ObjectiveWe aimed to analyze the differences and characteristics of sap flow of Populus hopeiensis among the different environment conditions in the water-wind erosion crisscross region on the Loess Plateau, and to clarify the relationship between sap flow density and environmental factors. This study provides a scientific basis for the water resource carrying capacity and tree species selection.

MethodThe Granier's thermal dissipation probe method and a micrometeorological monitoring system were used to measure the sap flow density of Populus hopeiensis during growing season in Wuqi County, Shaanxi Province of northwestern China.

ResultThe results showed that the diurnal process of sap flow density in P. hopeiensis showed a broad unimodal change and the maximum sap flow density reached about 2.5 hours before the maximum of solar radiation, and about 4 hours before the maximum of vapor pressure deficit (VPD). During the studying period, the nocturnal sap flow was noticeable in 00:00—06:00 and only in very few occasions sap flow was totally stopped. On sunny days, daily variation in sap flow density exhibited little amplitudes, whereas it exhibited large variation on rainy days. The peak value appearing time of sap flow density of trunk base was early apparently after being watered, and the peak sap flow density of trunk base and crown base increased by 73.62% and 66.66%, respectively after being watered. The results showed that the peak value in the sap flow density was greatly influenced by the soil moisture. And the mean daily sap flow density of EG (experimental group: watering treatment) and control check(CK) decreased by 2.21% and 21.89%, respectively after being watered. It showed that artificial irrigation can supplement the soil water deficit in the forest land in a short time, and restore the normal transpiration water consumption pattern of the tree. With respect to P. hopeiensis, sap flow density on sunny day was positively correlated with each of solar radiation, VPD, air temperature, wind speed (P < 0.01) and soil volumetric moisture content(P < 0.05), but negatively correlated with relative humidity(P < 0.01). On rainy day, sap flow density was positively correlated with solar radiation, air temperature, wind speed (P < 0.01) and VPD(P < 0.05). Compared with CK, the correlation coefficients of sap flow density of EG and meteorological factors were not higher.

ConclusionOur findings suggest that the meteorological factors and soil moisture had important impacts on sap flow density and the time lags of sap flow density to environmental variables were related with the discharge of stem water storage. Watering treatment can shorten the time lags of sap flow and meteorological factors.