Transpiration of <Emphasis Type="Italic">Cyclobalanopsis glauca</Emphasis> (syn. <Emphasis Type="Italic">Quercus glauca</Emphasis>) stand measured by sap-flow method in a karst rocky terrain during dry season

Seasonal drought may have a high impact on the karst ecosystem. The transpiration from Cyclobalanopsis glauca (syn. Quercus glauca) stand on a rocky hilly slope in South China was measured during the dry period of 2006 by using the Granier’s sap-flow method. During the experimental period, maximum sap flux density (J _s) ranged from 20 to 40 g H₂O m⁻² s⁻¹ according to diameter of breast height (DBH) of individual trees. On sunny days, daily transpiration varied between 3.4 and 1.8 mm day⁻¹. Transpiration of C. glauca was closely correlated to the radiation, air temperature, and vapor pressure deficit (VPD). Soil moisture was a very important factor influencing transpiration. The very low soil water content might result in low stand transpiration even when VPD is high, but high soil water content might also result in low transpiration if it was low VPD. However, VPD rather than soil moisture, affected largely the stand transpiration under high soil water content. The amount of transpiration was much more than that of the total soil moisture loss during the continuous sunny days, indicating that the dry shallow soils were probably not the only source for root-uptake water. C. glauca grows deep roots through the rock fissures of epikarst, indicating that epikarst might be another main source for sustaining transpiration in response to dry demand in autumn. Therefore, a large amount of deep roots of karst species would be a very important hydraulic connecting from the epikarst to above ground by transpiration, which would promote the biogeochemical process in a karst system.