Nighttime Sap Flow Research Articles

Comparative study of daytime and nighttime sap flow of Populus euphratica

In order to quantify and characterize the variance in desert riparian forest tree sap flow, we measured the sap flow from Populus euphratica and compared the daytime and nighttime patterns and responses to environmental variables. Results showed that daytime sap flow velocity was significantly higher (P < 0.05). Daytime and nighttime mean sap flow velocities were 7.65 and 4.01 cm h−1 in spring, 21.38 and 9.60 cm h−1 in summer, and 11.04 and 5.21 cm h−1 in autumn, respectively. Moreover, results indicated that the stoma remained partially open (15% minimum) throughout the night, providing sufficient evidence for the existance of nighttime transpiration. The vapor pressure deficit (VPD), stomatal conductance (Cs), photosynthetically active radiation (PAR), air temperature (Ta), wind speed (WS), and soil moisture (θ) all had significant positive effects on P. euphratica sap flow velocity (P < 0.05). Furthermore, the relationship between daytime sap flow velocity and VPD showed clockwise hysteresis, while the relationship between nighttime sap flow velocity and VPD showed counter-clockwise hysteresis. It was evident that PAR and VPD were the key factors impacting daytime sap flow velocity, while Cs and θ were the key factors impacting nighttime sap flow velocity. Furthermore, linear regression results showed that daytime sap flow had a significant positive effect on nighttime sap flow throughout the growing season (P < 0.05).

Habitat moisture is an important driver of patterns of sap flow and water balance in tropical montane cloud forest epiphytes.

Microclimate in the tropical montane cloud forest (TMCF) is variable on both spatial and temporal scales and can lead to large fluctuations in both leaf-level transpiration and whole plant water use. While variation in transpiration has been found in TMCFs, the influence of different microclimatic drivers on plant water relations in this ecosystem has been relatively understudied. Within the TMCF, epiphytes may be particularly affected by natural variation in microclimate due to their partial or complete disassociation from soil resources. In this study, we examined the effects of seasonal microclimate on whole plant water balance in epiphytes in both an observational and a manipulative experiment. We also evaluated the effects of different microclimatic drivers using three hierarchical linear (mixed) models. On average, 31% of total positive sap flow was recovered via foliar water uptake (FWU) over the course of the study. We found that precipitation was the greatest driver of foliar water uptake and nighttime sap flow in our study species and that both VPD and precipitation were important drivers to daytime sap flow. We also found that despite adaptations to withstand seasonal drought, an extended dry period caused severe desiccation in most plants despite a large reduction in leaf-level and whole plant transpiration. Our results indicate that the epiphytes studied rely on FWU to maintain positive water balance in the dry season and that increases in dry periods in the TMCF may be detrimental to these common members of the epiphyte community.

Seasonal variation of night-time sap flow of a young olive orchard: the unconsidered process for evapotranspiration estimations

Night-time sap flow (Sn), with transpiration as an important proportion of it at moderate to high vapor pressure deficits (VPD), is an important unconsidered factor that contributes significantly to total evapotranspiration (ET) of horticultural and fruit tree crops. This nocturnal process will be likely increased in a climate change scenario, with increases in night-time temperatures at higher rates compared to diurnal temperatures. The aim of this study was to characterise night-time water consumption over a commercial drip-irrigated young olive orchard (Olea europaea L. 'Arbequina') located in Pencahue valley, Maule Region, Chile (35°23-LS; 71°44-LW; 96 m a.s.l.) and its dynamics within the 2010/11 season. Four olive trees were selected for sap flow measurements using the Compensated Heat Pulse Velocity technique (CHPV). The canopy conductance (Gc) was calculated by inverting a modified Penman- Monteith equation. The aerodynamic conductance (ga) was calculated using an algorithm of the two-layer model proposed by Shuttleworth and Wallace (1985). An eddy covariance system was installed in the orchard to measure real ET. Results showed that Sn varied between 1.79 and 3.09 L tree-1 night-1 depending mainly of the atmospheric demand. The diurnal sap flow (Sd) measured was from 7.1 to 18.2 L tree-1 day-1. Parabolic shape curves described the relationship between Sn and Gc. Furthermore, the Sn/Sd ratio changed between 16 and 25% depending on the weather conditions, which it is not currently considered in ET models. It is clear that there is a nocturnal flow of water from soil to plant and water movement within the plant, but it is not yet clear the partition between the transpiration process and hydraulic redistribution. However, the former could be more likely due to the highly significant correlations found between VPD and Sn.

Tree water relations can trigger monoterpene emissions from Scots pine stems during spring recovery

Abstract. Tree canopies are known to emit large amounts of VOCs (volatile organic compounds) such as monoterpenes into the surrounding air. High VOC emission rates from boreal forests have been observed during the transition from winter to summer activity. The most important sources of these are considered to be the green foliage, understory vegetation and soil organisms, but emissions from the living stand woody compartments have so far not been quantified. We analyzed whether the non-foliar components could partially explain the springtime high emission rates. We measured the monoterpene emissions from Scots pine (Pinus sylvestris L.) stem and shoots during the dehardening phase of trees in field conditions in two consecutive springs. We observed a large, transient monoterpene burst from the stem, while the shoot monoterpene emissions remained low. The burst lasted about 12 h. Simultaneously, an unusual nighttime sap flow and a non-systematic diurnal pattern of tree diameter were detected. Hence, we suggest that the monoterpene burst was a consequence of the recovery of the stem from wintertime, and likely related to the refilling of embolized tracheids and/or phenological changes in the living cells of the stem. This indicates that the dominant processes and environmental drivers triggering the monoterpene emissions are different between the stem and the foliage.

The Sap Flow Dynamics and Response of Hedysarum scoparium to Environmental Factors in Semiarid Northwestern China.

Hedysarum scoparium is an important, fast-growing and drought-resistant shrub that has been extensively used for grassland restoration and preventing desertification in semiarid regions of northwestern China. The primary objective of this study was to investigate the diurnal and seasonal variations in stem sap flow (J s) and its relation to environmental factors. The stem heat balance method was applied to plants that were approximately 17 years old (with diameters of 25, 16, 13, and 9 mm at ground level and heights of 3.1, 1.8, 1.7 and 1.4 m) and growing under natural conditions. The vertical soil temperature profile (ST), soil surface heat flux (SoilG), volumetric soil moisture content (SWC) and meteorological variables such as solar radiation (R n), air temperature (T a), vapour pressure deficit (VPD), wind speed (W s) relative humidity (RH) and precipitation (P) were simultaneously measured at a meteorological station on site. Results indicated that J s varied regularly during the diurnal and seasonal term. The nocturnal J s was substantial, with a seasonal variation similar to the patterns of daytime J s. The magnitude of J s changed considerably between sunny and rainy days. Redundancy (RDA) and Kendall’s tau analysis suggested that daily J s in large plants was more sensitive to environmental factors, and the variation in daily J s during the growing season could be described by a multiple linear regression against environmental variables including T a, VPD, W s, RH, ST, and SoilG. While the nocturnal J s in smaller plants was more sensitive to meteorological factors. T a, VPD, and W s were significantly correlated with nighttime J s. The hourly nighttime sap flow rate of H. scoparium corresponded closely to T a and VPD following a non-linear pattern. The results of this study can be used to estimate the transpiration of H. scoparium.

Erratum to: 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 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.

Partitioning evapotranspiration in an intact forested watershed in southern China

AbstractAccurate estimates and partition of evapotranspiration (ET) are difficult but critical for understanding terrestrial ecosystem processes and primary productivity. A multi‐year, multi‐technique study including the catchment water balance (12 years), a semi‐empirical ET model (9 years), eddy covariance (9 years), canopy water balance (1 year) and sap flow (1 year) techniques was conducted to measure ET and its components within an intact forested watershed in the subtropical region of southern China. Over a 9‐year period, the estimates of annual ET using the catchment water balance, eddy covariance and semi‐empirical ET model techniques were in close agreement, averaging 809.9 ± 62.8, 803.8 ± 36.6 and 801.6 ± 46.9 mm per year, respectively, amounting to an average of approximately 50.2% of the mean annual precipitation. Qualitative similarities in seasonal and diurnal variation were observed between the sap flow and eddy covariance estimates of water flux. Sap flow estimates of transpiration were approximately 60.2% of the annual ET estimated with the eddy covariance technique. Interception evaporation was 31.7% of the total ET and was demonstrated to be a main contributor of total evaporation. Soil evaporation was calculated to be approximately 8.1% of the total annual ET and 4.7% of the annual precipitation. Furthermore, daytime transpiration exhibited a logarithmic increase with increases in the vapour pressure deficit (VPD) on daily scale in both the wet and dry seasons, and tended to level off when the VPD was &gt;1 kPa because of the stomatal regulation of transpiration. Correspondingly, nighttime sap flow amounted to an average of approximately 6% of the total daily sap flow, and no significant correlations with the VPD, air temperature or relative humidity were found. Essential differences among the mechanisms of evaporation and transpiration were demonstrated, suggesting that soil evaporation is primarily a climate‐driven process, with air temperature and wind speed as the predominant driving forces. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Seasonal dynamics of night-time stem water recharge of Schima superba and its relation to tree architecture and leaf biomass

Aims Our objectives are to study the effects of tree architecture and leaf biomass of Schima superba on seasonal variations of nocturnal stem water recharge and to elucidate the compensation mechanism for hydraulic limitation and response to the hydraulic limitation hypothesis proposed by Roan and Yoder. Methods Night-time sap flow of 15 trees in a S. superba plantation was measured with Granier-type sensors installed in the sapwood at a height of 1.3 m for calculating night-time stem water recharge. Single and multiple linear regressions were performed in data analysis. Important findings The ending time of nocturnal sap flow was not followed by photosynthetically active radiation (PAR) but was the active time, which was unrelated to diameter at breast height (DBH). Night-time stem water recharge (W) was strongly linearly related with DBH, canopy size, sapwood area and leaf biomass (r = 0.596 to 0.895), except in winter. The linear relationship between W and tree height was found only in spring. W was positively affected by canopy in spring and summer, but by DBH in autumn and by tree height in winter. The results show that tree architecture and leaf biomass had significant effects on W and the degree changed with seasons.

Night-time sap flow is parabolically linked to midday water potential for field-grown almond trees

To quantify night-time (Sn) and diurnal (Sd) tree water uptake, two sets of sap flow sensors (heat-pulse compensated) were installed per tree in the north-east and south-west sides of the trunk in three trees per treatment. There were two treatments: (1) control, irrigated with 100 % ETc (T100), and (2) deficit, irrigated at 60 % ETc (T60) with daily irrigations at the peak atmospheric demand (December-January). Normalised Sn by trees was in the range of 15-25 % throughout the season, compared to normalised Sd, for T100 and T60, respectively. Furthermore, Sn was parabolically correlated to plant water status from the previous day, measured as midday stem water potential. We also found strong correlations between Sn and noctur- nal vapour pressure deficit for T100 and T60, indicating that nocturnal transpiration was significant for both treatments. Differences in Sn were observed for the NE and SW sensors for T60, being significantly less for the NE side (sunny side) compared to the SW side (more shaded). No differences were observed for T100 regarding probe positioning.

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

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. 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. 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 (E-L) by transpiration of canopy leaves was only similar to 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 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.

Effect of foehn on nighttime sap flow of soybean

Foehn winds cause damage known as white head injury to paddy rice; and this damage is more severe at night than during the day. However, little has been reported about the nighttime effects of the foehn on other crops. We investigated the foehn's effects on soybean plants, and compared day- and nighttime responses. We measured sap flow, which is assumed to equal transpiration, by the heat balance method. The gauge conductance of sap flow sensors during the nighttime foehn was calibrated against values obtained in the pre- or post-foehn period. Stomatal conductance was calculated using sap flow data and meteorological data. At night, sap flow during foehns was greater than that in non-foehn conditions. During the foehns in 2007 and 2008, average nighttime sap flows were 4.3 g h-1 (2007) and 4.0 g h-1 (2008). The cumulative sap flow through the night during the foehns in 2007 and 2008 were 10.5% and 7.0% of their respective daytime values. In 2007, sap flow increased linearly with increasing vapor pressure deficit (Da) when Da exceeded 0.7 kPa. In 2008, however, sap flow increased rapidly when Da exceeded 0.8 kPa. These different responses of sap flow to Da in 2007 and 2008 were attributed to differences in regulation of stomatal conductance, which was low in 2007 but varied markedly in 2008. The reason for these differences may be the different soil moisture conditions in 2007 and 2008.

TDP MEASUREMENTS TO DETERMINE NIGHTTIME SAP FLOW AND THE IMPACT OF FOG IN TREE TRANSPIRATION OF A LAURISILVA FOREST

The evergreen ‘laurisilva’ forests of the Canary Islands (Spain) are frequently immersed in fog. However the role of fog precipitation remains yet unanswered, despite the belief that fog is important for the survival and distribution of endemic ‘laurisilva’ tree species. Granier’s sap flow measurements carried out in Erica arborea L. and Myrica faya Ait. trees of the Garajonay National Park (La Gomera), during a ten month period, show that diurnal tree transpiration is greatly reduced, down to nighttime values, during fog occurrence as compared to fog-free periods. Additionally, nighttime sap flow was detected in laurisilva tree species with an ad hoc method which combines evapotranspiration and Granier’s Thermal Dissipation Probes (TDP) measurements. To our knowledge this is the first time that a study shows under natural field conditions and for a long measuring period the effect of fog on whole-tree transpiration using sap flow techniques.

Effect of Foehn on Night-time Sap Flow of Soybean

Effect of Foehn on Night-time Sap Flow of Soybean

An alternative method to estimate zero flow temperature differences for Granier's thermal dissipation technique

Calibration of the Granier thermal dissipation technique for measuring stem sap flow in trees requires determination of the temperature difference (DeltaT) between a heated and an unheated probe when sap flow is zero (DeltaT(max)). Classically, DeltaT(max) has been estimated from the maximum predawn DeltaT, assuming that sap flow is negligible at nighttime. However, because sap flow may continue during the night, the maximum predawn DeltaT value may underestimate the true DeltaT(max). No alternative method has yet been proposed to estimate DeltaT(max) when sap flow is non-zero at night. A sensitivity analysis is presented showing that errors in DeltaT(max) 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 DeltaT(max). By rearranging Granier's original formula, an optimization method to compute DeltaT(max) from simultaneous measurements of diurnal DeltaT 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 DeltaT(max) values obtained by the proposed method may be similar in some instances to the DeltaT(max) 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 Cermák et al. (1973) heat balance method and the T-max heat pulse system of Green et al. (2003).

Water relations of baobab trees (Adansonia spp. L.) during the rainy season: does stem water buffer daily water deficits?

Baobab trees are often cited in the literature as water-storing trees, yet few studies have examined this assumption. We assessed the role of stored water in buffering daily water deficits in two species of baobabs (Adansonia rubrostipa Jum. and H. Perrier and Adansonia za Baill.) in a tropical dry forest in Madagascar. We found no lag in the daily onset of sap flow between the base and the crown of the tree. Some night-time sap flow occurred, but this was more consistent with a pattern of seasonal stem water replenishment than with diurnal usage. Intrinsic capacitance of both leaf and stem tissue (0.07-0.08 and 1.1-1.43 MPa(-1), respectively) was high, yet the amount of water that could be withdrawn before turgor loss was small because midday leaf and stem water potentials (WPs) were near the turgor-loss points. Stomatal conductance was high in the daytime but then declined rapidly, suggesting an embolism-avoidance strategy. Although the xylem of distal branches was relatively vulnerable to cavitation (P50: 1.1-1.7 MPa), tight stomatal control and minimum WPs near--1.0 MPa maintained native embolism levels at 30-65%. Stem morphology and anatomy restrict water movement between storage tissues and the conductive pathway, making stored-water usage more appropriate to longer-term water deficits than as a buffer against daily water deficits.

Diurnal and seasonal variability in the radial distribution of sap flow: predicting total stem flow in Pinus taeda trees

We monitored the radial distribution of sap flux density (v; g H2O m(-2) s(-1)) in the sapwood of six plantation-grown Pinus taeda L. trees during wet and dry soil periods. Mean basal diameter of the 32-year-old trees was 33.3 cm. For all trees, the radial distribution of sap flow in the base of the stem (i.e., radial profile) was Gaussian in shape. Sap flow occurred maximally in the outer 4 cm of sapwood, comprising 50-60% of total stem flow (F), and decreased toward the center, with the innermost 4 cm of sapwood (11-15 cm) comprising less than 10% of F. The percent of flow occurring in the outer 4 cm of sapwood was stable with time (average CV < 10%); however, the percentage of flow occurring in the remaining sapwood was more variable over time (average CV > 40%). Diurnally, the radial profile changed predictably with time and with total stem flow. Seasonally, the radial profile became less steep as the soil water content (theta) declined from 0.38 to 0.21. Throughout the season, daytime sap flow also decreased as theta decreased; however, nighttime sap flow (an estimate of stored water use) remained relatively constant. As a result, the percentage of stored water use increased as theta declined. Time series analysis of 15-min values of F, theta, photosynthetically active radiation (PAR) and vapor pressure deficit (D) showed that F lagged behind D by 0-15 min and behind PAR by 15-30 min. Diurnally, the relationship between F and D was much stronger than the relationship between F and PAR, whereas no relationship was found between F and theta. An autoregressive moving average (ARIMA) model estimated that 97% of the variability in F could be predicted by D alone. Although total sap flow in all trees responded similarly to D, we show that the radial distribution of sap flow comprising total flow could change temporally, both on daily and seasonal scales.

Sources of error in stem heat balance sap flow measurements

Whilst the importance of recognising and reducing the sources of error in sap flow measurements has been acknowledged, relevant information is widely dispersed in the experimental literature. This paper presents a critical review of the theory and reported sources of error for the constant power stem heat balance (SHB) method. The important influence of stem diameter and sap flow rate in determining the magnitude of errors is highlighted. The magnitude and relative importance of different errors were investigated through an analysis of data obtained from a large field experiment in Nigeria. Sap flow was measured in stems of 6-year-old trees of Acacia nilotica (subsp. adstringens) and Prosopis juliflora between July 1994 and January 1995. Commercially available constant power SHB gauges were used on different sized stems (19–50 mm diameter) under a wide range of sap flow and environmental conditions. The analysis demonstrates how stem size and night-time sap flow interact to cause error in determining the sheath conductance, the manufacturer's apparent understatement of the time constant, especially for large gauges, and how stem size, heat storage and patterns of ambient temperature variation combine to determine diurnal patterns of variation in measurement error. Results highlight the importance of considering a priori, for any given set of experimental conditions, the likelihood that underlying theoretical assumptions may be violated. Precautions to minimise potentially important sources of error are described for different sap flow rates and environmental conditions, and with respect to different experimental objectives.

Transpiration of well-watered and water-stressed Asian pear trees as determined by lysimetry, heat-pulse, and estimated by a Penman-Monteith model

Early season transpiration of 5-year-old Pyrus serotina trees receiving different amounts of irrigation was measured using lysimetry and the compensation heat-pulse technique. Withholding water resulted in a rapid decline in soil-water content within the lysimeters, and caused a 50% decrease in stomatal conductance and a 25–35% reduction in tree water use. Weekly transpiration rates, as determined by integrated heat-pulse measurements, were comparable with measurements by lysimetry for a well-watered tree, but overestimated water use by a water-stressed tree by 22%. The reason for this discrepancy was thought to be diurnal changes in sapwood water content, leading to an overestimation by the heat-pulse technique. Stomatal conductance and meteorological data obtained over 2 days were used to parameterize a Penman-Monteith model to estimate tree transpiration. On a daily basis, these estimates were within 8% of the sap flow determined by the heat-pulse technique. Substantial sap flow occurred in the stressed tree at night even when the stomata were closed. Obviously, this night-time sap flow could not be predicted by the Penman-Monteith model. Instead, it appeared to be a capacitance effect owing to the nocturnal refilling of the tree's internal store of water.