Abstract: Clarifying the variations of sap flow in trunks and their environmental responses is of great significance for understanding the driving mechanisms underlying transpiration patterns. In this study, the heat dissipation technique was employed to continuously monitor the sap flow in the trunks of 20-year-old poplar (Populus × xiaozhuanica) plantations at the southern edge of the Horqin Sandy Land during May to June in 2019. The characteristics of sap flow under different weather conditions and the hysteresis effect of sap flow under varying soil moisture conditions were analyzed, and the relationships between sap flow and environmental variables at different scales were established. The results showed that the sap flow rate of poplar exhibited a unimodal curve on sunny days and multimodal curves on cloudy and rainy days. The sap flow rate was highest on sunny days, followed by cloudy days, and lowest on rainy days. There was a significant hysteresis effect between the sap flow rate and meteorological variables, with the sap flow preceding vapor pressure deficit by 60 min and air temperature by 30 min, whereas it lagged solar radiation by 20 min under high soil moisture conditions and lagged solar radiation by 50 min under low soil moisture conditions, indicating that soil moisture significantly influenced the hysteresis effect of sap flow. The sap flow rate of poplar at different temporal scales was significantly positively correlated with air temperature, solar radiation, vapor pressure deficit, and soil moisture, but negatively correlated with air humidity and groundwater level. Results of principal component analysis indicated that the contribution rates of the first principal component (meteorological variables) at different time scales were 61.6%, 62.2%, and 57.6%, respectively, with solar radiation and vapor pressure deficit being the key environmental variables affecting sap flow. On the daily scale, solar radiation had a greater impact on sap flow than vapor pressure deficit. As the temporal scale increased (10 min, 1 h, and 1 d), the influence of meteorological variables on sap flow decreased, while the influence of soil moisture increased. Our findings provide a theoretical basis for understanding the pattern of transpiration of poplar plantations in the semiarid sandy regions and their responses to environmental changes.

Key words: Populus × xiaozhuanica, sap flow, time-lag effect, time scale, environmental variable