We observed the phenomenon of midday depression in the rate of tree root respiration. Diurnal changes in the root respiration rate of Quercus crispula and Chamaecyparis obtusa were measured under intact conditions using a closed chamber method and a soil respiration measurement system (LI-6400 with a root respiration chamber) in a forest in the foothills of Mt. Fuji. After the measurement of intact root respiration in the field, the root was excised and taken to the laboratory, and the temperature dependence on the respiration rate of the detached root was measured using an open-flow gas exchange system with an infrared gas analyzer (LI-6252). The measurement was conducted in September 2003, August and November 2005, and June 2006. Whereas the root respiration rate of both species under intact conditions increased with increasing soil and root temperatures from dawn to early morning, the respiration rate decreased around midday from 10:00 to 15:00 despite an increment of soil and root temperatures. There was no clear relationship between the intact root respiration rate and root temperature in either species, although the detached root respiration rate of both increased exponentially with the temperature. The amount of the CO2 efflux estimated using the temperature dependence of detached root respiration tended to underestimate the actual measurement value (intact respiration rate) by 20-50% in both species. These results indicate that evaluating midday depression in root respiration would be important for a more accurate estimation of the carbon cycle or net ecosystem production in forests.
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