Terrestrial biosphere models may overestimate Arctic CO₂ assimilation if they do not account for decreased quantum yield and convexity at low temperature

How terrestrial biosphere models (TBMs) represent leaf photosynthesis and its sensitivity to temperature are two critical components of understanding and predicting the response of the Arctic carbon cycle to global change. We measured the effect of temperature on the response of photosynthesis to irradiance in six Arctic plant species and determined the quantum yield of CO₂ fixation ( ${\varphi }_{{\mathrm{CO}}_2}$ ) and the convexity factor (θ). We also determined leaf absorptance (α) from measured reflectance to calculate ${\varphi }_{{\mathrm{CO}}_2}$ on an absorbed light basis ( ${\varphi }_{{\mathrm{CO}}_2.a}$ ) and enabled comparison with nine TBMs. The mean ${\varphi }_{{\mathrm{CO}}_2.a}$ was 0.045 mol CO₂ mol^-1 absorbed quanta at 25°C and closely agreed with the mean TBM parameterisation (0.044), but as temperature decreased measured ${\varphi }_{{\mathrm{CO}}_2.a}$ diverged from TBMs. At 5°C measured ${\varphi }_{{\mathrm{CO}}_2.a}$ was markedly reduced (0.025) and 60% lower than TBM estimates. The θ also showed a significant reduction between 25°C and 5°C. At 5°C θ was 38% lower than the common model parameterisation of 0.7. These data show that TBMs are not accounting for observed reductions in ${\varphi }_{{\mathrm{CO}}_2.a}$ and θ that can occur at low temperature. Ignoring these reductions in ${\varphi }_{{\mathrm{CO}}_2.a}$ and θ could lead to a marked (45%) overestimation of CO₂ assimilation at subsaturating irradiance and low temperature.