| The Granier-type thermal dissipation probes (TDP) were applied to measure the tree sap flow dynamics in a natural Quercus liaotungensis forest at Mount Gonglushan located in the southern suburb of Yan'an city of Shaanxi Province in the central part of the Chinese Loess Plateau (36°25.40'N, 109°31.53'E). Air temperature, relative air humidity, solar radiation, wind speed, and soil water content were monitored at the same time. In this paper, we investigated the diurnal course of sap flow characteristics and the relationships between sap flux density and environmental factors, azimuthal and radial variations of sap flux density on xylem trunk, and estimated stand transpiration in the growing season of 2009. The main conclusions were as follows:(1) 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-June), high in the mid and late period (July-September), and rapidly declining in the last stage (October). Exponential saturation function was applied to the data sets of sap flux density and vapor pressure deficit, and the fitted curves effectively reflected the sap flow characteristics for different months. The differences in fitted curves and parameters among months suggest that the transpiration process in these forest trees was also affected by soil moisture conditions or other environmental factors.(2) Sap flux densities at four aspects (north, south, east and west) on the trunk were significantly different, but were highly linearly correlated. Daily whole-tree transpiration throughout the growing season (May to October) could be well fitted to the corresponding daily total solar radiation and average daytime air vapor pressure deficit using exponential saturation functions. The differences relative to tree transpiration estimates based on sap flux densities for four aspects were typically 30% and 18% in accordance with the sap flux densities for one and two measurement aspects, respectively. The results suggest that azimuthal variations of sap flux density could be a large source of errors in tree transpiration estimates.(3) Radial variation (0-1 cm, 1-2 cm, 2-3 cm) of sap flux density were significantly different for both Q. liaotungensis and Platycladus orientailis, but were linearly correlated only in P. orientailis. Sap flux density in Q. liaotungensis and P. orientailis gradually decreased from cambium to heartwood. About 93% of the change in the sap flux density at the depth of 1-2 cm and 2-3 cm can be explained by that of 0-1 cm. The proportion of the maximum sap flux density for 1-2 cm and 2-3 cm in the maximum sap flux density of 0-1 cm varies by individuals.(4) The power function model can be used to fit the relationship between sapwood area and DBH of the dominant tree species in Q. liaotungensis forest. The stand average sap flux density of each tree species were differed among different months in growing season. The seasonal dynamics of stand average sap flux density in each tree species may be related to the differences in leaf phenology, meteorological factors and availability of soil water. Stand transpiration were higher from May to July. Total stand transpiration during the growing season of 2009 were roughly estimated to be 100.5 mm. Daily mean stand transpiration for Q.liaotungensis forest in the region were about 0.53 mm day-1. |
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