Stem Sap Flow Dynamics Of Pinus Tabulaeformis

Water plays a key role in growth and development of plants, so study on rules of transpiration variation of plants is of great significance. This research was carried out at Huoditang Forest Zone of the Qinling Mountains from May to October in 2009. The diurnal and seasonal dynamics of sap flow of Pinus tabulaeformis trees were measured under natural condition, and main meteorological factors affecting on the rate of sap flow were also recorded and analyzed. The main results are as follows:During the growth period of P. tabulaeformis trees, the curve of diurnal changes of rate of sap flow tended to be"single peak"pattern, however the peak value of the rate of dap flow and the time to be reached were varied in different periods. In July, the rate of dap flow was obviously impacted by environmental factors, and showed a trend of classical"single peak curve"with different peak values. The maximum rate of sap flow was 134cm.h^-1, appeared at 1:30 pm on July 5. The average rates of sap flow from July 5 to 15 were 46 cm.h^-1, 38 cm.h^-1, 33 cm.h^-1, 41 cm.h^-1, 24 cm.h^-1, 48 cm.h^-1, 49 cm.h^-1, 31 cm.h^-1, 22 cm.h^-1, 9 cm.h^-1, and 36 cm.h^-1 respectively. However, the diurnal dynamics of sap flow under different weather conditions (sunny, cloudy, and rainy day) were in the same pattern and all showed circadian rhythm changes. The pattern was as follows: at 8:00 am, the transportation of sap flow began, and then the rate of sap flow was increasing; at 5 pm, the rate of sap flow was decreasing, and reached its lowest level before 8:00 am on the next day. The time that the rate of sap flow reached its peak value and the value of it under different weather conditions were different: in sunny days, it was at 12 am, 1.5 hours later on cloudy days; and 3 hours later on rainy day. The peak rate of sap flow on rainy day was 1/3 of the rate in sunny day and 1/2 on cloudy day. By using the multiple regression equation, the correlations between the rates of sap flow and weather factors were effective solar radiation > atmospheric temperature > relative humidity > wind velocity > vapor pressureThe rules of monthly average evaporation of P. tabulaeformis trees during the growing season was July (3.86l.h^-1) > August (2.31l.h^-1) > June (1.32l.h^-1) > September (1.21l.h^-1) > October (0.80l.h^-1) > May (0.54l.h^-1). Solar radiation was weak, and temperature was low in May, so the evaporation was at a low standard. In June, July and August, with the increasing of solar radiation and rise of temperature, the trees began to grow and the evaporation of them was also increasing. In September and October, the life activities of trees were decreasing, leaves were fallen, and the evaporation activities became slow down.