Báo cáo khoa học: " Environmental and endogenous controls on leaf- and stand-level water conductance in a Scots pine plantation Neils Sturm Barbara Köstner Wolfram a John D. "

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Original article Environmental and endogenous controls on leaf- and stand-level water conductance in a Scots pine plantation Hartung Neils Sturm Barbara Köstner Wolfram a John D. Tenhunen a of Plant Ecology II, Bayreuth Institute for Terrestrial Ecosystem Research, Department University of Bayreuth, 95440 Bayreuth, Germany Julius-von-Sachs-Institut b für Biowissenschaften der Universität Würzburg, Lehrstuhl für Botanik I, Mittlerer Dallenbergweg 64, 97082 Würzburg, Germany (Received 12 March 1997; accepted 31 July 1997) Abstract - Measurements of leaf level gas exchange and conductance, tree transpiration via sapflow monitoring, soil moisture and water extraction, predawn water potential, and xylem abscisic acid (ABA) concentration were carried out over the course of the 1993 and 1994 sum- mer seasons at the Hartheim Pinus sylvestris plantation on the Upper Rhein Plain, Germany. Periodic leaf level conductance determinations with porometry established a maximum value of ca 280 mmol m s (13.6 mm s Half maximal conductance was attained at 40 μmol m -2-1 ). -1 -2 -1 sand 90 % of light saturation occurred at ca 500 μmol m s PPFD. Conductance decreased -2 -1 strongly with increases in vapor pressure deficit above 10 hPa, while the temperature optimum was 22 °C at light saturation. Strong restrictions on maximum conductance at both leaf and stand levels were apparent below a soil moisture content of 16 volume percent. Although less strongly, conductance also decreased with initial drying of the upper soil layers and decreases in predawn water potential from -0.4 to -0.6 MPa. In this range of water potential change, xylem ABA increased to between 200 and 500 nmol L Thus, an immediate leaf-level reaction to the onset . -1 of summer weather conditions is observed, i.e. leaf conductance and water use decrease. We hypothesize that ABA functions as a key control on water balance, transmitting information about soil water status and endogenously modifying canopy response in order to budget water and avoid extensive cavitation damage in most years. Transpiration potential of the stand was reduced by thinning during autumn 1993 in approximate proportion to changes in leaf area index and sapwood area. Simultaneous observations of sapflow and conductance have allowed us to view the effects of leaf conductance on whole plant water use, while thinning revealed the effects of stand level phenomena on conductance regulation. (© Inra/Elsevier, Paris.) conductance / transpiration / abscisic acid / drought / Pinus sylvestris * Correspondence and reprints Tel: (49) 921 55 5620; fax: (49) 921 55 5799; e-mail: john.tenhunen@bitoek.uni-bayreuth.de
Résumé - Contrôle environnemental et endogène de la conductance stomatique et du cou- plantation de pins sylvestres. Des mesures de l’échange du gaz et de la conduc- vert dans une tance stomatique de l’eau, de la transpiration par deux méthodes de mesure du flux de sève, de l’humidité du sol et de l’extraction de l’eau du sol, du potentiel hydrique foliaire de base et de la concentration en acide absissique (ABA) dans l’aubier ont été réalisées au cours des étés 1993 et 1994 dans une plantation de pins sylvestres dans la plaine rhénane au sud-ouest de l’Allemagne, près du village de Hartheim. Les mesures périodiques de la conductance stomatique ont montré une valeur maximale de 280 mmol m s (13,6 mm s Le demi-maximum de la conduc- -2-1 ). -1 tance stomatique était atteint pour un rayonnement de 40 μmol m s et la conductance était éta- -2 -1 blie à 90 % du maximum lors d’une exposition à 500 μmol m sLa conductance était dimi- -2 -1 . nuée rapidement dès que le déficit de saturation de l’air dépassait 10 hPa. L’optimum de la conductance était atteint pour une température de 22 °C, en condition de lumière saturante. Au- dessous d’une humidité volumique du sol de 16 %, la conductance foliaire ainsi que la conduc- du couvert étaient fortement limitées. La conductance diminuait aussi, mais moins fort, pour tance dessèchement initial des couches supérieures du sol, correspondant à une diminution de -0,4 un à -0,6 MPa du potentiel hydrique foliaire de base. Dans les limites de cette variation du poten- tiel hydrique, la concentration de l’ABA dans l’aubier est passée de 200 à 500 nmol l Ainsi, une . -1 réaction immédiate a pu être observée au niveau des feuilles au moment de l’installation des conditions estivales, c’est-à-dire une diminution de la conductance stomatique et de consom- mation en eau. Nous supposons que l’ABA occupe une position clé dans le bilan hydrique en trans- mettant des informations sur les conditions hydriques dans le sol et en modifiant la réponse du peu- plement à ces conditions pour maintenir le budget d’eau, et pour protéger les arbres contre des dommages durables causés par cavitation. La transpiration potentielle du couvert a été dimi- nuée par une éclaircie en automne 1993, approximativement proportionnellement aux modifications de la surface du bois d’aubier et de l’indice foliaire (LAI). Les mesures simultanées de flux de sève et de conductance nous ont permis d’examiner les effets de la conductance stomatique sur l’uti- lisation de l’eau à l’échelle de l’arbre, tandis que l’éclaircie révélait les effets des phénomènes à l’échelle du peuplement sur la régulation de la conductance stomatique. (© Inra/Elsevier, Paris.) stomatique / transpiration / acide abscissique / sécheresse / pin sylvestre conductance 1. INTRODUCTION increasing with increase in radiation, but decreasing with increase in leaf to air vapor pressure deficit and with decrease in The conductance for water vapor trans- soil water availability. fer from the vegetation to the atmosphere is a key parameter for describing ecosys- The strong correlation between leaf tem function and the environmental rela- CO and water vapor-exchange has been - 2 tions of plants. Due to tight atmospheric exploited to develop phenomenological coupling in forest stands, this conductance stomatal models [4-6, 31] which offer is dominated by time-dependent physio- promise in attempts to predict atmospheric coupling, forest stand growth and catch- logical processes governing the opening and closing of the stomata, which deter- ment water balances under conditions of mine patterns in water use, in energy bal- elevated atmospheric CO i.e. where car- , 2 ance, and in nutrient relations as well as bon allocation considerations have sug- the fixation of CO and uptake of pollu- gested the manner in which CO 2 -uptake 2 tants such as SO and O [25]. The rela- potentials may change. Tenhunen et al. 2 3 tionship or response of conductance at [45] demonstrated that the complex net both leaf and stand level to environmental photosynthesis response surface with variables is similar and reasonably well respect to radiation, temperature and vapor described [24, 26, 32, 51]; conductance pressure deficit along with an endogenous
’soil coupling’ influence could be effec- 2. MATERIALS AND METHODS tively related to daily, seasonal and annual changes in conductance of a Mediter- Measurements were conducted in a 35-year- ranean oak species subjected to a large old P. sylvestris L. (Scots pine) plantation in southwest Germany. The site is situated on the range in radiation, temperature and soil alluvial floodplain of the Rhine River 20 km availability. water west of Freiburg im Breisgau and close to the village of Hartheim. As a consequence of water Despite having gained knowledge dur- management measures in this region during ing recent decades of the primary factors the past 150 years, the bed of the Rhine River deepened by erosion and was subsequently influencing stomatal conductance in nat- sealed, such that vegetation on the alluvial ter- ural habitats, surveys demonstrate that races no longer has access to groundwater. Pre- large unexplained regional and continen- cipitation in the Upper Rhine Valley is strongly tal scale heterogeneity in response is found influenced by the north to south extension of for well-studied species (e.g. Ogink-Hen- the Vosges Mountains, which creates an obsta- cle to humid air masses from the main westerly drik [39], Peck and Mayer [41]and wind direction [40]. The shallow nature of the Alsheimer et al. [I]with respect to Nor- top soil layer and the high portion of coarse way spruce) which may be due to accli- textured soil increase the probability of extreme mation to natural gradients or to varying and extended drought exposure of the forest degrees of anthropogenic ecosystem [20]. Further information about the Hartheim impacts and manipulations [26, 42]. In plantation is given by Jaeger and Kessler [23]. Stand characteristics before and after thinning addition, species-specific sensitivity with in autumn 1993 are described in table I. respect to stress factors is poorly described, e.g. a literature search provided little infor- During the summers of 1993 and 1994, microclimate profiles were observed within mation on the shape of the response func- the Hartheim forest stand. Meteorological data tion for Pinus sylvestris with respect to above the canopy, such as air temperature, air soil water availability. humidity and global radiation, were provided by the Meteorological Institute, University of The purpose of the present study with Freiburg. A diffusion porometer (WALZ P. sylvestris was to define the response CQP130i, Effeltrich, Germany) with a H differential BINOS infrared gas anal- O/CO 2 sensitivities of conductance at both leaf yser (Leybold Heraeus, Hanau, Germany) was and stand levels to radiation, vapor pres- used on 38 d in 1994 to monitor transpiration, sure deficit and soil water availability. We assimilation and conductance of terminal chose to study a long-term site which is shoots. Observations were carried out in dif- regularly subjected to ferent crown levels of two Scots pine trees that drought, summer were accessible from a tower. During each but of varying degree. The comparison of experiment, gas exchange was observed con- leaf- and stand-level response provides tinuously on the same sample branch over the important baseline data for the develop- course of the day. Mean values of gas exchange ment of up-scaling gas exchange model were logged at 2-min intervals and these were hierarchies [13, 14]. The models can in then used to obtain 10-min mean values. Addi- turn be used to compare stands and to help tionally, a LI-COR H porometer (Li-1600, O 2 Lincoln, USA) was used in four crowns to mea- identify differences in Scots pine forest sure daily courses of shoot transpiration and controls on gas exchange along environ- water vapor conductance. The time increment mental gradients. Additionally, we exam- between measurements was 2 h for each branch ined the relationship between conductance sampled. and xylem sap abscisic acid (ABA) con- Xylem water potential was measured at centration which may act as an integra- predawn with a P70 pressure chamber (PMS, tive root to shoot signal, conveying infor- Corvallis, Oregon) with a sampling frequency mation on root system status [21, 46]. of I week in 1993 and 2-3 d in 1994. Each
28, 30]. With the Granier method, cylindrical observation time is recorded as the arithmetic mean value of 3-5 Scots pine shoots taken heating and sensing elements were inserted from the upper crown level. Xylem sap for into the trunks at breast height, one above the determination of ABA was obtained from the other ca 15 cm apart, and the upper element same branch samples as for water potential was heated with constant power. The temper- determinations by increasing the pressure ature difference sensed between the two ele- 0.2-0.3 MPa above the balancing pressure and ments was influenced by the sap flux density in collecting the exuded sap into a glass capil- the vicinity of the heated element. Sap flux lary. Samples were taken from approximately density was estimated via calibration factors half of the branches used for predawn potential established by Granier [16]. With the steady- observations. Sample volume was between 10 state, null-balance method, a constant temper- and 50 μL. The samples were immediately ature difference of 3 K was maintained between frozen in liquid nitrogen and freeze dried prior a sapwood reference point and a heated stem to determination of xylem sap ABA concen- segment. The mass flow of water through the tration with the highly specific and sensitive xylem of the heated area is proportional to the ELISA immunoassay test as described by energy required in heating. During 1993, 15 Mertens et al. [35]. null-balance sensors were used to measure sapflow, while during the summer of 1994, Two methods for measuring xylem sapflow five null-balance systems and ten installations employed to tree transpiration: were measure of the Granier-type were employed. Data were thermal flowmeters constructed according to logged every 10 s and averaged over 10-min Granier [16, 17] and the steady-state, null-bal- method of Cermák and co-workers intervals. To standardize the further processing [10, ance
3. RESULTS of the data, the output values for both systems were converted to sapflux density (sapflow in kg cm h As described in Köstner et al. -2 -1 ). Plotting of observed conductances from [28, 29] no difference was observed between daily courses as a function of a single envi- the range of flux densities and time-lag of the ronmental variable is extremely useful, sapflow systems. The arithmetic mean sapflux despite the difficulties imposed by actual density for all trees was multiplied by the stand response to simultaneous change in several sapwood area at the height of the sensor to obtain estimates of stand transpiration. factors. While a highly scattered collec- tion of points is obtained (figure 1), these Six time domain reflectometry (TDR) sen- plots reveal: (Trime P3EZ, IMKO, Germany) sors were used to determine short-term fluctuations in a) the dependency of stomatal conduc- soil moisture (5-min sampling intervals) in the the variable in ques- tance in response to upper soil layer and along one soil profile. In tion under conditions optimal for other addition, ten soil cores were taken weekly to variables influencing response. This is gravimetrically determine the spatial distribu- tion of soil moisture content (integrating the seen as the upper limit or borderline of water content from 0-40 cm) within the for- the plotted observations; est stand. b) the influence of a secondary filtered Canopy conductance was estimated as total variable on conductance, i.e. by limiting conductance assuming a tight atmo- water the range of observations selected for plot- spheric coupling and exclusive control by the ting with respect to a secondary variable, stomata [27, 33]. The time-lag between tran- a series of borderlines may be defined spiration and sapflow was variable (0-1 h) and considered for the calculation of conduc- which describe the interacting effects of not tance: the two variables; c) information about the response to environmental factors that are difficult or impossible to investigate under laboratory where g is total water conductance at the tw conditions, such as the influence of soil canopy level (mm s E is transpiration per ), -1 moisture on the leaf conductance of large time increment (mm s D is air saturation ), -1 a deficit (kPa), &a density of air (kg m G is ), -3 v rho; trees. = gas constant of water vapor (0.462 m kPa kg 3 -1 many observations Nevertheless, are ), -1 K and T air temperature (K). ais and sampling should be carried required Water vapor conductance at the leaf level out over long periods [39]. Figure 1 shows calculated according to Field et al. [15], was the distribution of observed shoot water assuming a negligible boundary layer in the vapor conductance values for P. sylvestris ventilated cuvette: as related to temperature (figure 1a), air saturation deficit (figure 1b), and photo- synthetically active photon flux density (PPFD; figure 1c, d). The plot of stomatal conductance against air temperature was is stomatal conductance for water s where g more triangular than bell-shaped. Maxi- E is measured transpiration in (mmol vapor, 1 mum conductance occurred at 22 °C -2 - i m s w water content of the air inside the ), is which corresponds to the mean daily max- leaf (mol mol and w is water content of ), -1 o imum temperature at the site from the the air outside the leaf in the chamber (mol beginning of May until October. The tem- ). -1 mol perature response curve at otherwise opti- All calculations of conductance at the leaf mum conditions may be approximated and at the stand scale are related to projected with two linear segments below and above leaf area which is total leaf area divided by a 22 °C. With decreasing PPFD, maximum factor of 2.57.
vpd value, the conductance decreases conductance at lower temperatures occurs suggested by the logarithmic regres- approximately logarithmically toward as sions applied to data in different PPFD zero. During clear nights and in early ranges in the figure (best estimates for the morning hours, condensation was occa- optimum with PPFD of 500 μmol m s -2 -1 sionally observed in the porometer cuvette at approximately 19 °C; at 200 μmol m-2 and tubing. For this reason, values -1 s approximately 17 °C). observed below 3 hPa have been excluded from the analysis. A shift in the maximum Ignoring the question of whether a conductance or shape of the relationship direct effect is observed or whether between saturation deficit and conduc- response is mediated via leaf water content tance with differing irradiance was not [2, 3, 36, 37], air vapor pressure deficit apparent. However, the maximum con- strongly influences conductance of P. ductance decreased from 280 mmol m -2 sylvestris. Stomatal conductance as related s at PPFD observations > 500 μmol m -2 -1 to water saturation deficit is left-skewed s to 250 mmol m s with PPFD from -2 -1 -1 with a maximum at 10 hPa. Above this
tively constant and in a range where max- 200-500 μmol m s and to 190 mmol -2 -1 imum conductance could be attained. As - 2 -1 m s with PPFD below 200 μmol m -2 seen in figure 2, the temporal maxima are -1 s (as judged from the upper limit of the not in phase with radiation changes but scattergram). are delayed by 8-15 min. Thus, with fre- The scatter obtained between stomatal quent change in PPFD in the early after- conductance and irradiance (figure 1c, d) noon, there is almost no stomatal response. was examined with respect to a saturation While conductance changed slowly, the response curve [i.e. g g1 + K s smax /( s / = effects of fluctuating light on net photo- PPFD)]. The value of g (140 mmol /2 smax synthesis were rapid, indicating that the -2 -1 m s is reached at K 40 μmol m -2 ) s = cuvette system itself did not substantially ; -1 s90 % of light saturation occurs at ca contribute to the time lags seen. Greater 500 μmol m sPartitioning the data set -2 -1 . conductance is observed during the mom- into temperature or saturation deficit ing hours than in the afternoon which can- classes reveals the expected decrease of not be explained as a response to above- conductance at high temperatures and high ground microclimate conditions. This values of vpd. There was no apparent general time-dependent effect seems change in the light saturation level of related to changes in internal water stores. 500 μmol m s among temperature and - 2 -1 classes. vpd The relationship of maximum stomatal conductance on individual days to Based on the 2-min values of gas mean observed soil moisture is shown in fig- exchange, hysteresis was observed in the ure 3a. The data suggest that maximum response of stomatal conductance to leaf level conductance without water stress changing light conditions (figure 2) as found by others for P. sylvestris [38, 50]. was the same during both years. Due to In the example shown for13 August 1994, the thinning of the Hartheim stand in air temperature and vpd remained rela- autumn1993 and due to higher precipita-
tion input, the soil moisture in 1994 At the limit of the scatter plot, maximum stand water conductance decreased lin- decreased only to ca 16 volume percent early with reduced soil water content (fig- and had a limited effect on conductance. ure 3b) below a soil moisture of ca 16 vol- Pooled data from 1993 and 1994 reveal a ume percent. Conductances at stand level strong limitation on maximum daily stom- atal conductance as soon as soil water are significantly lower than at the leaf level since they reflect the response of the aver- decreases below 16 volume percent. The age leaf under conditions of reduced light maximum stomatal conductance of ca 280 mmol m s obtained with the LI-COR -2 -1 intensity. The absolute values of maxi- null-balance porometer agreed well with stand conductance in 1993 mum mmol m s were in general much -2 -1) data from the WALZ measurement sys- (100 lower than in 1994 (200 mmol ms -2-1) tem. A more complete picture of response to water stress is obtained from the con- despite greater LAI due to the effects of strong drought (discussed further below). tinuous tree transpiration measurements.
The effects of successive reductions in In the driest situation observed same. on July 1993, stand conductance was 29 soil water availability on daily courses of essentially zero throughout the day. stand conductance are illustrated for the summer periods of 1993 and 1994 in fig- Seasonal changes in tree physiological ure 4. Four clear days with comparable parameters during 1993 and 1994 are meteorological conditions have been cho- shown in figures5 and 6. A long period of sen. Maximum conductance is reached in restricted water availability occurred dur- the morning hours and decreases as vpd ing July 1993 which was terminated with increases and as water is removed from thunderstorms at the beginning of August. plant internal storage over the course of Predawn water potential of the pines the day. Maximum conductance decreases decreased during drought to -1.5 MPa continously with decreasing water avail- (figure 5 upper panel), increased with the ability as illustrated in figure 3. The daily precipitation in August to -0.6 MPa, and recovered with additional precipitation to pattern of water use remains much the
the winter level of -0.4 MPa. While a gen- increases in these soil moisture measures. eral correlation is seen with soil moisture Xylem ABA concentration is strongly cor- related with predawn water potential (fig- measured at 20 cm and the store integrated 5). Maximum values of about from 0-40 cm, it is obvious that the trees ure 2000 nmol L were recorded at the begin- -1 reacting strongly to precipitation input are to the upper soil layer. Water potential ning of August during severe drought. After recovery from drought in the fall, recovery is much more rapid than are
morning hours in summer, daily tran- the ABA concentration remains between100 and 200 nmol L The observed maxi- . -1 spiration was severely restricted due to subsequent strong reductions in conduc- mum conductances at leaf level decreased to < 50 mmol m s during stress (fig- -2 -1 tance (figure 4). Stand transpiration was ure 3) and recovered to 250 mmol m s -2 -1 strongly correlated with predawn water potential and xylem ABA concentration. A in the fall. While nighttime water storage sensitive reaction to ABA appeared to permitted conductance values of 50 during
in available water may be amplified via between 0 and 1 000 nmol L . -1 occur the ABA control mechanism. Maximum Refilling of the soil water store in Septem- conductance at the leaf level decreased to ber 1993 increased daily transpiration to 150 mmol m s with moderate water -2 -1 about 1 mm d This relatively low value . -1 shortage. Despite reduction in LAI from 3 is due to low available energy during this to 2, water use (1.5 mm d remained ) -1 period. above the levels measured during summer 1993 due to greater leaf conductances and Due to frequent rains, soil moisture was large atmospheric demand. The impor- variable during summer 1994 (fig- more tance of integrating soil moisture effects on ure 6 ). The soil water store was rapidly conductance over daily courses is evident; emptied by transpiration but replenished coincidence between changes in the soil by rainfall events. Due to rainfall and thin- moisture store and the seasonal course of ning of the stand during the previous win- daily transpiration is high. ter, trees were not subjected to strong reductions in water availability. Predawn The most important correlations result- water potential remained between -0.4 from the seasonal observations are ing and -0.6 MPa throughout the summer. summarized in figure 7a-f. Figure 7a, b Nevertheless, soil moisture reductions in illustrates the overall relationship between mid-July and beginning to mid-August maximum water conductance, shoot are sensed by P. sylvestris as seen in the predawn water potential and soil moisture time course for xylem ABA. Effects of content. Based on the curvi-linear fits the small reductions in predawn water shown, a predawn potential value of potential are seen on stomatal conductance approximately -0.6 MPa can be viewed as previously reported for Mediterranean as separating two phases of stomatal shrubs [46]. The response to reductions response to drought (cf. [44]). Initial
decreases in water potential (-0.4 to -0.6 in 1993 (table I), stand transpiration (rate MPa with no obvious threshold) associ- of soil water extraction) in relation to soil ated with drying of the upper soil layers moisture content below 16 volume per- cent was approximately the same during lead to strong stomatal closure and sav- ings of water. The largest changes in con- both years. Thus, individual trees appeared ductance occur during this phase. During to conduct more water at this soil mois- the second phase as water potential ture level during 1994. It is not clear whether these observations are an artifact decreases below -0.6 MPa and even lower of the time sequence of change in condi- soil levels dry, ’very conservative behav- tions, internal regulatory phenomena, and ior’ is exhibited by the plants with stomata hysteresis effects, or whether the trees opening for only very brief periods dur- have actually adjusted their response ing the morning to replenish carbon pools. (greater conductance as shown in figure 6) At this point, physiological mechanisms to fully utilize the available water resource. are no longer able to stabilize predawn water potential and water relations. Changes in xylem ABA concentration at predawn similarly suggest two phases in 4. DISCUSSION response to drought. Increased ABA con- AND CONCLUSIONS centrations in the range 0 to 500 nmol L -1 occur as water balance is maintained The shape of stomatal response curves within a restricted range via reduced stom- derived by analyzing the upper limits of atal conductance. As soil drying results in scatter plots (figure I) are consistent with the inability to maintain predawn water results that have been obtained by other potential above -0.6 MPa (figure 7c), very methods for pine and coniferous species. strong increases in ABA are associated Granier and Lousteau [18] found that light with conservative controls on water use saturation of stomatal conductance for P. mentioned above (cf. [21, 46]). Assum- pinaster occurred at 400 μmol m s -2 -1 ing that plant available water is stored in PPFD. They estimated a maximum stom- the upper 40 cm of the soil, the wilting atal conductance of 12.9 mm s based on -1 point is at 11.7 volume percent and that projected leaf area, while we determined a field capacity is 31.4 volume percent (see value of 13.6 mm s (280 mmol ms -2-1 -1 ) table I), maximum extractable water is ca for P. svlvestris in the Hartheim stand. 79 mm. The transition or change in plant Bernhofer et al. [8] and Granier et al. [19] behavior discussed above occurs at a soil reported a maximum conductance of12.5 water storage of ca 17 mm. and13 mm s for a period in May 1992 at -1 Hartheim. Slightly higher maximum con- ductances of ca16mm s were reported -1 In terms of changes in stand transpira- tion, transitions in by Beadle et al. [7] for P. sylvestris grow- response are not observed at the same soil moisture levels ing in Thetford Forest, UK and by Cien- as for conductance and ABA. Instead a ciala et al. [11 ] for a 50-year-old stand of gradual change in daily water use occurs. P. sylvestris in central Sweden. Their mea- It must be remembered that over the surements also support light saturation for course of these measurements both vpd stomatal conductance at approximately 500 μmol m s A broader perspective -2 -1. and temperature are changing which results in a longer period of maximal tran- on intersite variability in the absolute level spiration despite initial restriction of water of maximum conductance is needed for use via stomatal closure. Despite thinning P. sylvestris, since even small differences which decreased stand density by half and cause large effects when incorporated into xylem sapwood area to two thirds of that models that integrate water use over time.
rapidly (cf. figure 2). At the stand While Beadle et al. [7] concluded that ates level, time-lag effects in response to air saturation deficit was the primary envi- changing irradiance were not obvious. ronmental factor explaining time-depen- Self-shading and the integrated response of dent changes in stomatal conductance in Thetford Forest, it was not possible in our all shoots apparently determine response studies at Hartheim to identify one spe- characteristics at this level, including hys- teresis effects [38]. Whitehead and Teskey cific environmental driver that played a [50] concluded that hysteresis in shoot dominant role, rather temperature, satu- ration deficit and irradiance showed strong response has a great influence on stom- interactions in determining response. Dur- atal behavior during short time periods but that the influence on daily transpira- ing periods of reduced soil water content, tional sums is negligible. These conclu- conductance was strongly coupled to the available soil water. Some stomatal mod- sions are also supported by the Hartheim els describe conductance as a product of data. individual correlation functions (e.g. [4, 34]). It is assumed in these that the shape Stand conductance values measured in of the response functions is invariable. 1993 and in 1994 show distinct differ- However, the correlation between stom- ences. Thinning of the stand and frequent atal conductance and air temperature indi- rain resulted in a filled water store into cates that maximum conductance is late June during 1994. Larger leaf level reached at lower temperatures when illu- stomatal conductance values occurred dur- mination is low. Such important interactive ing July and August 1994 and resulted in effects must be included in stomatal mod- higher daily water use than during sum- els in order to avoid large prediction errors. mer 1993. Under non-water-stressed con- An indirect means of including interac- ditions, Whitehead et al. [49] found that tive effects on conductance is to describe stand transpiration rates were significantly interactive effects of environmental fac- lower with lower tree density in a spac- tors on photosynthesis and to exploit the ing experiment for P. sylvestris even sev- correlative changes that occur in conduc- eral years after thinning. In contrast, Bréda tance and photosynthesis rate[6, 45 ]. et al. [9] found a significant decrease in stand transpiration of Quercus petraea in A time lag of as much as 15 min was the first year after thinning but transpira- tion rates equal to the unthinned stand in observed in the response of stomata of P. sylvestris to changes in irradiance. For this the second year. The measurements reason, good correlations between highly reported here were only begun during resolved stomatal conductance values and 1993 after the soil dried. In order to con- irradiance can only be expected with suf- sider the effects of thinning on gas ficient time for equilibration. A similar exchange capacity, we must refer to the time lag was reported for P. taeda [50]. observations of Granier et al. [ 19] in this A longer period of adjustment of two same stand during May 1992, where tran- spiration fluxes of 2-2.7 mm d occurred hours or more was reported by Ng and -1 Jarvis [38] for P. sylvestris. This could not with high available energy. Comparing be verified with our measurements because May 1992 to June 1994, we can conclude such long periods of constant illumination that transpiration potential of the stand was reduced by thinning in approximate do not occur under field conditions. In any case, time-lag effects are included in the proportion to change in leaf area index responses described in figure 1. Models and sapwood area. The low rates in fall based on these data may on averge per- of 1993 are the result of low available form well but fail when radiation fluctu- energy. Jackson et al. [22] found in two
ductance response surfaces and a defini- sylvestris stands that xylem relative P. tion of the range in response as well as the decreased after one cycle water content of drought as an effect of cavitation but identification of important time-depen- that xylem water conductance and tran- dent phenomena. Simultaneous observa- tions of sap flow and estimation of stand spiration rate were not affected. Although water use and conductance has allowed Hartheim, drought at stronger was our results do not contradict this conclusion. us to examine the effects of leaf conduc- tance on whole plant water use, while thin- Aspreviously reported for Mediter- ning during autumn 1993 revealed the shrubs [46], P. sylvestris exhibits a ranean effects of stand level phenomena on con- sensitive response to soil drying and initial ductance regulation. The observations pro- small changes in predawn water potential. vide a framework within which existing With initial drying, roots in the upper soil model hierarchies [13, 14] may be used layer appear to increase the concentration to quantify water use and carbon fixation of xylem ABA to between 200 and of pine stands as well as to examine 500 nmol L apparently as the root-shoot , -1 response in scenarios describing potential signal proposed by Davies and Zhang [12]. climate change. Long-term data records an immediate leaf-level reaction to Thus, at the Hartheim site will in fact allow us to the onset of summer weather conditions test such models for previous periods and is observed, i.e. leaf conductance and to examine the relationship of fluctuating water use decrease (cf. [43, 44, 46]). As carbon budgets to growth and production. drought continues and water potentials become more negative, a second phase in response is observed which is associated ACKNOWLEDGEMENTS with very conservative water use and much higher levels of ABA. We hypoth- Meteorological data above the canopy were esize that ABA functions as a key control kindly provided by Dr L. Jaeger, Meteorolog- on water balance, transmitting informa- ical Institute, University of Freiburg. We thank tion about soil water status and endoge- B. Dietrich for analyzing the ABA samples and R. Geyer for assistance with data evalua- nously modifying canopy response, such tion. Financial support was provided from the that extensive cavitation damage is Bundesministerium für Bildung, Wissenschaft, avoided in most years. Forschung und Technologie, Germany (BEO 51-0339476A) and by the Deutsche The intent of our simultaneous obser- Forschungsgemeinschaft SFB 251 (WH). vations of leaf-, tree- and stand-level con- ductances throughout the course of two summer seasons at Hartheim was to char- REFERENCES acterize temporal changes and scale effects on water use by P. sylvestris at a site fre- Alsheimer M., Köstner B., Falge E., Ten- [1] quently subjected to low water availability, hunen J.D., Temporal and spatial variation ideal with respect to fetch, and relatively in transpiration of Norway spruce stands within a forested catchment of the Fichtelge- homogeneous from the standpoint of stand birge, Germany, Ann. Sci. For. 55 (1998) structure. The site provides a natural lab- 103-123. oratory useful for clarifying aspects of Aphalo P.J., Jarvis P.G., Do stomata respond [2] both plant-soil and plant-atmosphere cou- to relative humidity? Plant Cell Environ. 14 pling. The current observations with P. (1991) 127-132. sylvestris permit us to view several aspects Aphalo P.J., Jarvis P.G., The boundary layer [3] and the apparent responses of stomatal con- of response to drought in relation to one ductance to wind speed and the mole frac- another. Porometery has allowed a deter- tions of CO and water vapour in the air, Plant 2 mination of the shape of stomatal con- Cell Environ. 16 ( 1993) 771-783.
[4] Aphalo P.J., Jarvis P.G., An analysis of Ball’s Granier A., Une nouvelle méthode pour la [16] empirical model of stomatal conductance, mesure du flux de sève brute dans le tronc Ann. Bot. 72 (1993) 321-327. des arbres, Ann. Sci. For. 42 (1985) 81-88. Baldocchi D.D., An analytical solution for [5] Granier A., Mesure du flux de sève brute dans [17] coupled leaf photosynthesis and stomatal con- le tronc du Douglas par une nouvelle méthode ductance models, Tree Physiol. 14 (1994) thermique, Ann. Sci. For. 44 (1987) 1-14. 1069-1079. Granier A., Loustau D., Measuring and mod- [18] [6] Ball J.T., Woodrow I.E., Berry J.A., A model elling the transpiration of a maritime pine predicting stomatal conductance and its con- canopy from sap-flow data, Agric. For. Mete- tribution to the control of photosynthesis orol. 71 (1994) 61-81. under different environmental conditions, in: Granier A., Biron P., Köstner B., Gay L.W., [19] Binggins I. (Ed.), Progress in Photosynthe- Najjar G., Comparisons of xylem sap flow sis Research Vol IV.5, Proceedings of the and water vapour flux at the stand level and VII International Photosynthesis Congress, derivation of canopy conductance of Scots 1987, pp. 221-224. pine, Theor. Appl. Climatol. 53 (1996) 115-122. Beadle C.L., Talbot H., Neilson R.E., Jarvis [7] P.G., Stomatal conductance and photosyn- Hädrich F., Stahr K., Die Bπden in der Umge- [20] thesis in a mature Scots pine forest. III. Vari- bung von Freiburg i. Br, Freiburger ation in canopy conductance and canopy pho- Geographische Hefte 36 (1992) 129-195. tosynthesis, J. Appl. Ecol. 22 (1985) 587-595. [21] Hartung W., Heilmeier H., Stomatal [8] Bernhofer Ch., Blanford J.H., Siegwolf R., responses to abscisic acid in natural environ- Wedler M., Applying single and two layer ments, in: Jackson M.B., Black C.R. (Eds.), canopy models to derive conductances of a Interacting Stresses on Plants in a Changing Scots pine plantation from micrometeoro- 116, Climate, NATO ASI Series Vol logical measurements, Theor. Appl. Climatol. Springer, Berlin, 1993, pp. 525-543. 53 (1996) 95-104. Jackson G.E., Irvine J., Grace J., Xylem cav- [22] [9] Bréda N., Granier A., Aussenac G., Effects of itation in two mature Scots pine forests grow- thinning on soil and tree water relations, tran- ing in a wet and a dry area of Britain, Plant spiration and growth in an oak forest (Quer- Cell Environ. 18 (1995) 1411-1418. cus petraea (Matt.) Liebl.), Tree Physiol. 15 Jaeger L., Kessler A., The HartX period May [23] (1995) 295-306. 1992, seen against the background of twenty Cermák J., Kucera J., Penka M., Improve- [10] years of energy balance climatology at the of the method of sap flow rate deter- ment Hartheim pine plantation, Theor. Appl. Cli- mination in adult trees based on heat balance matol. 53 (1996) 9-21. with direct electric heating of xylem, Biol. Jarvis P.G., The interpretation of the varia- [24] Plant. (Praha) 18 (2) (1976) 111-118. tions in leaf water potential and stomatal con- Cienciala E., Kucera J., Ryan M.G., Lindroth [11] ductance found in canopies in the field, Phil. A., Water flux in boreal forest during two Trans. R. Soc. London Ser. B 273 (1976) hydrologically contrasting years; species spe- 593-610. cific regulation of canopy conductance and [25] Jarvis P.G., Stomatal conductance, gaseous transpiration, Ann. Sci. For. 55 (1998). exchange and transpiration, in: Grace J., Ford [12] Davies W.J., Zhang J., Root signals and the E.D., Jarvis P.G. (Eds.), Plants and their regulation of growth and development of Atmospheric Environment, Oxford Univer- plants in drying soil, Ann. Rev. Plant Physiol. sity Press, Oxford, UK, 1981, pp. 175-204. Plant Mol. Biol. 42 (1991) 55-76. Kelliher F.M., Leuning R., Schulze E.-D., [26] [13] Falge E., Berechnung der Kronendachtran- and canopy characteristics of Evaporation spiration von Fichtenbeständen (Picea abies coniferous forests and grassland, Oecologia (L.) Karst.) mit unterschiedlichen Model- 95 (1993) 153-163. lierungsansätzen, Doctoral Thesis, Univer- Köstner B.M.M., Schulze E.-D., Kelliher [27] sity of Bayreuth, 199 p. F.M., Hollinger D.Y., Byers J.N., Hunt J.E., [14] Falge E.M., Ryel R.J., Alsheimer M., Ten- McSeveny T.M., Meserth R., Weir P.L., hunen J.D., Effects of stand structure and Transpiration and canopy conductance in a physiology on forest gas exchange: A simu- pristine broad-leaved forest of Nothofagus: lation study for Norway spruce, Trees 1 an analysis of xylem sap flow and eddy cor- (1998) 436-448. relation measurements, Oecologia 91 (1992) 350-359. Field C.B., Ball J.T., Berry J.A., Photosyn- [15] thesis: principles and field techniques, in: Köstner B., Biron P., Siegwolf R., Granier [28] Pearcy R.E. et al. (Eds.), Plant Physiological A., Estimates of water vapor flux and canopy Ecology: Field Methods and Instrumentation, conductance of Scots pine at the tree level London, New York, 1989. different xylem sap flow methods, utilizing
maximum stomatal conductance, ecosystem Theor. Appl. Climatol. 53 (1-3) (1996) surface conductance, carbon assimilation rate, 105-114. and plant nitrogen nutrition: a global ecol- Köstner B., Granier A., Cermák J., Sapflow [29] ogy scaling exercise, Ann. Rev. Ecol. Syst. 25 measurements in forest stands - methods and (1994) 629-660. uncertainties, Ann. Sci. For. 55 (1998) 13-27. Tardieu F., Lafarge T., Simonneau Th., Stom- [43] Kucera J., Cermák J., Penka M., Improved [30] atal control by fed or endogenous xylem ABA thermal method of continual recording the in sunflower: interpretation of correlations transpiration flow rate dynamics, Biol. Plant between leaf water potential and stomatal 19 (1977) 413-420. conductance in anisohydric species, Plant Leuning R., A critical appraisal of a com- [31] Cell Environ. 19 (1996) 75-84. bined stomatal-photosynthesis model for C 3 Tenhunen J.D., Hanano R., Abril M.A.I., [44] plants, Plant Cell Environ. 18 (1995) Weiler E.W., Hartung W., Above- and below- 339-355. ground environmental influences on leaf con- Lösch R., Tenhunen J.D., Stomatal responses [32] ductance of Ceanothus thyrsiflorus growing to humidity - phenomenon and mechanism, in a chaparral environment: Drought response in: (Jarvis P.G., Mansfield T.A. (Eds.), Stom- and the role of abscisic acid, Oecologia 99 atal physiology Cambridge University Press, (1994) 306-314. Cambridge, 1981,pp. 137-161. Reynolds J.F., Lange O.L., Tenhunen J.D., [45] McNaughton K.G., Black T.A., A study of [33] Dougherty R., Harley P.C., Kummerow J., evapotranspiration from a Douglas fir forest Rambal S., QUINTA: A physiologically- using the energy balance approach, Water based growth simulator for drought adapted Resour. Res. 9 (1973) 1579-1590. woody plant species, in: Pereira J.S., Lands- Massman W.J., Kaufmann M.R., Stomatal [34] berg J.J. (Eds.), Biomass Production by Fast- response to certain environmental factors: a Growing Trees, NATO ASI Series, Applied comparison of models for subalpine trees in Science Volume 166, Kluwer Academic Pub- the Rocky Mountains, Agric. For Meteorol. lishers, Dordrecht, The Netherlands, 1989, 54 (1991) 155-167. pp. 135-168. Mertens R.J., Deus-Neumann B., Weiler [35] Tenhunen J.D., Sala Serra A., Harley P.C., [46] E.W., Monoclonal antibodies for the detection Dougherty R.L., Reynolds J.F., Factors influ- and quantitation of the endogenous plant encing carbon fixation and water use by growth regulator, abscisic acid, FEBS Lett. mediterranean sclerophyll shrubs during sum- 160 (1985) 269-272. mer drought, Oecologia 82 (1990) 381-393. Monteith J.L., A reinterpretation of stomatal [36] Wedler M., Geyer R., Heindl B., Hahn S., [47] responses to humidity, Plant Cell Environ. Tenhunen J.D., Leaf-level gas exchange and 18 (1995) 357-364. scaling-up of forest understory carbon fixation Mott K.A., Parkhurst D.F., Stomatal [37] fluxes with a ’patch-scale’ canopy model, responses to humidity in air and helox, Plant Theor. Appl. Clim. 53 (1996) 145-156. Cell Environ. 14 (1991) 509-515. Wedler M., Heindl B., Hahn S., Kπstner B., [48] Jarvis P.G., Hysteresis in the [38] Ng P.A.P., Bernhofer Ch., Tenhunen J.D., Model-based response of stomatal conductance in Pinus estimates of water loss from ’patches’ of the sylvestris L. needles to light: observations understory mosaic of the Hartheim Scots pine and a hypothesis, Plant Cell Environ. 3 (1980) plantation, Theor. Appl. Clim. 53 (1996) 207-216. 135-144. Ogink-Hendrik M.J., Modelling surface con- [39] Whitehead D., Jarvis P.G., Waring H., Stom- [49] ductance and transpiration of an oak forest atal conductance, transpiration, and resistance in The Netherlands, Agric. For Meteorol. 74 to water uptake in a Pinus sylvestris spacing (1995) 99-118. experiment, Can J. For. Res. 14 (1984) 692-700. Parlow E., The Regional Climate Project [40] REKLIP - an overview, Theor. Appl. Cli- Whitehead D., Teskey R.O., Dynamic [50] matol. 53 (1996) 3-7. response of stomata to changing irradiance in Loblolly pine (Pinus taeda L.), Tree Phys- Peck A., Mayer H., Einfluß von Bestandespa- [41] iol. 15 (1995) 245-251. auf die Verdunstung von Wäldern, rametern Forstw Cbl 115 (1996) 1-9. Zeiger E., Farquhar G., Cowan I. (Eds.) Stom- [51] atal function. Stanford University Press, Stan- Schulze E.-D., Kelliher F.M., Körner C., [42] ford, California, 1987. Lloyd J., Leuning R., Relationships among