Effects of sap velocity on the daytime increase of stem CO2 efflux from stems of Schima superba trees

Stem CO2 efflux (Es) has been estimated from a temperature-related equation, but sap flux often affects measurements of Es, which leads to misunderstanding real stem respiration. In order to observe the relationship between Es and stem temperature and to analyze the effect of sap velocity on Es, stem temperature, Es and sap flux were measured from a subtropical Schima superba plantation in South China on three trees for consecutive 3 days in July and October 2009. Stem temperature, Es and sap velocity were significantly higher in July than in October. Stem temperature could explain 17–41 and 54–75% variations of Es in July and October, respectively. A negative relationship between Es and stem temperature was found during 1800–2300 hours in July. The daytime Es was 9.2, 4.3 and 2.4% higher than the predicted for three trees in July, and this occurred only on Tree 1 in October. Sap velocity was positively correlated with Es for three trees in July, and the increase of Es with the increase of sap velocity was only observed on Tree 1 in October. These results demonstrated that the occurrence of sap flux could account for the increase of daytime Es, and the effect of sap velocity on Es varied with the seasons from the S. superba stem.