The temperature effect on the Intrinsic quantum yield at the ecosystem level

The intrinsic quantum yield (φ₀) is a measure of the efficiency of photosynthesis at low light levels and it is a crucial parameter for modelling gross primary productivity using “light use efficiency” (LUE) models. These models often assume that φ₀ is constant, but there is evidence retrieved at leaf level in the lab, that it may depend on temperature in a bell-shaped curve, with a peak around 30°C. This temperature dependence of φ₀(T) is still not fully understood, thus, it is still unknown if the shape of φ₀(T) is universal or if the responses at the leaf and ecosystem levels widely differ. Here we derived φ₀(T) at the ecosystem level for different sites during their growing season. We used sub-daily above and below-canopy measurements of photosynthetic flux density, long-wave radiation measurements to derive surface canopy temperature, and eddy covariance measurements of CO₂ exchange. We then compared our estimations with empirical models found in the literature and propose a new empirical equation. We found that φ₀(T) at the ecosystem level also follows a bell-shaped curve, with a rapid increase after 5 °C to peak around 20 °C to 25 °C, and a slight decrease with further increasing temperature. Overall, our estimations show lower values than previous leaf-level observations reported in the literature.  The results suggest that this new formulation for φ₀(T) may improve the predictions of current LUE models, but further testing is needed.