| The transpiration has an important impact on ecohydrology processes.The climate change,which resulted in changeable of global temperatures and rainfall patterns in recent years,have caused environmental problems such as land desertification and soil erosion.Revegetation on the Loess Plateau plays an important role in maintaining the stability of the Yellow River Basin ecosystem.Due to the constraints of natural conditions and the long-term influence of human activities,some re-vegetation shrubs have degraded,leading to the limitation of their ecological functions and the sustainable development.Therefore,it is important to clarify the water use characteristics and environmental adaptations of re-vegetation shrubs to provide theoretical guidance for optimal vegetation management.At present,the water consumption characteristics and environmental adaptations of shrubs in the Loess Plateau region have not been studied in depth,and quantitative estimation and explanations of the mechanisms are still lacking.The study was carried out in the Yangjuangou catchment in Yan’an City,Shanxi Province,China（36°42′N,109°31′E）.The typical shrub plantation,which is Vitex negundo L.were selected in this study.This study used the heat balance method to measure sap flow density in the main growing season（from June to September）in a normal precipitation year（2021）and a wetter year（2022）.Sensors were set at the base and at the top of the stem.The basal sensor measures the total stem sap flow,the apical sensor measures the canopy sap flow,and the difference in sap flow between the two sensors is the water storage dynamics of the stem in this study,three replicate sample stems were set up.The meteorological factors,including solar radiation,air temperature,relative humidity,wind speed,precipitation and vapor pressure deficit（VPD）,as well as soil water content from 0-200 cm,were monitored simultaneously.The dynamic characteristics of daytime sap flow and nighttime sap flow and its components（nighttime water storage and nighttime transpiration）at different time scales and influencing factors were analyzed,and the main results are as follows:（1）The dynamics of sap flow of Vitex negundo L.varied on different time scales.On the hourly time scale,the sap flow showed a pattern of higher daytime and lower nighttime variation.Daytime and nighttime sap flow accounted for 94.82%and 5.18%of the total sap flow throughout the day,respectively.At the daily time scale,the sap flow showed an overall decreasing trend over time during the 2021 growing season,while sap flow showed irregular fluctuations in 2022.The average daily sap flow rates were 9.79±0.57 g cm-2h-1and 17.57±0.66 g cm-2h-1 in 2021 and 2022,respectively.On the monthly time scale,sap flow was highest in June,followed by July and August,lowest in September in 2021,while sap flow was highest in July,followed by September and August;lowest in June in 2022.（2）The response of sap flow to environmental factors differed at different time scales.（1）On the hourly and daily time scales,sap flow were significantly positively correlated with solar radiation,atmospheric temperature and VPD,and negatively correlated with relative humidity,and the response of sap flow to solar radiation and VPD showed a non-linear pattern.The main influencing factors of daytime sap flow were solar radiation and atmospheric temperature,while relative humidity and VPD were the main factors on nighttime sap flow.（2）Soil water content was a major influence factor for sap flow in addition to meteorological factors.Firstly,when soil water content was lower in 2021（relative to 2022）,soil water content had a greater effect on daytime sap flow than meteorological factors,with an explanation of 58%;when soil water content was higher in 2022,soil water content explained less of the daytime sap flow with an explanation of 14%,and meteorological factors had a greater effect on both daytime and nighttime sap flow than soil water content.（3）According to analyzing the composition of nocturnal sap flow components,it was found that:（1）The daily average nocturnal stem water storage and transpiration were 1.52±0.19 g cm-2 and 8.61±0.75 g cm-2,with average percentages of 15.47%-17.27%and 82.73-84.53%,respectively.（2）On the hourly time scale,nocturnal water storage and transpiration were positively correlated with atmospheric temperature and VPD and negatively correlated with relative humidity.Nocturnal water storage was little affected by meteorological factors,and nocturnal transpiration was affected by VPD.Nocturnal water storage and transpiration occurred simultaneously and the two variables were affected with each other.Besides,the variation of nocturnal water storage increased with VPD nearly zero and air temperature decreasing,and refilling water was higher than transpiration,which indicated that water recharge was affected by circadian regulation.（3）On a daily scale,in the dry season（July 2021）,nocturnal water storage was significantly positively correlated with deep soil water content（120-200 cm）,with more nocturnal sap flow being used for water storage.In the wet season（September 2021）,nocturnal transpiration was significantly positively correlated with soil water content above 120 cm,with more nocturnal sap flow being used for transpiration.Plant water storage at night promoted daytime transpiration in the following day,increased the pre-dawn stem water potential,which could alleviate the effects of dry conditions on pre-dawn carbon fixation.In summary,meteorological factors and soil water content influenced the water consumption characteristics of plants and their adaptation to the environment at different time scales.These results provided some basis for improving the restoration effect of vegetation in the area.It is suggested that their long-term ecohydrological effects need to be studied on longer time scales and in greater depth under climate change,which would influence the water balance process and plant survival in water limited regions. |
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