Báo cáo khoa học: "Tree water relations and climatic variations at the alpine timberline: seasonal changes of sap flux and xylem water potential in Larix decidua Miller, Picea abies (L.) Karst. and Pinus cembra L."

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Nội dung Text: Báo cáo khoa học: "Tree water relations and climatic variations at the alpine timberline: seasonal changes of sap flux and xylem water potential in Larix decidua Miller, Picea abies (L.) Karst. and Pinus cembra L."

Original article Tree water relations and climatic variations at the alpine timberline: seasonal changes of sap flux and xylem water potential in Larix decidua Miller, Picea abies (L.) Karst. and Pinus cembra L. Tommaso Anfodillo Stefano Rento Vinicio Carraro, Luca Furlanetto Carlo Urbinati Marco Carrer Dipartimento Territorio e Sistemi Agro Forestali, University of Padova, Agripolis, Via Romea, 16, 35020 Legnaro (PD), Italy (Received 15 January 1997; accepted 15 September 1997) Abstract - Trees growing at the alpine timberline very seldom undergo severe water stress because of high precipitation during the vegetative period. Since trees are adapted to moist con- ditions, moderate water deficit may lead to a strong reduction in transpiration. Transpiration and xylem water potential were measured in two individuals each of Pinus cembra, Larix decidua and Picea abies growing at the timberline (2 080 m a.s.l.) in the north-eastern Italian Alps. From June to October 1996 predawn water potential was between -0.29 and -1.0 MPa with moderate differences among species. Throughout the growing period L. decidua showed a progressive decrease in the minimum water potential (from -0.45 to -1.93 MPa); in P. abies and P. cembra variations were more correlated to weather conditions with minima (-1.2 and -1.49 MPa, respec- tively) during a mild L. decidua showed the mean daily maximum sap flux den- drought period. sity (about 3.3 dm dm h while mean maximum values in P. abies and P. cembra were 3 -2 -1 ) about 0.9 and 0.7, respectively. High daily fluctuations of sap flow were observed in relation to rapid variations in weather conditions, particularly in L. decidua. Regardless of species a very high stomatal sensitivity to vapour pressure deficit was recorded. The three species seem to have evolved different drought avoidance strategies. L. decidua maintained a relatively high transpi- ration even during moderate water deficit periods because of its high water uptake capacity. During the same drought period P. abies and P. cembra showed an evident reduction in sap flux, suggesting a water saving behaviour. These different responses should be taken into account when considering the effects of global change on timberline trees. (© Inra/Elsevier, Paris.) water relations / timberline / resistance / stomatal sensitivity / climate warming drought effects * Correspondence and reprints Tel: (39) 49 827 2697; fax: (39) 49 827 2686; e-mail: anfodill@uxl.unipd.it Abbreviations: Ψ: xylem water potential (MPa); Ψm: minimum xylem water potential (MPa); Ψpd: predawn xylem water potential (MPa); Fd: sap flux density (dm dmhMWDP: mild water 3 -2-1 ); deficit period; HC: hydraulic conductance (dm dm h Mpa 3 -2-1 -1 ).
Résumé - Relations hydriques des arbres et facteurs du climat à la limite forestière alpine : variations saisonnières du flux de sève et du potentiel hydrique chez Larix decidua Miller, Picea abies (L.) Karst. et Pinus cembra L. Les arbres situés à la limite forestière dans les Alpes sont rarement soumis à des contraintes hydriques sévères, car les précipitations durant la période de végétation sont élevées. Alors que ces arbres sont adaptés à des conditions de forte humi- dité, une contrainte hydrique modérée peut conduire à une forte réduction de leur transpiration. La transpiration et le potentiel hydrique ont été mesurés sur deux individus de chacune des espèces : Pinus cembra, Larix decidua et Picea abies dans la zone de la limite forestière (altitude 2 080 m), dans le nord-est des Alpes italiennes. De juin à octobre 1996, le potentiel hydrique de base a varié entre -0,29 et -1,0 MPa, avec peu de différences entre espèces. Au cours de la période de végétation, L. decidua a montré une diminution progressive de son potentiel hydrique minimum (passant de -0,45 Mpa à -1,93 Mpa). Chez P. abies et P. cembra, les variations de ce paramètre étaient plus fortement corrélées aux facteurs climatiques, les valeurs atteintes étant respectivement de -1,2 Mpa et de -1,49 Mpa pour ces deux espèces, lors d’une période de sécheresse modérée. Les valeurs les plus élevées de densité de flux de sève ont été observées chez L. decidua (environ 3,3 dm dm h contre 3 -2-1 ), 0,9 dm dm h chez P. abies et 0,7 dm dm h chez P. cembra. Des fortes variations 3 -2 -1 3 -2 -1 journalières de flux de sève ont été mises en évidence en relation avec les fluctuations rapides des conditions climatiques, notamment chez L. decidua. Une forte sensibilité des stomates au déficit de saturation de l’air a été observée pour chacune de ces espèces. Ces trois espèces semblent avoir développé différentes stratégies de réponse à la sécheresse : L. deciduca a maintenu un taux de transpiration relativement élevé, même lors d’une sécheresse, en relation avec une forte capacité d’extraction de l’eau dans le sol. Au cours de la même période de dessèchement, P. abies et P. cembra ont montré une nette réduction de leur flux de sève, ce qui indiquerait une stratégie d’évitement. Ces différentes réponses doivent être prises en compte lorsqu’on s’intéresse aux effets des changements climatiques dans cette zone de limite forestière. (© Inra/Elsevier, Paris.) régulation stomatique / relations hydriques / limite forestière / résistance à la sécheresse / réchauffement du climat have themajor impact 1. INTRODUCTION seem to peratures limiting physiological processes: cold on vascular system, sub- soil, frozen soil The altitude of alpine timberline is or freezing temperatures during both dor- controlled by temperature [14]. mainly mancy and growth periods strongly affect However, the general statement that heat relations of treeline species deter- deficiency during the short and cold grow- water mining severe stress conditions [13]. ing season affects the carbon budget of trees, decreasing dry matter production As there is a strong influence of abi- [31], is often inadequate to explain why otic factors (i.e. temperature, wind, pre- the timberline occurs in different climatic cipitation) on physiological responses of regions. In continental alpine timberlines trees at the timberline the effects of cli- (e.g. Austrian Alps) an incomplete devel- mate warming might be particularly pro- opment of needle cuticles during the short nounced [17]. growing period seems to play the most There is sound evidence that climatic important role in determining severe changes can affect the distribution of plant drought conditions in the following winter communities and shift the range of various [3, 12, 32]. In arctic, temperate-maritime alpine species [21, 22]. Recently, climate and tropical treelines (Alaska, Washington warming has been thought to be the cause Cascades, Venezuelan Andes) cold tem-
of altitudinal shift upwards in alpine have stronger impact on reducing tran- an a plants [8] and for displacement of the arc- spiration in Norway spruce (Picea abies) and Stone pine (Pinus cembra) than in tic treeline as well as for an increase in stem growth in the Krummholz zone [24]. European larch (Larix decidua). Palynological data have outlined the pos- The extent of potential assimilation sible migrations of European flora in rela- reduction will also depend on the change tion to climatic variations [15]. in precipitation regime associated with the rising temperature. Since the link between On the contrary, evident effects of no precipitation and temperature in the Alpine temperatures on alti- higher recent summer region is not yet fully understood [35] and tudinal range have been recorded in alpine future scenarios are still contrasting, the sylvestris and P. cembra [11]. Pinus true effects of higher temperatures on the Predictions of possible impact of yet uncertain. timberline are as temperatures upon the physiol- warmer Nevertheless, higher summer temper- of plants adapted to cold climates ogy lead, in the long run, to a com- atures may should consider both the effective varia- position change of timberline forests due tions in plant temperature (degree of aero- to different drought avoidance strategies dynamic coupling between the plant layer developed in Alpine timberline species. and free atmosphere) and different aspects of temperature-mediated processes (freez- ing resistance, soil temperature and min- 2. MATERIALS AND METHODS eral nutrient supply, photosynthetic rate, rate of cell division, rate of mitochondrial Experiments were conducted on a timber- respiration) [19]. line ecotone at 2 080 m a.s.l. in the north-east- Among these, dark respiration could ern Italian Alps (Dolomites, Cortina be crucial since high altitude plants exhibit d’Ampezzo). The site has S aspect and 30 % slope. Here the timberline is formed by rela- a much higher respiration rate than low- tively young L. decidua, P. cembra and P. land species do, and unless acclimation abies mixed stands invading edges of recently occurs, this can negatively affect the plant abandoned pasture lands [7]. June-September carbon balance [23]. mean precipitation is about 450 mm. Further, predictions are also dependent The experiment lasted from 29 May-6 the type of temperature values consid- October 1996. Six similar-featured trees were on selected (two each of the above-mentioned ered: it is important to distinguish annual, species). In each tree a sample core was col- seasonal, daily means and extremes [18]. lected at 1.30 m and height, conventional age Seasonal monitoring of the water status and sapwood width were measured (table I). in timberline trees in the southern Alps Differences among trees were expected as a result of severe environmental conditions. A has allowed their drought resistance mech- quite good growing potential of the specimen anisms to be better defined and to make appeared comparing tree age and diameter. hypotheses on some possible responses to Xylem water potential (Ψ) was measured climate warming. weekly with a pressure chamber on1-year-old Our aim is to demonstrate that, despite shoots in L. decidua and P. abies and 1- on regularly distributed precipitation (about year-old bundle needles in P. cembra. Four samples were collected at a height of 2 m (two 400-500 mm between June and Septem- on the south- and two on the north-facing ber), trees at the timberline may undergo crown) on each tree just before dawn (predawn moderate water stresses (i.e. reduction in water potential, Ψpd) until sunset at 2-h inter- stomatal conductance) due to their high vals. Data were then averaged for each species stomatal sensitivity to drought. Further- since no statistical difference was recorded more, these moderate water deficits may between the two trees and crown aspect.
Xylem sap flux density (Fd, dm dm h 3 -2 -1 ) measured in each tree using 2-cm-long was continuously heated sap flowmeters [9]. Sen- sors were inserted into the xylem (NW aspect) 1.5-2 m high in the stem. Protection from high solar radiation was ensured both by insulating shields placed over the sensors and for P. abies and P. cembra by the dense tree crowns, with ground reaching branches. No alterations in thermal signal due to resin emission or wood desiccation were recorded over the whole monitoring period. Sap flowme- ters were heated from 14 June except for two trees in which heating began 20 days later. Measurements were taken every I min, aver- showing a less conservative water transport aged and stored every 15 mins. (as indicated from the lower averaged regres- Sap flux and water potential data were used sion coefficients - table II) to estimate the global hydraulic conductance Standard meteorological factors were mon- roots-leaves. Neglecting the stem-branch itored every minute, averaged and stored every capacitance effect, the equation describing sap 15 min with a data logger (Campbell Ltd transport between roots and leaves can be writ- CR10) connected to two multiplexers (Camp- ten as follows [6]: bell AM32). Power was provided by a solar panel (Helios technology, 50 W) and batteries where is the roots-leaves resistance. (140 Ah). r Water potential when sap flux is null (Ψ Technical and logistic support was ensured ) 0 was deduced from predawn measurements or from the Centre of Alpine Environment of the estimated with linear regressions using water University of Padova located 20 km away in S. potential data and the corresponding sap flux Vito di Cadore. values. Specific hydraulic conductance (HC 1/r = 3 -2 -1 dm dm h ) -1 Mpa calculated as the 3. RESULTS was slope of the linear regression of sap flux (Fd) versus the drop in the water potential (Ψ) In this is the Alpine area summer throughout the day. In L. decidua data devi- (mean precipitation of the wettest season ated slightly from the regression line, indicat- last 30 years about 500 mm). In 1996, dur- ing a low stem-branche capacitance [6]. In ing the measurement period, we recorded P. abies and P. cembra loops were wider
decrease. July and August were moder- (figure 1). At the end of July there 621 mm ately cold compared with previous years. unusual dry period (10 days with was an rain less than 0.4 mm d that we will call -1 ’mild water deficit period’ MWDP) since only four similar periods were recorded potential variations 3.1. Shoot water from 1960 to 1990. Figure 2 shows the seasonal course of The maximum mean air temperature the predawn water potential (Ψpd) of reached at the beginning of June was selected trees (no ecophysiological mea- (about 16 °C), followed by a sharp
made in the warmest bra also had lower Ψm values probably surements were due to the sampling method (needles period). instead of twigs). L. decidua reached the highest Ψpd L. decidua showed a completely dif- (-0.29 MPa) after high precipitation at the ferent behaviour: Ψm decreased regularly end of June (day 174), it then decreased from June (-0.7 MPa) to the end of July until the end of August, when gradually (day 215), stabilizing at about -1.9 MPa the minimum was reached (-1.0 MPa, day until the end of August (day 237). After- 237). In September a new increase in Ψpd wards Ψm again increased, reaching the was recorded according with the variation values of the beginning of the season. In in Ψm when high precipitation, high soil this species no close relationship was water availability and low vapour pres- found between short-term variations in sure deficit (VPD) occurred. precipitation and Ψm. P. abies and P. cembra showed more parallel variations until the end of the MWDP. In P. cembra Ψpd appeared 3.2. and seasonal variations Daily lower than the other two species except in sap flux density (Fd) at the end of July. In contrast to L. decidua, both species exhibited a reduc- Examples of Fd and Ψ courses through- tion in Ψpd in relation to the MWDP typical day at the beginning of out a (about 0.3 MPa) and a slow recovery over August are shown in figure 4. 2-3 weeks. Due to frequent cloudiness variations The minimum water potential (Ψm) at high altitude, air temperature (and in P. abies and P. cembra are well curves VPD), as well as solar radiation, change related to precipitation variations (fig- accordingly. ure 3). The lowest values (-1.18 and -1.49 MPa, respectively) occurred at the L. decidua appeared strongly coupled end of the MWDP, the highest (-0.52 and with the variations in VPD. Fd increases -0.60 MPa) on 22 June (day 174). P. cem- very sharply reaching the daily maximum
Fd daily sum variations and the average (mean maximum range 3-3.5 dm dm 3 -2 diurnal VPD (from 6 a.m. to 8 p.m.) were ) -1 h couple of hours after sunrise. Ψ a calculated for the entire measurement decreases rapidly as well: 4 h later it can be period (figure 5). 1 MPa lower. Ψm is normally reached after noon and the recovery can be quite VPD throughout the growing The mean fast. quite low, as expected in a season was timberline environment. All species Fd in P. abies began later and the max- showed Fd variations coupled with VPD imum value is much lower than L decidua but, due to high stomatal sensitivity (see (mean maximum range 0.8-1.0 dm dm 3 -2 below) Fd is well correlated to VPD only ). -1 h Variations in Fd are less dependent below the treshold of 7-8 hPa. When VPD on VPD and the course of Ψ appeared is higher stomatal conductance decreases regular. more leading to a reduction in the expected Fd. P. cembra had the lowest Fd values (mean maximum range 0.6-0.8 dm dm 3 -2 During the MWDP no significant changes in Fd were recorded in L. ). -1 h Ψm is reached just after noon but decidua. On day 205 (VPD 6.8 hPa) daily subsequent recovery is the slowest among Fd was 33.5 dm dmd on day 210, 3 -2-1 ; the three species.
the end of the MWDP (VPD 5.8 hPa), Fd seemed not reduced if compared with at about 32 dm dm d These values were 3 -2 -1 . 196: 8.5 versus 9, respectively, but day close to those recorded on similar days after some days (day 210versus 222) Fd (e.g. day 196 Fd 34; day 222 Fd 32.5), appeared strongly reduced (-35 %). = = showing no influence of the soil drying In order to better define the effect of out. MWDP on Fd of studied trees a compar- On the contrary, P. abies showed an ison between the cumulated Fd over a 7-d evident reduction in Fd during the MWDP wet period versus 7 d during the MWDP (day 205 Fd 10 versus day 196 Fd has been made (table III). The effect of = = 12.5; day 210 Fd 7.5 versus day 222 Fd MWDP on cumulated Fd of the evergreen = 11) reaching about -35 % under the species is expressed in relation to cumu- = same VPD conditions. The extent to which lated Fd in L. decidua which is the only P. cembra Fd was influenced by MWDP species not affected by water shortage. Fd appeared slightly different from P. abies. is reduced by 25 % in P. abies and 32 % in At the beginning of the MWDP (day 205) P. cembra.
of Fd daily highest val- The seasonal maximum in Fd was on Scattergrams VPD at the same time high- 230-232 in all species, just after days ues versus lights the relationship between the former abundant rainfall even if very high tran- and stomatal control (figure 6). Regard- spiration rates were also recorded in July.
less of species and tree, Fd increases with except for two days after the end of the increasing VPD from 0 to 4-5 hPa, then MWDP. In P. cembra and P. abies HC tends to stabilize and over 8 hPa no rele- dropped to minimum values at the end of vant increase was recorded. The shape of the MWDP. With the following precipi- the scatters showed clearly that a strong tation HC in P. cembra rose quickly while stomatal control occurred, suggesting a P. abies needed 2 weeks to recover val- very high sensitivity of these species to ues comparable to the beginning of the water deficit. season. Differences between Larch &num;1 and &num;2 probably due to the position of the are 4. DISCUSSION probes in the stem. This may occur using single probe measurements [20]. Small variations in Ψpd and Ψm both in P. abies and P. cembra over the season 3.3. Seasonal variation in are due to the high frequency of precipi- hydraulic conductance (HC) tation but also showed that they are able to use the available moisture in an econom- ical way. Hydraulic conductance showed large variations throughout the season (figure 7). Values of Ψm in P. abies were signif- The highest values corresponded to high- icantly higher than in other studies [25] est precipitation at about the end of June suggesting that a more pronounced water when air temperature was also particularly saving behaviour was developed. More- low and soil evaporation was prevented. over Ψm appeared to be much higher than All three species showed a marked the turgor loss point which, at the alpine depression in HC during the MWDP. HC timberline, was found to be relatively con- values in L. decidua were always higher stant throughout the growing season at than in the other two evergreen species about -2.8 MPa [2]. Stomatal control
al. [30]). In fact, if the stem and branches therefore occurred well above the thresh- have little capacity to store water and old of both incipient plasmolysis and of desaturate the reservoirs the variations in significant loss of xylem functionality in Fd measured at 1.3 m will be strongly conifers (at least 2.5 MPa [5]). dependent on stomatal behaviour. This is The roughly constant decrease in Ψpd also partially confirmed looking at the and Ψm from the beginning of June until quite good coherence between variations the end of July in L. decidua (figure 3), in Ψ measured in shoots and Fd (figure 4). followed by the increase at the end of the Less variability in Fd of P. abies and season could be due to an osmotic adjust- P. cembra also suggests a more efficient ment (even if it seems to have little impor- water loss control. The lowest Fd are tance in conifers) [16]. Osmoregulation found in P. cembra and this is consistent should allow maintenance of physiologi- with the widespread belief that P. cembra cal activity (i.e. turgor maintenance) as Ψ is the most drought resistant species (with falls [28]. Hence, the species enhanced its water uptake ability in the mid-summer a water saving strategy) at the alpine tree- line [32]. A bigger time lag between the when it is more likely that moderate water start of sap flow and the decreasing of deficit occurs. In L. decidua the value of Ψm also appeared above the threshold for twig water potential confirmed a higher stem-branche capacitance of the two ever- inducing xylem dysfunctions. species. green It is well known that L. decidua devel- of seasonal Fd highlighted system which allows it The deep root a course ops to utilize water sources in the deepest and strong impact of MWDP on P. abies and a P. cembra (figure 5; table III). Both wettest soil layers as also demonstrated species seemed unable to maintain an ade- using hydrogen stable isotope analysis quate water supply to the leaves after a [34]. few days without rainfall. This led to a As expected, under non-limiting soil decrease in the assimilation rate when the moisture conditions, L. decidua exhibited supposed most ’favourable’ weather con- a Fd higher (up to about three times) than ditions (high temperature and radiation) the other two evergreen species. This occurred. depends mainly on its deciduous strategy, since the shorter assimilation period [26, Stomatal sensitivity appeared particu- larly pronounced since stomatal control 32] must be associated with a higher pho- tosynthetic capacity and hence with a more began at 4-5 hPa VPD. Lower sensitivity effective stomatal gas exchange. to soil drought and higher drought resis- has been demonstrated in lower ele- tance level of coupling between A high vation species compared to high moun- canopy and atmosphere was demonstrated tains species [4, 25]. Values of HC in P. for coniferous stands [27] and this is par- abies appeared similar to other measure- ticularly true in a less dense stand as ments at the same Ψpd conditions [10]. occurs at the timberline. In our trees, VPD appeared to be the major factor determin- During the MWDP the HC decreased ing Fd, and among the species L. decidua sharply in all three species but remained generally higher in L. decidua. Since trees showed the best degree of coupling (fig- ure 4). This may be due both to the less did not experience xylem water potential below the threshold of significant loss of dense crown structure which determines a more efficient air mixing and to the lower xylem functionality in conifers, at least 2.5 MPa [5] the drop in HC seemed stem-branch capacitance (in L. decidua mainly to be due to an increase of and P. abies water is stored mainly in hydraulic resistance between soil and root branches as demonstrated by Schulze et
interface. Hence, it appeared that soil the effect of higher CO concentration 2 as moisture could play an important role in growth [29]. on tree determining water stress conditions in If precipitation rate, regimes and cloudi- some species at the timberline. Due to the ness should change towards more xeric lowering of Ψm L. decidua was able to conditions, as has been hypothesized [35], take up water in drier conditions than did in the long term, a change in species com- P. abies and P. cembra, which appeared position in timberline should not be more susceptible to water shortage. The exluded. results showed that, despite high precipi- tation, soils at high altitude could become physiologically dry because they are shal- ACKNOWLEDGMENTS low, discontinuous and highly permeable. High temperatures and VPD not asso- This research was carried out with the finan- ciated with an adequate water supply cial support of the Ministry of University and Scientific and Technological Research appeared to have a negative effect on P. (MURST) funds ex40 %. The authors wish to abies and P. cembra growth. Since, in this thank the Regole of Cortina d’Ampezzo for zone, the growing period (considered as having allowed the study on their property. a period of wood formation at DBH) is Special thanks to Fausto Fontanella, Roberto about 50 d [1], any break in assimilation Menardi and Giuseppe Sala of the Centre of processes could have a considerable Alpine Environment for the precious technical impact on total annual growth. support. We also thank the Alberti family, owner of the 5 Torri Refuge, for the kind hos- Moreover, since trees adapted to cold pitality offered throughout the work. climates have a relatively low tempera- ture optimum for photosynthesis (between 10 and 14 °C [33]) high temperatures are REFERENCES not necessary to develop maximum assim- ilation rate. Anfodillo T., Carrer M., Rento S., Urbinati [1] C.. Accrescimento radiale di Picea abies In fact no significant variations in Fd Karst, Larix decidua Mill., Pinus cembra L. were recorded on bright and warm days e fattori climatici al limite superiore del bosco: primi risultati di un’indagine nelle Alpi ori- relatively cloudy days. compared to as entali, Proc. VII Congres S.It.E. Napoli, 1996, Trees seemed unable to make the most pp. 35-38. of the fine weather conditions. The early Anfodillo T., Casarin A., Variazioni stagionali [2] stomatal control when VPD approaches nelle relazioni idriche di rametti di abete rosso 4-5 hPa, irrespective of species studied, lungo un gradiente altitudinale, in: Anfodillo T., Urbinati C. (Eds.), Ecologia delle foreste undoubtedly affects growth potential. This di alta quota, Proc. XXX Corso di Cultura in high stomatal sensitivity was also reported Ecologia, University of Padova, 1993, pp. in other spruce species [13]. 143-171. Baig M.N., Tranquillini W., Studies on upper [3] The different response recorded in P. timberline: morphology and anatomy of Nor- abies and P. cembra as compared with L. way spruce (Picea abies) and stone pine decidua allows us to speculate that in the (Pinus cembra) needles from various habitat case of an increase in air temperature (and conditions, Can. J. Bot. 54 (1976) 1622-1632. VPD) the latter could be favoured in com- Barton A.M., Teeri J.A., The ecology of ele- [4] vational position in plants: drought resistance petition against the former. Predictions in five montane pine species in Southern Ari- on a change in stand composition must be zona, Am. J. Bot. 80 (1993) 15-25. carefully evaluated considering possible Cochard H., Vulnerability of several conifers [5] future scenarios of precipitation and to air embolism, Tree Physiol. 11 (1992) cloudiness due to climate warming as well 73-83.
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