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Some measured values of the ratio transpiration rate/assimilation rate (E/A) for a Sitka spruce stand in Scotland and an oak-hickory stand are presented and compared with E/A for some agricultural crops. Typical midday values of E/A (on a molar basis) range from 100 to 900. Analysis suggests that this variation can be explained by differences in climate, canopy conductance and degree of coupling of the vegetation to the atmosphere. During the day, E/A varied considerably in the tree stands at the same time as there were changes in radiation, vapor saturation deficit and temperature. A lumped parameter, 'big leaf' model of canopy transpiration and assimilation was used to provide insight into how this complex of variables might influence E/A. The model predicted little response to radiation, but increase in E/A with increasing vapor saturation deficit and temperature, in agreement with the trends in the data. It was concluded that there is a great need for measurements of E/A of stands in relation to nutrient and water stress.
Patrick Jarvis (Mon,) studied this question.