Development of a measurement system of whole-tree gas-exchange

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A field-system for measuring the whole-tree gas-exchange of CO_2and H_2O was developed. We compared transpiration rates on a leaf area basis of a hinoki (Chamaecyparis obtusa) tree 6.2 m high measured with the system to those measured with a porometer, so that it is concluded that the system is nearly the same as the porometer both in accuracy and response.Photocynthesis and transpiration were measured on five sample trees in a 20-year-old hinoki stand over one year using the system. Photosynthesis and transpiration were closely related to each other, i.e., photosynthesis increased with increasing transpiration. Water use efficiency was low in March to August when physiological activity of trees was high, whereas high in other months. The water use efficiency was different not only months, but also among sample trees. There existed a clear size-dependence of the transpiration rate of a tree to the square of stem girth at the bottom of the crown, which is in turn closely related to the foliage area of the tree. The relationship was approximated by the power function.