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Nighttime sap flow (Qn) is an important physiological activity under which trees manage drought stress. An in-depth understanding of the characteristics of Qn and its response to environmental and canopy conditions are of significance for arid area forest and water management. This study measured daily sap flow (Qs) of a Larix principis-rupprechtii plantation in the Liupan Mountains, northwest China during the 2017–2019 growing seasons, and separated Qs into daytime sap flow (Qd) and Qn. 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 Qn. The structural equation model and upper boundary line method determined the effects of compound and single factors on Qn. The daily mean Qn 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 Qs, respectively. Qn at different canopy development phases (leaf expanding, LG; leaf expanded, LD; and defoliation, DF) over three years was LD > LG > DF. Qn increased with increasing ETref, whereas the ratio of Qn to Qs decreased. Qn did not show regular variation in the three-year growing seasons under different soil moisture conditions. ETref and LAI mainly controlled Qn by affecting Qd, whereas RSWC had no significant effect on Qn. Qn had a positive and linear relationship with LAI and a quadratic relationship with ETref. Both explained 40% of variation in Qn. Meteorological and canopy conditions are important factors affecting Qn on the semi-humid study site. The application of the Qn model coupled with the impact of ETref and LAI furthers understanding of the impacts of climate and forest structure change on Qn. [ABSTRACT FROM AUTHOR]

An increasing body of evidence has shown that nighttime sap flux occurs in most plants, but the physiological implications and regulatory mechanism are poorly known. The significance of corticular photosynthesis has received much attention during the last decade, however, the knowledge of the relationship between corticular photosynthesis and nocturnal stem sap flow is limited at present. In this study, we divided seven tree species into two groups according to different photosynthetic capabilities: trees of species with (Castanopsis hystrix, Michelia macclurei, Eucalyptus citriodora, and Eucalyptus grandis × urophylla) and without (Castanopsis fissa, Schima superba, and Acacia auriculiformis) photosynthetic stems, and the sap flux (Js) and chlorophyll fluorescence parameters for these species were measured. One-way ANOVA analysis showed that the Fv/Fm (Maximum photochemical quantum yield of PSII) and ΦPSII (effective photochemical quantum yield of PSII) values were lower in non-photosynthetic stem species compared to photosynthetic stem species. The linear regression analysis showed that Js,d (daytime sap flux) and Js,n (nighttime sap flux) of non-photosynthetic stem species was 87.7 and 60.9% of the stem photosynthetic species. Furthermore, for a given daytime transpiration water loss, total nighttime sap flux was higher in species with photosynthetic stems (SlopeSMA = 2.680) than in non-photosynthetic stems species (SlopeSMA = 1.943). These results mean that stem corticular photosynthesis has a possible effect on the nighttime water flow, highlighting the important eco-physiological relationship between nighttime sap flux and corticular photosynthesis.

An increasing body of evidence has shown that nighttime sap flux occurs in most plants, but the physiological implications and regulatory mechanism are poorly known. The significance of corticular photosynthesis has received much attention during the last decade, however, the knowledge of the relationship between corticular photosynthesis and nocturnal stem sap flow is limited at present. In this study, we divided seven tree species into two groups according to different photosynthetic capabilities: trees of species with (Castanopsis hystrix, Michelia macclurei, Eucalyptus citriodora, and Eucalyptus grandis × urophylla) and without (Castanopsis fissa, Schima superba, and Acacia auriculiformis) photosynthetic stems, and the sap flux (Js) and chlorophyll fluorescence parameters for these species were measured. One-way ANOVA analysis showed that the Fv/Fm (Maximum photochemical quantum yield of PSII) and ΦPSII (effective photochemical quantum yield of PSII) values were lower in non-photosynthetic stem species compared to photosynthetic stem species. The linear regression analysis showed that Js,d (daytime sap flux) and Js,n (nighttime sap flux) of non-photosynthetic stem species was 87.7 and 60.9% of the stem photosynthetic species. Furthermore, for a given daytime transpiration water loss, total nighttime sap flux was higher in species with photosynthetic stems (SlopeSMA = 2.680) than in non-photosynthetic stems species (SlopeSMA = 1.943). These results mean that stem corticular photosynthesis has a possible effect on the nighttime water flow, highlighting the important eco-physiological relationship between nighttime sap flux and corticular photosynthesis. [ABSTRACT FROM AUTHOR]

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 Nighttime sap flow was substantial and showed seasonal variation similar to the patterns of daytime sap flow in 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 (EL) by transpiration of canopy leaves was only ∼2.6–8.5% of the total nighttime sap flow (Et) during the nights of July 17–18 and 18–19, 2006. Therefore, it is likely that the nighttime sap flow was mainly used for refilling water 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. [ABSTRACT FROM PUBLISHER]

Calibration of the Granier thermal dissipation technique for measuring stem sap flow in trees requires determination of the temperature difference (ΔT) between a heated and an unheated probe when sap flow is zero (ΔTmax). Classically, ΔTmax has been estimated from the maximum predawn ΔT, assuming that sap flow is negligible at nighttime. However, because sap flow may continue during the night, the maximum predawn ΔT value may underestimate the true ΔTmax. No alternative method has yet been proposed to estimate ΔTmax when sap flow is non-zero at night. A sensitivity analysis is presented showing that errors in ΔTmax may amplify through sap flux density computations in Granier's approach, such that small amounts of undetected nighttime sap flow may lead to large diurnal sap flux density errors, hence the need for a correct estimate of ΔTmax. By rearranging Granier's original formula, an optimization method to compute ΔTmax from simultaneous measurements of diurnal ΔT and micrometeorological variables, without assuming that sap flow is negligible at night, is presented. Some illustrative examples are shown for sap flow measurements carried out on individuals of Erica arborea L., which has needle-like leaves, and Myrica faya Ait., a broadleaf species. We show that, although ΔTmax values obtained by the proposed method may be similar in some instances to the ΔTmax predicted at night, in general the values differ. The procedure presented has the potential of being applied not only to Granier's method, but to other heat-based sap flow systems that require a zero flow calibration, such as the Čermák et al. (1973) heat balance method and the T-max heat pulse system of Green et al. (2003). [ABSTRACT FROM PUBLISHER]

• Nighttime sapflow per leaf area J s n exhibited thresholded-response to T a , VPD and G s. • Seasonal variation in T a and VPD controlled J s n in wet and G s and SWC in dry years. • Interannual variation in T a and G s directly controlled J s n , while indirectly by SWC. • J s n responses to biophysical factors underscores the acclimation to dry conditions. • J s n significantly contributed to the daily total sap flow with interannual mean of 11%. The impact of extreme desert conditions on long-term nighttime sap flow dynamics in desert-dwelling shrubs is poorly understood. Based on heat-balance measurements in a semiarid area of northwest China, we investigated the nighttime sap flow (transpiration) dynamics in Salix psammophila as affected by changes in site biophysical variables over a six-year period (2012-2017). Nocturnally, sap flow rate per leaf area (J s n) formed a positive linear relationship with air temperature (T a), water vapor pressure deficit (VPD) and stomatal conductance (G s), when T a < 17 °C, VPD < 1 kPa, and G s < 0.004 mol m−2 s−1, remaining unchanging or forming a negative linear relationship, when T a , VPD and G s were greater than these limits. This relationship was modified by soil water content (SWC) through its effects on G s. The multiple regression model showed that together T a , VPD and G s explained 63% of the variation in J s n. Seasonally, pairwise relationships between T a and VPD and G s and SWC explained 76 and 70% of the variation in J s n during wet and dry years, respectively. Interannually, T a and G s were able to explain a significant portion of observed variation in J s n , individually explaining about 74 and 83% of that variation. Interannual control on J s n also varied with SWC, T a and G s explaining 84% of the variation. The diminished sensitivity of J s n to both T a and VPD and increased sensitivity to SWC during dry years, suggested that the shrubs had a physiological capacity to adapt to dry soil conditions. Daytime sap flow rates (J s d) were significantly greater than those at night (i.e., J s n), with an interannual mean difference of 0.15 ± 0.04 g cm−2 d−1 and coefficient of variation of 27%. In addition, J s n formed a positive linear association with J s d during the growing season. Mean J s n contributed to the daily total sap flow (i.e., J s n /J s 24-h × 100) by 8-14% over the six years, with an interannual mean of 11%. Our findings underscore the importance of nighttime sap flow in S. psammophila and the importance of temperature and SWC in controlling nighttime water losses through transpiration. It is suggested that the proportion of nighttime sap flow could increase during plant acclimatization in response to increasing temperature and water shortages with future climate change. [ABSTRACT FROM AUTHOR]