Temporal variations of atmospheric CO₂ concentration in a temperate deciduous forest in central Japan

Abstract

In order to examine the temporal variation of the atmospheric CO₂ concentration in a temperate deciduous forest, and its relationship with meteorological conditions, continuous measurements of CO₂ and meteorological parameters have been made since 1993 on a tower at Takayama in the central part of Japan. In addition to an average secular increase in atmospheric CO₂ of 1.8 ppm yr⁻¹, diurnal variation with a maximum during the night-time to early morning and a minimum in the afternoon is observed from late spring to early fall; the diurnal cycle is not so clearly observed in the remaining seasons of the year. A concentration difference between above and below the canopy, and its diurnal variation, can also be seen clearly in summer. Daily mean concentration data show a prominent seasonal cycle. The maximum and the minimum of the seasonal cycle occur in April and from mid August to mid September, respectively. Day-to-day changes in the diurnal cycle of CO₂ are highly dependent on the day-to-day variations in meteorological conditions. However, CO₂ variations on longer time scales (>10 d) appear to be linearly related to changes in respiration. At Takayama, variations in the 10-d standard deviation of daily mean CO₂ data and 10-d averaged respiration show distinct relationships with soil temperature during spring and fall seasons. In spring, respiration has a stronger exponential dependence on soil temperature than in fall. Interestingly, in summer when soil temperature becomes greater than about 15 °C, biological respiration becomes more variable and independent of the soil temperature. Thus, at the Takayama site, the Q₁₀ relationship is seasonally dependent, and does not represent well the biological respiration process when the soil temperature rises above 15 °C.