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

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