Original article

Sapflow measurements in forest stands:methods and uncertainties

Barbara Köstnera André Granierb Jan Cermákc

aDepartment of Plant Ecology II, Bayreuth Institute for Terrestrial EcosystemResearch (BITÖK), University of Bayreuth, 95440 Bayreuth, Germany

bCentre de recherches forestières, Inra, Champenoux, BP 35, 54280 Champenoux, FrancecInstitute of Forest Ecology, Mendel’s Agricultural and Forest University,

Zemedelska 3, CS-61300 Brno, Czech Republic

(Received 15 January 1997; accepted 20 October 1997)

Abstract-This paper discusses the respective advantages and disadvantages of three sapflow tech-niques used for measuring tree transpiration in forests: heat pulse velocity, tissue heat balance (Cer-mák-Type), and radial flowmeter (Granier-Type). In the EUROFLUX programme, aiming atanalysing and modelling water and CO2 fluxes above European forests, the two latter techniquesare used at several sites. These two techniques were compared on the same trees, and resulted insimilar flux estimates. Principal problems of the methods are linked with the influence of natu-ral thermal gradients in the trunks and with effects of heat storage and conduction within thetissue. Sapflow probes can be typically left in place during one vegetation period, without anyapparent modification of water transfer properties of the xylem. Different sources of sap flux vari-ability related to temporal and spatial scale are discussed. Accuracy of sapflow estimates at thestand level can only be achieved by appropriate sample size of flux measurements and struc-tural scalars. In a homogeneous, untreated stand, the appropriate sample size is usually about tenbut increases depending on species, conducting type of the xylem and spatial heterogeneity of thesite. It is recommended to combine sapflow measurements with eddy covariance techniques in orderto separate tree transpiration from total forest water vapor flux and to examine spatial heterogeneityof fluxes within forest stands. (© Inra/Elsevier, Paris.)

xylem sapflow methods / calibration / comparison / scaling / forests

Résumé - Mesure du flux de sève dans les peuplements forestiers : méthodes et incerti-tudes. Cet article analyse les avantages et les inconvénients respectifs des trois principales mé-thodes de mesure du flux de sève dans les arbres: les impulsions de chaleur, et les méthodes à chauf-fage continu : bilan d’énergie du xylème (Cermák) et fluxmètre radial (Granier). Ces deux

* Correspondence and reprintsE-mail: barbara.koestner@bitoek.uni-bayreuth.de

dernières techniques sont utilisées en routine sur plusieurs sites dans le cadre du programmeEuroflux, qui porte sur l’analyse et la modélisation des flux d’eau et de CO2 au-dessus des forêts

européennes. Ces deux méthodes ont pu être comparées sur les mêmes arbres, et ont donné desrésultats très proches. Les problèmes majeurs dans l’utilisation de ces méthodes sont liés aux modi-fications du signal par les gradients thermiques qui existent naturellement dans le tronc desarbres, et par les phénomènes de stockage et de transfert de la chaleur dans les tissus. En géné-ral, ces capteurs peuvent rester en place dans les troncs pendant une saison de végétation, sansinfluence apparente sur les propriétés de transfert hydrique de la zone du xylème mesurée. Les dif-férentes sources de variabilité temporelle et spatiale des mesures de flux de sève sont discutées ;on constate en général que la variabilité intraarbre est du même ordre de grandeur que celle entreles arbres. La précision dans l’estimation de la transpiration à l’échelle de la parcelle dépend dela taille de l’échantillon pour les mesures de flux et de la variable de changement d’échelle. Lenombre de capteurs de mesure à mettre en œuvre pour avoir une estimation acceptable de latranspiration d’une population homogène est de l’ordre de 10, mais ce nombre peut augmenter selonl’espèce étudiée, le type de tissu conducteur et avec l’hétérogénéité de la parcelle. En conclusion,il est recommandé, dans ces projets de recherche sur la mesure des flux d’eau et de carbone dansles écosystèmes forestiers, de combiner les mesures de flux de sève à la méthode des corrélationsturbulentes, pour pouvoir séparer la transpiration des arbres du flux total de vapeur d’eau, etpour analyser l’hétérogénéité spatiale des flux hydriques dans les peuplements forestier.(© Inra/Elsevier, Paris.)

flux de sève / étalonnage / comparaison / échelle / forêt

1. INTRODUCTION

Xylem sapflow techniques provide amean at the tree level to estimate foreststand transpiration [37, 63]. Sapflow ratesof trees scaled to forest canopy transpira-tion are used to compare tree transpirationin relation to water vapor flux from theforest floor and to total water vapor fluxmeasured above the forest canopy [27, 35,38, 51, 52, 80]. Total evaporation of a Scotspine plantation estimated from tree tran-spiration using different sapflow techniquesplus forest floor evapotranspiration wasnot different from above-canopy surfaceevaporation and varied in the same range(mean coefficient of variation, CV = 13 %)as total water vapor fluxes measured simul-

taneously by several eddy-covariance sys-tems above the canopy (CV = 16 %; seetable I). Apart from comparative mea-surements estimating water vapor flux offorest stands, sapflow techniques demon-strate tree vegetation activity separatelyfrom total surface evaporation and con-ductance. Especially in old forest stands,tree transpiration is often found to be sig-

nificantly lower than expected from totalevaporation rates [54, 55] and maximumtree stomatal conductance is only ca. 1/3 ofmaximum surface conductance [71]. Fur-ther, sapflow estimates demonstrate smallscale heterogeneity of water fluxes due tostand parameters such as age, size, densityof trees and species composition [2, 7, 22,62]. Combined measurements of eddycovariance and tree xylem sapflow canalso show coherent short-term fluctuations

(10-3 to 10-2 Hz) of sapflow and atmo-spheric momentum, temperature and airhumidity. Observations of emergentNothofagus trees suggested that the maxi-mum size of eddies to which the trees

responded were ca. 100 m and according tothe displacement events, fluctuations werebest correlated among neighboring trees[46, 70].

Within the frame of EUROFLUX, long-term eddy-covariance measurements willbe combined with tree sapflow monitoringat several experimental sites. Tree level esti-mates of water fluxes will aid us to 1) esti-mate the contribution of tree transpiration,2) verify estimates of forest floor evapo-

transpiration as the residual component oftotal stand and tree level fluxes, 3) examinespatial heterogeneity within forest stands, 4)assess temporal difference between soilwater uptake and canopy evaporation, and5) bridge missing data of above canopymeasurements. For this purpose, appropri-ate sapflow methods, various methodolog-ical and technical assumptions and prob-lems, as well as scaling procedures (cf.Jarvis [49]) are summarized. This paperaddresses the current discussion on sapflowmonitoring methods applied in forest stands(cf. Tenhunen et al. [75]).

2. APPROPRIATE METHODSAND TECHNIQUES

For continuous long-term measurementsof xylem sapflow in trees two differentthermal principles of sapflow methods areappropriate: the heat pulse velocity (HPV)and the tissue (THB) or stem surface(SHB) heat balance methods (for reviews

see Cohen [25] and Swanson [74]). HPVmethods (early descriptions in Huber andSchmidt [47], Marshall [60] and Swanson[73]) measure sap velocity by deliveringheat pulses from an active electrode andregistering temperature increase by ther-mocouples shortly above and below thepulsing electrode [24, 50, 74]. Absoluteflux rates are estimated by transformingsap velocity to sap flux density (for defi-nition of measures see Edwards et al. [30])via specific wood density and multiplyingflux density with conducting sapwood area.THB methods deliver heating current con-tinuously to a volume of xylem tissue,which is either an undefined volume of tis-sue surrounding a needle-type sensorinserted radially into the stem [331, 48, 61,76] or a better defined volume of tissueincluded between heating plates [26] orseveral heating elements placed in parallelinto the xylem [16-18]. To derive sap fluxdensity, an empirical calibration of tem-perature change versus the amount of waterflowing can be used. In figure I the rela-

tionship between temperature change andflux density observed for stems of varioustree species and an artificial stem (sawpowder) are shown (cf. Granier [31]).Absolute flux rates are obtained by mul-tiplying flux density with conducting sap-wood area [31, 32].

In the other case, mass of water flowingthrough a defined volume of xylem tissuecan be directly calculated via the physi-cal heat capacity of water [16]. Heatingpower can be applied constantly whilevariable change of temperature differenceis registered as analysed in detail byKucera et al. [56]. Advantages of variableheating systems are due to the fact thataccuracy does not depend on absolute fluxrates and that the thermal equilibrationrate between heated stem and surroudingsremains constant [40]. Disadvantages ofvariable heating systems are connectedwith higher power requirements and highercosts of the equipment. SHB systems mea-sure total stem water flux using a heatingshield wrapped around the stem [4, 19,57, 68, 72]. These systems use differentapproaches of calculating sapflow. They

have the advantage of an already inte-grated measurement of the total stem. Fur-ther, no heating elements need to beinserted into the wood. However, SHB

systems are only appropriate for smallstems or branches.

All methods require assumptions onheat losses due to conduction. Techniquesaccording to Cermák using several heatingplates or Granier using needle-type sen-sors are more adapted to measurements inlarge than in small trees. The first onemeasures sapflow within a larger xylemvolume than the latter, which is morelocally dependent on xylem fluxes withinthe small volume surrounding the sensor.Therefore, the Cermák-Type system, espe-cially the electronically controlled, vari-able heating system, needs more energyfor heating than the Granier-Type. Bothsystems can be powered by batteries. Infield measurements, the techniques agreein the range of flux densities measured

[54] and in the daily course of sapflowwhen both techniques are applied in par-allel in the same tree (see figure 2). Com-parisons were conducted in seven trees

during periods up to 60 days. Average dif-ference in cumulative sapflow rate

between the techniques was ± 9 %. Nosignificant difference in daily flux ratesof both techniques was found (t-test ofpaired samples). Flux rates measured bydifferent techniques varied in the samerange as flux rates measured by systems ofthe same technique within a tree [1].

Most users of sapflow techniques agreethat there exists no unique technical solu-tion for all tree sizes and all tree species.The techniques exhibit advantages or dis-advantages in different directions [38, 74].Tree xylem differs in conducting type, e.g.tracheids in conifers, diffuse- or ring-porous types in deciduous trees. Forinstance, Granier-Type systems, whenused on species bearing narrow sapwood,such as Fraxinus excelsior (Granier andPeiffer, unpublished) showed an underes-timation of the actual tree transpiration.Otherwise, when used in species with deepsapwood such as pine trees, sensors at dif-ferent depths have to be used for an accu-rate estimate [37, 53, 62]. At the tree level,THB/SHB techniques are inherently moreappropriate for quantification than HPVtechniques. Uncertainties of HPV meth-ods are associated with point sampling,probe separation, and estimation of wounddiameter and volumetric water content

[29, 44]. Otherwise, HPV systems are lessaffected by thermal imbalances, requirelowest power supply and, therefore, maybe left unattended for longer periods.

Commercial distributers of HPV-sys-tems are for instance GreenSpan (Victo-ria, Australia) and IMKO GmbH (Ettlin-gen, Germany). THB or SHB systemsaccording to Cermák and Kucera [ 16, 18,54] using an electronic controller of bothconstant power or constant temperaturedifference are distributed by Kucera(Environmental Measuring Systems Inc.,Brno, Czech Republic). THB-systemsaccording to Granier [30, 31] are com-mercially distributed by UP GmbH (Land-

shut, Germany) and in a new technicalversion by Dynamax Inc. (Huston TX,USA).

3. TECHNICAL PROBLEMSASSOCIATED WITH THERMALPROPERTIES OF THE TISSUE

The quantification of sapflow withTHB systems relies on the assumption thattotal heat dissipation by conduction of thewood and by convection of sapflow isknown. Heat dissipation by conduction isdetermined as basic heating (Qfictive, cf.Cermák et al. [17]) or as maximum tem-perature difference (ΔTmax, cf. Granier[32]) during periods when sapflow isabsent (zero-flux). However, temperaturesources other than heating sensors suchas sun beams, temperature changes of thexylem water from root to above-groundlevels, or effects of heat storage in thestem can affect the artificially establishedtemperature gradients. Effects of heat stor-age in the stem or differences in heat con-ductance between day and night can falsifythe zero-flux determination. Also, freezingof the xylem provokes troubles due tolarge heat exchanges during the freez-ing-sawing phases [69]. Therefore, inaddition to means such as insulation mate-rial and compensating thermocouples [11],it is suggested that temperature control beintroduced into heat balance methods [40,79]. This seems practicable at least insmall trees. Up to now, investigations inlarge forest trees have not been sufficientto assess the potential error of sapflowestimates related to heat storage effects.The correction of heat storage terms

according to the results from an artificialtree model (acrylic glass filled with saw-dust) as shown by Herzog et al. [45] doesnot seem appropriate because maximumflux densities obtained in the model treereached only 20 % of realistic flux densi-ties in coniferous trees. Considering suc-cessful comparisons of sapflow measure-

ments at the stand level, heat storageeffects do not seem to be of major impor-tance. However, improvements made onthis subject could decrease variability insapflow rates.

Problems of natural thermal gradients inthe stem seem to be more pronouncedwhen sapflow is measured close to the soilsurface (especially in open stand condi-tions as in orchards or in thinned stands),where steep temperature gradients betweentrunk and soil usually develop. Naturaltemperature gradients in the stem areobserved for minutes or hours when colder

xylem water from roots passes the lowerreference sensor and gradients declineagain when the water reaches the uppersensor [18, 64]. During an experiment inan old Norway spruce stand in the Bay-erische Wald/Germany, eight spruce andfour beech trees were measured with avariable heating system (Cermák-Type).Natural temperature gradients were mon-itored between days of sapflow measure-ments [65]. Xylem sapflow rates were cor-rected according to natural temperaturedifferences monitored between days ofactive sapflow measurements. On anhourly basis corrected sapflow rates relatedto uncorrected rates changed up to ± 25 %in spruce and up to ± 100 % in smallerbeech trees. On a daily basis, average dif-ferences of corrected related to uncorrected

sapflow rates amounted to ± 14 % inspruce and ± 25 % in beech. In most cases,the coefficient of variation (CV) of thedaily rates could be reduced from ca.± 20 % of the uncorrected to ± 10 % ofthe corrected flow rates. On the same daycorrected flow rates of individual treeswere both higher or lower than uncorrectedones. Therefore, the effect of correctionon absolute flux rates was more or less

compensated at the stand level and overperiods of several days (Köstner et al.,unpublished). Similar observations weremade using the Granier system in a beechstand, corrections being made from the

natural gradients either measured duringthe previous days when sensors were notheated, or by using a switched power sup-ply allowing the measurement of temper-ature difference between the two probesfor 30 min under constant heating, and for30 min without heating (Granier, unpub-lished).

The experience of many users of theGranier-Type sensor has shown that sen-sors can practically be used for one vege-tation period or even longer. However, theinjury of the cambium modifies its activ-ity and probes may need to be replacedevery year. This is also true of the Cer-

mák-Type system. Although, histologicalinvestigations of wood tissue from Piceaabies measured for one vegetation periodby the variable heating system revealedthat xylem damage and increased resinflow was confined to a few tracheid rows

along the heating plates, cambium activitywas seriously affected around and betweenthe heating plates, obviously due to highelectrical tension (up to 100 V; Köstnerand Mehne-Jakobs, unpublished).

4. SCALING OF SAPFLOWMEASUREMENTS

The general question of accuracy ofsapflow techniques refers both to the levelof the sensed area or individual plant andto the level of plant stands. At the plantlevel, quite successful comparison stud-ies of sapflow techniques with other inde-pendent methods (gravimetry, potometry,isotope tracing, porometry) could bedemonstrated (e.g. Dugas et al. [28]), butconditions assessed during the compar-isons (variation in sap flux density andconducting sapwood area, thermal prop-erties of the tissue) may change in largetrees and during long-term measurements.Sapflow measurements in large trees haveto be proved at the stand level by com-parison with micrometeorological flux

estimates. At the stand or watershed level,however, variability increases with anincrease in spatial heterogeneity and addi-tional sources of evaporation or transpi-ration. Also, stand-level sap flux estimatesand eddy-correlation measurements orwater balance methods do not refer to thesame time and spatial scale. For instance,in a catchment study of Norway spruce,Peschke et al. [65] could show simultane-ously the increasing phase-shifts in thediurnal oscillation of irradiance, leaf tran-spiration, trunk sapflow, trunk radius, soilmatrix potential and runoff at the weir.Despite these difficulties related to tem-poral and spatial scales, comparative mea-surements derived from different scalesare very useful for improving the quality ofresults and their interpretation andstrengthening the plausibility of quantita-tive estimates.

4.1. From sensor to tree

Variation in xylem sap flux within treesdepends on xylem structure and sapwoodcross-sectional area. Sapwood depth andnumber of growth rings within sapwoodare important factors affecting flux den-sity [29]. Depending on site conditionsand tree species, sapwood cross-sectionalarea within trees (and within the stand)may vary from rather regular (e.g. plan-tations, constant growing conditions) tovery irregular (cf. Phillips et al. [66]).

Some sapflow techniques (HPV andGranier-Type THB) require the determi-nation of cross-sectional sapwood area oftrees. In most cases, sapwood depth canbe determined by fresh cores showing dif-ferences in color or in transparency of sap-wood and heartwood due to differencesin water content. Radial profiles of sapflowvelocity characterizing the conducting sap-wood were determined by the stainingtechnique in large willow, oak, eucalyptusand spruce trees [ 19, 20] showing a range

of velocity patterns including those rapidlydeclining from maximum close to cam-bium to the heartwood and those withmaximum in mid sapwood (see summaryin Phillips et al. [66]). Further, computertomography [2, 41] can assess variabilityin sapwood cross-sectional distribution atthe living tree and thermal imaging anal-ysis [3] demonstrates changes in flux den-sities with sapwood depth.

As previously observed in oak speciesby a combination of short sapflowmeterswith thermal IR-imaging [36], prelimi-nary results from measurements in largediffuse-porous beech trees showed thatflux density is exponentially reduced fromouter to inner sapwood (figure 3). Noreduction in flux density up to a 4-cm sap-wood depth was observed in two hard-wood species (Quercus alba, Liquidambarstyraciflua) by Phillips et al. [66]. Never-theless, relationships of flux density andsapwood depth have to be investigatedwhen total flux of trees with large cross-sectional sapwood area is estimated.

The trunk base of a tree often exhibitsa more heterogeneous sapwood structureand geometry than higher parts of thetrunk. Accordingly, azimuthal variationin stem sapflow usually decreases in anupward direction. Experiments on Pinuspinaster using two sets of four sensorsinserted at two heights in the trunk showedCV in sap flux density of 35 % whensapflow was measured at 1.3 m, while CVdecreased to 14 % when measured belowlive-crown [59]. Studies on spruce, larch,pine and beech confirmed dependence ofsuch variation also on soil water supply -drought significantly increased variation insapflow within stems [13, 21, 25]. In somespecies, infections by fungi decreased tran-spiration rate and increased variation insapflow [14]. Hatton et al. [44] found thegreatest potential source of error in scalingfrom sensor to tree. To estimate the fluxrate of a single Eucalyptus tree (HPVmethod) 12 probes were needed to keep

the CV at 15 % when probes were placedrandomly, while sample size could bereduced to six when the probes were strat-ified by depth and quadrant within the tree.

4.2. From tree to stand

In order to scale sapflow rates from treeto stand, the sampling strategy should berelated to stand structure such as tree sizeand tree species distribution within thestand. If there is no information available,pre-studies are necessary to determine treesize distribution and tree/stand sapwoodarea and to assess the appropriate samplesize or plot area. Structural scaling fac-tors usually used are tree basal area, vol-ume or tree circumference [11, 18], thetree sapwood area [37] or leaf area [43].Cermák [10] could demonstrate thatsapflow rates related to solar equivalentleaf area were less prone to systematic

errors than flow rates related to stem orcrown size parameters. In dry habitats,leaf area may be a worse scalar owing topronounced changes in transpiration perleaf area during drought [44].

Within stand variability in sapflowdepends on species distribution, standstructure and soil properties. For instance,increased variability between trees wasobserved during drought and after thin-ning [7] and in stands exhibiting symp-toms of forest decline [37]. Scaling offluxes should be based on an appropriatenumber of trees representing the spatialdistribution of tree types or classes withina stand. A tree class can be related to

dimension, social position, leaf area orvitality of tree species. In various

monospecific stands, flux rates of samplemeans deviated from population mean inthe range of ± 7 to 22 % for a sample sizeof 8 to 12 trees [22, 24, 25, 42, 53, 54,58]. Oren et al. [62] report sample sizes

between 4 and 48 required for a CV of15 % of mean sapflux in various conifer-ous and broad-leaved stands.

5. SAPFLOW MEASUREMENTSIN ROOTS AND BRANCHES

Sapflow measurements can be used tostudy repartitioning and temporal changesof water fluxes in branches or roots oftrees. This allows investigations of spa-tial flux variation within the tree canopy orthe root system. Differences in diurnaldynamics and specific sapflow occurredat the stem base compared to branches ofa large willow tree. Within branches,sapflow rate increased from shaded to sun-exposed parts of the crown [19, 72]. Fur-ther, it was observed on spruce that theuppermost quarter of the crown (consid-ering both length and needle dry weight)transpired as much as the residual crownbelow [15]. Similarily, it was shown onspruce, [1, 33] as well as on pine [61] thati) temporal variation in sap flux densitywas faster in branches than in the trunk,ii) higher flux density and higher flowrates were found in sun-compared toshade-exposed branches [19, 32, 72], andiii) increase in sapflow in the morning wasearlier in branches than in the trunk. For a

40-year-old spruce tree, leaf transpiration,branch sapflow and stem sapflow at dif-ferent heights are shown in figure 4.Whole branch transpiration was calculatedby modelling light interception and gas-exchange based on porometer measure-ments at the branch tip (Falge, unpub-lished). Total needle biomass of the branchwas about 0.5 kgdw, which equalled a nee-dle surface (total area) of ca. 7 m2.Depending on the assumptions on needleclumping of the branch, both slightlyhigher or lower rates of modelled transpi-ration were obtained compared to mea-sured branch sapflow. The time-lagbetween leaf transpiration measured at thebranch tip and sapflow at the branch base

was more pronounced (ca. 70 min,figure4B) than the time-lags between sapflowmeasurements at the branch base and dif-ferent heights of the stem. This is

explained by low absolute transpiriatonrates (max. daily rate ca. 0.1 kg h-1branch-1). During the first 70 min of themorning a total of only 0.02 kg branch-1were transpired which equals about10-20 % of the assumed capacity of easyavailable water in the branch.

For comparison of water fluxes in rootsand in the trunk of an apple tree, sapflowwas monitored in 14 main roots close tothe trunk base [8, 9]. Good agreement wasfound between the sum of fluxes in rootsand sapflow measured in the trunk. Fur-thermore, spatial variation in the rate ofwater absorbed by the roots was observedaround the trunk. This distribution wasmodified after irrigation.

6. CONCLUSIONS

Within the context of physical andchemical environmental change, materialfluxes in forest ecosystem became a focusof research. Xylem sapflow measurementsin trees are increasingly used to identifythe contribution of indivdual trees or theforest canopy to total water vapor fluxmeasured concurrently by eddy-correla-tion. For this reason, methods and uncer-tainties of sapflow monitoring methodsand scaling procedures applied in foreststands were described. From current

knowledge we make the following con-clusions.

For continuous long-term measure-ments of tree xylem sapflow both method-ological principles, THB as well as HPVmethods, are appropriate. While HPVmethods show higher variation due topoint measurements and higher relationof wound area to measured xylem tissue,integrative THB methods are more prone

to errors due to thermal imbalances, atleast during shorter time-constants.

Problems of accuracy associated withthermal properties of the wood are not suf-ficiently investigated in large trees of var-ious conducting types. More research isneeded on this subject.

Accuracy of sapflow measurementsscaled from sensor to tree can be provedby comparison with independent mea-surements. Such comparisons are usuallylimited by tree size.

Accuracy of sapflow estimates at thestand level can only be achieved by appro-priate sample size of flux measurementsand structural scalars. Analysis of waterflux components derived from differentlevels of integration should be based onsound statistical evaluation.

Finally, flux programmes such asEUROFLUX are encouraged to includesapflow measurements in order to have acomplementary, analytical tool for sepa-rating tree transpiration from total water

flux and assessing the range and dynamicsin temporal variability and spatial hetero-geneity of transpiration within the stud-ied plot.

ACKNOWLEDGEMENTS

We thank all contributors to the discussionon the referred subject during the workshopon ’Water Flux Regulation in Forest Stands’from 14-18 September 1996 at the UniversityBayreuth/Thurnau. Especially, we thank RamOren for valuable comments on the manuscript.Financial support to B.K. was provided by theGerman Ministry for Research and Technol-ogy (BEO 51-0339476 A).

REFERENCES

[1] Alsheimer M., Charakterisierung räumlicherund zeitlicher Heterogenität von unter-schiedlichen Fichtenbeständen (Picea abies(L.) Karst.) durch Xylemflußmessungen, Doc-toral thesis, University Bayreuth, 1997.

[2] Alsheimer M., Köstner B., Falge E., Ten-hunen J.D., Temporal and spatial variationin transpiration of Norway spruce standswithin a forested catchment of the Fichtelge-birge, Germany, Ann. Sci. For. 55 (1998)103-123.

[3] Anfodillo T., Sigalotti G.B., Tomasi M.,Semenzato P., Valentini R., Applications of athermal imaging technique in the study of theascent of sap in woody species, Plant, Cell.Environ. 16 (1993) 997-1001.

[4] Baker J.M., van Bavel C.H.M., Measurementof mass flow of water in the stem of herba-ceous plants, Plant, Cell Environ. 10 (1987)777-782.

[5] Berbigier P., Diawara A., Loustau D., Amicroclimatic study of the effects of droughton evapotranspiration in a maritime pine standand its understorey, Ann. Sci. For. 48 (1991)157-177.

[6] Bernhofer C., Gay L.W., Granier A., Joss U.,Kessler A., Köstner B., Siegwolf R., Ten-hunen J.D., Vogt R., The HartX-Synthesis:An experimental approach to water and car-bon exchange of a Scots pine plantation, The-oret. Appl. Climatol. 53 (1-3) (1996)173-183.

[7] Bréda N., Granier A., Aussenac G., Effects ofthinning on soil and tree water relations, tran-spiration and growth in an oak forest (Quer-

cus petraea (Matt) Liebl), Tree Physiol. 15(1995) 295-306.

[8] Cabibel B., Continuity of water transfer insoil-plant system: the case of fruit trees,Agronomie 14 (1994) 503-514.

[9] Cabibel B., Do F., Thermal measurement ofsap flow and hydric behavior of trees. 2. Sapflow evolution and hydric behavior of irri-gated and non-irrigated trees under trickleirrigation, Agronomie 11 (1991) 757-766.

[10] Cermák J., Solar equivalent leaf area: an effi-cient biometrical parameter of individualleaves, trees and stands, Tree Physiol. 5(1989) 269-289.

[11] Cermák J., Kucera J., The compensation ofnatural temperature gradient at the measur-ing point during the sap flow rate determina-tion in trees, Biol. Plant. 23 (1981) 469-471.

[ 12] Cermák J. , Kucera J., Transpiration of fullygrown trees and stands of spruce (Picea abies(L.) Karst.) estimated by the tree-trunk heatbalance method, in: Swanson R.H., BernierP.Y., Woodward P.D. (Eds.), Proc. Forest.Hydrology and Watershed Measurements,Vancouver, Canada, August 1987, Publ.No. 167, IAHS-AISH, Wallingford, UK,1987, pp. 311-317.

[13] Cermák J., Kucera J., Scaling up transpira-tion data between trees, stands and water-

sheds, Silva Carelica 15 (1990) 101-120.

[14] Cermák J., Kucera J., Changes in transpirationof healthy mature trees due to environmen-tal conditions and of those with damagedwater conductive system, in: Klimo E.,Materna J. (Eds.), Proc. Cs MAB Natl.Comm-IUFRO Internat Workshop "Verifi-cation of hypotheses and possibilities ofrecovery of forest ecosystems", Beskydy Mt,Czechoslovakia, 4-8 Sept 1989, Agr. Univ.Brno, 1990, pp. 275-286.

[15] Cermák J., Kucera J., Extremely fast changesof xylem water flow rate in mature trees,caused by atmospheric, soil and mechanicalfactors, in: Raschi A., Borghetti M. (Eds.),Proc. CEC Internat Workshop "Methodolo-gies to assess the impact of climatic changeson vegetation: Analysis of water transport inplants and cavitation of xylem conduits",29-31 May 1991, Firenze, Italy.

[16] Cermák J., Deml M., Penka M., A newmethod of sap flow determination in trees,Biol. Plant 15 (1973) 171-178.

[17] Cermák J., Kucera J., Penka M., Improve-ment of the method of sap flow rate deter-mination in adult trees based on heat balancewith direct electric heating of xylem, Biol.Plant (Praha) 18 (2) (1976) 111-118.

[18] Cermák J., Ulehla J., Kucera J., Penka M.,Sap flow rate and transpiration dynamics in

the full-grown oak (Quercus robur L.) infloodplain forest exposed to seasonal floodsas related to potential evapotranspiration andtree dimensions, Biol. Plant (Praha) 24 (6)(1982) 446-460.

[19] Cermák J., Jenik J., Kucera J., Zidek V.,Xylem water flow in a crack willow tree(Salix fragilis) in relation to diurnal changesof environment, Oecologia (Berlin) 64 (1984)145-151.

[20] Cermák J., Cienciala E., Kucera J., LindrothA., Hallgren J.-E., Radial velocity profiles ofwater flow in stems of spruce and oak and

response of spruce trees to severing, TreePhysiol. 10 (1992) 367-380.

[21] Cermák J., Matyssek R., Kucera J., Rapidresponse of large drought stressed beech treesto irrigation, Tree Physiol. 12 (1993)281-290.

[22] Cermák J., Cienciala E., Kucera J., LindrothA., Bednarova E., Individual variation of sapflow rate in large pine and spruce trees andstand transpiration: A pilot study at the cen-tral NOPEX site, J. Hydrol. 168 (1995)17-27.

[23] Cienciala E., Lindroth A., Cermak J., Hall-gren J.-E., Kucera J., The effect of water

availability on transpiration, water potentialand growth of Picea abies during a growingseason, J. Hydrol. 155 (1994) 57-71.

[24] Cohen Y., Determination of orchard waterrequirement by a combined trunk sap flowand meteorological approach, Irrig. Sci. 12(1991) 93-98.

[25] Cohen Y., Thermoelectric methods for mea-surements of sap flow in plants, in: Stanhill G.(Ed.), Advances in Bioclimatology Vol 3,Springer-Verlag, Berlin, Heidelberg, NewYork, 1994, pp. 63-89.

[26] Daum C.R., A method for determining watertransport in trees, Ecology 48 (1967)425-431.

[27] Diawara A., Loustau D., Berbigier P., Com-parison of two methods for estimating theevaporation of a Pinus pinaster (Ait.) stand:sap flow and energy balance with sensibleheat flux measurements by an eddy covari-ance method, Agric. For. Meteorol. 54 (1991)49-66.

[28] Dugas W.A., Wallace J.S., Allen S.J., RobertsJ.M., Heat balance, porometer, and deuteriumestimates of transpiration from potted trees,Agric. For. Meteorol. 64 (1993) 47-62.

[29] Dye P.J., Olbrich B.W., Poulter A.G., Theinfluence of growth rings in Pinus patula onheat pulse velocity and sap flow measure-ment, J. Exp. Bot. 42 (1991) 867-870.

[30] Edwards W.R.N., Becker P., Cermák J., Aunified nomenclature for sap flow measure-

ments, Tree Physiol. 17 (1996) 65-67.

[31] Granier A., Une nouvelle méthode pour lamesure du flux de sève brute dans le troncdes arbres, Ann. Sci. For. 42 (1985) 193-200.

[32] Granier A., Evaluation of transpiration in aDouglas-fir stand by means of sap flow mea-surements, Tree Physiol. 3 (1987) 309-320.

[33] Granier A., Claustres J.P., Water relations ina Spruce (Picea abies L.) under natural con-ditions: spatial variation, Acta Œcol., Œcol.Plant 10 (1989) 295-310.

[34] Granier A., Loustau D., Measuring and mod-elling the transpiration of a maritime pinecanopy from sap-flow data, Agric. For. Mete-orol. 71 (1994) 61-81.

[35] Granier A., Bobay V., Gash J.H.C., Gelpe J.,Saugier B., Shuttleworth W.J., Vapour fluxdensity and transpiration rate comparisons ina stand of Maritime pine (Pinus pinaster Ait.)in Les Landes forest, Agric. For. Meteorol.51 (1993) 309-319.

[36] Granier A., Anfodillo T., Sabatti M., CochardH., Tomasi M., Valentini R., Bréda N., Axialand radial water flow in the trunk of oak trees:a quantitative and qualitative analysis, TreePhysiol. 14 (1994) 1383-1396.

[37] Granier A., Biron P., Bréda N., PontaillerJ.Y., Saugier B., Transpiration of trees andforest stands: short and long-term monitor-ing using sapflow methods, Global ChangeBiology 2 (1996) 265-274.

[38] Granier A., Biron P., Köstner B., Gay L.W.,Najjar G., Comparisons of xylem sap flowand water vapour flux at the stand level andderivation of canopy conductance for Scots

pine, Theor. Appl. Climat. 53 (1996)115-122.

[39] Grime V.L., Morison J.I.L., Simmonds L.P.,Sap flow measurements from stem heat bal-ances: a comparison of constant with vari-able power methods, Agric. For. Meteorol.74 (1995)27-40.

[40] Grime V.L., Morison J.I.L., Simmonds L.P.,Including the heat storage term in sap flowmeasurements with the stem heat balancemethod, Agric. For. Meteorol. 74 (1995)1-25.

[41 Habermehl A., Hüttermann A., Lovas G., Rid-der H.-W., Computer Tomographie von Bäu-men, Biologie in unserer Zeit 4 (1990)193-200.

[42] Hatton T.J., Vertessy R.A., Variability ofsapflow in a Pinus radiata plantation and therobust estimation of transpiration, Hydrol-ogy and Water Resources Symposium,Christchurch, New Zealand, 1989, pp. 6-10.

[43] Hatton T.J., Wu H., Scaling theory to extrap-olate individual tree water use to stand wateruse. Hydrol. Proc. 9 (1995) 527-540.

[44] Hatton T.J., Moore S.J., Reece P.H., Esti-mating stand transpiration in a Eucalyptuspopulnea woodland with the heat pulsemethod: measurement errors and samplingstrategies, Tree Physiol. 15 (1995) 219-227.

[45] Herzog K.M., Thum R., Zweifel R., HäslerR., Heat balance measurements - to quantifysap flow in thin stems only? Agric. For. Mete-orol. 83 (1997) 75-94.

[46] Hollinger D.Y., Kelliher F.M., Schulze E.-D., Köstner B.M.M., Coupling of tree tran-spiration to atmospheric turbulence, Nature371 (1994) 60-62.

[47] Huber B., Schmidt E., Eine Kompensations-methode zur thermoelektrischen Messunglangsamer Saftströme, Ber. Dtsch. Bot. Ges.55 (1937) 514-529.

[48] Ittner E., Der Tagesgang der Geschwindigkeitdes Transpirationsstromes im Stamm einer75-jährigen Fichte. Ecol. Plant. III (1968)177-183.

[49] Jarvis P.G., Scaling processes and problems,Plant, Cell Environ. 18 (1995) 1079-1089.

[50] Jones H.G., Harmer P.J.C., Higgs K.H., Eval-uation of various heat-pulse methods for esti-mation of sap flow in orchard trees: compar-ison with micrometeorological estimates ofevaporation, Trees (1988) 250-260.

[51] Kelliher F.M., Köstner B.M.M., HollingerD.Y., Byers J.N., Hunt J.E., McSeveny T.M.,Meserth R., Weir P.L., Schulze E.-D., Evap-oration, xylem sap flow, and tree transpirationin a New Zealand broad-leaved forest, Agric.For. Meteorol. (1992) 62, 53-73.

[52] Köstner B.M.M., Schulze E.-D., KelliherF.M., Hollinger D.Y., Byers J.N., Hunt J.E.,McSeveny T.M., Meserth R., Weir P.L.,Transpiration and canopy conductance in apristine broad-leaved forest of Nothofagus:an analysis of xylem sap flow and eddy cor-relation measurements, Oecologia 91 (1992)350-359.

[53] Köstner B., Biron P., Siegwolf R., GranierA., Estimates of water vapor flux and canopyconductance of Scots pine at the tree levelutilizing different xylem sap flow methods,Theor. Appl. Climatol. 53 (1996) 105-113.

[54] Köstner B., Alsheimer M., Falge E., GeyerR., Tenhunen J.D., Tree canopy water usevia xylem sap flow in an old Norway SpruceForest and a comparison with simulation-based canopy transpiration estimates, Ann.Sci. For. 55 (1998) 125-139.

[55] Köstner B., Alsheimer M., Wedler M., Schar-fenberg H.-J., Zimmermann R., Falge E., JossU., Tenhunen J.D., Controls on evapotran-

spiration in spruce forest stands, in: TenhunenJ.D., Lenz R., Hantschel R., Eds., Processesin Managed Ecosystems, Ecological Studies(1998), in press.

[56] Kucera J., Cermák J., Penka M., Improvedthermal method of continual recording thetranspiration flow rate dynamics, Biol. Plant.19 (1977) 413-420.

[57] Lindroth A., Cermak J., Kucera J., CiencialaE., Eckersten H., Sap flow by the heat bal-ance method applied to small size salix treesin a short-rotation forest, Biomass and Bioen-ergy 8 (1) (1995) 7-15.

[58] Loustau D., Berbigier P., Roumagnac P.,Arruda-Pacheco C., David J.S., Ferreira M.I.,Pereira J.S., Tavares R., Transpiration of a64-year-old maritime pine stand in Portugal.1. Seasonal course of water flux through mar-itime pine, Oecologia 107 (1996) 33-42.

[59] Loustau D., Domec J.C., Bosc A., An analy-sis of the spatial variability of sap flow den-sity in maritime pine, Ann. Sci. For. (1998)??-??.

[60] Marshall D.C., Measurement of sap flow inconifers by heat transport. Plant Physiol. 33(1958) 385-396.

[61] Nadezhdina N., Sazonova T.A., KaibiainenL.K., Specifications of the dynamics of sapflows and water potentials in leaves of appletrees under different water supply (in Russ),8th All-Union Symp. on Water Regime,Tashkent, 1984, p. 104.

[62] Oren R., Phillips N., Katul G., Ewers B.E.,Pataki D.E., Scaling xylem sap flux and soilwater balance, and calculating variance: amethod for partitioning water flux in forests.Ann. Sci. For. 55 (1997) 191-216.

[63] Pallardy S.G., Cermák J., Ewers F.W., Kauf-mann M.R., Parker W.C., Sperry J.S., Watertransport dynamics in trees and stands, in:Smith W.K., Hinckley T.H. (Eds.), ResourcePhysiology of Conifers, Academic Press,San Diego, New York, 1995, pp. 301-389.

[64] Penka M., Cermak J., Deml M., Water trans-port estimates in adult trees based on mea-surement of heat transfer by mass flow, ActaUniv. Agric. (Brno), Ser C, 42 (1973) 3-23.

[65] Peschke G., Rothe M., Scholz J., Seidler C.,Vogel M., Zentsch W., Experimentelle Unter-suchungen zum Wasserhaushalt von Fichten(Picea abies (L.) Karst.) Forstw Cbl 114(1995) 326-339.

[66] Phillips N., Oren R., Zimmermann R., Radialpatterns of xylem sap flow in non-, diffuse-and ring-porous tree species, Plant Cell Env-iron. 19 (1996) 983-990.

[67] Poschenrieder W., Die Bestimmung der Tran-spiration eines Fichtenaltbestandes (Piceaabies (L.) Karst.) mit der Konstant-Temper-

aturdifferenz-Methode unter Vergleich mitdem Eddy-Korrelationsverfahren, thesis, Uni-versity Bayreuth (1992).

[68] Sakuratani T., A heat balance method formeasuring water flux in the stem of intactplants, J. Agric. Meteorol. 37 (1981) 9-17.

[69] Saugier B., Granier A., Pontailler J.Y.,Dufrêne E., Baldocchi D.D., Transpirationof a boreal pine forest measured by branchbags, sapflow and micrometeorological meth-ods, Tree Physiol. 17 (1997) 511-519.

[70] Schulze E.-D., The regulation of plant tran-spiration: interactions of feedforward, feed-back, and futile cycles, in: Schulze E.-D.(Ed.), Flux Control in Biological Systems.From Enzymes to Populations and Ecosys-tems, Academic Press Inc, San Diego, 1994,pp. 203-237.

[71] Schulze E.-D., Kelliher F.M., Körner C.,Lloyd J., Leuning R., Relationships amongmaximum stomatal conductance, ecosystemsurface conductance, carbon assimilation rate,and plant nitrogen nutrition: A global ecol-ogy scaling excercise, Ann. Rev. Ecol. Syst.25 (1994) 629-660.

[72] Steinberg S.L., McFarland M.J., Worthing-ton J.W., Comparison of trunk and branchsap flow with canopy transpiration, J. Exp.Bot. 41 (1990) 653-659.

[73] Swanson R.H., Water transpired by trees isindicated by heat pulse velocity, Agric. Mete-orol. 10 (1972) 277-281.

[74] Swanson R.H., Significant historical devel-opments in thermal methods for measuringsap flow in trees, Agric. For. Meteorol. 72(1994) 113-132.

[75] Tenhunen J.D., Valentini R., Köstner B., Zim-mermann R., Granier A., Variation in forestgas exchange at landscape to continentalscales, Ann. Sci. For. (1997).

[76] Vieweg G.H., Ziegler H., ThermoelektrischeRegistrierung der Geschwindigkeit des Tran-spirationsstromes, Ber. Dtsch. Bot. Ges. 73(1960) 221-226.

[77] Vogt R., Gay L.W., Tenhunen J.D.,Bernhofer C., Kessler A., Eds., HartX’92 -Vegetation-Atmosphere Coupling of aScots Pine Plantation, Theor. Appl.Climatol. 53 (1996).

[78] Wedler M., Heindl B., Hahn S., Köstner B.,Bernhofer C., Tenhunen J.D., Model-basedestimates of water loss from ’patches’ of theunderstory mosaic of the Hartheim Scots pineplantation, Theor. Appl. Climatol. 53 (1996)135-144.

[79] Weibel F.P., Boersma K., An improved stemheat balance method using analog heat con-trol, Agric. For. Meteorol. 75 (1995) 191-208.

[80] Zidek V., Actual and potential evapotran-spiration in the floodplain forest, Ekologia(CSSR) 7 (1) (1988) 43-59.