Báo cáo khoa học: "Field comparison of transpiration, stomatal conductance and vulnerability to cavitation of Quercus petraea and Quercus robur under water stress"

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Original article Field comparison of transpiration, stomatal conductance and vulnerability to cavitation of Quercus petraea and Quercus robur under water stress A Granier N Bréda H Cochard, E Dreyer, INRA, Laboratoire de Bioclimatologie et Écophysiologie, Champenoux, F54280 Seichamps, France January 1993; accepted 2 June 1993) 6 (Received Summary — Water relations were analysed in adult oaks (Quercus petraea and Q robur) during a period of water shortage in a simplified lysimeter. Sap flux densities and stomatal conductance were reduced by 70% at maximal drought intensity. Predawn leaf water potential then ranged from -1.7 = to -2.0 MPa. The slightly lower transpiration observed in pedunculate oaks could be ascribed to their smaller crown development. Nevertheless, no significant difference in stomatal conductance could be observed between the two species. They also had the same percent loss of conductivity (= 80%) in petioles at maximal drought intensity when midday leaf water potential had dropped to -3.0 = MPa. Finally, good agreement was found between observed losses of hydraulic conductivity during in situ dehydration and the vulnerability curves obtained under laboratory conditions. The shifts in maximal conductivity observed in some droughted trees probably accentuated discrepancies be- tween field and laboratory data. However, a correction procedure helped overcome these artifacts. drought/ xylemI cavitation/ Quercus petraea/ Quercus ro- stomatal conductance / sap flux / bur Résumé — Comparaison en conditions naturelles de la transpiration, de la conductance sto- matique et de la vulnérabilité à la cavitation de Quercus robur et Q petraea soumis à un stress hydrique en forêt de Champenoux (France). L’étude compare le comportement hydrique de chênes sessiles (Quercus petraea) et pédonculés (Q robur) adultes en conditions de dessèche- ment du sol. Les mesures de flux de sève et de conductance stomatique ont montré une diminution de 65 à 70% de ces paramètres au maximum de la sécheresse. Les potentiels de base atteints * Correspondence and reprints. Abbreviations: F sap flux density (dm g midday stomatal conductance to water vapor ); -1 ·h -2 : · 3s : d (cm·s k initial hydraulic conductivity of petioles (kg·m·s K maximal hydraulic ); : -1 i ); ·MPa : -1 max conductivity of petioles after 2 flushes at high pressure (kg·m·s ψ midday leaf water ); ·MPa : -1 wm potential (MPa); ψ predawn leaf water potential (MPa). : wp
étaient alors compris entre -1,7 et -2,0 MPa. Une transpiration légèrement plus faible observée pour le chêne pédonculé a été interprétée comme résultant de différences dans le statut social des 2 es- pèces. Toutefois, aucune différence significative de conductance stomatique n’a pu être mise en évi- dence entre les 2 espèces, qui apparaissent toutes 2 comme assez tolérantes à la sécheresse. Au plus fort de la sécheresse, les 2 espèces ont montré des pourcentages d’embolie de l’ordre de 70 à 80% dans leurs pétioles, alors que le potentiel hydrique foliaire minimum atteignait -3,0 MPa. Enfin, une bonne concordance entre les mesures de perte de conductivité réalisées lors du dessè- chement progressif in situ, et les courbes de vulnérabilité établies au laboratoire a été mise en évi- dence. Cependant, des dérives de conductance maximale en cours de sécheresse ont été à l’origine de certaines des différences observées. Dans ce cas, une procédure de correction du pourcentage d’embolie a permis de contrebalancer cet effet. chêne sessile / chêne pédonculé / flux de sève / cavitation / sécheresse / conductance stomati- que / xylème Furthermore, periods of oak decline and INTRODUCTION dieback occurred following the 1976 The drought affected mainly pe- drought. in the genus The distribution of species dunculate oaks (Becker and Lévy, 1982). Quercus (oaks) depends partly on water Apparently, this species appears to be availability. Large differences in drought more sensitive to dry periods. On the other tolerance are found among oak species. hand, we concluded recently (Bréda et al, Among western European oak species, 1993) that sessile oak was rather drought- sessile oak (Quercus petraea) is known to tolerant, as are most North American oaks be more tolerant to water shortage and to (Abrams, 1990). require less fertile soils than pedunculate for these frequently ob- An explanation oak (Quercus robur) (Becker et al, 1982). served differences in the ecological re- In the northern half of France, deep quirements of both species may be related soils with high fertility and periods of to water transport efficiency, and to possi- waterlogging, due to the presence of a ble involvement of cavitation and embolism clay layer, are common (Pardé, 1942). On in stress reactions. Cochard et al (1992) these sites, sessile and pedunculate oaks showed that Q robur was more prone to can grow together. They are found in water-stress-induced embolism than Q pe- mixed stands comprised of small groups of traea. However our measurements were each species rather than being intermixed. made on branches rapidly dehydrated un- der laboratory conditions. These observa- Becker (1986) showed differences in vigor tions have to be confirmed with adult trees and growth rates between species, with under natural conditions, and the impor- sessile having a clear advantage over pe- tance of cavitation in drought reactions of dunculate oak. This observation is also trees in the stand has to be assessed (Co- confirmed by forest managers. When both chard et al, 1992). Q robur and Q petraea grow together in the same site, sessile oak is always taller, This paper presents a comparative anal- larger in diameter and healthier than pe- ysis of water relations between trees of dunculate oak. Some forest management these 2 species growing in a natural mixed texts even suggest replacing the latter by stand. Sensitivity of mature trees to the former whenever possible (Poskin, drought was assessed using an imposed water shortage in a simplified lysimeter. 1934).
Seasonal time-course of water relations of both watered and droughted trees was monitored and analysed. MATERIAL AND METHODS Experimental plots Effects of water stress on Q petraea and Q ro- bur were compared in 2 groups of 8 trees (4 of each species) in a 30-yr old, 16-m high mixed stand in the Forest of Champenoux, near Nan- cy, France (48°44N, 6°14E, elevation: 237 m). Two scaffolding towers allowed measurements in the crowns, each giving access to 4 trees of Measurements each species. These experimental plots have been extensively described elsewhere (Bréda et al, 1993) and consist of a control plot and a dry Leaf water potential was measured weekly on 2 plot. The dry plot consists of a 5 x 5 m square leaves of each study tree using a pressure that includes 17 trees and is surrounded by a chamber. Leaves were sampled in the upper 1.4-m deep trench. A water-tight roof covered third of the crown just prior to dawn (predawn the soil below the crowns. The comparative leaf water potential, ψ and at 1 pm solar time ) wp carried out 2 successive sea- study during was during sunny days (midday leaf water potential, sons: ψ Predawn leaf water potential (ψ was ). wm ) wp During 1991, the control plots was left under used as an index of mean soil water in the root - natural conditions during the first part of the sea- zone. son and watered by manual irrigation at the end Sap flow was monitored on all study trees us- of August (d 241, 2 irrigations of 60 mm each). a continuously heated radial flowmeter all ing In the dry plot, water supply was withheld since over the growing season (Granier, 1985, 1987). end of June (day 170). Unfortunately, a late frost This device allows measurement of sap flux in spring immediately after leaf emergence com- density (Fd, dm along a radial axis ) -1 ·h -2 ·dm 3 pletely killed the bursting buds and induced a 3- (2 cm long) in the xylem. Total sap flux (dm ) -1 ·h 3 wk delay in leaf flushing. A limited rehydration was calculated by multiplying sap flux density occurred in this treatment as a consequence of (F by the sapwood cross-section at the same ) d leaks which occurred during a thunderstorm height in the trunk. Stand transpiration was com- (d 278, Oct 4). The whole lysimeter was com- puted from sap flow measurements by taking pletely rewatered in late autum, after all the into account the statistical weight of the sampled leaves had fallen (d 317, Nov 13), by manually trees in the stand. This experimental procedure adding 90 mm water and removing the roof. has been described by Bréda et al (1993). During winter 1991-1992, natural rainfall com- - stomatal conductance of water vapor Midday pletely resaturated the soil. measured between 11 and 12 am solar was s g During 1992, the control plot was kept well wa- time each week with a Li-Cor 1600 porometer - tered by natural and manual irrigation during the 5 to 10 (Lincoln, NE, USA) sun-exposed on measurement period. The lysimeter was cov- leaves on different branches from the upper half ered before bud-break (d 60, end of February). of the crown. The rewatering occurred on October 8 (d 282), Soil water content was measured weekly in 8 before litter-fall, by applying 150 mm water. (3 in the control plot and 5 in the dry one) 1.6-m The number of trees studied in each plot has long deep aluminium access tubes via a neutron been presented in table I. probe (NEA, Denmark).
Assessment of embolism for the 2 oak spe- RESULTS cies was made on excised petioles. Two or 3 2- yr-old branches were cut from the upper canopy Time-course of leaf water potential of each study tree during the early morning and brought into the laboratory. All measurements were performed within 4 h on 5 to 8 petioles re- Figure 1 shows the seasonal time-course cut under water (Cochard et al, 1992). Hydraulic of predawn and midday leaf water poten- conductivity was measured on 2-cm long sam- ples using the technique described by Sperry et tials (ψ and ψ for each treatment and ) wm wp al (1988) and Cochard and Tyree (1990). Acidi- species during the 2 study seasons. Dur- fied and de-aerated water was forced through ing the first part of 1991 (fig 1 a), and until the samples at a low pressure (7 kPa), the flow the irrigation of the control plot (d 241), measured with a balance, and the initial conduc- there was no significant difference be- tivity (K calculated from the flow/pressure ratio. ) i tween species in the control plot, neither Two successive periods of overpressure flush- for predawn nor for midday leaf water po- ing (0.1 MPa, over a 20-min period) allowed the embolized vessels to refill. The resulting con- tentials. ψ of control trees showed a wp ductivity (maximal conductivity) was calculated strong decline from -0.5 to -1.3 MPa be- as previously described. The ratio between ini- tween the first part of the season until the tial (K and maximal conductivity (K yields ) i ) max end of August (d 240). In fact, control trees the loss of conductivity according to: were water-stressed for a month till the re- d 240. % loss of watering conductivity 1 ) max / i (K on = -
In the dry plot (fig 1b), ψ was initially wm slightly higher in pedunculate oak than in sessile oak (d 180-210). The difference between ψ and ψ (w increased wm ) Δψ wp more gradually in the former than in the lat- ter species. This was related to the delay in leaf area index development in the for- mer species, due to a higher sensitivity to spring-frost. Later on, drought induced a gradual and parallel decline in ψ and wp ψ until September 20 (d 263). On Sep- wm tember 23 (d 266), a thunderstorm pro- moted a non-controlled and deep rewater- ing leading to an increase of leaf water po- tential. During the greatest periods of stress, values of ψ and ψ were slightly wm wp but consistently lower in sessile than in pe- dunculate oaks. A similar seasonal varia- tion was observed during 1992, except that, as control trees were kept well wa- tered, ψ never dropped below -0.60 wp MPa (fig 1c). During 1992, the difference increased the variability in F within each d species. Again, F was lower in the 2 between sessile and pedunculate drought- d ed trees were greater and significant for smallest trees. ψ and ψ (fig 1 d). wm wp Seasonal variations of the mean daily sap flow of the 3 dry pedunculate and 3 dry sessile oaks, averaged over 10-d peri- Effects of restricted water supply ods, have been shown in figure 3. A strong on sap flux density drought-related decrease in total transpira- tion occurred in both species, as compared The daily time-course of sap flux density in droughted trees did not display ) d (F interspecific difference at the beginning of the drought period (d 210, July 29 1991; 3 trees per species, fig 2). These values were not significantly different from the mean of control trees. Nevertheless, the 2 smallest trees (one of each species) showed a lower F that was already ob- d served on other suppressed trees (Bréda et al, 1993). On d 262 (September 19), drought induced a strong decline in F for d both species. This decline appeared to be greater for the pedunculate oaks, despite their slightly higher predawn leaf water po- tential -1.54 MPa), compared to wp (ψ = sessile oaks -1.75 MPa). Drought wp (ψ =
with control trees. During stress, sessile ations during 1991 and 1992. No differ- oaks maintained slightly higher sap flows appeared at the beginning of the 2 ence than pedunculate oaks. This difference, seasons between dry and control plots and even if not always statistically significant be- between each species. g increased grad- s ually in both species with a large variability cause of high within-tree variability, was nevertheless maintained during the whole between leaves. This may be ascribed to period. Variations in soil water content were leaf maturation. Maximal values were 0.6≈ -1 cm·s for both species during 1991 (fig computed during the 2 seasons. The maxi- mum extracted water in the lysimeter was 4a,b) and somewhat higher during 1992 (0.8 cm·s fig 4c,d). Higher maximal val- , -1 141 mm during 1991 and 148 mm during 1992. Soil water depletion as detected in ues of g measured in 1992 may be as- s the vicinity of root systems of both species cribed to the better irrigation of the control plot during this year. A strong decline in g to a 1.60-m depth was rather similar (data s was observed in the control trees (fig not shown). Nevertheless, water content profiles at the end of the dry period showed 4a,b), which was reversed after rewatering that extraction had occurred in even deeper by irrigation (d 240) and was followed by a soil layers near sessile oak roots. relative stability during late summer. In contrast, trees in the stressed plot during 1991 showed much lower values af- Stomatal conductance ter d 240. g stabilised around minimal val- s ues of 0.05 cm·s until accidental and par- -1 Seasonal time-course of midday stomatal tial rewatering (d 268) occurred. It increased slightly later on. This increase conductance g (fig 4) displayed large vari- s
was larger in Q robur. During 1992, mini- Development of embolism in the field mal values of the magnitude were same (< 0.1 cm·s but were reached earlier d ) -1 Figure 6 shows an example of the season- 220) for Q robur and Q petraea (fig 4c,d). progession of embolism on petioles of al A general plot of g (values of 1991 and s one dominant tree of each species. A sig- 1992) as a function of ψ is presented in nificant reduction in conductivity was ob- wp figure 5. For a statistical analysis of inter- served in petioles after the first measure- specific differences, data were separated ment performed in late spring 1991, when into 2 classes according to their value of drought had not yet begun. During 1991 ψ (below and above -0.6 MPa). Differ- (fig 6a), embolism increased after the date wp ences between species were tested (t-test) when ψ was -1.8 MPa for both trees, at wp within each class. Neither mean values nor which time ψ was -3.3 MPa for Q pe- wm regressions (linear model for g were sig- ) s traea and -2.6 MPa for Q robur. At this nificantly different between species. A time, loss of conductivity reached 40% for sharp decrease associated with a large Q petraea and 10% for Q robur. During dispersion for predawn leaf water potential 1992 (fig 6b), embolism reached 80% for values ranging between -0.25 and -0.6 Q petraea and 30% for Q robur at maxi- MPa was observed. Between -0.6 and mum stress intensity. The same minimal - 2.0 MPa the decrease in g was more s values of ψ were observed during 1992 wm gradual. Under most severe water stress as well as during 1991 (-3.3 and -2.6 MPa conditions, stomatal conductance still re- mained at significant and constant levels of about 0.10 cm·s thereby allowing signifi- , -1 cant rates of leaf transpiration to continue.
respectively for both species). We attribute the 100% loss of conductivity that oc- curred on d 286 in 1992 to the first frost event (-2°C). In situ observed embolism compared to vulnerability curves as We plotted losses of hydraulic conductivity observed in situ during 1991 and 1992 on petioles against the minimum value of mid- day leaf water potential recorded prior to each estimate of embolism (fig 7). The re- sulting plot was compared with vulnerabili- ty curves obtained on excised branches dehydrating under laboratory conditions (Cochard et al, 1992). Despite a higher variability for in situ dehydration, we ob- served good agreement between both sets of results in sessile oak (fig 7a). However, in the case of pedunculate oak (fig 7b), the losses of conductivity measured on peti- oles in situ seemed to remain below the vulnerability curve between -2.5 and -3.0 MPa. But at the same time, during 1992 we observed a large decrease in the maxi- mal hydraulic conductivity K for pedun- max culate oak in the dry plot from d 233 (Au- gust 20) on: K decreased from 6.6 x max -7 10 (± 5.3 x 10 to 3.5 x 10 (± 3.3 x -7 ) -7 ) ·MPa -7 -1 10 kg·m·s (in 1991, K dis- max played a mean value of 6.1 x 10 ± 2.9 x -7 ). -7 10 Such a decrease was not observed in sessile oak, where K remained con- max stant during the entire season (11 x 10 ± -7 2.6 x 10 kg·m·s The tech- -7 -1 ). ·MPa agreed well with the vulnerability ob- curve nique used to restore maximal conductivity tained in the laboratory. in the petioles did not fully resaturate the embolized vessels during late summer and led to a value of K which was signifi- max DISCUSSION AND CONCLUSION cantly lower than the pre-stress maximal conductivity. We recalculated the percent- age of embolism using the average values Although oak transpiration was reduced of K measured before the decrease be- max under drying soil conditions, it remained gan. As shown in figure 8, corrected val- quite high even for ψ-1.7 MPa: it was wp ≤ ues of losses of hydraulic conductivity reduced by≈ 75% when water stress was
near sessile oaks (160 to deeper layers 200 cm). These observations (higher leaf area and deeper soil water extraction) could help explain the slightly higher sap- flow and lower ψ in the 3 individuals wp from this species that we observed. But these observed differences may not be an intrinsic species-related feature. Rather, they could be due to the favorable compet- itive status of the sessile oak individuals in mixed stands containing pedunculate oaks. This competitive advantage of Q pe- traea vs Q robur in mixed stands of 30-60 yr has frequently been reported by forest practitioners and ecologists (Lévy et al, 1992). We did not find any difference in maxi- mal stomatal conductance (g between ) s species in well-watered trees. Restricted water supply had a strong effect on stoma- tal conductance: g was reduced by 70% s = between -0.6 and -2.0 MPa predawn leaf water potential (ψ with no interspecific ), wp difference. On the other hand, no clear re- maximum. We have shown in a recent lationships between g and neither the ra- s paper (Bréda et al, 1993) that sessile oak diation nor the vapor pressure deficit could was characterized by an efficient and deep explain the large dispersion of g between s root system. We concluded that Q petraea 0 and -0.6 MPa. In fact, ψ seemed to be wp was a rather drought-tolerant species be- a poor indicator of stress intensity when cause of its ability to maintain significant soil began to dry out, because it could not daily transpiration rates even under de- help explain the early decrease in leaf creasing soil water availability. stomatal conductance. Instead of ψ the , wp Seasonal time-course of predawn leaf soil water potential measured in the 30 cm potential showed a similar pattern upper soil profile, which contains 60% of water during the 2 yr of measurement: lower val- the fine roots, would probably be a better ues were observed for sessile oak than for characteristic to relate to g A recent hy- . s pedunculate during the periods of water pothesis for stomatal regulation involves a hormonal signal from roots, which is influ- shortage. We attributed this to a slightly higher transpiration rate in sessile than in enced by soil water status. As reported by Schulze (1986) and Davies and Zhang pedunculate oaks. However, stomatal con- ductance was identical in both species. (1991),soil water stress could trigger root Higher sapflow in sessile oak could be ex- signals stimulating stomatal reactivity. As a plained by higher leaf area of individual matter of fact, ψ may not represent the wp trees. The total water extraction from water potential in the driest soil layers, 1.60 m depth was very similar in the vicini- from where root signals could proceed, but ty of roots of pedunculate and sessile probably of the wettest and deeper rooting oaks. We also observed extraction from layers.
The consequences of differing LSCs From this drought-induced course of area. leaf water potential and probability of stomatal conductance and total transpira- on cavitation occurrence have been dis- tion, we have concluded that the 2 studied cussed by Jones and Sutherland (1991). species of oaks are water stress tolerant, We observed a slight difference in LSC be- and that no major difference between both tween species: Q robur seemed to have exists under natural conditions. lower LSC in petioles than Q petraea (data However, under laboratory conditions, a not shown) which could increase its sus- difference in vulnerability to cavitation was ceptibility to cavitation. observed between the species; Q robur is spite of a difference in vulnerability, In more sensitive than Q petraea (Cochard et species reached approximately the both al, 1992). Cavitation began when water same levels of losses in hydraulic conduc- potential reached -2.2 MPa, and a 50% of tivity (80%) under field conditions. In fact, embolism was measured at -2.7 MPa for dominant trees of Q petraea had lower leaf Q robur and -3.2 MPa for Q petraea. We water potentials. It is worth noting that showed a good agreement between the % stomatal conductance was significantly re- loss of hydraulic conductivity measured duced at ψ -0.6 Mpa, corresponding to under field conditions and those predicted = wp -2.0 MPa. This value is also the by vulnerability curves when K was wm ψ max = threshold for which embolism can signifi- stable over the season. For pedunculate cantly increase. Maximum stomatal clo- oak, we showed that K decreased, max ψ -1.5 MPa. At sure occurred when leading to an underestimation of the actual = wp this time, ψ -3.0 MPa and the loss of wm percentage of embolism. Two successive = hydraulic conductivity is close to 30%. high pressure perfusions of samples did Stomatal regulation was able to control the allow a complete dissolution of embolism degree of embolism and to restrict it to this (replaced the air by water) but the conduc- value for 1 month, despite decreasing = tivity was not restored because of plugging soil water availability. Later on, with great- of the vessels (tyloses, pit membrane oc- er drought, stomatal regulation was not clusion, etc). The good stability of K be- max able to prevent a sharp increase of embo- tween the first and the second flushes of lism. Loss of conductivity reached 80% high pressure reveals that air blockage of within a few d. Such a situation is in agree- embolized vessels was not involved. The ment with the model suggested by Tyree formation of tylosis as reported by Zimmer- et al (1988, 1989, 1991). It seemed sur- man (1979) that occurs in many trees at prising to us that such a large loss of con- the end of the growing season and that oc- ductivity in the petioles (and probably also curs in Q rubra and Q alba (Cochard and in the youngest twigs) did not strongly af- Tyree, 1990) could presumably be respon- fect the total sap flow of the trees. Total sible. A similar decrease in apparent K max transpiration remained constant below - has been detected with potted saplings of 2.5 MPa. This may be an illustration of the Q robur during increasing drought (Simo- fact that the main resistance to liquid water nin et al, 1994). flow from roots to leaves is probably locat- If embolism is directly dependent on ed between the soil-root interface and the leaf water potential, then leaf water poten- branches. As a consequence, strong in- tial is strongly related to another character- creases in the minor resistance like that in petioles or twigs have only limited conse- istic of hydraulic function: the leaf specific quences on the total resistance to water conductivity (LSC) of the petiole, which is flow (Tyree et al, 1994). calculated as the ratio of K and the leaf max
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