Some effects of variation in weather and soil water storage on canopy evapotranspiration and net photosynthesis of a young douglas-fir stand
Creators: name: Price, David Thomas
Publisher: name: University of British Columbia
Year: 2010

description: Measurements of the energy balances and net photosynthesis rates of two low productivity coniferous forest canopies (12 and 22 years old), were made successfully during both wet and dry growing seasons, using a modified Bowen Ratio method. Canopy conductances (gc) were calculated from canopy evaporation rates (E) using the Penman-Monteith equation. A model was developed to predict canopy growth and evaporation rates from basic soil and weather data, and compared with the measured data. The photosynthesis model was physiologically based, derived from recent work of Farquhar and coworkers. The canopy conductance model used an empirical approach, based on simple relationships with recorded environmental variables, while canopy E was predicted from the Penman-Monteith equation. Findings were: (1) Daytime E and canopy net photosynthesis rates (Fc) were generally lower in the younger canopy. (2) In the old canopy, E was more strongly decoupled from net irradiance (Rn) and more dependent on the atmospheric vapour pressure deficit (D) in accordance with the predictions of McNaughton and Jarvis (1983). (3) In the old canopy, Fc was significantly reduced by low soil water potential (Ψs) within the range of soil water storages at which measurements were made, while gc was less dependent on Ψs. From consideration of changes in intercellular C0₂ concentration, gc was not found normally limiting to Fc. (4) No simple relationship was apparent between solar irradiance (S) and F at the canopy level. However highest Fc and canopy water use efficiency ratios occurred on cloudy days with low air temperature and low D. (5) Night-time Fc measurements indicated that canopy respiration rates are generally very high and hence air temperature was a major factor limiting overall forest productivity. (6) The computer model could predict gc from four variables (D, S, root-zone soil water storage, W and time since dawn, t) with reasonable 2 success (r² 0.75). However, on days when gc was low, due to high D, E was occasionally significantly in error, because the Penman-Monteith equation is more sensitive to gc when the latter is low. Best agreement between measured and modelled E occurred on cloudy days when D was low and gc consequently high. (7) Values for the maximum rates of carboxylation, as limited by foliar carboxylase activity and electron transport rate, were set at one third of those reported by Farquhar and coworkers, in order to obtain best overall agreement between measured and modelled data. This requirement indicated that poor nutrition was also limiting to stand productivity. (8) Model prediction of canopy net photosynthesis was not satisfactory (r² 0.50), attributed mainly to using too simple an approach to estimating irradiance at the individual leaf level, and partly to unexplained variation in the measurements of Fc. In spite of its limitations, the model was found to respond realistically to changes in weather and Ψs, suggesting the approach was valid, and might be more successful with further development.