Báo cáo khoa học: "Estimating tree canopy water use via xylem sapflow in an old Norway spruce forest and a comparison with simulation-based canopy transpiration"

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Original article Estimating tree canopy water use via xylem sapflow in an old Norway spruce forest and a comparison with simulation-based canopy transpiration estimates Barbara Köstner Eva M. Falge, Martina Alsheimer. Ralf Geyer, John D. Tenhunen Bayreuth Institute for Terrestrial Ecosystem Research (BITÖK), Department of Plant Ecology II, University of Bayreuth, 95440 Bayreuth, Germany 15 20 October 1997) (Received January 1997; accepted Abstract - Tree xylem sapflow rates of 140-year-old Norway spruce (Picea abies) were scaled to the stand level and compared to canopy transpiration predicted by the stand gas exchange model STANDFLUX. Variation in sapflux densities between individual sensors was high (coef- ficient of variance 0.4) and included both variation within and between trees, but it was not dif- = ferent between two applied sapflow methodologies (radial flowmeter according to Granier, vari- able heating tissue heat balance method according to Cermák and Kucera). During the morning, a time-lag of typically 2 h elapsed between sapflow (E and predicted canopy transpiration rate ) f (E During this time total water iusehe as as high as 0.3 mm, which was l14 Lthan the estimated ess ). p of w capacity easily available water n t tree canopy (0.45 mm, on average per tree). Canopy conductance derived from stand sapflow rates (g and from STANDFLUX (g was in the same ) f ) p deficit of the air range (g 10 mm s but a stronger decline with increasing vapor pressure ), -1 : tmax * Correspondence and reprints Abbreviations: CBH: tree circumference at breast height; CV: coefficient of variation; DBH: tree diameter at breast height; D: vapor pressure deficit of the air; D daily maximum half-hour : max average vapor pressure deficit of the air; D average vapor pressure deficit during night; dw: : dark dry weight; E forest canopy transpiration rate; E forest canopy transpiration rate derived with : c : f time shift in xylem sapflow; E forest canopy transpiration rate predicted from STANDFLUX : p total conductance derived from shifted Model; g canopy conductance; g xylem sapflow; g : c : f : ns total conductance derived from non-shifted xylem sapflow; g canopy conductance predicted from : p o measurement STANDFLUX model; g total conductance from canopy t height of D; g : t : tmax maximum total canopy conductance; J: sapflux density (sapflow rate per sapwood area); LAI: pro- jected canopy leaf area index; LS: total leaf surface; LW: leaf dry weight; PPFD: photosynthetic photon flux density; SD: standard deviation; SE: standard error; SWA sapwood area at breast : bh height; SWA sapwood area below live crown; SWV: sapwood volume. : blc
(D) was observed for g as compared to g with current model parameterization. Tree water f p uptake measured by xylem sapflow was higher during spring and somewhat lower during sum- mer compared with E Seasonal sums of transpiration from April to October amounted to . p 108 and 103 mm seasonfor E and E respectively. Estimated tree water uptake during night -1 f , p increased with D up to 0.5 mm per dark period (on average 16 L per tree) which was 10-140 % of total daily flux. Because this flow rate did not increase with further increases in D during night, it is concluded that it reflects the refilling of easily exchangeable water in the trees rather than a rate of night transpiration. (© Inra/Elsevier, Paris.) forest transpiration / forest conductance / night water uptake / stand gas exchange model / Picea abies Résumé - Estimation de la consommation en eau des arbres à partir de la mesure du flux de sève brute dans un peuplement âgé d’épicéa, et comparaison avec un modèle de trans- piration du couvert. Les mesures de flux de sève brute réalisées dans un peuplement d’épicéa (Picea abies) âgé de 140 ans ont été extrapolées à l’échelle du peuplement et comparées à la transpiration du couvert prédite par le modèle Standflux. La variabilité des densités de flux entre les mesures individuelles était élevée (coefficient de variation de 0,4), liée aussi bien à la varia- bilité intraarbre qu’interarbres, mais les mesures ne différaient pas entre les deux méthodes uti- lisées (fluxmètre radial de Granier, et bilan d’énergie à chaleur variable de Cermak et Kucera). Au cours de la matinée, un déphasage, atteignant typiquement 2 h, se produisait entre le flux de sève (E et la transpiration prédite (E L’équivalent en eau correspondait à 0,3 mm pour cette ) f ). p durée, ce qui est inférieur à la quantité d’eau facilement disponible dans le couvert des arbres (0,45 mm, soit en moyenne 14 L par arbre). La conductance de couvert, calculée à partir des mesures de flux de sève du peuplement (g et du modèle Standflux (gétaient du même ordre ) f ), p de grandeur (g 10 mm s-1), mais une décroissance plus forte, en relation avec l’augmenta- max: tion du déficit de saturation de l’air (D), était observée pour g comparé à g avec la paramétri- f , p sation actuelle du modèle. La consommation en eau par les arbres mesurée à partir du flux de sève était plus élevée au printemps, et relativement plus faible en été, par rapport à p m c sai- E Les mumul . 103 saisonniers de transpiration entre avril et octobre ont atteint 108 mm saison et -1 -1 sonpour E et E respectivement. La consommation en eau par les arbres durant la nuit aug- fp , mentait avec D jusqu’à 0,5 mm par nuit (soit en moyenne 16 L par arbre), ce qui correspondait à 10 à 140 % du flux total journalier. Comme ce flux n’augmentait pas notablement au-delà d’un certain seuil de D pendant la nuit, il a été conclu que ce flux reflétait plus le remplissage du réservoir d’eau facilement échangeable des arbres plutôt qu’une véritable transpiration nocturne. (© Inra/Elsevier, Paris.) transpiration de la forêt / conductance / absorption hydrique nocturne / modèle de trans- piration / Picea abies 1. INTRODUCTION close to total system evapotranspiration [3]. In intensively managed forest ecosys- tems which show a patchy mosaic of Tree xylem sapflow rates scaled to the stands varying in age and structure, such as stand level provide an independent esti- the Lehstenbach catchment in our study mate of forest water use which can be [1], comparisons of old forest canopy referred to above canopy water vapor flux water use with water vapor fluxes mea- to separate the contribution of trees from other components [8, 20, 21, 31]. Tree sured by eddy covariance are difficult due transpiration estimated with a dry canopy to the small surface occupied by the old and added to a careful estimate of the for- forest stands in the catchment and because est floor component [56] sums to values the understory contribution is large. Dur-
ing 1994, water vapor fluxes from the for- 2. MATERIALS AND METHODS floor and from patches of the under- est story vegetation (Deschampsia flexuosa, Calamagrostis villosa, Vaccinium myr- 2.1. Study site and sample tillus) in a 140-year-old Norway spruce tree characteristics stand were estimated by lysimeters and chamber gas exchange techniques [57]. The study sites is located in the Lehsten- On summer days, areally integrated water bach catchment in the Fichtelgebirge (Northeast vapor fluxes below the tree canopy Bavaria/Germany; latitude 50° 9’N, longitude 11 ° 52’E) which comprises an area of ca 4 km 2 reached values of up to 1.1mm d which -1 with altitudinal variation from 877 m a.s.l. at equalled ca 40 % of total stand water loss. the Waldstein summit to 700 m at the discharge weir. About 90 % of the catchment is covered Furthermore, tree water storage changes with Norway spruce (Picea abies (L.) Karst.) in large trees during periods when tran- varying in age from young regrowth to stands spiration is observed via sapflow or at the up to 160 years [36]. Average annual temper- atures typically range between 5 and 6.5 °C leaf level [11, 44]. Storage changes and annual precipitation between 950 and 1 050 dynamically on a daily basis [10, 25, 47] mm. A relatively high number of foggy days well seasonal basis as continu- as as on a (100-200 per year) and a short vegetation and recharge of water con- depletion ous period is typical for the region ([40]; for general tent in the trees occurs from spring to win- infomation on climate of the Fichtelgebirge, ter in correlation with soil drying and see also Eiden [12]). wetting [6, 54, 55]. While diurnal changes Six stands ranging in age from 40 to 140 in tree water storage depend on a rela- years were chosen for transpiration studies tively small pool of easily available water [1].In this paper, data from the oldest site (Coulissenhieb) are presented. Characteristics in extensible tissues, seasonal changes in of sample trees used for sapflow measure- water content are related to the total ments are shown in table I; for stand charac- amount of extractable water in woody tis- teristics see table II. Stand density and stand [53]. sues basal area were determined for all 803 trees within the study area (2.5 ha; Gerstberger, In the following, unpublished). Leaf biomass (LW), total leaf we compare canopy surface (LS) and sapwood area below live estimated by xylem sapflow water use crown (SWA were determined by harvest ) blc methods with canopy transpiration pre- of five trees (LW(kg) = 27.56*CBH(m) , 2.51 dicted by a three-dimensional gas 2 r 0.96; LS(m 347.9*CBH(m) ), 2 2.35 2 r = = = exchange model STANDFLUX [13, 15]. 0.95; SWA r blc 2 , 1.83 )=0.017*CBH(m) 2 (m = STANDFLUX uses information on three- 0.97; Köstner and Fischer, unpublished). Total dimensional tree structure and temporal leaf surface was converted to projected leaf by division of 2.57 [39]. The average rela- variation in the profile of atmospheric fac- area tion of SWA was 0.52 (cf. 0.5 for bh /SWA blc tors to calculate spatial light interception Douglas fir in [54]), the average relation of and process-based gas exchange of three- tree height height of 25 trees was 0.58. /total blc dimensional canopy units. Estimates of Due to the relatively low cumulative sapwood stand xylem sapflow and modelled canopy area of the 140-year-old stand, the leaf transpiration are used to 1) investigate area/sapwood area ratio was highest at this principle differences in the water uptake site as compared to the younger sites in the catchment [1] . Sapwood area at breast height and canopy transpiration at various tem- (SWA was determined by two or three stem ) bh poral scales, 2) compare estimates of cores on 45 trees and by stem disks from the canopy conductance derived from both ) 2 (m bh (SWA five harvested trees = approaches, and 3) estimate tree water 1.98 2 0.032*CBH(m) r 0.82; n 50). Values , = = uptake during the night in relation to total from stem disks agreed with average values canopy transpiration. from stem cores. Good agreement between
methods also found using computer 2.2. Meteorological data was for non-destructive determina- tomography tion of total sapwood area of the trees [1]. Meteorological data were obtained from a Cumulative sapwood area of the stand was 30 m telescopic mast [30] located within the determined by the equation above using the stand. Photosynthetic photon flux density CBH of all trees (n 803) from the site. (PPFD) was measured at the top of the mast =
with linearized photodiodes (G1118, Hama- 2.3. Xylem sapflow matsu) calibrated against a LiCor quantum (Li 190SB, LI-COR, Inc., Lincoln, sensor Xylem sapflow of eight trees was measured Nebraska, USA). Air humidity, air temperature an electronically controlled constant tem- by and wind speed were measured at three perature difference system (tissue heat balance heights (30, 17 and I m) using VAISALA system, THB) constructed according to Cer- HMP-35 UTA humidity sensors (Vaisala, Fin- mák and co-workers [5, 35]. Sapflux density (J) land) with linearized thermistors and three- of five additional trees was measured by con- stant heating flowmeters according to Granier dimensional anemometers (ONZ-Windmesser, [18, 19]. The flux signals were measured every MeteolaborAG, Wetikon, Switzerland) with 10 s and 10-min averages were stored by a data high resolution propellers (YOUNG, Traverse logger. The sensors of the constant tempera- City, Michigan, USA). Data from 30 m height ture difference system covered the average sap- used as driving variables for the STAND- were wood depth (4 cm) while sensors of the con- FLUX model and to analyse dependencies of stant heating system covered 2 cm of sapwood stand sapflow on environmental variables. depth. No significant change in J measured in different depths (0-2, 2-4 cm) was observed Vapor pressure deficit (D) was calculated during the season. Sapflux density of the THB using the MAGNUS formula [7] with con- system was calculated by dividing tree xylem stants from Smithsonian Meteorological sapflow by estimated sapwood area of the tree. Tables [50]. Standard meteorological data were Maximum J of individual measurements was in also provided by the Department of Climatol- the same range for both methods (figure 1). (BITÖK, University Bayreuth; Gerchau, Accordingly, no systematic difference was ogy found between methods on a daily basis [1]. unpublished).
species for a required CV of 15%. A rel- Scaling from tree to stand level 2.4. tree ative deviation of ± 15 and 22 % from the mean was determined for 12 sample trees of old Scots scaling sapflow measurements from For pine and old Norway spruce [4]. forest stand xylem sapflow rates are sensor to related to structural scalars of the trees or stand. Due to high variation at the sensor level (see below), we calculated mean J from non-strat- 2.5. Estimation of canopy ified samples and used cumulative sapwood conductance from stand sapflow area of the stand (cumul. SWA to derive ) bh forest canopy transpiration (E ): f Canopy conductance derived from sapflow comprises the total water vapor measurements transfer conductance (g from the ’average’ ) t Variation in J of all forest sites measured canopy to the measurement stomata of the tree in the catchment was high and independent height of D [52], which includes both aerody- from tree size of codominant or emergent trees namic (g components of momentum and sur- : a (figure 2). This high variation in J was referred face boundary layer; e.g. [27]) and stomatal to within tree variation in sapwood distribu- components (g It follows: l/g l/g + l/g tca , ). c = tion, sapwood density or activity (highest ratio see Köstner et al. [32]. Because g is usually an a of two sensor records within one tree at breast order of magnitude larger than g in conifer- c height =1:3), and between tree variation in ous stands, E is controlled by g rather than c c tree size or leaf area. For a selection of five by g and, therefore, differences between g t a summer days with mean J ranging between and g are small. c 0.08 and 0.11kg cmd and a number of -2-1 To account for the delay of sapflow rates 55-58 codominant and emergent sample trees compared to transpiration rates, the onset of measured in the catchment, the coefficient of stand sapflow (E was simply fitted as a first ) f variation (CV) ranged between 0.41 and 0.46 approximation to the onset of predicted tran- independent of sapflow methodologies. spiration (E which corresponded to the onset ) p According to the corresponding t-value (two- of irradiance (PPFD > 25 μmol m s on dry -2-1 ) sided), e.g. t the sample size required for 55;0.05 days. Total canopy conductance was calcu- a CV of 15 % would amount to 30-38 while a lated from sapflow as follows [32]: usual sample size of between 11 and 9 trees corresponds to a CV of 25 to 30 %. Oren et al. [38] report sample sizes from 7 to 48 of various
Conversion factor k = G * T G gas 3. RESULTS AND DISCUSSION v Kv; = constant of water vapor (4.62 m hPa kg K 3 -1 -1 ), T = air temperature (Kelvin); values of D < 1 K Daily courses of E and g derived from ct hPa were excluded. different approaches are compared in fig- E increased strongly with ure 3. While p photosynthetic photon flux density 2.6. The STANDFLUX model (PPFD), the course of E was more similar f to the course of vapor pressure deficit of the air (D) (figure 3A, B). A time-lag of The STANDFLUX model [13, 15] inte- typically about 2 h on dry days elapsed grates three-dimensional information on stand between the onset of PPFD or E and E pf . structure and vertical information on stand This time-lag is related to the contribu- microclimate to compute spatial light inter- tion to transpiration of easily available ception and spatial canopy gas exchange. It consists of three nested components with a leaf water extracted from extensible tissue of or branch gas exchange module [ 14], a three- needles, bark and young xylem [9, 42, 45, dimensional single-tree light interception and 53, 59]. Rapid diurnal depletions of water gas exchange module, and the resulting three- are mainly related to changes in water con- dimensional forest stand gas exchange model. tent of the crown biomass, while seasonal depletions of stored water can be observed Gas exchange of foliage elements is in the stem [6, 54, 55]. described according to Harley and Tenhunen [24] based on estimates of leaf carboxylation, For the old spruce stand, a potential RuBP regeneration and respiratory capacities of 9 mm (280 L per tree) [ 16,17], and an empirical formulation for leaf amount extractable water in the stem (154 m 3 conductance [2]. The application to needled blc branch segments is described in Falge et al. -1 ha blc SWV * 0.6, for conversion of total [14]. Stomatal conductance is calculated as: SWV into available water according to Waring and Running [54]) and 2 mm (sum of water content in needles and branches) in the crown biomass is estimated. About 0.45 mm (on average 14 L per tree) of the with net CO fixation rate, NP (μmol m s -2 -1 ), 2 crown pool would be easily available relative humidity, h (decimal fraction), CO s 2 water (assuming 120 % rel. water content (ppm), empirically deter- partial pressure, Cs mined minimum conductance, g (mmol m -2 of needle dry mass, 80 % rel. water content min ), -1 s and gfac (dimensionless), describing the of branch dry mass and 10 % of total water empirically determined sensitivity of stomata to content as easily extractable water; see changes in NP, h and C [51]. Leaf conduc- s s table II for biomass estimates). Time-lags tance in subsections was scaled to the canopy between leaf transpiration and water flow by leaf area of subsections and tree classes, sensed in the xylem are determined by tis- defined by similarity in size, structure and phys- sue storage capacity while hydraulic iology, and based on structural measurements resistences influence the flux rate (e.g. at the site [13, 15]. A boundary layer conduc- tance (g is considered per canopy subsection, ) a [29]). Higher hydraulic resistances are estimated according to Nobel [37], modified usually observed in branches compared for conifers as suggested by Jarvis et al. [28] to the stem of Norway spruce [49]. and adopted to the given leaf area distribution Roberts [43] reported that hydraulic resis- in the canopy subsection [15]. From total tance of cut trees (Pinus sylvestris) placed canopy conductance (gp) canopy transpiration in water pots was only half that of control was calculated by multiplying g with D mea- p sured above the canopy [see equation (2)]: trees with intact root systems. Further, contribution of water stored in the trunk to transpiration was less for the trees in
water pots, obviously due to the lacking [22, 34, 58] play an important role. The of root resistance. However, a temporary variable power input of the THB system as removal of stored water in the upper stem well as the low power input of the con- was also observable in the cut trees, sug- stant heating system are probably less sen- gesting that most easily available reser- sitive to artificial thermal effects com- voirs of water are transpired first. pared to systems which apply constant heat around the whole stem [23]. There In our case, the sum of E during the p was also no apparent difference between first 2-3 h of summer days did not exceed time-lags measured with the constant heat- 0.2-0.3 mm (on average 6-9 L per tree), ing system and the variable heating sys- which is less than the estimated amount tem. of easily available water in the crown. For calculation of canopy conductance There is no strong evidence that artificial from stand sapflow, the course of E was f time-lags of thermoelectric heat balance shifted to the onset of E (figure 3B, E). p systems caused by heat storage in the stem
Conductance values derived from non- In figure4 daily courses of E and E f p shown for May and August 1995. Pro- shifted E (g result in significantly lower f ns ) are nounced differences between measured values during morning and midday (fig- and predicted values occurred during May. ure 3C, F). We are aware that simple shift- Water uptake of trees during spring could ing of the sapflow values cannot account be referred to refilling of storage capacities for effects of internal water storage on [54, 55]. In spring after rainy days up to 20 canopy transpiration during the whole April, initial sapflow started with increas- daily course. This would require direct ing temperature (> 20 °C) and increasing measurements and appropriate modelling D (> 15 hPa). During this period, sapflow of changes in water content and potential did not reach zero during night compared gradients [10]. Since conductance derived to lowest or no apparent flux on cold from sapflow inherently includes stom- (< 5 °C) or rainy days in the middle of atal, hydraulic and aerodynamic features, May. During August, hourly maxima of it should be understood as a particular, measured flux rates were lower than pre- specific measure. Nevertheless, for a use- dicted ones while more similar flux rates were obtained during July. In August, ful practical description good qualitative sapflow rates sensed during nights with characteristics of g are obtained in com- f relatively high D (10-15 hPa) did not parison to g of the gas exchange model. p reach minimum values as observed dur- Maximum values (see below) or values ing rainy days. of higher temporal integration [41] are useful for comparative or complementary Differences between E and Ep f studies. decreased with increasing temporal inte-
of the data. While discrepancies was restricted by soil drought, although gration effects of increased soil resistance on tree remained quite large on a daily basis (fig- water uptake during summer cannot be ure 5A), differences declined on a monthly fully excluded. Maximum soil suction basis (figure 5B). High water uptake of (400-600 hPa) occurred for short periods trees measured by xylem sapflow during in late July and August in the upper soil spring resulted in 26 mm month mea- -1 horizon (20 cm depth) but remained low sured in May compared to18 mm month -1 during the rest of the year (< 200 hPa), the model. In contrast, E was predicted by f while soil suction never exceeded 100 hPa slightly lower in August compared to E p in 90 cm depth (Lischeid, pers. comm.). (21 and 25 mm month for measured and -1 predicted values). Very similar estimates The relationship between g and D t of E were obtained in June (13), July (28 c derived from stand sapflow and predicted and 29), September (8 and 7) and Octo- from STANDFLUX is shown in figure 6. ber (4 mm month for measured and pre- -1 Generally, g from both approaches was t dicted values, respectively). Over the in the same range. In some cases, higher whole season from April to October, E c tree water uptake measured by sapflow in of both approaches agreed well but was May (cf. discussion on figure 4) resulted in generally relatively low (108 and 103 mm higher maximum conductances compared -1 seasonfor measured and predicted val- to modelled conductance. Different ues). Low rates of Ep resulted from low responses of g to D between May and tmax predicted light interception due to needle August are also explained by lower air clumping in the modelled canopy sections. temperature in May resulting in lower val- No seasonal changes in leaf physiology ues of predicted photosynthesis and g in p were included in the model and no drought May for the same value of D as compared effects were considered in the model pre- August. Daily mean temperature ranged to diction. There is no strong evidence that E c from 2 to 17 °C and from 13 to 21 °C in
May and August, respectively. In August, during night when stomata are not com- g was more reduced with increasing D pletely closed. Predicted night transpira- f than g showing a stronger curvilinear tion of STANDFLUX, based on empirical p decline of g Although the determination . f estimates of minimum conductance (g ) min of canopy conductance from stand sapflow [14] was not necessarily zero during the remains critical without correction of night but it was generally much less than changes in capacity throughout the daily measured sapflow during the night (see course (cf. discussion on figure 3 C, F), below). The amount of water taken up by the values are comparable to the range of trees during the dark period (E was ) dark values for Picea abies summarized by calculated for the different seasons by Schulze et al. [48]. adding the flow rate from sunset to sunrise (defined as PPFD < 25 μmol m s-2 -1). During this study of old Norway spruce Typical amounts of xylem sapflow dur- frequent sapflow was monitored during ing the night of dry summer days ranged night (E This water uptake is related ). dark between 0.2 and 0.4 mm. E was posi- dark to refilling of tissues and to transpiration
range of E values from10 to 140 % correlated with the total daily tively f /E dark of E (figure 7A) but did not (figure 7C). However, the absolute amount f amount exceed a certain threshold (ca 0.5 mm) in of E was not related to the amount of dark summer. Although, the estimation of E dark precipitation indicating no strong effect is critical owing to uncertainties of ther- of soil moisture on tree water uptake dur- moelectric methods in determining the ing the night. Under conditions of higher zero line of sapflux, the values of E dark soil water depletion, rain events during are reasonable in relation to estimations the night may play a more important role of easily available water storage in exten- in stem refilling [41 ]. Further, an increase sible tissue. E was also correlated with dark of g with increasing E ratio was f /E dark fmax increasing D in the night (D up to ca ) dark observed (figure 7D). The relationship 5 hPa but no further increase of E with dark also holds if E is correlated to g f /E dark tmax increasing D was observed (figure 7B). dark of the following day. This demonstrates The ratio of E during dry summer f /E dark that the status of actual tree water storage days (for E > 0.5 mm d was on average ) -1 f could control maximum conductance 30 % while low rates of n rainy, foggy f oE or cold days were associated with a high reached during the day [53].
4. CONCLUSIONS sapflow methodology monitoring water uptake during the night to investigate decoupling of bulk water flow in large Tree sapflow rates scaled to the stand from canopy water vapor flux driven trees level and canopy transpiration predicted by short-term changes in atmospheric con- by a stand-level model based on canopy ditions. gas exchange were used to analyse prin- ciple differences in tree water uptake and canopy transpiration. Considering uncer- ACKNOWLEDGEMENTS tainties in estimating stand sapflow both approaches agreed on a daily basis We thank A. Suske and G. Müller for their throughout the season. Relative differ- technical assistance and two reviewers for their ences between the approaches occurred helpful comments on the manuscript. Finan- over the season. We conclude that differ- cial support was provided by the German Min- ences in spring are influenced by changes istry for Research and Technology (BEO 51- in tree water storage due to higher tree 0339476 A) and the Bavarian Climate Research Program BayFORKLIM. uptake compared to canopy tran- water spiration. On the other hand, model param- eterization might not correctly reflect sea- REFERENCES sonal trends in leaf physiology. Changes in tree water storage are also Alsheimer M., Köstner B., Tenhunen J.D., [1] involved in estimating canopy conductance Canopy transpiration of Norway spruce stands (Picea abies [L.] Karst.): seasonal trends and from stand sapflow compared to conduc- stand differences, Ann. Sci. For. 55 (1998). tance derived from canopy photosynthesis. Ball J.T., Woodrow I.E., Berry J.A., A model [2] As a practical approximation the course of predicting stomatal conductance and its con- tribution to the control of photosynthesis sapflow rates can be shifted to the onset of under different environmental conditions, in: transpiration to obtain useful estimates of g f Binggins I. (Ed.), Progress in Photosynthe- comparable to predicted estimates. But sis Research, Vol IV.5, Proc. of the VII Inter- because conductance from sapflow data national Photosynthesis Congress, 1987, pp. 21-224. inherently includes stomatal, hydraulic and Bernhofer C., Gay L.W., Granier A., Joss U., [3] aerodynamic features, it should be under- Kessler A., Köstner B., Siegwolf R., Ten- stood as a specific measure complemen- hunen J.D., Vogt R., The HartX-Synthesis: tary to leaf or surface conductance. An experimental approach to water and car- bon exchange of a Scots pine plantation, The- In large trees water uptake was oret. Appl. Climatol. 53 (1-3) (1996) 173-183. recorded during the entire night period Cermák J., Cienciala E., Kucerá J., Lindroth [4] indicating refilling of xylem and extensi- A.. Bednárová, Individual variation of sap- ble tissues, and possible transpiration dur- flow rate in large pine and spruce trees and ing the night. Because maximum water stand transpiration: a pilot study at the central NOPEX site, J. Hydrol. 168 (1995) 17-27. uptake during night increased with D, but Cermák J., Deml M., Penka M., A new [5] confined to a certain threshold (ca was method of sap flow determination in trees, 0.5 mm), we conclude that it indicates Biol. Plant 15 (1973) 171-178. storage capacities rather than night tran- Clark J., Gibbs R.D., Studies in tree physiology. [6] IV. Further investigations of seasonal changes spiration. The high ratio of E (up to f /E dark in moisture contents of certain Canadian forest 50 %) during dry summer days stresses trees, Can J. Botany 35 (1957) 219-253. the importance of storage capacities. In Deutscher Wetterdienst (Ed.), Aspirations- [7] future research more emphasis should be Psychrometer-Tafeln, 5, Auflg., Vieweg, Braunschweig, 1976. laid on the dynamics and quantification Diawara A., Loustau D., Berbigier P., Com- [8] of storage capacities at the tree and stand parison of two methods for estimating the level [10] and on the improvement of evaporation of a Pinus pinaster (Ait.) stand:
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