alone and with O₃: a meta-analysis

Atmospheric carbon dioxide (CO₂) concentrations are expected to increase throughout this century, potentially fostering tree growth. A wealth of studies have examined the variation in CO₂ responses across tree species, but the extent of intraspecific variation in response to elevated CO₂ (eCO₂) has, so far, been examined in individual studies and syntheses of published work are currently lacking. We conducted a meta-analysis on the effects of eCO₂ on tree growth (height, stem biomass and stem volume) and photosynthesis across genotypes to examine whether there is genetic variation in growth responses to eCO₂ and to understand their dependence on photosynthesis. We additionally examined the interaction between the responses to eCO₂ and ozone (O₃), another global change agent. Most of the published studies so far have been conducted in juveniles and in Populus spp., although the patterns observed were not species dependent. All but one study reported significant genetic variation in stem biomass, and the magnitude of intraspecific variation in response to eCO₂ was similar in magnitude to previous analyses on interspecific variation. Growth at eCO₂ was predictable from growth at ambient CO₂ (R² = 0.60), and relative rankings of genotype performance were preserved across CO₂ levels, indicating no significant interaction between genotypic and environmental effects. The growth response to eCO₂ was not correlated with the response of photosynthesis (P > 0.1), and while we observed 57.7% average increases in leaf photosynthesis, stem biomass and volume increased by 36 and 38.5%, respectively, and height only increased by 9.5%, suggesting a predominant role for carbon allocation in ultimately driving the response to eCO₂. Finally, best-performing genotypes under eCO₂ also responded better under eCO₂ and elevated O₃. Further research needs include widening the study of intraspecific variation beyond the genus Populus and examining the interaction between eCO₂ and other environmental stressors. We conclude that significant potential to foster CO₂-induced productivity gains through tree breeding exists, that these programs could be based upon best-performing genotypes under ambient conditions and that they would benefit from an increased understanding on the controls of allocation.