Nadezhda Nadezhdina

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‐mediated hydraulic redistribution in large roots on opposing sides of a Douglas‐fir tree following localized irrigation

*Increasing evidence about hydraulic redistribution and its ecological consequences is emerging. Hydraulic redistribution results from an interplay between competing plant and soil water potential gradients. In this work, stem-mediated hydraulic redistribution was studied in a 53-year-old Douglas-fir tree during a period of drought. *Sap flux density measurements using the heat field deformation method were performed at four locations: in two large opposing roots and on two sides of the tree stem. Hydraulic redistribution was induced by localized irrigation on one of the measured roots, creating heterogeneous soil water conditions. *Stem-mediated hydraulic redistribution was detected during night-time conditions when water was redistributed from the wet side of the tree to the nonirrigated dry side. In addition to stem-mediated hydraulic redistribution, bidirectional flow in the dry root was observed, indicating radial sectoring in the xylem. *It was observed that, through stem-mediated hydraulic redistribution, Douglas-fir was unable to increase its transpiration despite the fact that sufficient water was available to one part of the root system. This resulted from the strong water potential gradient created by the dry soil in contact with the nonirrigated part of the root system. A mechanism of stem-mediated hydraulic redistribution is proposed and its possible implications are discussed.

Scots pine root distribution derived from radial sap flow patterns in stems of large leaning trees

This study characterizes whole tree root system distribution in a non-destructive way based on its functional parameters, particularly the sap flow patterns in stems. This approach particularly considers sap flow variation across stems, both radial and circumferential patterns of flow that are usually used for a better integration of sap flow density at the whole tree level. We focused at: (1) Showing examples of sap flow variation across stems at a defined situation (high midday values at the period of non-limiting water supply; (2) Analyzing radial flow patterns in terms of root distribution; (3) Validating these results at the stand level (mean data of series of individual trees) using results of classical biometric methods used at the same site; and (4) Applying the results for evaluation of root distribution around leaning trees. Sap flow rate was measured by the heat deformation method on a set of 14 trees at an experimental pine forest stand in Brasschaat (Belgium) during the growing season of 2000. Sap flow variation across stems was measured at a total of 700 points. Amounts of water supplied by superficial (horizontally oriented) and sinker (vertically oriented) roots were estimated from sap flow patterns. The vertical distribution of absorbing roots as derived from the analysis of sap flow patterns in stem sapwood was very similar to the distribution determined by the classical biometric analysis of fine roots. Trees leaning to the East had stem radii at the stump level and crown radii enhanced in the leaning direction. Sinker roots showed higher absorption activities in the leaning direction, but superficial roots were more absorbing in the opposite direction. The application of the above-described method allows for a better evaluation of the whole-tree behavior and facilitates the evaluation of tree and stand properties in traditional forest stands, which are not equipped for detailed scientific research. This may also facilitate practical applications in landscape-level studies.

Integration of water transport pathways in a maple tree: responses of sap flow to branch severing

Abstract• It has been known for a long time that sectored and integrated patterns of vascular systems exist in different species and even within the same tree, depending on its age and history. However, very few publications consider the topology of the vascular pathways between roots and branches.• Some results on this important aspect of the vascular system are presented in this paper. They have been obtained with adult maple trees by directly studying the water movement in the stem and root xylem with the heat field deformation (HFD) method for sap flow measurements.• Multi-point HFD sensors were installed at different heights of a Norway maple tree (Acer platanoides L.) along its stem axis. Single-point HFD sensors were installed in three small lateral roots of another sample maple. Experimental treatments (branch severing) triggered changes in sap movement in the stem and root sapwood.• The sample trees belong to the group with an integrated transport system (“integrated pipes”), sharing stem space on both sides of the tree to supply two large parts of the crown with water from each root sector. Nevertheless, conducting pathways had their autonomy for axial transport and the pipe model theory describes the vascular system of the studied trees well. Thus, the integration of axial transport in the stem xylem should presumably occur through the cross-grained network of axial vessels.Résumé• Il est connu depuis longtemps que des modèles sectorisés et intégrés de système vasculaire coexistent chez différentes espèces et même dans un arbre unique, en fonction de son âge et de l’histoire. Cependant, très peu de publications ont examiné la topologie du parcours vasculaire entre les racines et les branches.• Quelques résultats sur cet aspect important du système vasculaire sont présentés ici. Ils ont été obtenus avec des érables adultes en étudiant directement le mouvement de l’eau dans le xylème de la tige et de la racine avec la méthode de déformation du champ de chaleur (HFD) pour mesurer le flux de sève.• Des capteurs HFD-multi-points ont été installés à différentes hauteurs dans un érable plane (Acer platanoïdes L.) le long de la tige. Des capteurs HFD-simple-point ont été installés dans trois petites racines latérales d’un autre érable. Des traitements expérimentaux (ablation de branche) ont déclenché des changements dans la circulation de la sève dans l’aubier de la tige et de la racine.• Les arbres échantillonnés présentent un système vasculaire intégré (« tuyaux intégrés »), avec un tronc partagé en deux secteurs alimentant les deux grandes parties de la couronne avec de l’eau à partir des racines de chaque secteur. Néanmoins, les parcours conducteurs de la sève ont leur autonomie pour le transport axial et la théorie du pipe model décrit bien le système vasculaire des arbres étudiés. L’intégration du transport axial dans le xylème de la tige se fait probablement par l’intermédiaire d’un réseau interconnecté de vaisseaux axiaux.

Instrumental Approaches for Studying Tree-Water Relations Along Gradients of Tree Size and Forest Age

There is an increasing demand for data on tree structural and functional attributes that can be collected simultaneously at multiple sites and integrated across landscapes. Here, we present several examples of approaches applicable for measuring tree and stand structures and for characterizing spatial and temporal variation in important functions of the above- and belowground parts of both large and small trees. Special attention is given to explanations of the theoretical basis for several sap flow techniques and to the types of information that can be gleaned from carefully planned measurements of sap flow in stems and roots. A variety of approaches for characterizing the structure and function of root systems: the hidden half of trees, are also described.

Sap flow index as an indicator of water storage use

Abstract Symmetrical temperature difference also known as the sap flow index (SFI) forms the basis of the Heat Field Deformation sap flow measurement and is simultaneously collected whilst measuring the sap flow. SFI can also be measured by any sap flow method applying internal continuous heating through the additional installation of an axial differential thermocouple equidistantly around a heater. In earlier research on apple trees SFI was found to be an informative parameter for tree physiological studies, namely for assessing the contribution of stem water storage to daily transpiration. The studies presented in this work are based on the comparative monitoring of SFI and diameter in stems of different species (Pseudotsuga menziesii, Picea omorika, Pinus sylvestris) and tree sizes. The ability of SFI to follow the patterns of daily stem water storage use was empirically confirmed by our data. Additionally, as the HFD multipointsensors can measure sap flow at several stem sapwood depths, their use allowed to analyze the use of stored water in different xylem layers through SFI records. Radial and circumferential monitoring of SFI on large cork oak trees provided insight into the relative magnitude and timing of the contribution of water stored in different sapwood layers or stem sectors to transpiration.

Field Studies of Whole-Tree Leaf and Root Distribution and Water Relations in Several European Forests

Practically oriented field studies of different structures important for environmental purposes are dealing with wide levels of biological organization, starting e.g. with micro- structure (cells, molecules) and continuing to macro-structures up to regional, continental or even global levels. Naturally, only very extensive research teams can cover the whole range of these problems. Situation is a bit simplified, when limiting too complex physiological studies only to selected ones. We described here some phenomena related to water relations and corresponding structures, starting from individual organs (roots or branches), through whole trees and stands and when using appropriate models up to landscapes. Measurements of leaf and root area distributions help to evaluate other eco-physiological processes and also to up scale the data (e.g., allowing connections with remote sensing, serve for calibration etc.). Sap flow measurement provides a tool suitable for short- as well as long-term studies. Whole tree water storage helps trees to overcome critical periods of time in summer and specify periods when actually trees grow. Combination of anatomical analysis with sap flow measurement serves for evaluation of the efficiency of the water conducting system. Analysis of hydraulic tree architecture including water redistribution can explain tree survival under drought. Application of biometric parameters helps to up-scale the sap flow data from trees to entire stands. Combination of maps and remote sensing data enable to work up to the landscape level. Present long-term experience shows, that the approach based on field-applicable mobile instrumentation, is possible to adapt for solving different practical problems e.g., such considering tree or stands survival, stem growth, water balance and others.

Estimating the absorptive root area in Norway spruce by using the common direct and indirect earth impedance methods

AimsEstimates of root absorption magnitude are needed for the balanced management of forest ecosystems, but no methods able to work on the whole tree and stand level were available. Modified earth impedance method was developed recently and here it was tested, by comparing the results with those obtained by combination of several classical methods.MethodsWe used direct (soil cores, scanning and microscopy) and indirect (sap flow patterns and modified earth impedance) methods in an attempt to estimate the absorptive root area indexes (RAI) at two sites of about 25 and 40-years-old Norway spruce. We considered the geometric surfaces of all scanned fine roots to be equal to the fine root absorptive area (RAIscan). To estimate the potentially physically permeable area of fine roots, we microscopically evaluated the point of secondary xylem appearance and calculated the geometric area of root portions with primary structure (RAImicro). We termed the area of electrically conductive root surface as the active (ion) absorptive area (RAImei) and measured its extent by the modified earth impedance (MEI) method.ResultsThe highest values for absorptive root areas at the two experimental sites we obtained with the scanning method (RAIscan was considered to be 100%), followed by the RAImicro (51%) and RAImei (32%). RAImei reached about 2/3 of RAImicro. The surface area of the ectomycorrhizal hyphae was an order of magnitude larger than that of all fine roots, but the MEI did not measure such increase. ConclusionsWe showed that the absorptive root area, indirectly estimated by the MEI, provides consistent results that approach the values obtained for fine roots with a primary structure estimated by traditional direct methods. The similar range of the values for the absorptive root surface area obtained by microscopy and by the MEI method indicates that this method is feasible and that it could be used to determine the extent of active absorptive root surface areas in forests.

Measuring and modelling forest transpiration

Two transpiration models were tested, one with evaporation control, and another without evaporation control. The principle of the model with evaporation control is that the physical mechanism of transpiration is evaporation, which is actively controlled by plants. The supposed mechanism is: part of the heat (heat equivalent to the energy absorbed from solar radiation) that would cause heating of the plant above 25 °C is dissipated by evaporation. The model has four physical parameters, which are in principle measurable independently. The model without evaporation control is based on the assumption that evaporation dissipates a constant fraction of the incoming solar radiation. In this model, no physical mechanism of transpiration is given. The latter model needs only one parameter, which can be found by means of parameter search. Both models were tested by comparing their results with transpiration measured in the floodplain forest growing along the bank of the Dyje River close to Pohansko (Moravia, Czech Republic). Both models produced a typical difference of about 1.5 mm d-1 between measured and calculated daily transpiration totals. We can therefore say that both models are equally valid.