Differences in maximum assimilation rates (ΔA_max μmol m⁻² s⁻¹) between CO₂ partial pressure treatments (ΔA_max = A_{max − C} − A_{max − A}) in relation to elevation (m) for both species. A_{max − C} : maximal rate of assimilation measured at constant- low-elevation P_CO2 (37.5 Pa); A_{max − A} : maximal rate of assimilation measured at ambient P_CO2 . Values correspond of the mean of 5 to 19 individuals per site ± standard errors