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

Abstract

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.

Keywords

Nighttime sap flow / Reference evapotranspiration / Leaf area index / Soil moisture / Multi-factor impact