Improving sap flux density measurements by correctly determining thermal diffusivity, differentiating between bound and unbound water

Testing of a custom, portable drill press to minimize probe misalignment in sap flow sensors

The accurate estimation of plant transpiration is critical to the fields of hydrology, plant physiology and ecology. Among the various methods of measuring transpiration in the field, the sap flow methods based on head pulses offers a cost-effective and energy-efficient option to directly measure the plant-level movement of water through the hydraulically active tissue. While authors have identified several possible sources of error in these measurements, one of the most common sources is misalignment of the sap flow probes due to user error. Though the effects of probe misalignment are well documented, no device or technique has been universally adopted to ensure the proper installation of sap flow probes. In this paper we compare the magnitude of misalignment errors among a 5 mm thick drilling template (DT), a 10 mm thick DT, and a custom designed, field-portable drill press. The different techniques were evaluated in the laboratory using a 7.5 cm wood block and in the field, comparing differences in measured sap flow. Based on analysis of holes drilled in the wood block, we found that the portable drill press was most effective in assuring that drill holes remained parallel, even at 7.5 cm depth. In field installations, nearly 50% of holes drilled with a 5 mm template needed to be redrilled while none needed to be when drilled with the drill press. Widespread use of a portable drill press when implementing the heat pulse method would minimize alignment uncertainty and allow a clearer understanding of other sources of uncertainty due to variability in tree species, age, or external drivers or transpiration.

Sap flow monitoring of two Australian native tree species in a suburban setting: Implications for tree selection and management

Sap flow, the transport of fluid in the water-conducting xylem tissues of plants, is commonly measured in studies of plant-water relationships by the heat pulse velocity method. Publications have been rare of long-term sap flow measurements for individual trees in a suburban environment. Plant-water relations in urban settings are essential for promoting urban greening where there is a perceived danger to infrastructure and buildings from planting trees in streets on clay sites. The function of residential houses built on reactive clays can be significantly impaired and walls of buildings cracked if considerable amounts of water are extracted from the soil by the root system of a tree or a group of trees in close proximity, leading to localised soil shrinkage settlement. This part of the wider study aimed to monitor sap flow of eight individual Australian native trees from two species using the heat ratio method (HRM) in the field over 12 months. The analysis of monthly sap flow volume showed a similar pattern for all monitored trees, although daily water demand varied substantially. Methods for estimating tree leaf surface area, crown shape and crown volume were investigated and the equation for calculating thermal diffusivity (k) and sap flow velocity on the basis of the HRM was reviewed. It has been proposed that k may vary substantially depending on how thermal conductivity (K) is estimated, which could lead to significant discrepancies for estimations of plant transpiration. Two K models (K_Hog and K_Van) were investigated and it was found that the impact on mean daily sap volume was negligible for the trees in this study.

A double-ratio method to measure fast, slow and reverse sap flows

Sap velocity measurements are useful in fields ranging from plant water relations to hydrology at a variety of scales. Techniques based on pulses of heat are among the most common methods to measure sap velocity, but most lack ability to measure velocities across a wide range, including very high, very low and negative velocities (reverse flow). We propose a new method, the double-ratio method (DRM), which is robust across an unprecedented range of sap velocities and provides real-time estimates of the thermal diffusivity of wood. The DRM employs one temperature sensor upstream (proximal) and two sensors downstream (distal) to the source of heat. This facilitates several theoretical, heat-based approaches to quantifying sap velocity. We tested the DRM using whole-tree lysimetry in Eucalyptus cypellocarpa L.A.S. Johnson and found strong agreement across a wide range of velocities.

The importance of conduction versus convection in heat pulse sap flow methods

Heat pulse methods are a popular approach for estimating sap flow and transpiration. Yet, many methods are unable to resolve the entire heat velocity measurement range observable in plants. Specifically, the Heat Ratio (HRM) and Tmax heat pulse methods can only resolve slow and fast velocities, respectively. The Dual Method Approach (DMA) combines optimal data from HRM and Tmax to output the entire range of heat velocity. However, the transition between slow and fast methods in the DMA currently does not have a theoretical solution. A re-consideration of the conduction/convection equation demonstrated that the HRM equation is equivalent to the Péclet equation which is the ratio of conduction to convection. This study tested the hypothesis that the transition between slow and fast methods occurs when conduction/convection, or the Péclet number, equals one, and the DMA would be improved via the inclusion of this transition value. Sap flux density was estimated via the HRM, Tmax and DMA methods and compared with gravimetric sap flux density measured via a water pressure system on 113 stems from 15 woody angiosperm species. When the Péclet number ≤ 1, the HRM yielded accurate results and the Tmax was out of range. When the Péclet number > 1, the HRM reached a maximum heat velocity at approximately 15 cm hr -1 and was no longer accurate, whereas the Tmax yielded accurate results. The DMA was able to output accurate data for the entire measurement range observed in this study. The linear regression analysis with gravimetric sap flux showed an r2 of 0.541 for HRM, 0.879 for Tmax and 0.940 for DMA. With the inclusion of the Péclet equation, the DMA resolved the entire heat velocity measurement range observed across 15 taxonomically diverse woody species. Consequently, the HRM and Tmax are redundant sap flow methods and have been superseded by the DMA.

Bamboo Water Transport Assessed with Deuterium Tracing

Bamboo water transport comprises the pathway rhizomes-culms-leaves as well as transfer among culms via connected rhizomes. We assessed bamboo water transport in three big clumpy bamboo species by deuterium tracing. The tracer was injected into the base of established culms, and water samples were collected from leaves of the labeled culms and from neighboring culms. From the base of labeled culms to their leaves, the average tracer arrival time across species was 1.2 days, maximum tracer concentration was reached after 1.8 days, and the tracer residence time was 5.6 days. Sap velocities were high (13.9 m d−1). Daily culm water use rates estimated by the tracer method versus rates measured by a calibrated sap flux method were highly correlated (R2 = 0.94), but the tracer estimates were about 70% higher. Elevated deuterium concentrations in studied neighbor culms point to deuterium transfer among culms, which may explain the difference in culm water use estimates. We found no differences in deuterium concentrations between neighbor-established and neighbor freshly sprouted culms of a given species. In two species, elevated concentrations in both neighbor-established and neighbor freshly sprouted culms were observed over an extended period. An applied mixing model suggests that five neighbor culms received labeled water. In contrast, for the third species, elevated concentrations in neighbor culms were only observed at the earliest sampling date after labeling. This could indicate that there was only short-term transfer and that the tracer was distributed more widely across the rhizome network. In conclusion, our deuterium tracing experiments point to water transfer among culms, but with species-specific differences.

The Dual Method Approach (DMA) Resolves Measurement Range Limitations of Heat Pulse Velocity Sap Flow Sensors

Sap flow, the movement of fluid in the xylem of plants, is commonly measured with the heat pulse velocity (Vh) family of methods. The observable range of Vh in plants is ~−10 to ~+270 cm/h. However, most Vh methods only measure a limited portion of this range, which restricts their utility. Previous research attempted to extend the range of Vh methods, yet these approaches were analytically intensive or impractical to implement. The Dual Method Approach (DMA), which is derived from the optimal measurement ranges of two Vh methods, the Tmax and the heat ratio method (HRM), also known as the “slow rates of flow” method (SRFM), is proposed to measure the full range of sap flow observable in plants. The DMA adopts an algorithm to dynamically choose the optimal Vh measurement via the Tmax or HRM/SRFM. The DMA was tested by measuring sap flux density (Js) on Tecoma capensis (Thunb.) Lindl., stems and comparing the results against Js measured gravimetrically. The DMA successfully measured the entire range of Vh observed in the experiment from 0.020 to 168.578 cm/h, whereas the HRM/SRFM range was between 0.020 and 45.063 cm/h, and the Tmax range was between 2.049 cm/h and 168.578 cm/h. A linear regression of DMA Js against gravimetric Js found an R2 of 0.918 and error of 1.2%, whereas the HRM had an R2 of 0.458 and an error of 49.1%, and the Tmax had an R2 of 0.826 and an error of 0.5%. Different methods to calculate sapwood thermal diffusivity (k) were also compared with the kVand method showing better accuracy. This study demonstrates that the DMA can measure the entire range of Vh in plants and improve the accuracy of sap flow measurements.

Water relations in silver birch during springtime: How is sap pressurised?

Positive sap pressures are produced in the xylem of birch trees in boreal conditions during the time between the thawing of the soil and bud break. During this period, xylem embolisms accumulated during wintertime are refilled with water. The mechanism for xylem sap pressurization and its environmental drivers are not well known. We measured xylem sap flow, xylem sap pressure, xylem sap osmotic concentration, xylem and whole stem diameter changes, and stem and root non-structural carbohydrate concentrations, along with meteorological conditions at two sites in Finland during and after the sap pressurisation period. The diurnal dynamics of xylem sap pressure and sap flow during the sap pressurisation period varied, but were more often opposite to the diurnal pattern after bud burst, i.e. sap pressure increased and sap flow rate mostly decreased when temperature increased. Net conversion of soluble sugars to starch in the stem and roots occurred during the sap pressurisation period. Xylem sap osmotic pressure was small in comparison to total sap pressure, and it did not follow changes in environmental conditions or tree water relations. Based on these findings, we suggest that xylem sap pressurisation and embolism refilling occur gradually over a few weeks through water transfer from parenchyma cells to xylem vessels during daytime, and then the parenchyma are refilled mostly during nighttime by water uptake from soil. Possible drivers for water transfer from parenchyma cells to vessels are discussed. Also the functioning of thermal dissipation probes in conditions of changing stem water content is discussed.

Xylem hydraulic safety and construction costs determine tropical tree growth

Faster growth in tropical trees is usually associated with higher mortality rates, but the mechanisms underlying this relationship are poorly understood. In this study, we investigate how tree growth patterns are linked with environmental conditions and hydraulic traits, by monitoring the cambial growth of 9 tropical cloud forest tree species coupled with numerical simulations using an optimization model. We find that fast-growing trees have lower xylem safety margins than slow-growing trees and this pattern is not necessarily linked to differences in stomatal behaviour or environmental conditions when growth occurs. Instead, fast-growing trees have xylem vessels that are more vulnerable to cavitation and lower density wood. We propose the growth - xylem vulnerability trade-off represents a wood hydraulic economics spectrum similar to the classic leaf economic spectrum, and show through numerical simulations that this trade-off can emerge from the coordination between growth rates, wood density, and xylem vulnerability to cavitation. Our results suggest that vulnerability to hydraulic failure might be related with the growth-mortality trade-off in tropical trees, determining important life history differences. These findings are important in furthering our understanding of xylem hydraulic functioning and its implications on plant carbon economy.

Daytime depression in temperature-normalised stem CO2 efflux in young poplar trees is dominated by low turgor pressure rather than by internal transport of respired CO2

Daytime decreases in temperature-normalised stem CO₂ efflux (E_{A_D} ) are commonly ascribed to internal transport of respired CO₂ (F_T ) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure and F_T , sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO₂ fluxes was applied to estimate F_T . Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of F_T , were reliable predictors of E_{A_D} . After tree manipulation, only turgor pressure was a robust predictor of E_{A_D} . Moreover, F_T accounted for < 15% of E_{A_D} . Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of E_{A_D} in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models.

A single-probe heat pulse method for estimating sap velocity in trees

Available sap flow methods are still far from being simple, cheap and reliable enough to be used beyond very specific research purposes. This study presents and tests a new single-probe heat pulse (SPHP) method for monitoring sap velocity in trees using a single-probe sensor, rather than the multi-probe arrangements used up to now. Based on the fundamental conduction-convection principles of heat transport in sapwood, convective velocity (V_h ) is estimated from the temperature increase in the heater after the application of a heat pulse (ΔT). The method was validated against measurements performed with the compensation heat pulse (CHP) technique in field trees of six different species. To do so, a dedicated three-probe sensor capable of simultaneously applying both methods was produced and used. Experimental measurements in the six species showed an excellent agreement between SPHP and CHP outputs for moderate to high flow rates, confirming the applicability of the method. In relation to other sap flow methods, SPHP presents several significant advantages: it requires low power inputs, it uses technically simpler and potentially cheaper instrumentation, the physical damage to the tree is minimal and artefacts caused by incorrect probe spacing and alignment are removed.

Influence of stem temperature changes on heat pulse sap flux density measurements

While natural spatial temperature gradients between measurement needles have been thoroughly investigated for continuous heat-based sap flow methods, little attention has been given to how natural changes in stem temperature impact heat pulse-based methods through temporal rather than spatial effects. By modelling the theoretical equation for both an ideal instantaneous pulse and a step pulse and applying a finite element model which included actual needle dimensions and wound effects, the influence of a varying stem temperature on heat pulse-based methods was investigated. It was shown that the heat ratio (HR) method was influenced, while for the compensation heat pulse and Tmax methods changes in stem temperatures of up to 0.002 °C s(-1) did not lead to significantly different results. For the HR method, rising stem temperatures during measurements led to lower heat pulse velocity values, while decreasing stem temperatures led to both higher and lower heat pulse velocities, and to imaginary results for high flows. These errors of up to 40% can easily be prevented by including a temperature correction in the data analysis procedure, calculating the slope of the natural temperature change based on the measured temperatures before application of the heat pulse. Results of a greenhouse and outdoor experiment on Pinus pinea L. show the influence of this correction on low and average sap flux densities.

Environmental controls in the water use patterns of a tropical cloud forest tree species, Drimys brasiliensis (Winteraceae)

Trees from tropical montane cloud forest (TMCF) display very dynamic patterns of water use. They are capable of downwards water transport towards the soil during leaf-wetting events, likely a consequence of foliar water uptake (FWU), as well as high rates of night-time transpiration (Enight) during drier nights. These two processes might represent important sources of water losses and gains to the plant, but little is known about the environmental factors controlling these water fluxes. We evaluated how contrasting atmospheric and soil water conditions control diurnal, nocturnal and seasonal dynamics of sap flow in Drimys brasiliensis (Miers), a common Neotropical cloud forest species. We monitored the seasonal variation of soil water content, micrometeorological conditions and sap flow of D. brasiliensis trees in the field during wet and dry seasons. We also conducted a greenhouse experiment exposing D. brasiliensis saplings under contrasting soil water conditions to deuterium-labelled fog water. We found that during the night D. brasiliensis possesses heightened stomatal sensitivity to soil drought and vapour pressure deficit, which reduces night-time water loss. Leaf-wetting events had a strong suppressive effect on tree transpiration (E). Foliar water uptake increased in magnitude with drier soil and during longer leaf-wetting events. The difference between diurnal and nocturnal stomatal behaviour in D. brasiliensis could be attributed to an optimization of carbon gain when leaves are dry, as well as minimization of nocturnal water loss. The leaf-wetting events on the other hand seem important to D. brasiliensis water balance, especially during soil droughts, both by suppressing tree transpiration (E) and as a small additional water supply through FWU. Our results suggest that decreases in leaf-wetting events in TMCF might increase D. brasiliensis water loss and decrease its water gains, which could compromise its ecophysiological performance and survival during dry periods.

Evaluating the potential of a novel dual heat-pulse sensor to measure volumetric water use in grapevines under a range of flow conditions

The aim of this study was to validate a novel, dual sap-flow sensor that combines two heat-pulse techniques in a single set of sensor probes to measure volumetric water use over the full range of sap flows found in grapevines. The heat ratio method (HRM), which works well at measuring low and reverse flows, was combined with the compensation heat-pulse method (CHPM) that captures moderate to high flows. Sap-flow measurements were performed on Vitis vinifera L. (cvv. Thompson seedless, Chardonnay and Cabernet Sauvignon) grapevines growing in a greenhouse and in three different vineyards, one of which contained a field weighing lysimeter. The combined heat-pulse techniques closely tracked diurnal grapevine water use determined through lysimetry in two growing seasons, and this was true even at very high flow rates (>6L vine-1h-1 for Thompson seedless vines in the weighing lysimeter). Measurements made with the HRM technique under low flow conditions were highly correlated (R2 ~ 0.90) with those calculated using the compensated average gradient method that is used to resolve low flow with the CHPM method. Volumetric water use determined with the dual heat-pulse sensors was highly correlated with hourly lysimeter water use in both years (R2=0.92 and 0.94 in 2008 and 2009 respectively), but the nature of the relationship was inconsistent among replicate sensors. Similar results were obtained when comparing grapevine water use determined from sap-flow sensors to miniaturised weighing lysimetry of 2-year-old potted vines and to meteorological estimates for field-grown vines in two additional vineyards. The robust nature of all of the correlations demonstrates that the dual heat-pulse sensors can be used to effectively track relative changes in plant water use, but variability of flow around stems makes it difficult to accurately convert to sap-flow volumes.

Sap-flux density measurement methods: working principles and applicability

Sap-flow measurements have become increasingly important in plant science. Since the early experiments with dyes, many methods have been developed. Most of these are based on the application of heat in the sapwood which is transported by the moving sap. By measuring changes in the temperature field around the heater, sap flow can be derived. Although these methods all have the same basis, their working principles vary widely. A first distinction can be made between those measuring the sap-flow rate (gh-1) such as the stem heat balance and trunk sector heat balance method and those measuring sap-flux density (cm3cm-2h-1). Within the latter, the thermal dissipation and heat field deformation methods are based on continuous heating, whereas the compensation heat pulse velocity, Tmax, heat ratio, calibrated average gradient and Sapflow+ methods are based on the application of heat pulses. Each of these methods has its advantages and limitations. Although the sap-flow rate methods have been adequately described in previous reviews, recent developments in sap-flux density methods prompted a synthesis of the existing but scattered literature. This paper reviews sap-flux density methods to enable users to make a well founded choice, whether for practical applications or fundamental research questions, and to encourage further improvement in sap-flux density measurement techniques.

Using electrical resistivity tomography to differentiate sapwood from heartwood: application to conifers

Estimating sapwood area is one of the main sources of error when upscaling point scale sap flow measurements to whole-tree water use. In this study, the potential use of electrical resistivity tomography (ERT) to determine the sapwood-heartwood (SW-HW) boundary is investigated for Pinus elliottii Engelm var. elliottii × Pinus caribaea Morelet var. hondurensis growing in a subtropical climate. Specifically, this study investigates: (i) how electrical resistivity is correlated to either wood moisture content, or electrolyte concentration, or both, and (ii) how the SW-HW boundary is defined in terms of electrical resistivity. Tree cross-sections at breast height are analysed using ERT before being felled and the cross-section surface sampled for analysis of major electrolyte concentrations, wood moisture content and density. Electrical resistivity tomography results show patterns with high resistivities occurring in the inner part of the cross-section, with much lower values towards the outside. The high-resistivity areas were generally smaller than the low-resistivity areas. A comparison between ERT and actual SW area measured after felling shows a slope of the linear regression close to unity (=0.96) with a large spread of values (R(2) = 0.56) mostly due to uncertainties in ERT. Electrolyte concentrations along sampled radial transects (cardinal directions) generally showed no trend from the centre of the tree to the bark. Wood moisture content and density show comparable trends that could explain the resistivity patterns. While this study indicates the potential for application of ERT for estimating SW area, it shows that there remains a need for refinement in locating the SW-HW boundary (e.g., by improvement of the inversion method, or perhaps electrode density) in order to increase the robustness of the method.

Using the compensated heat pulse method to monitor trends in stem water content in standing trees

Studying the dynamics of stem water content (θ) in living trees has an outstanding physiological interest but all the available techniques to measure θ exhibit major drawbacks. In this work, we present a new methodology to estimate variations in θ along with sap velocity using the compensated heat pulse (CHP) technique. One lab experiment was performed on several wooden blocks obtained from three different tree species. Samples were slowly dried and their moisture loss was monitored by both gravimetric approaches and time-domain reflectometry (TDR) or CHP probes in order to contrast the validity of our methodology (volumetric specific heat (VSH)-CHP) over a range of water contents. In addition, a field experiment was conducted to monitor θ fluctuations in standing olive trees (Olea europaea L. cv. 'Arbequina') growing under three different irrigation regimes. In the lab test, the actual θ values deduced gravimetrically differed from the estimates yielded by the VSH-CHP method. However, it could successfully track relative changes in the water stored for the range of θ expected in living wood. Furthermore, the field experiment showed a seasonal change in θ, which was similar in shape and magnitude to those reported in the literature for olive and other Mediterranean tree species. On the other hand, differences in the seasonal patterns of θ between irrigation treatments strongly corresponded with those of sap flow and some leaf water potential measurements. The results of this work suggest that the CHP technique could be employed to monitor the dynamics of both θ and sap flow simultaneously in standing trees and evidence that seasonal changes in θ might be used as a long-term water status indicator.

Sapflow+: a four-needle heat-pulse sap flow sensor enabling nonempirical sap flux density and water content measurements

• To our knowledge, to date, no nonempirical method exists to measure reverse, low or high sap flux density. Moreover, existing sap flow methods require destructive wood core measurements to determine sapwood water content, necessary to convert heat velocity to sap flux density, not only damaging the tree, but also neglecting seasonal variability in sapwood water content. • Here, we present a nonempirical heat-pulse-based method and coupled sensor which measure temperature changes around a linear heater in both axial and tangential directions after application of a heat pulse. By fitting the correct heat conduction-convection equation to the measured temperature profiles, the heat velocity and water content of the sapwood can be determined. • An identifiability analysis and validation tests on artificial and real stem segments of European beech (Fagus sylvatica L.) confirm the applicability of the method, leading to accurate determinations of heat velocity, water content and hence sap flux density. • The proposed method enables sap flux density measurements to be made across the entire natural occurring sap flux density range of woody plants. Moreover, the water content during low flows can be determined accurately, enabling a correct conversion from heat velocity to sap flux density without destructive core measurements.