Báo cáo khoa học: "Retrieving leaf conductances from sap flows in a mixed Mediterranean woodland: a scaling exercise"

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Original article Retrieving leaf conductances from sap flows in mixed Mediterranean woodland: a a scaling exercise Serge Rambal* José Teixeira Filho Claire Damesin Richard Joffre CEFE CNRS (UPR9056), 34293 Montpellier cedex 5, France (Received 31 July 1995; accepted 7 December 1995) Abstract-Xylem sap-flux densities were monitored continuously using Granier-type sensors on five Quercus ilex, four Arbutus unedo and one Quercus pubescens from June 1993 to October 1994. Half-hourly measurements of incoming solar radiation, air temperature and humidity, horizon- tal wind speed and precipitation were carried out at the top of a tower at a height of 12 m, about 2 m above the canopy. Leaf physiological measurements (stomatal conductance, water potential) on individual sunlit leaves from each of the three tree species were obtained on seven complete or partial diurnal time courses. For these three species, to estimate leaf stomatal conductance, we used the big-leaf approach of Penman-Monteith. We have divided the leaves into sunlit and shaded. The model sums the individual-leaf model for only the sunlit fraction to produce the whole-canopy predictions. Transpiration was deduced from sap flux through a transfer function taking into account stem water storage. Stomatal conductance for a given species was evalu- ated half-hourly from transpiration and microclimate data inverting the Penman-Monteith equa- tion. An empirical model was identified that related stomatal aperture to simultaneous varia- tions of microclimate and plant water potential for the 1993 period. The predicted leaf conductances were validated against porometer data and those of the 1994 period. The diurnal patterns of pre- dicted and measured transpiration indicated that stomatal conductance was accurately predicted. The leaf conductance models were also compared with already published literature values from the same tree species. In spite of the simplifications inherent to the big-leaf representation of the canopy, the model is useful for predicting interactions between Mediterranean mixed wood- land and environment and for interpreting H exchange measurements. (© Inra/Elsevier, Paris.) O 2 mixed Mediterranean woodland / stomatal and canopy conductances / Penman-Monteith equation / sap flow / Quercus ilex / Quercus pubescens / Arbutus unedo Present address: Departamento de Água e Solo, Faculdade * de Engenharia Agrícola, Unicamp, C.P. 6011, CEP 13083-970, Campinas, SP, Brasil ** Correspondence and reprints
Résumé - Estimation des conductances foliaires à partir des flux de sève dans une forêt mixte méditerranéenne : un exercice de changement d’échelle. La densité de flux de sève a été mesurée en continu à l’aide de capteur de type Granier sur cinq Quercus ilex, quatre Arburus unedo mesures ont été complétées par des Quercus pubescens de juin 1993 à octobre 1994. Ces et un microclimatiques bihoraires de rayonnement global, de température et d’humidité de l’air, mesures de vitesse du vent et de hauteur de précipitation. Ces mesures sont faites au sommet d’une tour de 12 m dominant le couvert forestier d’environ 2 m. Sept suivis journaliers complets ou partiels de conductance stomatique et de potentiel hydrique pour des feuilles exposées au soleil des trois espèces d’arbre ont été réalisés. Pour ces trois espèces, nous avons estimé la conductance sto- matique à l’aide du modèle simple feuille de Penman-Monteith. Les feuilles sont subdivisées en feuilles de lumière et d’ombre. Seule les feuilles de lumière sont supposées contribuer à la trans- piration totale. La transpiration est dérivée des mesures de flux de sève à l’aide d’une fonction de transfert qui tient compte du stockage de l’eau dans le tronc. La conductance stomatique est déduite de l’inversion du modèle de Penman-Monteith compte tenu de la transpiration et des conditions microclimatiques. Un modèle empirique multiplicatif de ces conductances a été ajusté sur les données acquises en 1993. Il les relie aux conditions microclimatiques et au potentiel hydrique foliaire. Ce modèle de conductance a été validé à l’aide des données acquises en 1994 et à des mesures de conductances réalisées au poromètre. Ce modèle a été comparé aux modèles de la littérature proposés pour ces espèces. En dépit des simplifications inhérentes à la repré- sentation simple feuille du couvert, ce modèle est utile pour prédire les interactions entre les forêts mixtes méditerranéennes et leur environnement et pour interpréter les mesures de trans- piration. (© Inra/Elsevier, Paris.) forêt mixte méditerranéenne / conductances stomatique et de couvert / équation de Penman- Monteith / flux de sève / Quercus ilex / Quercus pubescens / Arbutus unedo Measurements and modelling are dif- 1. INTRODUCTION ficult in the mixed evergreen canopies that are very common in Mediterranean land- Modelling terrestrial ecosystem func- tions at watershed, region or larger scales scapes. In these areas, natural vegetation has to cope with a strong seasonality in demands the development of generalized environmental conditions where cold wet representations of the most relevant eco- logical and biophysical processes. Mass winters alternate with hot dry summers. and energy exchanges in forest canopy are However, it is probably drought that has most dramatically shaped vegetation and key factors in photosynthesis, net primary controlled plant functions. If attempts are production, growth and some ecosystem functions and regional forest canopy phys- made to study mass and energy exchanges or even water yield of forested watersheds, iology may influence climate and hydro- one must take into account the interac- logical cycle. The links among canopy tions between soil or plant status, atmo- physiology, surface energy exchange, and sphere and leaf regulation. This control water and carbon dioxide exchanges have can be considered at different time-scales. been long recognized. Some models Scaling from leaf to canopy is not only a explicitly include this linkage [2, 3]. As problem of changing spatial scale but also emphasized by Bonan [6]: "A future chal- lenge (...) is not to merely show that cli- a problem of integrating temporal scales. Scaling is used here in the Norman [46] mate change affects terrestrial ecosystems, but rather to considered what level of sense, i.e. "scaling implies an intuitive physiological and biophysical detail is leap that provides a quantitative connec- tion between distant phenomena - a short needed to accurately model climate change cut". impact".
are between Septem- winter, and ca 80 % To the extent that is possible, mea- April. Mean annual precipitation ber and surements at different time and spatial at Vailhan, 1.5 km south of the study site, scales are necessary to validate modelling is 755 mm recorded over the previous 15 scaling efforts. A continuous sap flow and years. Mean monthly temperatures at leaf ecophysiology measurement program Bédarieux 10 km north (1951-1994 conducted in a Mediterranean wood- was period, elevation 195 m) range from 5.7 °C land. These data link the local scale envi- in January to 21.9 °C in July with a mean ronmental conditions with micro-scale leaf annual value of 13.2 °C. Penman estimates functioning, and consequently afford the of potential evapotranspiration (PET) opportunity to propose and test a model range between 920 and 1020 mm ha . -1 of canopy physiology. In this context, the big-leaf approach of Penman-Monteith [44] provided, if not quantitatively at least conceptually, a useful simplified descrip- 2.2. Vegetation measurements tion and the basis to explore stomatal effects on canopy transpiration with Dominant species are two evergreen respect to tree species. The present study trees, holm oak (Quercus ilex) and straw- was undertaken to: 1) examine tree xylem berry tree (Artutus unedo), which together sap flow and stomatal responses in a make up 90 % of the total 36 m habasal 2 -1 mixed evergreen Mediterranean wood- area. Pubescent oak (Quercus pubescens), land; 2) derive canopy conductance val- a deciduous species, is also present, but ues from the inversion of the Penman- represents less than 3 % of the 8 870 stems Monteith equation; and 3) identify and . -1 haUnderstorey species are mainly validate a multi-constraint empirical model Viburnum tinus (2 650 individuals ha ) -1 of leaf conductance for each tree species. and Erica arborea (270 individuals ha). -1 Stem densities of Q. ilex, A. unedo and Q. pubescens were 5 280, 3 360 and 230 2. SITE DESCRIPTION stems per hectare, respectively, and the AND METHODS corresponding mean diameters at breast height (DBH) were 7.0 ± 2.9, 6.7 ± 2.5 and 13.8 ± 4.8 cm (see table I). The cor- 2.1. Site description responding numbers of stems per stool are 2.2 ± 0.9, 3.0 ±1.2 and 1.7 ± 1.0, respec- tively. New leaves of the deciduous Q. The study site was located in the Peyne pubescens grew at the end of March and watershed about 45 km west of Montpel- senesced during October. We consider the lier, southern France (43°34’ N 3°18’ E, April-October period as the only active elevation186 m) at the bottom of a south transpiration period for this species. eastern facing 35 % slope. The woodland, composed of resprouted trees following a Estimates of leaf area index (L) were clear cut in 1945, has reached a height of made in the same plot using a LAI-2000 ca 10 m and supports a leaf area that we plant canopy analyser (LI-Cor Inc., Lin- estimated by satellite remote sensing of coln, NE, USA). This instrument mea- between 5 and 6 m m throughout the 2 -2 sures the gap fraction of the canopy based year [63]. The soil is a shallow, stony, on diffuse blue light attenuation at five loamy clay developed on schists (lithic zenith angles simultaneously. Measure- xerorthent). ments were made at the nodes of a 6 x 6 grid within a 30 x 30 m area. Reference Mediterranean-type cli- has The a area reading of sky brightness could be during autumn and mate. Rainfall occurs
obtained quickly at the top of the tower. with photochopper output anemometer Because direct sunlight on the canopy A100R). causes errors exceeding 30 % in the LAI- 2000 measurements, we collected data only on cloudy days. LAI maps for the 2.4. Sap flow measurement plot have been obtained by punctual krig- ing, as in Joffre et al. [34], using the We used simple radial sap flow sen- SURFER package [35]. Measurements were repeated in October 1993, March applicable to trees [21-23]. A pair of sors 2 1994 and August 1994. cm long probes separated vertically by 10-15 cm are implanted in the sap wood. The top probe is heated with constant power and the temperature difference 2.3. Meteorological data between the probes monitored. The probes were installed in freshly bored holes in A Campbell Scientific weather station the outermost 2 cm of sap wood and installed at the top of a 12 m scaf- moved every 3-4 months. The sensors was folding tower, 2 m above the top of the were shielded from rain with a thin film of forest canopy. Data were stored on a plastic and the stem was thermally insu- CR21X datalogger. Throughout the inves- lated with 6 cm polystyrene sheet extend- tigation period, the system logged 30 min ing approximately 0.25 m above and mean air temperature and relative humid- below the sensors. The sensors were con- ity measured with a MP100 Rotronic nected to a CR21X datalogger. The data probe (platinium resistance thermometer logger scanned the probe signals every 1 and polymer humidity sensors) inside a min and recorded half-hourly means after model 41004-5 Gill radiation shield. Aux- converting probe voltage to °C. Ten trees located close to the meteorological tower iliary meteorological measurements included solar radiation (silicon cell pyra- were selected (table I). Temperature dif- nometer SKS 1110 Skye Inst. Ltd), 30 min ference between the two sensors is related rainfall intensities (tipping bucket rain to sap flux density (i.e. sap flow per unit of sap wood area, expressed in mm mm3 -2 gauge ARG 100 calibrated for a 0.2 mm ) -1 h by a relationship proposed by Granier tip) and horizontal wind speed (cup
evaporation [55]. This els for canopy [21] and that we applied for these tree discussion in Cabibel and Do species (see approach simulating canopy physiol- to [8] and Goulden and Field [20]). These ogy is based on the hypothesis that leaf sensors average the sap flux density across properties can be quantitatively scaled up a sap wood radius of 2 cm. For a given canopy. As a result, with respect to to tree species, sap flow for the site was esti- energy and water flux, the canopy can be treated as a ’big-leaf’. The evaporation is mated by multiplying its sap flux density averaged over the sampled trees by its total then given by the Penman-Monteith [equa- sap wood area. Measurement were car- tion (1)] [44]: ried out continuously from June 1, 1993 to September 30, 1994. 2.5. Ecophysiological measurements where E and R are, respectively, the flux n densities of water vapour and net irradi- A steady state parameter (LI1600, LI- ance per unit ground (we neglected here COR Inc., Lincoln, Nebraska, USA) was heat flux into the air between the trees and used to measure leaf stomatal conduc- storage in the biomass as well as soil heat tance. Data were collected on three to five flux), D is the air saturation deficit at a mature leaves per species chosen at ran- reference height above the canopy,ϵ is dom in the sunny part of the canopy from the ratio of latent to sensible heat increase dawn to ca 2 000 hours on 7 days (18 June with temperature for saturated air, λ is the and 7 July 1993; 11March, 28 April, 23 air density and λ the latent heat of vapor- June, 4 August and 15 November 1994). isation of water. Here, g and g are, a c respectively, the bulk aerodynamic con- Xylem potential (Ψ ) p water was mea- ductance for the water vapour flux Scholander-type sured with a standard between the evaporating leaf surfaces and pressure chamber (PMS 1000, PMS Inst., the reference height and the bulk canopy Corvallis, Oregon, USA). A short shoot conductance. In our case, because of high with a minimum of three leaves was cut leaf area index and leaf litter covering the and from which water potential was imme- soil, we neglected direct soil evaporation. diately measured in the field. On three The canopy conductance, g can then be , c trees per species, we measured two shoots calculated from the inversion of equation per tree, if the difference between them (1 ): was more than 0.2 MPa we measured a third twig. 3. ESTIMATION OF LEAF CONDUCTANCES case, R was assumed to be lin- In n our related to incoming solar radiation early R with an absorption coefficient of 0.8 g and a constant net loss of thermal radiation 3.1. Theoretical background of 50 W m (data not shown) was calcu- -2 lated using equation (3) with z and d The principles of combined energy and 0 diffusion control have been generalised being assumed to be proportional to the stand height h and arbitrarily chosen as d by numerous workers to produce the so- = 0.75h and z 0. lh ([68]; see also Ram- called ’combination equation’, the basis o = for both single-layer and multilayer mod- bal et al. [54]):
We fit stomatal conductances g with sw following multiple-constraint function the [72]: where z is the surface roughness, d is the o zero plane displacement, k is the von Kar- man’s constant and u is the wind speed at height z. To take into account the lag These response functions have been between E and the sap flux F we assume successfully incorporated into semi-empir- the damping effect due to stem storage to ical models. The functions f ranging , i be represented by a linear differential between 0 and I, account for the con- equation analogue to a resistance-capaci- straints on g imposed by light, air satu- sw tance network [70]: ration deficit D and plant water status R used here as a surrogate through . &p is Psi; g for photosynthetically active radiation, the dominant regulator of stomatal opening. Solving equation (4) yields a numerical It is usually considered that stomatal con- filter [equation (5)] that gives E at time t ductance shows a hyperbolic response to function of F in the same time interval , g R so: and of F in the previous time interval The stomatal response to air humidity could be linear or curvilinear depending on the control system involved, a direct feedforward response results in a linear The parameter k is adjusted by trial d relationship, whereas a feedback response and error particularly at dusk when xylem via plant water status leads to a non-linear sap continues to flow after stomatal clo- relationship [18]. We used here a two- sure when E = 0. We retained a time con- parameter linear feedforward relationship stant for water transport k of 1 500 s close d of the form: to those already reported in the literature [48, 70]. Canopy stomatal conductance can be down scaled to the leaf level using meth- ods developed for similar scaling of carbon assimilation [31, 38]. For canopies with 3.2. Calibration of the leaf a spherical leaf angle distribution (see dis- conductance model cussion of this assumption in Rambal et al. [54]), the sunlit leaf area index L* is: The parameters that describe stomatal opening in response to the dependent vari- L* 2 exp(-0.5L / cos &thetas;)](6) &thetas;[ 1 cos = - ables were estimated by non-linear least squares regression using Marquardt’s where &thetas; is the zenith angle of the and sun method (see limitation of this approach in L the leaf area index. Jarvis [32]). Estimations of g were arbi- sw With estimates of canopy conductance g trarily shared in two data sets, the 1993 c and L*, averaged stomatal conductance period is used for calibration of the param- g was calculated for the three dominant eters andthe 1994 period reserved for val- sw already mentioned tree species as: idation of the model. Specifically, these
258). The potentials for Q. ilex and Q. split into subsets based on two sets were predawn potential classes of 0.25 MPa on the same day reached pubescens wide. For each subset we estimated k and b -1.66 0.14 and -1.6 ± 0.10MPa, respec- ± g f that we assumed to be swmax 3 ) p (Ψ tively. In 1994, the summer drought did related to Ψand kand k assumed to be pac not have the same intensity because of the independent of Ψ . p rainfall in July (17.6 mm on DOY 209 more than 24.4 mm on DOY 212). As a result, the minimum values reached on 21 4. RESULTS September (DOY 263) were only -1.28 ± 0.04, -1.09 ± 0.19 and -0.95 ± 0.03 MPa for A. unedo, Q. ilex and Q. pubescens, the 2 years of measurements, During respectively. Outside the summer drought reach very negative values (fig- p Ψdid not period and in the absence of any water ure 1). In 1993, A. unedo was the species was between -0.2 and -0.35 that had the lowest Ψ 0.22 (SD) &p psi; stress, , -1.72 ± p MPa in all three species. MPa on 15 September (day of year, DOY
comparison was made between the A 2 with r -1 ) p Ψ 2a), g = (1.09 - 3.25 ure swmax daily sap flow densities of each of mean 0.985 (P < 0.001) for A. unedo (fig- = the tree species, between April and Octo- -1 ) p 2.90 Ψ ure 2b) and g swmax (1.67 - = Q. ber 1994, a period chosen to take into (P with r 0.983 2 0.001) for < = account the deciduous nature of Q. pubescens (figure 2c). The decreases in pubescens. The mean flows were 3.67 ± maximum conductance for the three 0.36 dm d for Q. ilex and 2.10 ± 3 -1 species were significantly described by 3 -1 dm d for A. unedo. The 0.36 corre- these reciprocal functions. The relation- coefficients of variation were sponding ships between the parameter k [see equa- b 10 and 17 %. The mean flow for the single were of a sigmoid tion (10)] and &p Psi; individual of Q. pubescens sampled was nature. These relationships were fitted to 2.7 dm d Furthermore no significant 3 -1 . equations of the form k = a / (1 + b exp (c b relation was observed between the mean where k was expressed in kPa and b )) p Ψ &pisP ; sap flow density and DBH (r -0.44 ns = in MPa. We obtained k = 1.77 / (1 + b andr = 0.62 ns for Q. ilex and A. unedo, 29.6 exp (5.14Ψ with r = 0.969 (P < p2 ) respectively). 0.001) for Quercus ilex (figure 3a), k b = 1.9 / (1 + 21.8 exp (3.59 Ψ with r )2 punedo = The area-averaged leaf area indices of 0.971 (P < 0.001) for Arbutus (fig- the study site were 5.51 ± 0.64 in Octo- ure 3c) and k 1.82/ (1 + 8.91 exp b = ber 1993, 5.16 ± 0.65 in March1994 and p2 (3.84Ψ with r = 0.944 (P < 0.001) for ) 5.60 ± 0.44 in August 1994. The com- Quercus pubescens (figure 3c). bined analysis of maps of leaf area indices (data not shown) and the position of the For validation, we used data from 1 individuals sampled showed that there was January to 30 September 1994. Compar- little or no overlap between crowns. The isons were made for: 1) the measured and functioning of each species could there- simulated daily time courses of canopy fore be considered to be separate. The conductance; 2) the stomatal conductances overall functioning of the ecosystem deduced from both the canopy conduc- would therefore be the linear combination tances and the area of leaf subjected to of each of its three compartments. The direct solar radiation and to porometer analyses that follow concern the stomatal measurements of leaf conductance; and functioning analysed species by species. 3) the measured and simulated daily tran- The values of the parameters identified spirations for the three species taken into for each Ψclass and for each species are account and their cumulative, that is p shown in table II. k values thus identi- ecosystem transpiration. The simulation a fied were 116, 132 and 100 W m Q. -or f2 of the canopy conductances gave satis- ilex, A. unedo and Q. pubescens, respec- factory results. The example of three con- tively. g values that were reached in secutive days for the Q. ilex component swmax of the ecosystem is shown in figure 4. The the absence of water stress, i.e. when Ψ p was close to zero, were 0.9, 0.65 and 0.5 same was true when the simulated stom- cm s respectively, for the same species. , -1 atal conductances were compared with The relations between g and Ψ those obtained independently by porome- , p swmax fixed at the median value for each class, try (figure 5). The measured and simu- could be fitted to hyperbolic curves. These lated daily transpirations were compared relationships were fitted to equations of for Q. ilex (figure 6a), A. unedo (figure 6b) -1 ) p +bΨ the form g (a and the ecosystem (figure 6c). The results where g = swmax swmax was expressed in cm s and -1 &p in MPa. for Q. pubescens are not shown because its Psi; -fig- ) p Ψ (1 We obtained g (0.77 - 2.35 contribution to the total was low. At this = swmax 2 with r = 0.942 (P < 0.001) for Q ilex daily scale the correlation coefficients
amplitudes to what has been recorded in between the measured and simulated val- ues were 0.83, 0.76, 0.94 and 0.85 for Q. certain tropical rainforests [24]. They were also evident in 13 isotope content of Q. C ilex, A. unedo, Q. pubescens and the ecosystem, respectively. These values ilex and Q. pubescens leaves collected were all very highly significant (P < 0.01). from the site in October1993, and there- The model did, however, underestimate fore correlated with the intrinsic water use the measured values at low rates, i.e. at efficiency (see [19]). On ten individuals values of less than1 mm per day. of each of these two species the δ con- C 13 tent varied from -29.1 and -24.7 &permil; in Q. ilex and -28.8 and -25.7 &permil; in Q. 5. DISCUSSION pubescens [14]. These ranges are much greater than those normally found within Spatial variations in daily sap flows in natural ecosystems, but are less than those Q. ilex were similar in their A. unedo and recorded by Mooney et al. [45] and
Kohom et al. [39]. The sources of varia- nant Q. ilex in a seasonal pattern of canopy tions are many. The lack of consistent conductance (figure 4) which varied between 4.0 and 2.0 cm sThis is of the . -1 trends observed for water consumption versus tree size supports the observations same order of magnitude as that measured of Doley and Grieve [15]and Hatton and by Valentini et al. [71] over a Mediter- Vertessy [26]. Hatton and Vertessy [26] ranean macchia canopy dominated by Q. ilex (3.3 cm s and is in agreement with ) -1 showed differences in daily flows that could reach 70 % between two Pinus radi- those proposed in the reviews of Kelliher ata trees despite the fact that they were et al. [37] and of Schulze et al. [61] for separated by only 3 m, occupied almost the superclasses temperate deciduous for- est (2.07 ± 0.65 cm s and sclerophyl- ) -1 the same area of ground and had similar lous shrubland (2.2 ± 0.2 cm s It is ). -1 stem diameters and heights. Doley and Grieve [ 15] analysed differences in flows however greater than that of the super- between13 Eucalyptus marginata of sim- class temperate evergreen broadleaf for- ilar size. They observed that "the water est which was based on only a single value, 1.4 cm s . -1 consumption could not be closely related with their diameters, heights or crown The low amplitude of &p made it Psi; exposure although observation of the impossible analyse the mechanism by to crowns suggested that the amount of leaf which transpiration is regulated over the material may have had an important influ- entire range of functioning in each of the ence". This remark supports the conclu- three species studied. The analysis con- sions of pioneer works of Ladefoged [41 ]. cerned range of over which the tran- &p Psi; a He found that crown shape had a marked rate remained relatively high. spiration influence on transpiration in 39 trees in For example on 25 August 1994, in a several stands within mixed deciduous neighbouring study site, Damesin and forests in northern Europe dominated by Rambal [ 12] recorded mean values of &p Psi; Quercus petraea, Fagus silvatica and that reached -2.7 and -3.5 MPa for Q. Fraxinus excelsior. This remark also cor- pubesceus and Q. ilex. The latter value is roborates the recent results of Le Goff et slightly lower than the empirical limit of al. [43] and Sala et al. [60] who found an -3.4 MPa found in the review made on almost linear relation between the leaf area Mediterranean evergreen oaks by Ram- of an individual tree and its cumulative bal and Debussche [50]. As for Arbutus water consumption in dense stands of unedo, it seems probable that its root sys- Fraxinus excelsior and of salt cedar, tem is superficial in nature, since it not respectively. Le Goff et al. [43] also only experienced the lowest but also &p Psi; demonstrated the importance of the social showed a sharp rise in the &pfollowing Psi; position of the tree. The importance of this rainfall events in July 1994. Beyschlag et social position has also been stressed by al. [4] measured a leaf water potential of Kelliher et al. [36] in explaining varia- -5 MPa in A. unedo, whereas two co- tions in water consumption within a occurring evergreen oaks did not exceed broadleaf forest of Nothofagus spp. The -3 MPa. The comparison made by Castell demonstration of the existence of these et al. [9] of co-occurring deep-rooted Q. variations does not however call into ques- ilex compared to shallow-rooted A. unedo tion the work we conducted on averaged showed that were -2.5 and -3.4 MPa, &p Psi; flows of individual species and for the at the peak of the summer respectively, whole ecosystem. drought. Inversion of the Penman-Monteith In this latter study, the maximum stom- equation resulted in the case of the domi- atal conductances, averaged over the light
saturated phase of the day, that were ertheless, in Q. ilex at less than &p Psi; reached in the absence of any drought -0.5 MPa, there was convergence between stress were lower than 0.5 cm s in A. -1 our observations and those in the litera- unedo and at the same time reached ture. All of these response curves are 0.75 cm s in Q. ilex. The same ranking of -1 shown in figure 7a, b. It can be seen that conductances was found by Tenhunen et the evergreen oak maintained a high con- The same is true of ductance at low al. [67] who compared A. unedo with two . &p Psi; Mediterranean evergreen oaks Q. suber other Mediterranean evergreen oaks: Q. and Q. coccifera: g was 0.35 cm s -1 dumosa had g of 0.2 cm s at -1 smax swmax -3.3 MPa; Q. coccifera, 0.375 cm sat -1 in the former, whereas it was between 0.75 -3 MPa and Q. suber, 0.25 - 0.3 cm sat -1 and1 cm s for the two oaks. Rhi- -1 zopoulou and Mitrakos [57] recorded val- -3 MPa [67]. ues reaching 0.7 cm s in Q. ilex, whereas -1 The parameter k relative to the stom- a Sala and Tenhunen [59] found a value of atal response to sunlight is in agreement 0.65 cm s (see also the review by -1 with published values. An irradiance Acherar and Rambal [1 ] in which g swmax threshold of 200 W m for temperate -2 ranged between 0.7 and 1.05 cm s ). -1 deciduous oaks was proposed by Thomp- These comparisons are however of indica- son and Hinckley [69] and Simpson et al. tive value only because these studies were [62]. A net irradiance greater than 75 W conducted under environmental conditions -2 m was adopted by Pitaco and Gallinaro and on plant material (age of trees and [49] for Q. ilex. Lower threshold values, leaves, nitrogen content) that differed and ranging between 70 and 200 (mol m s -2 -1 at leaf water potentials that were more or of photosynthetic active radiation, have less close to zero. been found in seedlings of Q. ilex and two Mediterranean deciduous oaks (Acherar For the species studied, the observa- and Rambal, unpublished data). tions on daily changes in stomatal con- ductance (figure 5) showed that the pat- Tenhunen et al. [64-66] and Lange et terns were similar to those proposed by al. [42] observed a depression in stomatal Hinckley et al. [29, 30]. During the onset conductance at solar midday in several of drought, a decrease in maximum con- species of Mediterranean woody plants, ductance was recorded which closely particularly Q. ilex and A. unedo. This related to the predawn water potential and depression was considered to be a char- fitted to an inverse function. A relation of acteristic of Mediterranean species allow- the same type was obtained by Pereira et ing them to reduce water losses when the al. [47] on Eucalyptus globulus, by evaporative demand is highest. It is prob- Acherar and Rambal [1 ] on four Mediter- ably controlled by the saturation vapour ranean oaks including Q. ilex and by pressure deficit of the air. In contrast, Damesin [13] for Q. ilex and Q. Hinckley et al. [30] thought that it depends pubescens. This decrease in maximum on a combination of several factors and conductance was also observed in situ in especially the instantaneous water poten- other oak species by Reich and Hinckley tial, whereas Correia et al. [11] suggested [56] and in Q. ilex by Sala and Tenhunen that it results from the inhibitory effects [59] in two contrasting ecological situa- of intense photosynthetic radiation on the tions: ridge and valley locations. How- chloroplasts. The depressions at solar mid- ever, these latter authors found that it could day were not very pronounced in our results (see figure 5). This was also the be described by a linear relation. Such a case for observations made in situ on adult dissimilarity could be related to the rate at which soil moisture was depleted. Nev- Q. ilex trees by Sala and Tenhunen [59]
and on Q. ilex and Q. pubescens by in the evergreen Mediterranean oak, Q. Damesin [13] which also demonstrated agrifolia and in the deciduous, Q. Iobata. the slight amplitude of this phenomenon. In A. unedo, the response observed by Although direct effects of air humidity Tenhunen et al. [65] indicates that the surrounding leaves on stomatal aperture behaviour of A. unedo is similar to that of were limited, they do nevertheless com- Q. coccifera. A clear midday stomatal clo- ply with published results. The effect of sure occurred although it was more pro- D on stomatal opening has been demon- nounced than that observed in Quercus. strated in several deciduous temperate In A. unedo subjected to a water potential of between -2 and -2.5 MPa, Beyschlag et oaks with a dg ratio ranging from /dD sw al. [4] found a sensitivity of stomatal con- -0.19 to -0.38 [7, 10, 16, 28]. That is the ductance dg close to -0.2, i.e. a value same order of magnitude as we recorded /dD sw (0.26) in our two Mediterranean oaks. This similar to that of our observations. sensitivity is much lower than that recorded by Sala and Tenhunen [59] and The approach we used enabled us to Pittaco and Gallinaro [49] in Q. ilex but take into account the functional relation- similar to that observed by Hollinger [31] ships controlling stomatal behaviour and to
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