Analysis of the net ecosystem exchange of carbon dioxide in a 56-year-old coastal Douglas-fir stand: Its relation to temperature, soil moisture and photosynthetically active radiation

The primary goal of this thesis was to investigate the relationship of canopy photosynthesis (P) to photosynthetically active radiation (PAR) in a 56-year-old coastal Douglas-fir stand (DF49) located on Vancouver Island. Canopy P was calculated as daytime NEP + daytime Re, where NEP and Re are net ecosystem production of CO2 and ecosystem respiration, respectively. Half-hourly values of NEP were obtained using an EC (eddy covariance) system consisting of a 3-D sonic anemometer-thermometer and a closed-path infrared gas (CO2/H2O) analyzer, and daytime Re was inferred by obtaining the intercept of the relationship between half-hourly values of NEP and PAR. Daytime Re thus obtained was approximately 71--75% of that calculated by applying the logarithmically-transformed relationship between nighttime NEE (-NEP) and soil temperature (Ts) to daytime half hours. Values of R10 (the rate of Re at Ts = 10°C), obtained from both annual nighttime and daytime Re--Ts relationships, increased linearly with increasing soil moisture when averaged over the active growing season (April 1--Sept 30). However, the effect of soil moisture on Re shown on the multi-year scale could not be detected on the seasonal or annual scale probably as a result of the confounding effects of other environmental factors on Re.; The effective PAR (Qe) contributing to canopy P in this Douglas-fir canopy was well described as Q d0 + kQb0, with Q d0 and Qb0 being sky diffuse and direct PAR, respectively. The parameter k, which accounts for the total scattering of Qb0 and the non-scattering effect (e.g., penumbral light spreading) of the solar rays, was found to be approximately 0.22 for this stand. While the Michaelis-Menten equation (the MM model) (i.e., P = alphaQt0 Amax/(alphaQt0 + Amax), where Qt0 = Qd0 + Qb0) results in significant overestimation of P in sunny conditions and significant underestimation of P in cloudy conditions, its modification into P = alphaQeA max/(alphaQe + A max) (the Qe-MM model) eliminated these systematic errors. When k = 1, the Qe-MM model reduces to the MM model. The Qe-MM model is a single big-leaf model, but it avoids the type of errors made in earlier generations of single big leaf models of canopy P, i.e., using APAR (the total absorbed PAR by the canopy) to calculate P. The simplicity of the Qe-MM model makes it convenient to be incorporated into large-scale carbon climate models.; This study also shows that the widely used sun/shade model developed by de Pury and Farquhar (1997) is inadequate, mainly because the sun/shade model fails to account for the incidence angle between the solar beam and individual sunlit leaves. As with the P modeled using the MM model, the modeled P obtained using the sun/shade model has significant systematic errors with respect to Qd0/Q t0 (the ratio of Qdo to Qt0). In contrast, using the Qe-MM model to estimate canopy P for this Douglas-fir stand eliminated these systematic errors with respect to Qd0/Qt0. In addition, the Qe-MM model developed in this study agrees with the detailed multilayer model of canopy P developed by Norman and Arkebauer (1991) for agricultural crops (i.e., soybean and corn).