Rising atmospheric CO₂ greatly enhances plant production, but its effect on biomass allocation, particularly in the presence of environmental stresses, is not well understood. Here, we used meta-analysis and pairwise analysis to examine root mass fraction (RMF) as affected by elevated CO₂ and environmental stresses.

Results/Conclusions

Our results showed lower soil fertility increased RMF and the magnitude was similar for ambient and elevated CO₂-grown plants. Lower soil water also increased RMF, but to a greater extent at elevated than at ambient CO₂. While CO₂ enrichment had little effect on the magnitude of O₃-caused reduction in RMF in herbaceous species, it alleviated the adverse effect of higher O₃ on root production in woody species. These results demonstrate that CO₂ has less pronounced effects on RMF than other environmental factors. Under abiotic stresses, e.g., drought and higher O₃, elevated CO₂-grown plants will likely increase biomass allocation below-ground. Because of the non-uniform changes in drought and O₃ projected for different parts of the world, we conclude that elevated CO₂ will have regional, but not global, effects on biomass allocation under various global change scenarios.