Biological and environmental controls on nighttime water-use behaviors in Populus simonii in semi-arid areas of the Chinese Loess plateau.

• Atmospheric and soil drought stimulate the nighttime water-use behaviors of trees. • The RE makes up a major part of WU n in semi-arid areas with a mean RE % of 65.7–68.0%. • The Ѱ pd and VPD n affect RE % in the soil dry and wet stages, respectively. Recent studies have revealed that the active nighttime water-use behaviors by trees have critical physio-ecological significance in survival and growth in adversity. However, the characteristics and biophysical control of nighttime water-use of silvicultural trees are not clear. We continuously monitored sap flow and leaf water potential (predawn and midday) of Populus simonii of different tree sizes as well as environmental factors during the 2021 and 2022 growing seasons (May-September) in a semi-arid area of Loess Plateau. Our results showed that both nighttime water-use (WU n) and the ratio of nighttime to daily water-use (WU n %) increased significantly with tree size, and had larger values in the drought year than in the normal year. The WU n % of different tree sizes ranged from 10.24 to 13.01% and can reach up to 36.49% during prolonged soil drought. Besides, the WU n % of different tree sizes were significantly positively and negatively correlated with nighttime vapor pressure deficit (VPD n) and soil water content, respectively. By partitioning the components of WU n , we found that as tree size increased, the stem water refilling (RE) and nighttime transpiration increased obviously, but the ratio of RE to WU n (RE %) did not change. The RE of different tree sizes constituted the major part of WU n with an average RE % of 65.7–68.0%, and the RE % was larger in the drought year than in the normal year. Additionally, the RE % of different tree sizes increased with decreasing predawn leaf water potential (Ѱ pd) during the soil dry stage and with increasing VPD n during the soil wet stage. Given the increased risk of atmospheric drought and soil drought, it can be expected from our findings that the nighttime water-use behaviors of trees become more active in the future, which represents a significant performance of drought adaptation strategies of silvicultural species. [ABSTRACT FROM AUTHOR]