Host Plant Physiology and Mycorrhizal Functioning Shift across a Glacial through Future [CO2] Gradient

Rising atmospheric carbon dioxide concentration ([CO₂]) may modulate the functioning of mycorrhizal associations by altering the relative degree of nutrient and carbohydrate limitations in plants. To test this, we grew Taraxacum ceratophorum and Taraxacum officinale (native and exotic dandelions) with and without mycorrhizal fungi across a broad [CO₂] gradient (180-1,000 µL L^-1). Differential plant growth rates and vegetative plasticity were hypothesized to drive species-specific responses to [CO₂] and arbuscular mycorrhizal fungi. To evaluate [CO₂] effects on mycorrhizal functioning, we calculated response ratios based on the relative biomass of mycorrhizal (M_Bio) and nonmycorrhizal (NM_Bio) plants (R_Bio = [M_Bio - NM_Bio]/NM_Bio). We then assessed linkages between R_Bio and host physiology, fungal growth, and biomass allocation using structural equation modeling. For T. officinale, R_Bio increased with rising [CO₂], shifting from negative to positive values at 700 µL L^-1 [CO₂] and mycorrhizal effects on photosynthesis and leaf growth rates drove shifts in R_Bio in this species. For T. ceratophorum, R_Bio increased from 180 to 390 µL L^-1 and further increases in [CO₂] caused R_Bio to shift from positive to negative values. [CO₂] and fungal effects on plant growth and carbon sink strength were correlated with shifts in R_Bio in this species. Overall, we show that rising [CO₂] significantly altered the functioning of mycorrhizal associations. These symbioses became more beneficial with rising [CO₂], but nonlinear effects may limit plant responses to mycorrhizal fungi under future [CO₂]. The magnitude and mechanisms driving mycorrhizal-CO₂ responses reflected species-specific differences in growth rate and vegetative plasticity, indicating that these traits may provide a framework for predicting mycorrhizal responses to global change.