Dynamic Changes Of Populus Davidiana Sap Flow In Hilly-gully Region Of Loess Plateau

In this study, the thermal dissipation probe (TDP) was applied to measure the sap flow velocity of Populus davidiana in Yan'an City on the Loess Plateau of China from April 14 to May 22, 2009. Soil water content, photosynthetic active radiation (PAR), air temperature, relative air humidity, and wind speed were measured at the same time. Study the law of transpiration water consumption of those two forest types has important reference value to the water management of soil and water conservation forests as well as to the analysis of forest hydrologic effect. After the study of dynamic changes of Populus davidiana sap flow velocity,and the relationship between sap flow and the environmental factors, major conclusions got were as follows:1. Sap flow velocity dynamics of Populus davidianaDiurnal variation of sap flow velocity displayed unimodal curve with fast-rising and slow-declining trends in the whole growth season. The sap flow velocity didn't stop after reaching its peak, but there were minor fluctuations, a number of small peaks formed a "peak platform", and then began to decline. The sap flow also exists in night though it is small. The unimodal curve is different from others in different seasons. In spring, the sap flow started to increase at 6:00—7:30, reached the peak value at 13:30—15:30, and descend at 18:00. In summer, the sap flow started to increase at 5:00—7:00, but the time of sap flow velocity reached the peak value has a greatly differences. From June 1 to June 19, the sap flow velocity reached the peak value at 14:00—16:00. From June 20 to July 7, the sap flow reached the peak value at 9:00—11:00 because the drought. From July 8 to August 31, the sap flow reached the peak value at 12:00—14:00. In autumn, the time of sap flow started to increase and reached the peak value both delay to 6:30—10:00 and 12:30—17:00.2. The relationship between sap flow velocity and soil factorsSoil water content was gradually declined in spring, but its'influence on the sap flow was not significant. The sap flow velocity declining goes with the soil water content in June and middle of July. Because there was plentiful rainfall, the soil water content was recruited quickly. At the same time, the sap flow velocity was also increasing gradually and maintained a high-speed level. In autumn, the soil water content was recruited quickly because of the rainfall. The soil water content was so excessive that the sap flow velocity was restrained. In this study we conclude that the soil temperature has nothing to do with the sap flow velocity.3. The relationship between sap flow velocity and soil factorsIn spring, the sap flow velocity was significantly correlated with air temperature and water vapor pressure deficit (VPD), and the correlated coefficients were highly significant at 1% safety. Soil water content was gradually declined in this period, but its'influence on the sap flow velocity was not significant. The peak of sap flow velocity in the upper trunk was higher than that in the lower trunk apparently in fine days, while it was lower in clouded and rainy days. The sap flow velocity peak became smaller day-by-day as the drought became seriously in both upper and lower trunks, and the appearing time of upper trunk's sap flow velocity peak was getting earlier while that of lower trunk's sap flow velocity peak was getting later. In summer, the sap flow velocity was significantly correlated with photosynthetic active radiation (PAR) and air temperature in fine days, and the peak of sap flow velocity in the upper trunk was higher than that in the lower trunk apparently. But in clouded and rainy days, the sap flow velocity of the upper trunk was significantly correlated with photosynthetic active radiation (PAR) and air temperature, and the sap flow velocity of the lower trunk was significantly correlated with water vapor pressure deficit (VPD). The peak of sap flow velocity in the upper trunk and the lower trunk was no apparently change. Pearson correlation coefficient of sap flow velocity to climatic factors in summer is less than that in spring and autumn.In autumn, as same as in spring the sap flow velocity was significantly correlated with air temperature and water vapor pressure deficit (VPD). The peak of sap flow velocity in the upper trunk was higher than that in the lower trunk apparently both in fine days and clouded days.