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

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(Qd)and Q_(n).Meteorological conditions(reference evapotranspiration,ETref),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 ETref,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.ETrefand LAI mainly controlled Q_(n)by affecting Qd,whereas RSWC had no significant effect on Q_(n).Q_(n)had a positive and linear relationship with LAI and a quadratic relationship with ETref.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 ETrefand LAI furthers understanding of the impacts of climate and forest structure change on Q_(n).

Nighttime sap flow and its driving forces for Populus euphratica in a desert riparian forest, Northwest China

Nighttime sap flow is a potentially important factor that affects whole-plant water balance and water-use efficiency （WUE）. Its functions include predawn disequilibrium between plant and soil water potentials as well as between the increments of oxygen supply and nutrient uptake. However, main factors that drive nighttime sap flow remain unclear, and researches related to the relationship between nighttime sap flow velocity and environmental factors are limited. Accordingly, we investigated the variations in the nighttime sap flow of Populus euphratica in a desert riparian forest of an extremely arid region, Northwest China. Results indicated that P. euphratica sap flow occurred throughout the night during the growing season because of the partial stomata opening. Nighttime sap flow for the P. euphratica forest accounted for 31%-47% of its daily sap flow during the growing season. The high value of nighttime sap flow could be the result of high stomatal conductance and could have significant implications for water budgets. Throughout the whole growing season, nighttime sap flow velocity of P. euphratica was positively correlated with the vapor pressure deficit （VPD）, air temperature and soil water content. We found that VPD and soil water content were the main driving factors for nighttime sap flow of P. euphratica.

Nighttime sap flow of Acacia mangium and its implications for nighttime transpiration and stem water storage

Aims Nighttime sap flow of trees may indicate transpiration and/or recharge of stem water storage at night.This paper deals with the water use of Acacia mangium at night in the hilly lands of subtropical South China.Our primary goal was to reveal and understand the nature of nighttime sap flow and its functional significance.Methods Granier’s thermal dissipation method was used to determine the nighttime sap flux of A.mangium.Gas exchange system was used to estimate nighttime leaf transpiration and stomatal conductance of studied trees.Important Findings Nighttimesap flowwas substantial and showed seasonal variation similar to the patterns of daytime sap flowin A.mangium.Mean nighttime sap flow was higher in the less precipitation year of 2004(1122.4 mm)than in the more precipitation year of 2005(1342.5 mm)since more daytime transpiration and low soil water availability in the relatively dry 2004 can be the cause of more nighttime sap flow.Although vapor pressure deficit and air temperature were significantly correlated with nighttime sap flow,they could only explain a small fraction of the variance in nighttime sap flow.The total accumulated water loss(E_(L))by transpiration of canopy leaves was only;2.6–8.5%of the total nighttime sap flow(E_(t))during the nights of July 17–18 and 18–19,2006.Therefore,it is likely that the nighttime sap flow was mainly used for refillingwater in the trunk.The stem diameter at breast height,basal area and sapwood area explained much more variance of nighttime water recharge than environmental factors and other tree form features,such as tree height,stem length below the branch,and canopy size.The contribution of nighttime water recharge to the total transpiration ranged from 14.7 to 30.3%depending on different DBH class and was considerably higher in the dry season compared to the wet season.