Maurits W. Vandegehuchte

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.

Sap flux density measurements based on the heat field deformation method

Accurate measurements of whole tree water use are needed in many scientific disciplines such as hydrology, ecophysiology, ecology, forestry, agronomy and climatology. Several techniques based on heat dissipation have been developed for this purpose. One of the latest developed techniques is the heat field deformation (HFD) method, which relies on continuous heating and the combination of a symmetrical and an asymmetrical temperature measurement. However, thus far the development of this method has not been fully described in the scientific literature. An understanding of its underlying principles is nevertheless essential to fully exploit the potential of this method as well as to better understand the results. This paper therefore structures the existing, but dispersed, data on the HFD method and explains its evolution from an initial ratio of temperature differences proportional to vapor pressure deficit to a fully operational and practically applicable sap flux density measurement system. It stresses the importance of HFD as a method that is capable of measuring low, high and reverse flows without necessitating zero flow conditions and on several sapwood depths to establish a radial profile. The combination of these features has not been included yet in other heat-based sap flow measurement systems, making the HFD method unique of its kind.

Stem diameter variations as a versatile research tool in ecophysiology

High-resolution stem diameter variations (SDV) are widely recognized as a useful drought stress indicator and have therefore been used in many irrigation scheduling studies. More recently, SDV have been used in combination with other plant measurements and biophysical modelling to study fundamental mechanisms underlying whole-plant functioning and growth. The present review aims to scrutinize the important insights emerging from these more recent SDV applications to identify trends in ongoing fundamental research. The main mechanism underlying SDV is variation in water content in stem tissues, originating from reversible shrinkage and swelling of dead and living tissues, and irreversible growth. The contribution of different stem tissues to the overall SDV signal is currently under debate and shows variation with species and plant age, but can be investigated by combining SDV with state-of-the-art technology like magnetic resonance imaging. Various physiological mechanisms, such as water and carbon transport, and mechanical properties influence the SDV pattern, making it an extensive source of information on dynamic plant behaviour. To unravel these dynamics and to extract information on plant physiology or plant biophysics from SDV, mechanistic modelling has proved to be valuable. Biophysical models integrate different mechanisms underlying SDV, and help us to explain the resulting SDV signal. Using an elementary modelling approach, we demonstrate the application of SDV as a tool to examine plant water relations, plant hydraulics, plant carbon relations, plant nutrition, freezing effects, plant phenology and dendroclimatology. In the ever-expanding SDV knowledge base we identified two principal research tracks. First, in detailed short-term experiments, SDV measurements are combined with other plant measurements and modelling to discover patterns in phloem turgor, phloem osmotic concentrations, root pressure and plant endogenous control. Second, long-term SDV time series covering many different species, regions and climates provide an expanding amount of phenotypic data of growth, phenology and survival in relation to microclimate, soil water availability, species or genotype, which can be coupled with genetic information to support ecological and breeding research under on-going global change. This under-exploited source of information has now encouraged research groups to set up coordinated initiatives to explore this data pool via global analysis techniques and data-mining.

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

Several heat-based sap flow methods, such as the heat field deformation method and the heat ratio method, include the thermal diffusivity D of the sapwood as a crucial parameter. Despite its importance, little attention has been paid to determine D in a plant physiological context. Therefore, D is mostly set as a constant, calculated during zero flow conditions or from a method of mixtures, taking into account wood density and moisture content. In this latter method, however, the meaning of the moisture content is misinterpreted, making it theoretically incorrect for D calculations in sapwood. A correction to this method, which includes the correct application of the moisture content, is proposed. This correction was tested for European and American beech and Eucalyptus caliginosa Blakely & McKie. Depending on the dry wood density and moisture content, the original approach over- or underestimates D and, hence, sap flux density by 10% and more.

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.

Woody tissue photosynthesis in trees: salve on the wounds of drought?

Drought-induced tree stress has gained increasing interest because of the recent coupling between forest decline and global change associated droughts (Allen et al., 2010; Anderegg et al., 2012, 2013; Martinez-Vilalta et al., 2012; McDowell et al., 2013b; Zeppel et al., 2013; IPCC, 2014; Doughty et al., 2015; Hartmann et al., 2015). To synthesize existing knowledge on drought stress and mortality mechanisms, McDowell et al. (2008) proposed the widely applied hydraulic failure and carbon starvation hypotheses. Hydraulic failure manifests when plants irreversibly desiccate due to uncontrolled air intrusion in the water transport system. Air intrusion, or cavitation, has dual consequences: at moderate level it may improve plant water status by local tension release and water supply to the transpiration stream (Vergeynst et al., 2015), but progressive cavitation and excessive conductivity loss will ultimately lead to mortality (Tyree & Sperry, 1988). Trees may minimize the risk of hydraulic failure by closing their stomata, but this also limits CO2 uptake. Prolonged stomatal closure may eventually lead to a negative plant carbon balance and ultimately carbon starvation (Zhao et al., 2013). A distinction has been made between anisohydric tree species that operate at narrow hydraulic safety margins and are more susceptible to hydraulic failure, and isohydric tree species that prevent lethal cavitation by tightly regulating stomatal conductance, which makes them more susceptible to carbon starvation (McDowell et al., 2008). For both functional tree types, pests and biotic agents such as insects or pathogens may either weaken the trees before droughtinduced tree mortality or accelerate the actual mortality process (Gaylord et al., 2013; Oliva et al., 2014). While providing a good framework, the hydraulic failure and carbon starvation hypotheses have been the focus of intense debate and further research. Tree mortality experiments indicate that a much more complex reality exists, in which hydraulic and carbon dynamics are strongly interlinked (Adams et al., 2009; McDowell, 2011; Sala et al., 2012; McDowell et al., 2013a,b; Mitchell et al., 2013; O’Grady et al., 2013; Sevanto, 2014; Sevanto et al., 2014; Hartmann et al., 2015). Recent studies indicate that isohydric and anisohydric tree species show both hydraulic failure and carbon starvation characteristics, evoking the image of a drought response continuum rather than a strict distinction between isohydricity and anisohydricity (Mitchell et al., 2013; Sevanto et al., 2014). Furthermore, it has been suggested that more isohydric species have the lowest hydraulic safety margins and the highest capacity to repair xylem cavitation (Meinzer & McCulloh, 2013). These examples indicate the high complexity of intertwined mechanisms supporting tree life under drought, and, when failing, leading to death. As indicated by Zeppel et al. (2011) and Martinez-Vilalta et al. (2012), the research field of tree mortality is, even though rapidly progressing, still in its infancy. While drought has been shown to rapidly affect the plant water balance adversely (Hartmann et al., 2013), carbon processes must not be neglected in mortality studies as water and carbon processes are closely linked (McDowell, 2011; Steppe et al., 2015). Besides the possible limitation of CO2 uptake due to stomatal closure, drought may induce hydraulic constraints on the transport of nonstructural carbon (NSC) between the different plant compartments, inducing local carbon deficits (Ruehr et al., 2009; Sala et al., 2010; Sevanto, 2014). However, even with local presence of carbohydrates, other drought-induced processes might deplete these local pools to maintain the hydraulic integrity and negate the effects of xylem cavitation, such as osmoregulation (Sevanto et al., 2014), embolism refilling (Secchi&Zwieniecki, 2012) and sensing (Zwieniecki & Holbrook, 2009). These carbohydrates are then unavailable for regularmetabolicmaintenance processes, leading to carbon starvation (McDowell & Sevanto, 2010). Hence, carbon starvation is not just a question of the carbon storage pool size, but primarily of the local availability of carbon for cell survival in sink tissues. Surprisingly, woody tissue photosynthesis as a means of providing carbon locally by recycling respired CO2 via photosynthesis in chlorophyll containing woody tissues (stem recycling photosynthesis; Avila et al., 2014) has received little to no attention throughout the mortality discussion.

Sap flow as a key trait in the understanding of plant hydraulic functioning

1Laboratory of Plant Ecology, Department of Applied Ecology and Environmental Biology, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, 9000 Gent, Belgium; 2Dipartimento di Bioscienze e Territorio, Universita’ degli Studi del Molise, 86090 Pesche, Italy; 3School of GeoSciences, University of Edinburgh, Crew Building, West Mains Road, Edinburgh EH9 3JN, UK; 4ICREA at CREAF, Universidad Autonoma de Barcelona, Cerdanyola del Valles, Barcelona, Spain; 5Corresponding author (kathy.steppe@UGent.be)

Use of the correct heat conduction–convection equation as basis for heat-pulse sap flow methods in anisotropic wood

Heat-pulse methods to determine sap flux density in trees are founded on the theory of heat conduction and heat convection in an isotropic medium. However, sapwood is clearly anisotropic, implying a difference in thermal conductivity along and across the grain, and hence necessitates the theory for an anisotropic medium. This difference in thermal conductivities, which can be up to 50%, is, however, not taken into account in the key equation leading to the currently available heat-pulse methods. Despite this major flaw, the methods remain theoretically correct as they are based on derivations of the key equation, ruling out any anisotropic aspects. The importance of specifying the thermal characteristics of the sapwood according to axial, tangential or radial direction is revealed as well as referring to and using the proper anisotropic theory in order to avoid confusion and misinterpretation of thermal properties when dealing with sap flux density measurements or erroneous results when modelling heat transport in sapwood.

Changes in stem water content influence sap flux density measurements with thermal dissipation probes

Key messageStem WC may decline during the day. Zero-flowdTmincreases when WC decreases. Use of nighttimedTmin the calculation of sap flux density during the day might introduce errors.AbstractThere is increasing evidence of diel variation in water content of stems of living trees as a result of changes in internal water reserves. The interplay between dynamic water storage and sap flow is of current interest, but the accuracy of measurement of both variables has come into question. Fluctuations in stem water content may induce inaccuracy in thermal-based measurements of sap flux density because wood thermal properties are dependent on water content. The most widely used thermal method for measuring sap flux density is the thermal dissipation probe (TDP) with continuous heating, which measures the influence of moving sap on the temperature difference between a heated needle and a reference needle vertically separated in the flow stream. The objective of our study was to investigate how diel fluctuations in water content could influence TDP measurements of sap flux density. We analysed the influence of water content on the zero-flow maximum temperature difference, dTm, which is used as the reference for calculating sap flux density, and present results of a dehydration experiment on cut branch segments of American sycamore (Platanus occidentalis L.). We demonstrate both theoretically and experimentally that dTm increases when stem water content declines. Because dTm is measured at night when water content is high, this phenomenon could result in underestimations of sap flux density during the day when water content is lower. We conclude that diel dynamics in water content should be considered when TDP is used to measure sap flow.Stem WC may decline during the day. Zero-flow dT m increases when WC decreases. Use of nighttime dT m in the calculation of sap flux density during the day might introduce errors. There is increasing evidence of diel variation in water content of stems of living trees as a result of changes in internal water reserves. The interplay between dynamic water storage and sap flow is of current interest, but the accuracy of measurement of both variables has come into question. Fluctuations in stem water content may induce inaccuracy in thermal-based measurements of sap flux density because wood thermal properties are dependent on water content. The most widely used thermal method for measuring sap flux density is the thermal dissipation probe (TDP) with continuous heating, which measures the influence of moving sap on the temperature difference between a heated needle and a reference needle vertically separated in the flow stream. The objective of our study was to investigate how diel fluctuations in water content could influence TDP measurements of sap flux density. We analysed the influence of water content on the zero-flow maximum temperature difference, dT m, which is used as the reference for calculating sap flux density, and present results of a dehydration experiment on cut branch segments of American sycamore (Platanus occidentalis L.). We demonstrate both theoretically and experimentally that dT m increases when stem water content declines. Because dT m is measured at night when water content is high, this phenomenon could result in underestimations of sap flux density during the day when water content is lower. We conclude that diel dynamics in water content should be considered when TDP is used to measure sap flow.

Corrigendum to: 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 (g h-1) such as the stem heat balance and trunk sector heat balance method and those measuring sap-flux density (cm3 cm-2 h-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.