Midday depression of tree root respiration in relation to leaf transpiration

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AbstractThe root respiration rate often shows an exponential or a linear relationship with temperature under laboratory conditions. However, under intact conditions in the field, the root respiration rates of some tree species decreased around midday despite an increment of the root temperature (Bekku et al. 2009). To clarify the cause of midday depression, we examined the relationships between the intact root respiration and parameters of leaf gas exchange through the simultaneous field measurement of the gas exchange in the leaf and root of Quercus crispula and Chamaecyparis obtusa, which are canopy trees. There were no significant relationships between the root respiration rates (Rr) and the parameters of leaf gas exchange in the field. However, in C. obtusa, the relationships between Rr and the transpiration rates (E) at 1 h before the measurement of Rr were fitted by logarithmic function with a determination coefficient of 0.60–0.89. In the light‐manipulation experiments using saplings, Rr had significant positive correlations with E at 20 min before the measurement of Rr, root temperature (Tr), and the photosynthesis (Pn) at 20 min before the measurement of Rr. We examined which factor, Pn or E, affects the root respiration rate through a manipulation experiment using a growth chamber regulating the ambient CO2 concentration and relative humidity independently under constant air temperature and photosynthetic photon flux density. As a result, the root respiration rates changed corresponding to E and not Pn. These results suggest that the root respiration rate of trees changes significantly in the daytime and is affected by the leaf transpiration rate as well as the temperature.