CO_2 and H_2O gas-exchanges of trees in changeable environments

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The effect of elevated atmospheric CO_2 concentrations on the nighttime respiration were examined for two sample branches of a hinoki cypress (Chamaecyparis obtusa (Sieb.et Zucc.) Endl.) tree growing in the field with an open gas exchange system. The branches were of a similar size and located at a similar position within the crown. One branches was subjected to ambient air which had a CO_2 concentration of about 370 mu mol mol^<-1>. The nighttime respiration rate was higher in elevated CO_2 level than in ambient CO_2 level. The relationship between hourly nighttime respiration and the corresponding nighttime air temperature was fitted with the exponential function in every month. The segregation of the regression lines between the two CO_2 treatments increased gradually as the season progressed. The Q_<10> values for nighttime respiration were lower in elevated CO_2, i.e.1.9 to 3.7, than in ambient CO_2, i.e.2.4 to 4.5 in every month. The Q_<10> was inversely related to the monthly mean nighttime air temperature in both elevated and ambient CO_2. The estimated daily dark respiration rate showed a similar seasonal pattern in both CO_2 treatments, which almost synchronized with the temperature change. The respiration ratio of elevated CO_2 to ambient CO_2 increased gradually from 1.0 to 1.5. The present results indicated that the CO_2 level and the temperature have a strong interactive effect on respiratory activities.