Terrestrial biosphere models may overestimate Arctic CO 2 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 (ϕCO 2 ) and the convexity factor (θ). We also determined leaf absorptance (α) from measured reflectance to calculate ϕCO 2 on an absorbed light basis (ϕCO 2.a ) and enable comparison with nine TBMs. The mean ϕCO 2.a was 0.045 mol CO2 mol-1 absorbed quanta at 25°C and closely agreed with the mean TBM parameterization (0.044) but as temperature decreased measured ϕCO 2.a diverged from TBMs. At 5°C measured ϕCO 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 parameterization of 0.7. These data show that TBMs are not accounting for observed reductions in ϕCO 2.a and θ that can occur at low temperature. Ignoring these reductions in ϕCO 2.a and θ could lead to a marked (45%) overestimation of CO2 assimilation at sub-saturating irradiance and low temperature. This article is protected by copyright. All rights reserved.