| Summary: | 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). === Land and Food Systems, Faculty of === Graduate
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