Rising atmospheric CO(2) 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 combined with pairwise techniques to examine root mass fraction (RMF; i.e., the fraction of root to total biomass) as affected by elevated CO(2) and environmental stresses. Our results showed that lower soil fertility increased RMF and the magnitude was similar for ambient and elevated CO(2)-grown plants. Lower soil water also increased RMF, but to a greater extent at elevated than at ambient CO(2). While CO(2) enrichment had little effect on the magnitude of O(3)-caused reduction in RMF in herbaceous species, it alleviated the adverse effect of higher O(3) on root production in woody species. These results demonstrate that CO(2) has less pronounced effects on RMF than other environmental factors. Under abiotic stresses, e.g., drought and higher O(3), elevated CO(2)-grown plants will likely increase biomass allocation below-ground. Because of the non-uniform changes in drought and O(3) projected for different parts of the world, we conclude that elevated CO(2) will have regional, but not global, effects on biomass allocation under various global change scenarios.