Figure 1.

[CO₂] is predicted to mediate nutrient effects on mycorrhizal associations by altering relative resource limitation in host plants. Specifically, the mycorrhizal response ratio (R_Bio) is predicted to increase with rising [CO₂] due to simultaneous increases in plant carbohydrate production and plant nutrient limitation. R_Bio is calculated from the biomass of mycorrhizal (M_Bio) and nonmycorrhizal (NM_Bio) plants (R_Bio = [M_Bio − NM_Bio]/NM_Bio). The dashed line represents a neutral association (no difference in plant size). R_Bio may be negative (solid line) or marginally positive (dotted line) at low [CO₂], depending on mycorrhizal effects on plant physiological responses to a CO₂-limiting environment.