Impacts of environmental and canopy conditions on the nighttime sap flow of larch plantations in the Liupan Mountains, China

Nighttime sap flow ( Q n ) is an important physiological activity under which trees manage drought stress. An in-depth understanding of the characteristics of Q n and its response to environmental and canopy conditions are of significance for arid area forest and water management. This study measured daily sap flow ( Q s ) of a Larix principis-rupprechtii plantation in the Liupan Mountains, northwest China during the 2017–2019 growing seasons, and separated Q s into daytime sap flow ( Q d ) and Q n . Meteorological conditions (reference evapotranspiration, ET ref ), canopy structure (leaf area index, LAI), and soil moisture (relative soil water content, RSWC) were considered as the main biophysical factors affecting Q n . The structural equation model and upper boundary line method determined the effects of compound and single factors on Q n . The daily mean Q n values during the growing seasons in 2017, 2018, and 2019 were 0.024, 0.026, and 0.030 mm d −1 , accounting for 6.2, 11.2, and 10.1% of Q s , respectively. Q n at different canopy development phases (leaf expanding, LG; leaf expanded, LD; and defoliation, DF) over three years was LD > LG > DF. Q n increased with increasing ET ref , whereas the ratio of Q n to Q s decreased. Q n did not show regular variation in the three-year growing seasons under different soil moisture conditions. ET ref and LAI mainly controlled Q n by affecting Q d , whereas RSWC had no significant effect on Q n . Q n had a positive and linear relationship with LAI and a quadratic relationship with ET ref . Both explained 40% of variation in Q n . Meteorological and canopy conditions are important factors affecting Q n on the semi-humid study site. The application of the Q n model coupled with the impact of ET ref and LAI furthers understanding of the impacts of climate and forest structure change on Q n .