Responses of nighttime sap flow to atmospheric and soil dryness and its potential roles for shrubs on the Loess Plateau of China

Nocturnal sap flow (Enight) has been observed in a variety of plant species and environmental conditions. In water-limited environments, Enight is important in the regulation of plant water's physiology. This study was designed to evaluate the way in which Enight (defined as sap flow from 20:30 to 06:00) responded to changes in the atmospheric vapor pressure deficit (VPD) and the soil water content (SWC), and explored its potential physiological significance for different plant species in a semi-arid area. We selected three shrub species: Vitex negundo L. (VN), Hippophae rhamnoides L. (HR) and Spiraea pubescens Turcz (SP) in the semi-arid Loess Plateau of China. The plots of the three communities dominated by each of three species were on the same hill slope. Half-hourly sap flow density was measured in six to seven sample stems for each species during the main growing season (June to August 2015). VPD, SWC, leaf water potential (Ψleaf) and stomatal conductance (Gs) were measured at the same time. Regression analyses were conducted to determine the relationships between Enight, Eday, Enight/Edaily, VPD and SWC at half-hourly and daily time scales, as well as between Enight, Eday and Ψleaf. The mean values of Enight and Eday were higher, but Enight/Edaily values were lower for VN compared to HR and SP. The responses of sap flow density to VPD and SWC varied at different temporal scales. VPD was the dominant factor that affected Enight and Eday at the half-hourly scale for all three species. In contrast, SWC was the key factor that influenced Eday at the daily time scale. The values of Eday and Enight/Edaily correlated negatively with SWC because the effect of SWC was stronger on Eday than on Enight. Although the low fraction of Enight/Edaily (4%–7%) may indicate a minor short-term effect of Enight on the standing water balance, Enight had ecophysiologically significance to the plants. The discrepancy in Ψleaf between sunset and the following day's predawn (ΔΨ) indicated that stem refilling occurred during this period. SP had the higher fraction of recharge water among the three species, as it had the relatively higher value of ΔΨ/Enight. Enight had positive relationship with predawn Gs. The increased Gs facilitated rapid onset of photosynthesis in the early morning. In addition, the positive correlation between Enight and Eday from 06:00 to 10:30 suggested that Enight was beneficial for transpiration in the following morning. The responses of Enight to VPD and SWC indicated differences in plant adaptation to drought stress, which provides important information for our understanding of the reactions to climate changes among species in semi-arid areas.