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Analysis of the net ecosystem exchange of CO₂ in a 56-year-old coastal Douglas-fir stand : its relation to temperature, soil moisture and photosynthetically active radiation Cai, Tiebo

Abstract

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 R[sub e], where NEP and R[sub e] are net ecosystem production of CO[sub 2] 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 (CO[sub 2]/H[sub2]O) analyzer, and daytime R[sub e] was inferred by obtaining the intercept of the relationship between half-hourly values of NEP and PAR. Daytime R[sub e] thus obtained was approximately 71-75% of that calculated by applying the logarithmically-transformed relationship between nighttime NEE (-NEP) and soil temperature (T[sub s]) to daytime half hours. Values of R[sub 10] (the rate of R[sub e] at T[sub s] = 10 °C), obtained from both annual nighttime and daytime R[sub e] – T[sub s] 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 R[sub e] 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 R[sub e]. The effective PAR (Q[sub e]) contributing to canopy P in this Douglas-fir canopy was well described as Q[sub d0] + kQ[sub b0], with Q[sub d0] and Q[sub b0] being sky diffuse and direct PAR, respectively. The parameter k, which accounts for the total scattering of Q[sub b0] 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 = αQ[sub t0])A[sub max]/(αQ[sub t0]+A[sub max]), where Q[sub t0] = Q[sub d0] + Q[sub b0]) results in significant overestimation of P in sunny conditions and significant underestimation of P in cloudy conditions, its modification into P = αQ[sub e])A[sub max]/(αQ[sub e]+A[sub max]) (the Q[sub e]-MM model) eliminated these systematic errors. When k = 1, the Q[sub e]-MM model reduces to the MM model. The Q[sub e] - 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 Q[sub e]-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 Q[sub d0]/Q[sub t0] (the ratio of Q[sub d0] to Q[sub t0]). In contrast, using the Q[sub e]-MM model to estimate canopy P for this Douglas-fir stand eliminated these systematic errors with respect to Q[sub d0]/Q[sub t0]. In addition, the Q[sub e] -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).

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