Abstract: The 13 years old Betula platyphylla from 12 provenances in Caohekou and Maoershan were used as experimental materials. In the growing season, three diameter classes roots <0.5 mm, 2.0-5.0 mm and >5.0 mm were taken, respectively. The CO₂ efflux rate of root system was measured by infrared gas analysis in vitro, in order to study the effect of root diameter, experiment site and provenance on CO₂ efflux of B.platyphylla root. The root diameter and environment had significant(P<0.01) effects on root CO₂ efflux rate, while provenance had no significant(P>0.05) effects on root CO₂ efflux rate. Root CO₂ efflux rate decreased significantly(P<0.01) with the increase of diameter class. All diameter class root CO₂ efflux rate in Caohekou was greater than that in Maoershan(P<0.01). The root CO₂ efflux rates of B.platyphylla in Caohekou and Maoershan with diameters <0.5 mm were 104.21 and 81.07 mol·g^-1·s^-1, the root CO₂ efflux rates with diameters 2.0-5.0 mm were 41.08 and 30.91 mol·g^-1·s^-1, and the root CO₂ efflux rates with diameters >5.0 mm were 24.87 and 11.37 mol·g^-1·s^-1, respectively. There were significant(P<0.05) differences in tree height and DBH among 12 provenances of B.platyphylla, and both tree height and DBH were negatively correlated with root CO₂ efflux. The results of relevant studies will provide an important basis for more accurate assessment of CO₂ efflux fluxes from tree roots.

Key words: root respiration, CO₂ efflux, provenances, Betula platyphylla