Temperature effects on the global patterns of photosynthetic quantum efficiency

The photosynthetic quantum efficiency ( φ ₀ ) is a key input parameter for modelling gross primary productivity in terrestrial biosphere models. Historically, these models assumed φ ₀ to be constant, based on leaf measurements under unstressed conditions and within a narrow temperature range. However, increasing evidence suggests a temperature-dependent φ ₀ on temperature, though it remains unclear whether this response is generalized or if it propagates to the ecosystem. Here, we derived φ ₀ ( T ) at the ecosystem level for sites distributed globally, using sub-daily eddy covariance measurements of CO ₂ exchange and above/below-canopy measurements of photosynthetic flux density to derive the fraction of absorbed photosynthetically active radiation (fAPAR). We found that φ ₀ ( T ) shows a consistent bell-shaped response curve with temperature in all the sites we analysed. These patterns held when analysed with a larger global dataset using remotely sensed fAPAR. Furthermore, we observed that the values of φ ₀ ( T ) are not markedly different among biomes, instead, there is a gradual transition of the peak φ ₀ ( T ) which decreases following an aridity gradient. Additionally, we noted varying sensitivity to temperature among the different sites, with sensitivity increasing as growth temperature decreases.