Abstract:

Granier-type thermal dissipation probes (TDP) were applied to measure tree sap flow dynamics in a naturally-regenerated Quercus liaotungensis forest in semiarid Loess Plateau region near Yanan, Shaanxi Province. Air temperature, relative air humidity, solar radiation, and soil water content were monitored at the same time. In this paper, we measured diurnal courses of sap flow characteristics and analyzed relationships between sap flux density and environmental factors with three dominant trees during the growing season of 2009. The results showed that sap flux densities in Q. liaotungensis reached their daily peaks earlier than solar radiation and vapor pressure deficit, usually around 10:00 am, though the diurnal courses of sap flux density were generally similar to the changes of environmental factors. As the season and leaf phenology progressed, the overall performance of sap flux density was relatively low at early stage (April to June), high in the mid and late stage (July to September), and rapidly declining in the last stage (October). Exponential saturation function was applied to fit the data sets of sap flux density and vapor pressure deficit, and the fitted curves effectively reflected the sap flow characteristics in different months. Differences in the fitted curves and parameters among months suggested that the transpiration process in these trees was also affected by soil moisture conditions or other environmental factors.

Key words: Loess Plateau, Quercus liaotungensis, sap flow, seasonal variation, thermal dissipation probe (TDP)