Stem Sap Flow Dynamics And Transpiration Of A Poplar Plantation

Poplar (Populus×euramericana cv. "74/76") is the most important plantation species in northern China. To fully understand the changes in sap flow density, We installed a field experiment in a 4-year plantation(2×2m) in DaXing District of Beijing in 2006. From May to October, we measured sap flow density using Thermal Dissipation Method, environmental factors including air temperature, air relative humidity, photosynthetically active radiation (PAR), soil temperature, wind speed and soil water content were also measured simultaneously with sap flow. We found that diurnal course of each tree stem sap flow density showed irregular diurnal course in the growing season, There was significant peak value during day time. The tree biological and environmental factors influencing the diurnal and seasonal stem sap flow were discussed, and mathematical models on the responses of the diurnal sap flow to the related environmental factors(photosynthetically active radiation (PAR), soil temperature, wind speed and soil water content)were developed during the growing season. PAR was the dominant environmental variables affecting the diural variation of sap flow density in difference growth period. In this article, sapwood area is used as a scalar to extrapolate the stand water use from whole tree water use measured by Thermal Dissipation Method. A significant correlation between sapwood area and diameter at breast height was found. The predication model is well fitted by power model. On the basis of the prediction model, sapwood area can be calculated by Thermal Dissipation Method. The sap flow density can be used to extrapolate stand water use by a series of mathematical models.