Statistical analyses of inter-annual variations in the vertical profile of atmospheric CO2 mixing ratio and carbon budget in a cool-temperate deciduous forest in Japan

Using the 1994–2002 continuous data of CO2 mixing ratio, CO2 flux and meteorological parameters observed at the Takayama site located in a cool-temperate deciduous forest in central Japan, various statistical analyses are performed to examine inter-annual variations of annual mean height-dependent vertical CO2 mixing ratio differences; these are then compared to annually integrated carbon budget components estimated from the flux measurement. The mixing ratio measurements are made at 27 and 18 m (located above the canopy), and 8.8 and 5.8 m (located inside the canopy) on a tower situated at the site. Relative to 27 m, the annual mean mixing ratios in the lower layers show a significant positive correlation (>90 or >95% confidence level) with the annually integrated soil respiration, while a significant negative correlation (>95% confidence level) between the 18- and 27-m mixing ratio difference for the daytime data and the annually integrated gross primary production is obtained. Significant long-term trends (>95% confidence level) of the annual mean vertical mixing ratio differences are found, with an increasing trend for the daytime and nighttime data at the lower levels inside the canopy and a decreasing trend for the daytime data near the canopy height. Seasonal analysis shows that the increase in trend occurs mostly in mid-summer of the growing season, while the decrease in trend occurs in late spring. It is therefore suggested that the inter-annual variation and the long-term trend of the vertical CO2 mixing ratio difference obtained from high-precision mixing ratio measurements on a tower in a forest could provide us with useful information on the carbon budget components.