Heat Field Deformation Method Research Articles

Sap Flow Analyzer: A tool to standardize sap flow estimation and scaling to whole‐tree water use using the HFD method

Abstract Sap flow measurements are fundamental to understanding water use in trees and could aid in predicting climate change effects on forest function. Deriving knowledge from such measurements requires empirical calibrations and upscaling methods to translate thermometric recordings to tree water use. Here, we developed a user‐friendly open‐source application, the Sap Flow Analyzer (SFA), which estimates sap flow rates and tree water use from the heat field deformation (HFD) instruments. The SFA incorporates four key features to ensure maximum accuracy and reproducibility of sap flow estimates: diagnosis diagrams to assess data patterns visually, regression models implemented to increase accuracy when estimating K (the main HFD parameter), three approaches to upscale sap flow rates to whole‐tree water use and visualization of the input parameters' uncertainty. Thirteen participants were given three raw datasets and assigned data processing tasks using the SFA user guide, from estimating sapwood depth to scaling sap flow rates to whole‐tree water use to assess the reproducibility and applicability of the SFA. Participants' results were reasonably consistent and independent of their background in using the SFA, R, or HFD method. The results showed lower variability for high flow rates (SD: mean 1% vs. 10%). K estimates and sapwood depth differentiation were the primary sources of variability, which in turn was mainly caused by the user's chosen scaling method. The SFA provides an easy way to visualize and process sap flow and tree water use data from HFD measurements. It is the first free and open software tool for HFD users. The ability to trace analysis steps ensures reproducibility, increasing transparency and consistency in data processing. Developing tools such as the SFA and masked trials are essential for more precise workflows and improved quality and comparability of HFD sap flow datasets.

Repeated summer drought changes the radial xylem sap flow profile in mature Norway spruce but not in European beech

Water consumption of trees is one of the most important processes connected to their survival under ongoing climate change and extreme events such as drought. Radial profiles of xylem sap flow density are an integral component to quantify the water transport for the level of an individual tree and that of ecosystems. However, knowledge of such radial profiles, in particular under stress, is very scarce. Here we show the radial profile of the xylem sap flow density in mature European beech (Fagus sylvatica L) and Norway spruce (Picea abies (L) Karst.) under repeated summer drought induced by throughfall exclusion (TE) and subsequent recovery compared to untreated control trees (CO). We measured xylem sap flow density (udaily in L dm−2 d−1) down to 8 cm sapwood depth at breast height using two different approaches, a thermal dissipation system and the heat field deformation method. In beech, repeated throughfall exclusion did not affect the radial xylem sap flow profile. However, in spruce, udaily was strongly reduced across the profile under repeated drought, changing the profile from a linear to a logarithmic regression. Even two years after drought release, the xylem sap flow profile did not fully recover in TE spruce. The reduction of udaily along the radial profile was accompanied by a reduction of the leaf area in TE spruce by c. 50%, while sapwood depth remained constant. The reduction of the xylem sap flow density along the profile reduced the calculated water consumption of TE spruce trees by more than 33% compared to CO trees, also after drought release. The impact of stressors such as repeated drought on the xylem sap flow density across the radial profile and its consequences for trees’ and stands’ water consumption needs to be addressed in more detail to minimize uncertainties in quantifying ecosystem water cycles.

Water transport secrets of the dragon’s blood trees revealed through sap flow measurements following partial stem incision

Dragon’s blood trees have long been prized for their red resin, and are consequently subjected to traditional practices for its harvesting which involves inflicting numerous, deep stem wounds. Nevertheless, these iconic monocots have continued to survive such drastic treatments. Partial stem incision (PSI) treatment effectively imitates stem wounding, which when combined with sap flow measurements monitored in a close distance of few centimeters above PSI, may significantly improve our understanding of water transport efficiency after stem damage in monocots. As it is impossible to conduct these experiments on protected adult Dracaena species native to remote sites, we demonstrated this approach on young Dracaena draco L. and Dracaena cinnabari Balfour f. plants ex-situ. The goal of this study was to simulate the effects of serious stem wounding on water transport of these woody monocots by applying PSI under permanent sap flow monitoring by the heat field deformation (HFD) method using multi-point sensors. This allowed us to simultaneously monitor sap flow in both wounded and intact stem parts. It was observed that PSI applied in stems up to one-half of the total stem diameter did not reduce sap flow in D. species in any of the monitored stem part indicating efficient bypassing interrupted axial xylem transport by lateral water movement. The dense three dimensional network of interconnected vascular bundles and abundant ground tissue with large simple pits could play an important role in such high transport effectivity of Dracaena species. Synchronous sap flow and nuclear magnetic resonance measurements would be especially useful for the revealing of water flow complexity of Dracaena species in future.

Sap Flow Measurements in a Socotra Dragon’s Blood Tree (Dracaena cinnabari) in its Area of Origin

For the first time, in situ field measurements of sap flow were conducted in adult Dracaena cinnabari plant native to the arid tropical climate of Socotra Island. The heat field deformation (HFD) method was applied using both single and multi-point sensors to study azimuthal and radial sap flow variability in stem, roots and first-order branches over two weeks during a winter monsoon. The main aim of this work was to monitor sap flow in adult D. cinnabari in-situ to better understand its physiological adaptation to extreme arid environments. The second aim was to compare our results with earlier sap flow measurements in adult D. draco uing the same HFD method. The last question we wanted to answer was comparison of sap flow measurements in both, young and adult Dracaena species. We found that sap flow magnitude is low and of a similar range in all observed D. species. High sap flow variability was recorded in different parts of adult D. cinnabari plant which changed throughout the day responding to interplay between intrinsic and extrinsic water potential gradients induced by sunlight. Maximum sap flow levels had variable pattern around stem in response to sun exposure, similarly as it was observed in adult D. draco plant. Sensors installed tangentially in stem xylem showed that water transport in adult D. cinnabari may move in lateral direction. This work also presents several methodological aspects detected from earlier observations of dicots which proved to be more pronounced in adult D. species. These methodologies relate to interpreting negative sap flow rates in conjunction with established axial flow reversal during hydraulic redistribution usually occurring under low evaporative demands and dry soil. Conversely, flow reversal during the day under high evaporative demands and wet soil may designate lateral water movement induced by internal water redistribution.

Revisiting the Heat Field Deformation (HFD) method for measuring sap flow

Abstract: The Heat Field Deformation (HFD) technique is a thermodynamic method for measuring sap flow. Based on continuous heating the HFD method allows for high time resolution measurements which are highly important when studying plant responses to abrupt environmental changes. This work provides a succinct review of previously described features of the HFD methodology. Analyzing symmetrical and asymmetrical temperature differences around a measured linear heater (dTsym and dTas) relative to their ratio dTsym/dTas (so called a K-diagram) is at the heart of this methodology. This key concept, however, has to date only been generally described in previous works on the HFD technique. My objective here is to provide a comprehensive overview describing different types of K-diagrams, their interpretation and application for determining K-values or dTas for a zero flow condition. The K-value is a measured parameter which is particularly important for objectively characterizing heat conducting properties at the sensor insertion point under specific local measurement conditions. Correctly determining the K-value is critical for accurately calculating sap flow based on recorded temperature measurements. I have included in this review several examples demonstrating how the K-value is dependent upon changes to the environment and its important role in sap flow estimation.

Effects of earlywood and latewood on sap flux density-based transpiration estimates in conifers

Heat-based sap flux density (SFD) methods have been widely used to estimate the water use by conifers, but complexities arise due to the heterogeneous nature of conifer sapwood with annual rings of earlywood (EW) and latewood (LW), which differ in water- and heat-conducting properties. Laboratory-based controlled flow experiments using freshly cut stem segments from 11 pine trees were undertaken to evaluate the potential impact of hydraulic architecture of conifer sapwood on tree water use estimates from the Heat Ratio Method (HRM) and Heat Field Deformation (HFD) method, by considering different scenarios regarding the hydraulic conductivity and thermal diffusivity of EW and LW. The results show that the actual water flux was systematically underestimated in Scenario 1 (assuming only EW was water-conductive but thermal diffusivity was mean of EW and LW) and Scenario 3 (assuming equal hydraulic conductivity of LW and EW but thermal diffusivity was that of only EW). However, the mean sap flux densities obtained from 11 sample trees after correction by the LW/EW ratios were pretty close to the gravimetrical flow. Assuming equal hydraulic conductivity of LW and EW and mean thermal diffusivity of EW and LW led to either overestimation or underestimation of water use by individual trees, but the mean tree-scale water use was unbiased when including all this variance in the study system. The observed heterogeneous radial SFD variability from the HFD measurements was closely linked with patterns of successive EW and LW, especially in the central parts of the sapwood where higher SFD values were generally observed. The decreasing SFD patterns towards the cambium and heartwood were partially attributed to the decrease in moisture content, tracheid diameter and the increase in wood density of EW and LW compared with the central sapwood. The results indicated that the LW/EW ratio in stems where sap flow probes have been inserted can be measured a posteriori to correct HRM-based sap flow measurements. The sap flux is recommended to be radially corrected using the SFD patterns from HFD sensors measured at the same location of the HRM measurements in the same tree.

Calibration and comparison of thermal dissipation, heat ratio and heat field deformation sap flow probes for diffuse-porous trees

Sap flow probes are routinely used in forest and horticulture hydrology for estimating tree water use. This requires unbiased measurements when upscaling from tree to stand level, but accuracy and comparability of different thermometric methods have been questioned. Three sap flow measuring techniques were compared against gravimetric flow measurement in cut stem segments: ‘Granier-type’ thermal dissipation probes (TDP; three different sensor types), the heat field deformation method (HFD), and the heat ratio method (HRM). For the empirical methods (TDP and HFD), new calibration parameters were estimated using a nonlinear hierarchical modelling approach. 66 stem segments from five temperate, diffuse-porous tree species (9–16cm stem diameter, 100cm stem length) were exposed to a wide range of flux densities by applying subatmospheric pressure (−50 to −650hPa) analogous to natural flow conditions in the field.All TDP probes underestimated flux density by 23–45% when calculated with Granier's original calibration parameters, with the deviation increasing with flux rate. The accuracy was significantly improved by estimating new calibration parameters, especially for probes differing from Granier's original sensor design. Species-specific parameters further improved accuracy, although the species differences might partially be explained by variation in the observed ranges of sap flux. The HFD sensor overestimated gravimetric flow by ∼11%; empirical calibration did not improve its accuracy compared to the manufacturer's equation. At low to medium flow rates, the HRM system achieved higher accuracy than the other probes (0.8% underestimation), while performing poorly at high flux rates under our measurement settings (energy input of 25J). Both for TDP and HFD sensors, we observed a surprisingly large variability in calibration parameters between different stems of the same species.We conclude that (i) TDP and HFD sensors require species-specific calibration to measure sap flux with high accuracy, (ii) the original Granier equation cannot be used for TDP probes with deviating design, and (iii), at low to medium flow rates, the highest accuracy can be achieved with HRM sensors. Our results help to increase the accuracy of tree sap flow measurements with thermal dissipation probes, and to assess various levels of errors related to the different thermometric methods. This is important when synthesizing forest transpiration data on regional and global scales.

Are Dracaena nebulophytes able to drink atmospheric water?

In arid and semi-arid environments, fog interception as a water acquisition mechanism has been long recognized as an important factor for plant survival. The “narrow-leaf syndrome” increases water absorption from horizontal precipitation and is typical of nebulophytes characterized by dense rosette type crowns, to which also Dracaena species belong. In this paper, we demonstrate that Dracaena nebulophytes are able to direct water intercepted from fog through the leaf axils into their succulent woody organs to be stored for later use. We conducted leaf axil watering (LAW) experiments in four young Dracaena plants (two Dracaena cinnabari and two Dracaena draco) while simultaneously measuring sap flow in plant stems using the heat field deformation method. It was assumed that inducing water potential within stems closed to zero would initiate simultaneous bidirectional water transport from the stem to the crown and roots, and that this moment would be reflected in corresponding sap flow. Three hypothetical scenarios of induced water transport imbalance were confirmed by analyzing measured temperature gradients around heated probes and calculating sap flow. Sap flow responses to LAW clearly appeared to be dependent on flow direction prior to water treatments, on the strength of forces driving upward and downward water movement and the quantity of water applied. Intrinsic sap flow changes depicted in the results confirm the hypothesis that the Dracaena species are able to direct atmospheric water through the axils of their leaves to stem tissues. This mechanism of bypassing soil water represents an alternative means of water uptake in plants and is especially important in foggy areas of arid and semi-arid climates.

Sap flux – a real time assessment of health status in Norway spruce

ABSTRACTTop dieback of Norway spruce (Picea abies), triggered by drought in 2004–2006, has been observed in Southeast Norway and trees died within four years after appearance of the first symptoms. The aim of our study was to use sap flux measurements as a diagnostic method for assessment of tree vitality. We used the heat field deformation method to monitor the sap flux density (SFD) in four pairs of healthy and declining trees in situ. To provide retrospective information on hydraulic performance of the trees we took samples for wood anatomical analysis. After felling the trees we used the modified differential translucence method (MDT) as a proxy for the SFD measurements. Healthy trees had three times higher SFD values as declining trees. In some healthy trees we detected decreasing SFD with time. The MDT method agreed with the SFD measurements. In conclusion, we detected sap flux dysfunction in declining trees and showed that the SFD reduction may occur during a short period, prior to occurrence of any visual symptoms. We suggest incorporating the SFD measurements into the repertoire of diagnostic tools in forest pathology.

A comparative structural and functional study of leaf traits and sap flow in Dracaena cinnabari and Dracaena draco seedlings.

Water relations for two remote populations of Dracaena tree species from the dragon tree group, Dracaena cinnabari Balfour f. and Dracaena draco (L.) L., were studied to test our hypothesis that morphological and anatomical differences in leaf structure may lead to varied functional responses to changing environmental conditions. Sap flow measurements were performed using the heat field deformation method for four Dracaena seedlings grown in one glasshouse and two greenhouses, and leaf traits related to plant-water relationships were characterised. All traits studied confirmed that D. cinnabari leaves are more xeric in their morpho-anatomical structure compared with D. draco leaves. No radial sap flow variability was detected in D. draco plant stems, whereas sap flow was found to be higher in the inner part of D. cinnabari stems. The regular occurrence of reverse sap flow at night in both Dracaena species was consistent with a staining experiment. Vapour pressure deficit (VPD) was found to be the main driver for transpiration for both Dracaena species. However, the relationship between VPD and sap flow appeared to be different for each species, with a clockwise or no hysteresis loop for D. draco and a counter-clockwise hysteresis loop for D. cinnabari. This resulted in a shorter transpiration cycle in D. cinnabari. The observed superior water-saving strategy of D. cinnabari corresponds to its more xeric morpho-anatomical leaf structure compared with D. draco.

Xylem hydraulic properties in subtropical coniferous trees influence radial patterns of sap flow: implications for whole tree transpiration estimates using sap flow sensors

A high spatial resolution dataset of sap flux density in subtropical conifers is used to assess the minimum number and location of sap flow sensors required to monitor tree transpiration accurately. Tree transpiration is commonly estimated by methods based on in situ sap flux density (SFD) measurements, where the upscaling of SFD from point measurements to the individual tree has been identified as the main source of error. The literature indicates that the variation in SFD with radial position across a tree stem section can exhibit a wide range of patterns. Adequate capture of the SFD profile may require a large number of point measurements, which is likely to be prohibited. Thus, it is of value to develop protocols, which rationalize the number of point measurements, while retaining a satisfactory precision in the tree SFD estimates. This study investigates cross-sectional SFD variability within a tree and successively for six individual trees within a stand of Pinus elliottii var. elliottii × caribaea var. hondurensis (PEE × PCH). The stand is part of a plantation in subtropical coastal Australia. SFD is estimated using the Heat Field Deformation method simultaneously for four cardinal directions with measurements at six depths from the cambium. This yields a reference value of single tree SFD based on the twenty-four point measurements. Large variability of SFD is observed with measurement depth, cardinal direction and selected tree. We suggest that this is linked to the occurrence of successive narrow early and latewood rings with contrasting-specific hydraulic conductivities and wood water contents. Thus, an accurate placement of sensors within each ring is difficult to achieve in the field with the sensor footprint covering several rings of both early and latewood. Based on the reference dataset, we identified both an “ideal” setup and an “optimal” setup in terms of cost effectiveness and accuracy. Our study shows the need of using a systematic protocol to optimize the number of sensors to be used as a trade-off between precision and cost. It includes a preliminary assessment of the SFD variability at a high spatial resolution, and only then based on this, an appropriate placement of sensors for the long-term monitoring.

Influence of temporospatial variation in sap flux density on estimates of whole-tree water use in Avicennia marina

Our study shows that sap flow in Avicennia marina varies significantly throughout the sapwood and that spatial patterns in sap flux density are dependent on meteorological conditions. Sap flux density measurements are used worldwide as a relatively inexpensive means to provide estimates of whole-tree and whole-stand water use in forest ecosystems. However, erroneous upscaling from point measurements to the entire sapwood area remains an issue, since sap flow is hardly ever constant throughout the tree. In this study, two widely used sap flow methodologies (the Heat Ratio or HR method and the Heat Field Deformation or HFD method) are used to assess radial and azimuthal variations in sap flux density in three mature trees of the mangrove species Avicennia marina in Brisbane, Australia. The genus Avicennia is characterised by secondary growth via successive cambia, resulting in an atypical sapwood pattern of xylem patches braided with phloem strings. Water use estimates were calculated in different ways. At first, spatial variation was ignored when upscaling from point measurements. Then, radial and azimuthal variations were incorporated subsequently by measuring at different depths and aspects around the tree. Ignoring azimuthal variation led to over- or underestimations of up to 102 %, while radial variation accounted for discrepancies of up to 25 %. Furthermore, the influence of changing meteorological conditions was assessed, which showed that radial profiles changed in shape during rain events, such that maximum sap flow rates occurred at different depths compared to dry periods. Our study thus indicates that spatial variation in sap flux density is highly unpredictable in A. marina due to its hydraulic architecture, and that changing meteorological conditions alter the pattern of this variation. These two factors should be accounted for when assessing whole-tree water use.

Water content measurement in tree wood using a continuous linear heating technique

This paper deals with the analytical and experimental analysis of heat transfer in the xylem of trees and the determination of stem water content (WC). The aim of this paper was to derive and verify a new approach to WC calculation from measured temperature differences around a continuously heated needle.The configuration of heater and thermocouples of the heat field deformation (HFD) method for sap flow measurements was taken for this purpose. The suggested formula is derived from the heat conduction equation with a continuous linear source of heating. The ability of the HFD method to non-destructively determine the radial distribution of the local conditions of sap flow measurements under zero flow (referred to as K-value) was used to derive the radial distribution of WC.Examples of the application of our method on several contrasting species with different plant hydraulic architecture are given where calculated WC along stem xylem radius was compared with its direct gravimetrical measurements or with relative determination of wetness of cross-sectional stem discs using a modified differential translucence method.The proposed method enables water content measurements during continuous linear heating and is important for information about WC and sap flow together per day or season when there is possible to extrapolate the temperature difference for zero flow condition.

Water uptake and hydraulic redistribution under a seasonal climate: long‐term study in a rainfed olive orchard

AbstractHydraulic redistribution plays a relevant role in the water relations of trees in climates with alternation between warm/dry and cold/wet periods. We aim to illustrate the ability of Mediterranean deep‐rooted rainfed olive trees to maintain transpiration during the hot dry season and to redistribute soil water through roots, tending to temporarily homogenize soil moisture vertically. Sap flow was monitored by the heat field deformation method for 2·5 years in the stem, lignotuber, medium and shallow roots of an olive tree tracing the long‐term variations in the patterns of transpiration, water uptake and hydraulic redistribution. During the same period, soil water content and meteorological data were measured and related to sap flow. Results show that hydraulic redistribution within the rhizosphere buffers the seasonal and long‐term water deficits. Under high evaporative demand during the dry summer, the deeper roots connected to stem through the lignotuber uptake water supporting transpiration needs and reducing the intensive drying of the upper soil layers. Copyright © 2014 John Wiley & Sons, Ltd.

Potencials of sap flow evaluation by means of acoustic emission measurements

The work deals with measurement techniques of water conducting system in the trees. Water conducting system (including xylem and phloem) indicates its importance for related physiological processes. There are still problems how to measure its functioning (which variables and how), especially in the open field (e.g., forests and orchards) in order to get maximum information about it. Simple band dendrometers measuring seasonal dynamics of stem growth have been already applied for many years, being gradually replaced by their more sophisticated electronic versions most recently. The sap flow is a suitable variable, because it links roots and crowns and provide information about transporting the largest amount of mass in plants, which can be decisive for their behavior. Following pioneering work in the last century (Huber, 1932), many types of sap flow measurement methods based on a variety of principles (e.g., thermodynamic, electric, magneto-hydrodynamic, nuclear magnetic resonance, etc.) have been described. Only a few of these, particularly those based on thermodynamics, have been widely used in field-grown trees. E.g., heat pulse velocity system developed by Green (1998) and Cohen et al. (1981). Heat ratio method also works with pulses, but interpreted the data in more sophisticated way (Burgess, 2001). Widely used is a simple heat-dissipation method (Granier, 1985). Direct electric heating and internal sensing of temperature was applied in the trunk heat balance method (Čermák et al., 1973, 1976, 1982, 2004; Kučera et al., 1977; Tatarinov et al., 2005). The heat field deformation method is based on measurement of the deformation of the heat field around a needle-like linear heater (Nadezhdina et al., 1998, 2002, 2006; Čermák et al., 2004).Another important variable is water potential, which could be measured in the past only periodically on selected pieces of plant material using pressure (Scholander) bomb, but most recently also continuous measurements became possible due to application of psychrometric method (Dixon and Tyree, 1985). There exist also other physical variables carrying important information, which can be measured using different principles. This includes e.g., acoustic methods, which can detect quantitative variation of pulses occurring during cavitation events, associated with interruptions of water columns in vessels. This must not necessarily be a single source of acoustic emissions. In this study we are focused on a general description of acoustic events measurable in a wide range of their spectrum. The first aim was to detect such signals and the second to learn them and gradually analyze in order to better understand the associated processes causing their occurrence and their relations to plant life.

Comparative study of long-term water uptake of Norway spruce and Douglas-fir in Moravian upland

Abstract Long-term water uptake of Douglas-fir and Norway spruce trees, growing in condition of Moravian upland, was studied with aim of comparing sap flow in small roots with flow in stems. Sap flow was measured by the heat field deformation method using multi-point sensors for stems and single-point sensors for roots. Differences between species were found in relationships between sap flow in tree stems and water uptake by roots, suggesting that Douglas-fir is able to take water from deeper soil more efficiently than spruce. This allows Douglas-fir to transpire more water especially during drought and grow faster than spruce. These biological features should be taken into account for future forest species compositions because they may have impact on both, forestry and hydrology.

Root functioning, tree water use and hydraulic redistribution in Quercus suber trees: A modeling approach based on root sap flow

Mediterranean evergreen oaks have to survive a long summer drought. Roots may play a relevant role under these conditions. We studied their structure and function in a mature Quercus suber L. tree in central Portugal. The root system was mapped till the lowest water table level (4.5m depth). Xylem anatomy was analyzed in a vertical profile belowground. Sap flow was continuously monitored for 1.5yrs in the stem and roots of this intensively studied tree (heat field deformation method) and in the stem of four trees (Granier method), in relation to environmental variables and predawn leaf water potential. The sources of water uptake were assessed by stable isotope analyses in summer. Results showed a dimorphic root system with a network of superficial roots linked to sinker roots, and a taproot diverting into tangles of deep fine roots submerged for long periods, with parenchyma aerenchyma. Transpiration was not restricted in summer due to root access to groundwater. The isotopic δ18O signature of twig xylem water was similar to that of groundwater in the dry season. Two functional types of superficial roots were identified: shallow connected and deep connected roots. A modeling approach was built considering that each superficial root was linked to a sinker, with part of the root deep connected (between the stem and the sinker) and part shallow connected (between the sinker and topsoil). This conceptual framework simulated tree stem sap flow from root sap flow with a high efficiency (R2=0.85) in four plot trees.On an annual basis, soil water and groundwater contributions were 69.5% and 30.5% of stem flow, respectively. Annual hydraulic lift and hydraulic descent were 0.9% and 37.0% of stem flow, respectively. The trees maximize the exploitation of the environmental resources by using the topsoil water during most of the year, and groundwater together with hydraulic lift (nutrient supply) in the dry summer. This study shows that a dimorphic root system, with roots reaching groundwater, is an efficient strategy of Q. suber trees to cope with seasonal drought. Knowledge of the functional behavior of Q. suber trees under shallow water table conditions may contribute to the definition of better adapted management practices and to anticipate their responses to climate change.

SAP FLOW DYNAMICS AS A DIAGNOSTIC TOOL IN NORWAY SPRUCE

Top-dieback of Norway spruce (Picea abies) trees has been observed in SE Norway. Trees usually die within 1-4 years after the first symptoms become visible and the dieback cause is unknown. The aim of our study was to establish when the irreversible spiral to tree death occurs. We assumed that hydraulic dysfunction, exemplified here by the sap flow reduction, was the final trigger of tree decline. We used the non-destructive heat field deformation method (HFD) to monitor the sap flow density (SFD) in non-symptomatic trees. After felling the trees we used the modified differential translucence method (MDT) to evaluate the sapwood translucence as an additional indicator of its functionality. In this study we show three examples of non-symptomatic trees with widely different sap flow densities detected by the HFD; tree A with unchanged sap flow throughout the monitoring period, tree B with decreasing sap flow in the last formed annual rings and minimal sap flow detected in tree C, which also started to show the first visible signs of dieback. In contrast, the MDT method detected in all cases functional (translucent) sapwood. In conclusion, we show that the sap flow reduction may occur during a relatively short period, prior to occurrence of any visual symptoms. Also, dysfunctions in sap flow could not be detected by the MDT method, as the sapwood still looked translucent and thus functional, although minimal sap flow was detected in that area by HFD method. Here we demonstrate the sensitivity of the HFD method in detecting the quantity of sap flow and thus a suitable diagnostic tool to evaluate the vitality in Norway spruce trees before the onset of visible disease symptoms.

INFLUENCE OF SPATIAL VARIATION IN SAP FLUX DENSITY ON ESTIMATES OF WHOLE-TREE WATER USE IN AVICENNIA MARINA

Sap flux density measurements are used worldwide as a relatively inexpensive means to provide an estimate of whole-tree and whole-stand water use in forest ecosystems. However, erroneous up-scaling from point measurements to the entire sapwood area remains an important issue, since sap flux density is hardly ever constant throughout the cross-section of a tree. In this study, two widely used sap flow methodologies (the Heat Ratio Method or HRM and the Heat Field Deformation method or HFD) are used to assess radial and azimuthal variations in sap flux density in the mangrove species Avicennia marina or grey mangrove, a species that is characterised by a secondary growth via successive cambia, resulting in an atypical sapwood pattern of xylem patches braided with phloem strings. Sap flux density values were scaled up to whole-tree water use in different ways. Initial estimates of sap flux were made from point measurements, ignoring spatial variations. We then adjusted these measurements by including radial and azimuthal variations assessed by measurements of radial profiles and multiple sensors around the circumference of the tree, respectively. Our results showed a discrepancy of 237+/-26% between the lowest and highest water use estimate, as a consequence of the high radial and azimuthal variations in sap flux density in this species. Our study indicates that caution is required when interpreting sap flux density values, hence it is crucial to account for radial and azimuthal variation when scaling up from point measurements to whole-tree water use.

MAPPING THE WATER PATHWAYS IN STEM XYLEM BY SAP FLOW MEASUREMENTS DURING BRANCH SEVERING EXPERIMENTS

The Heat Field Deformation (HFD) sap flow method proved to be a useful tool for analyzing whole-tree hydraulic architecture in trees. It measures sap flow rates, at different sapwood depths, with high time and space resolution. The response of multi-point sensors inserted in tree stems to manipulation experiments (such as branch severing) unravels the link between conducting xylem of stem and different foliage cohorts. The changes in the sap flow profile upon branch cuttings are easy to follow and interpret, especially when conducted under clear sky conditions. Based on experiments carried out in trees with different hydraulic architectures, we demonstrate the potential of the HFD method in the fast direct evaluation of the extent of the stem sapwood connected to each part of the crown and hence in the mapping of the water pathways between stem and crown. Such xylem maps may substantially contribute to the modeling of water transport in trees.