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.
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.