Soil-Moisture-Dependent Nocturnal Water Use Strategy and its Responses to Meteorological Factors in a Seasonal-Arid Poplar Plantation

Abstract

Nocturnal water use ( Q n ) significantly affects global water budgets and diurnal water cycles, which is, at present, suffering from restrictions of soil drought and soil water depletion caused by climate change and worldwide greening. Recent studies have recognized the important influence of soil water availability on Q n , but responses of nocturnal water use strategy to soil moisture variation were not fully understood. For example, the effect of soil water content ( θ ) on the trade-offs between two components of Q n , stem water refilling ( R e ) and nocturnal transpiration ( T n ), was rarely involved and remained unclear. This study analyzed the nocturnal sap flow of typical afforestation species, poplar plantation, under different soil water conditions in a seasonal arid region of northern China. Results indicated that higher θ significantly promoted Q n through enhancing the stomatal conductance but had less influence on the proportion of nocturnal water use to daily water use ( Q n %) (7%~10%). With the decline of soil water conditions, significant linear correlations between Q n and θ shifted from deep soil layers to the soil surface. The influence of meteorological factors on Q n depended on soil water condition, and the explanation of VPD n to Q n became weaker with increasing soil moisture. Negative linear relations between the proportion of stem water refilling to Q n ( R e %) and θ were shown within treatments, but a significantly higher R e % was observed under more sufficient soil water. This opposite responses of R e % to θ reflected the different adaptions of Q n to soil water availability on short- and long-term scales. This study highlights the crucial role of soil moisture in the nocturnal water use and balance strategy of a seasonal-arid poplar plantation. Our results help better understand the nocturnal transpiration processes in the context of global climate change.

Keywords: nocturnal sap flow, Trade-offs, Soil water content, Meteorological drivers, Populus tomentosa