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Thermal Dissipation Probe Abstracts
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Clearwater, M.J., Meinzer, F.C., et al. 1999, 'Potential Errors in Measurement of Nonuniform Sap Flow using Heat Dissipation Probes.', Tree Physiology, vol. 19, pp. 681-687.
The empirical calibration of Granier-type heat dissipation sap flow probes that relate temperature difference (ΔT) to sap velocity (v) was re-evaluated in stems of three tropical species. The original calibration was confirmed when the entire heated probe was in contact with coducting xylem, but mean v was under estimated when part of the probe was in contact with monoconducting xylem or bark. Analysis of the effects of nonuniform sap velocity profiles on heat dissipation estimates showed that errors increased as v and the proporion of the probe in the monoconducting wood increased. If half of a 20-mm probe is in sapwood with a v of 0.15 mm s-1 and the other half is nonconducting wood, then mean v for the whole probe can be underestimated by as much as 50%. A correction was developed that can be used if the proportion of the probe in nonconducting wood is known. Even with the entire heated probe in contact with conducting xylem, v would be underestimated when radial velocity gradients are present. In this case, the error would be smaller exvept when velocity gradients are very steep as can occur in species with ring-porous wood anatomy. Errors occur because the relationship between ΔT and v is nonlinear. Mean ΔT along the probe is there for not a measure of mean v, is intergrated along the length of the probe. The same type of error can occur when ΔT is averaged through time while v is changing, but the error is small unless there are sudden, step changes between zero and high sap velocity. It is recommended that relatively short probes (20 mm or less) be used and that probes longer than the depth of conducting sapwood be avoided. Multiple probes inserted to a range of depths should be used in situations where steep gradients in v are expected. If these conditions are met, heat dissipation probes remain useful and widely applicable for measuring sap flow in woody stems.
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Granier, A. 1985, 'Une Nouvelle méthode pour la Mesure du Flux de Sève Brute Dans le Tronc des Arbres (A New Method of Sap Flow Measurement in Tree Stems)', Annals of Forest Science, vol. 42, no. 2, pp. 193-200.
The method described in this paper is based on a thermal sensor composed of two probes radially inserted in the sapwood of the trunk. One of those probes is heated at a constant energy and the other considered as a temperature reference. A simple equation enables us to calculate the sapflow as a function of the difference of the temperature between the two elements. A calibration has been made on pieces of trunk of different species. Owing to its sensitivity and its low cost, this system may fit for the quantitative measurement of forests transpiration.
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Granier, A., Biron, P., et al. 1996, 'Comparisons of Xylem Sap Flow and Water Vapour Flux at the Stand Level and Derivation of Canopy Conductance for Scots Pine', Theoretical and Applied Climatology, vol. 53, pp. 115-122.
Simultaneous measurements of xylem sap flow and water vapour flux over a Scots pine (Pinus syvestris) forest (Hartheim, Germany), were carried out during the Hartheim Experiment (HartX), an intensive observation campaighn of the international programme REKLIP. Sap flow was measured every 30 min using both radial constant heating (Granier, 1985) and two types of Cermak sap flow meters installed on 24 trees selected to cover a wide range of the diameter classes of the stand (min 8cm; max 17.5cm). Available energy was high during the observation period (5.5 to 6.9 mm day-1), and daily cumulated sap flow on a ground area basis varied between 2.0 and 2.7 mm day-1 depending on climate conditions. Maximum hourly values of sap flow reached 0.33 mm h-1, ie, 230W m-2. Comparisons of sap flow with water vapour flux as measured with two OPEC (One Propeller Eddy Correlation, University of Arizona) systems showed a time lag between the two methods, sap flow lagging about 90 min behind vapour flux. After taking into account this time lag in the sap flow data sed, a good agreement was found between both methods; sap flow=0.745* vapour flux, r2=0.86. The difference between the two estimates was due to the understory transpiration. Canopy conductance (gc) wass calculated from sap flow measurements using the reverse form of Penman-Monteith equation and climatic data measured 4 m above the canopy. Variations of gc were well correlated (r2=0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitave expression for gc=f(R,vpd) was very similar to that previously found with maratime pine (Pinus pinaster) in the forest of Les Landers, South Western France.
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Granier, A., Huc, R., et al. 1996, 'Transpiration of Natural Rain Forest and its Dependence on Climatic Factors', Agricultural and Forest Meteorology, vol. 78, pp. 19-29.
Sap flow was measured on several species from the tropical rain forest in French Guiana during two successive years over the dry season. On bright days, sap flow densities (i.e. sap flow per unit of sapwood area) exhibited high variations from one species to another. Higher rates (3 to 4kg dm-2 h-1) were observed on late stage forest species like Dicorynia guianensis, Eperua falcata or E. grandifolia, and lower rates on Vouacapoua americana and Carapa procera (1.0 to 1.5kg dm-2 h-1). Calculated stand sap flow (F) was closely dependant on air vapour pressure deficit and less correlated to global radiation. A simple model of canopy conductance variations and hence of stand transpiration was derived from these measurements. Sap flow was linearly related to Penman Evapotranspiration (PET), the ratio F/PET being close to 0.75 under dry canopy conditions, as previously reported by Shuttleworth et al. (1984) in Central Amazon.
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Köstner, B., Biron, P., et al. 1996, 'Estimates of Water Vapor Flux and Canopy Conductance of Scots Pine at the Tree Level Utilizing different Xylem Sap Flow Methods', Theoretical and Applied Climatology, vol. 53, pp. 105-113.
During the Hartheim Experiment (HartX) 1992 conducted in the upper Rhine Valley, Germany, three different methods were used to measure sap flow in Scots pine trees via heating of water transported in the xylem; (1) constant heating applied redially in the sap wood ("Granier-system" -G), (2) constant heating of a stem segment ("Cermák-system" -C), and (3) regulated variable heating of a stem segment that locally maintains a constant temperature gradient in the trunk (Cermák / Schulze-system" -CS). While the constant heating methods utilize changes in the induced temperature gradient to quantify sap flux, the CS-system estimates water flow from the variable power requirement to maintain a 2 or 3 degree Kelvin temperature gradient over a short distance between inserted electrodes and referance point. The C- and CS- systems assume that all transported water is ecompassed and equally heated by the electrodes. In this case, flux rate is determined from temperature difference into sap flow density. Estimates of sapwood area are used to calculate the total flux. All three methods assume that the natural fluctuation in temperature of the trunk near the point of insertion of heating and sensing elements is the same as that where reference thermocouples are inserted.
Using all three systems, 24 tree were simutaneously monitored during the HartX campaign. Tree size within the stand ranged between 18 and 61 cm circumference at breast height, while sample trees ranged between 24 and 55 cm circumference. The samllest trees could only be measured by utilizing the G-system. Sap flow rates of individual trees measured at breast height increased rapidly in the morning along with invreases in irradiance and vapor pressure deficit (D), decreased slowly during the course of the afternoon with continued increase in D, and decreased more slowly during the night. Ignoring potential effects introduced by he different methods, maximum flow rates of individual trees ranged between 0.5 and 2.5kg H2O h-1 tree-1 or 0.3 and 0.6 mm h-1 related to projected crown area of trees and daily sums of sap flow for individual trees varied between 4.4 and 24 kg H2O tree-1 d-1 or 1.1 and 6.0 mm d-1. Maximum sap flow rates per sapwood area of trees varied least for the G-system (11 - 17 g cm-2 h-1) and was of similar magnitude as the C-(8 - 21 g cm-2 h-1) and CS-system (4 - 14 g cm-2 h-1). Regressions of total tree conductannce (gt) derived from sap flow estimates demonstrated the same linear increase of conductance with increasing irradiance, however decrease of conductance with invreasing D under non-limiting light conditions was different for the three systems with strongest reduction of gt measured with the CS-system followed by the C- and G-system. This led to different estimates of daily sap flow rates especially during the second part of the measurement period. Variation in sap flow rates is explained on the basis of variation in leaf area index of individual trees, heterogeneity in soil conditions, and methodological differences in sap flow measurements. Despite the hightly uniform plantation forest at the scale of hectares, the heterogeneity in tree size and soil depth at the scale of sqare meters still make it difficult to appropriately and efficiently select sample trees and to scale-up water flux from individual trees to the stand level.
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Lu, P., Biron, P., et al. 1995, 'Water Relations of Adult Norway spruce (Picea abies (L.) Karst) under Soil Drought in the Vosges Mountains: Water Potential, Stomatal Conductance and Transpiration.', Annals of Forest Science, vol. 52, pp. 117-129.
The effects of soil water depletion on sap flow, twig water potential, stomatal and canopy conductance were analysed in 2 plots of a 30-year-old stand of Norway spruce. One was subjected to an imposed drought; the other was watered by irrigation. Predawn water potential in trees from the dry plot decreased to -1.2 MPa. In the watered plot, a low between-tree variability of sap flux density was observed, with maximum values of 1.2-1.9 dm3 dm-2 h-1, corresponding to about 0.5 mm h-1. Tree transpiration and stomatal conductance showed a strong reduction in association with drought development, during which the predawn water potential decreased from -0.4 to -0.6MPa. Canopy conductance was calculated from the reverse of the Penman-Monteith equation assuming that vapour flux over the stand was equal to the estimated stand sap flow. Effects of climatic factors and drought on canopy conductance variations were take into account in a multi-variable transpiration model.
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Ringersma, J., Mechergui, M., et al. 1996, Transpiration Measurements in Date Palms using the Granier Method, American Society of Agricultural Engineers, Proceedings of the International Conference. 3-6 Nov. 1996 pp. 141-145.
This paper describes the use of the Granier method for determination of the sap flow in date palms. Measurements were taken over two periods, one in the summer and one in the autumn. The aim of the research was to determine whether the Granier method is suitable for determining of the transpiration of date palms. Measurements were conducted at places both high and low on the trunk and both deeply and superficially in the xylem. The results so far have produced knowledge of the sap flow pattern, and a reasonable indication of absolute values for water movement in the whole palm.
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Simpson, D.G. 2000, 'Water Use of Interior Douglas-fir', Canadian Journal of Forest Research, vol. 30, pp. 534-547.
Water use of individual Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) trees was measured in two plots at a forest site in southern British Columbia, Canada. Average daily early summer water use by trees with diameters of 7.5-70 cm varied from 1.8 to 166 L. Sap flux density (cm2 water/cm2 sapwood per hour) was linearly related to shoot xylem pressure potential and was found to invrease with increasing vapour pressure deficit (VPD) and short-wave irradiance (I), reaching maximum rates with VPD>0.6 kPa and I>200 W m-2. Daily sap flux density varied among trees but was not related to tree diameter, so an average value of 113704 L m-2 sapwood area was used to estimate average early summer stand trarnspiration for the two plots of 1.08 and 1.5mm d-1. A close curvilinear relationship (r2=0.85) was found between stem coss-sectional area increment and sapwood area. The relationshop was only slightly better (r2=0.89) between are increment and early summer individual tree water use. Stand volume growth for 1988-1998 for the two plots was 36-47 m3 ha-1. Stem volume relative growth rate over this 10-year period is estimated at 0.027 and 0.029 m3 m-3 a-1.
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Smith, D.M. and Allen, S.J. 1996, 'Measurement of Sap Flow in Plant Stems', Journal of Experimental Botany, vol. 47, no. 305, pp. 1833-1844.
Transpiration rates for the whole plants, individual branches or tillers can be determined by techniques which measure the rate at which sap ascends stems. All of these methods use heat as a tracer for sap movement, but they are fundamentally different in their operating principles. Two methods commonly employed, the stem heat balance and trunk sector heat balance methods, use the heat balance principle; the stem is heated electrically and the heat balance is solved for the amount of heat taken up by the moving sap stream, which is then used to calculate the mass flow of sap in the stem. In the heat-pulse method, rather than using continuous heating, short pulses of heat are applied and the mass flow of sap is determined from the velocity of the heat pulses moving along the stem. In addition, rates of sap flow can be determined empirically, using the thermal dissipation technique, from the temperature of sapwood near a continuously-powered heater implanted in the stem. Users must understand the therory underlying each of these methods, so that they can select the method most appropriate to their application and take precautions against potential sources of error. When attempting to estimate transpiration by stands of vegetation from measurements of sap flow in individual plants, users must also select an appropriate sampling strategy and scaling method.
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