Incorporating diffuse photosynthetically active radiation in a single-leaf model of canopy photosynthesis for a 56-year-old Douglas-fir forest

A simple top-down model of canopy photosynthesis (P) was developed and tested in this study. The model (referred to as the Qe-MM model) is P = αQ e P max/(αQ e + P max), α and P max are quantum-use efficiency and potential P, respectively. Q e is given by Q d ₀ + kQ b ₀, where Q d ₀ and Q b ₀ are the diffuse and direct photosynthetically active radiation (PAR) incident on the canopy, respectively. Q e can be considered to be the effective incident PAR contributing to P and k is a measure of the contribution of Q b ₀ to Q e. When k = 1, the Qe-MM model becomes the regular Michaelis-Menten type model of P (referred to as the MM model). A major objective of this study was to determine how well the Qe-MM model could estimate P of a 56-year-old coastal Douglas-fir stand. To this end, we parameterized the Qe-MM model using five and half years of eddy-covariance measurements of CO₂ flux above the Douglas-fir stand. The Qe-MM model, with the incorporation of a function of air temperature, accounted for 74% of the variance in over 34,000 half-hourly P measurements. P estimated using the Qe-MM model had no systematic errors with respect to Q d ₀. Although the Qe-MM model has only one more parameter than the MM model, it accounted for 30% more variance in P than the latter when total incident PAR exceeded 900 μmol m⁻² s⁻¹. On average, k was found to be 0.22. We show that this small value of k reflects the significant effect of the scattering of the solar beam and the fraction of light-limited sunlit leaves. We also show that the success of the Qe-MM model was due to the fact that a large fraction of the sunlit leaves were light-limited as a result of their orientation to the solar beam.