Báo cáo khoa học: " Water flux in boreal forest during two hydrologically contrasting years; species specific regulation of canopy conductance and transpiration"

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Original article Water flux in boreal forest during two hydrologically contrasting years; species specific regulation of canopy conductance and transpiration Jiri Kucera Michael G. b Emil Clenciala a c Ryan Anders Lindroth d a Department of Soil Sciences, Swedish University of Agricultural Sciences, Box 7014, 750 07 Uppsala, Sweden b Laboratory of Environmental Measuring Systems, Turisticka 5, 621 00 Brno, Czech Republic c Mountain Experiment Station, USDA Forest Service, Fort Collins, CO, USA Rocky d Department for Production Ecology, Swedish University of Agricultural Sciences, Box 7042, 750 07 Uppsala, Sweden (Received 15January 1997; accepted 21 October 1997) Abstract - We estimated the reduction of transpiration from drought for tree species in a mixed boreal 60-year-old stand in central Sweden. Actual transpiration was estimated from direct mea- surements of sap flow rate in Pinus sylvestris and Picea abies trees during two consecutive years with contrasting precipitation. Drought-induced reduction of transpiration (transpiration deficit) was quantified as the difference between the measured sap flow and the transpiration calculated for non-limiting soil water conditions. The drought-free transpiration was estimated on an hourly basis from Penman-Monteith equation with the parameterized canopy conductance (g func- ) c tions for individual species. The values of g for fitting a two-parameter function of radiation and c vapour pressure deficit were obtained for a 3-d period by inverting the Penman-Monteith equa- tion. Canopy conductance of pine was similar relative to spruce on ground area basis. This made g of pine larger relative to spruce per leaf area unit, since pine tree foliage mass was about one c third that of spruce. Transpiration deficit was small in the growth season of 1995. It reached about 10 % for spruce during the summer months. In 1994, however, the transpiration deficit was large for both species and extended throughout most of the growth season. During summer 1994, the decreased canopy conductance caused a 20 and 22 % reduction in gross photosynthesis for pine and spruce, respectively, indicating a loss of production of at least that proportion. Pines were less sensitive to drought spells as compared to the more shallow-rooted spruces. On the other hand, spruce utilised the precipitation incoming in small quantities more effectively and responded faster. Species composition of boreal forest can affect stand scale fluxes and this should be recognised by process models. (© Inra/Elsevier, Paris.) transpiration deficit / sap flow / spruce / pine / drought * Correspondence and reprints (46) 18 671168; fax: (46)18 672795; e-mail: emil.cienciala@mv.slu.se Tel:
Résumé - Flux d’eau dans une forêt boréale pendant deux années à pluviométrie contras- tée ; régulation spécifique de la conductance du couvert et de la transpiration. La réduction de la transpiration sous l’effet de la sécheresse a été estimée dans une forêt boréale mélangée de 60 ans dans le centre de la Suède. La transpiration réelle a été estimée à partir des mesures directes de flux de sève chez Pinus sylvestris et Picea abies pendant deux années successives mar- quées par des précipitations contrastées. La réduction de transpiration (ou déficit de transpiration) liée à la sécheresse a été quantifiée par la différence entre le flux de sève mesuré et la transpira- tion calculée en conditions non limitantes de disponibilité en eau. La transpiration maximale a été estimée au pas de temps horaire à partir de l’équation de Penman-Monteith avec des paramètres de la fonction de conductance du couvert (g calibrés pour chacune des deux espèces. Les ) c valeurs de g pour ajuster une fonction à deux paramètres du rayonnement et du déficit de satu- c ration de l’air ont été obtenues sur une période de 3 j par inversion de l’équation de Penman-Mon- teith. La conductance du couvert ramenée à l’unité de surface au sol du pin était du même ordre de grandeur que celle de l’épicéa. Mais sachant que la biomasse foliaire des pins n’était environ que d’un tiers de celle des épicéas, g était plus grande chez le pin par unité de surface foliaire. c Le déficit de transpiration a été faible pendant la saison de végétation 1995, atteignant environ 10 % chez l’épicéa pendant les mois d’été. En 1994, le déficit de transpiration a été important pour les deux espèces étudiées, et a duré une grande partie de la saison de végétation. Pendant l’été 1994, la réduction de la conductance du couvert a causé 20 % de réduction de photosynthèse brute chez les pins, et 22 % chez les épicéas, ce qui correspond à une perte de production sensiblement du même ordre. Les pins se sont montrés moins sensibles à la sécheresse que les épicéas, en liaison avec un système racinaire plus superficiel chez ces derniers. Toutefois, les épicéas ont mon- tré une plus forte aptitude et une plus grande rapidité à utiliser les faibles précipitations. Ainsi, la composition en espèces de la forêt boréale peut influencer les flux à l’échelle du peuplement, ce qui doit être pris en compte dans les modèles. (© Inra/Elsevier, Paris.) déficit de transpiration / flux de sève/ sécheresse / Picea abies / Pinus sylvestris 1. INTRODUCTION coniferous tree major species Norway spruce (Picea abies (L.) Karst.) and Scots There is growing evidence of a higher pine (Pinus sylvestris L.) constitute about frequency of climatic extremes on many 85 % of the forested land, with respective places of the Earth [17, 36]. Though the shares of 47 and 38 %. These species are long-term precipitation mean may not be grown in mixed stands over a range of cli- changing, the occurrence of extremely dry matic and edaphic conditions, despite sev- or wet years may affect the stability of eral obvious differences in their ecophys- forest ecosystems and cause a loss of pro- iology and architecture. Pine is a more duction. Therefore, there is a need for light demanding species and forms low long-term experiments, where ecophysio- density crowns with a sparse foliage con- logical performance of forest ecosystems centrated in the upper part of the stem. is observed in situ for a range of soil water Spruce tolerates shade better than pine and and climatic conditions with a detailed does well as an understory species. Spruce resolution. This is also extremely useful forms dense canopies extending often to for providing sufficient material for vali- lower parts of the stem. The species also dation of ecosystem models. differ in root architecture: pine is a deep- rooting species and it is thereby predis- The higher frequency of climatic posed to perform better under dry spells also impose a change in extremes can relative to the shallow-rooted spruce. It is species composition when some species known that deep rooting helps to main- may accommodate to changing conditions better than others. In Sweden, the two tain a sufficient water supply under water
are distinguished by different stands that deficit conditions (e.g. Kramer [21 ], spruce-pine quotients and age classes. The Hinckley et al. [16] and Teskey and rotation period for stands in the area is typi- Hinckley [35]). However, a shallow root cally 100 years. The soil is a deep boulder-rich system may be advantageous when pre- sandy till of glacial origin. At the site, the soil cipitation comes in small quantities. These was podzolized and classified as Dystric differences raise questions on species-spe- Regosols [34]. cific performance as regards water uptake, water economy and growth. Does pine really cope with drought better than 2.2. Meteorological variables spruce? How is the drought-induced reduc- tion of canopy conductance manifested in A continuous climatic data set for both 1994 and 1995 at the Norunda (NOPEX central) site, the carbon budget? Are the species-spe- where the sap flow measurements were made, cific differences in ecophysiology also was not available. The data from the central important on a stand and regional level? NOPEX site available for this study (SINOP This paper addresses these questions database) included solar radiation and air tem- perature for a part of the period evaluated here. the long-term continuous by analysing We have therefore used air temperature and measurements of sap flow in a mixed sub- relative humidity data from a climatic station in boreal forest in central Sweden. We com- Siggefora, about 15 km away. That station was bine the actual measurements with a sim- collecting data above a forest of similar age ple modelling tool to quantify transpiration and structure and the comparison of available deficit for tree species. Our previous study temperature and radiation records showed that the discrepancies were mostly below 3 % and from the site identified the uncertainty of therefore neglected. Net radiation was calcu- transpiration deficit quantification when lated as a simple linear function of short-wave performed on a daily basis [9]. Therefore, radiation with intercept and slope parameters of we worked here with an hourly time step. 23.8 and 0.77 W m respectively, as found , -2 Our measurements extended over 2 years over a stand of similar age and species com- with largely contrasting precipitation, illus- position in Siggefora. Daily precipitation data trating the climatic variation typical for were collected at the site for most of the season; the missing periods were filled with an average the area. We discuss species-specific eco- of the gauge measurements from three neigh- physiological performance based on the bouring sites in the region. quantified actual and potential water use and also assess effects of drought-induced limitation to canopy conductance on pho- 2.3. Stand description, sap flow tosynthesis. and transpiration The studied stand was 50 years old, with 2. MATERIALS AND METHODS the basal area of 29.3 m ha and a maximum 2-1 stand height of 23 m. The canopy was closed with occasional openings. The projected leaf- area index (LAI) was about 4-5. The stand was 2.1. Site description composed of Norway spruce (Picea abies (L.); 66 % of the stand basal area) and Scots pine The detailed description of the NOPEX (Pinus sylvestris (L.); 33 %) with a few spec- region can be found in Halldin et al. [15]. The imens of birch (Betula alba (L.)). central tower site (60°5’N, 17°29’E, alt. 45 m) Sap flow rate was measured on 12 trees is located in the Norunda Common about 30 with two measuring points on each. We used km north of Uppsala. Forests in the area are the standard equipment from Environmental mixtures of Norway spruce and Scots pine with Measuring Systems (P690.2), which is based the occasional occurrence of birch. They have on the technique described by Cermak et al. been managed by forestry practices for over [6] and Kucera et al. [22]. Two instruments 200 years. Today, forests are a rich mosaic of
provided 24 measuring channels equally dis- using the regression between daily sap flow of tributed between pine and spruce trees. Mea- species and potential evaporation according to Turc [37] at the start and end of the missing surements were performed throughout two periods. These values are identified by a sym- growth seasons. For the second growth sea- bol if applicable. son, a new set of sample trees was selected. The tree selection in 1994 was aimed at cov- ering the frequency distribution of stem diam- eters in the stand for individual species. In 2.4. Parameterization of canopy 1995, the selection of trees was similar, but a conductance weight was given to the upper diameter classes with trees whose contribution to total stand transpiration was more important. The breast Canopy conductance (g was calculated ) c height diameter over bark of the measured trees and parameterized for hypothetical monospe- ranged from 17 to 36 cm. cific stands of either pine or spruce on an hourly basis. The period of three sunny days in Stand transpiration was estimated from the July (9-11th) was selected for these calcula- measured tree sap flow using the ratio of a tions. The selection was made to avoid limi- foliage biomass supported by the set of the tations to transpiration flux by soil water deficit measured trees and that of the stand. This was and soil hydraulic limitations and/or very high performed individually for pine and spruce; evaporative conditions with a potential partial foliage mass was calculated using the Swedish embolism of conductive tissues. The species- biomass functions of Marklund [26]. The use of specific sap flow was cross-correlated with the the foliage mass was required to weight the product of short-wave radiation and vapour differences in mean tree diameters when select- pressure deficit to estimate an average time ing tree samples in the two measurement years. delay of the sap flow course behind the likely The procedure accounts for the non-linearity course of transpiration. For this, only relative of the relationship between stem diameter and values are considered and the magnitude of supporting foliage mass, which is important variables are not of any importance in this when the mean stem diameter of the sample phase. The mean time lag valid for the 3-d tree set differs from the corresponding mean parameterization period was 15 and 30 min for of all trees in a stand. pine and spruce, respectively. With this time lag, the correlation between sap flow and the To enable species-specific analyses, we product of VPD and radiation was tight and expressed water uptake of pine and spruce trees reached r 0.92 and r 0.91 for pine and = = separately to represent a flux of hypothetical spruce, respectively. Sap flow was thereby monospecific stands of either pine or spruce. accordingly shifted in time to mimic the rate of These stands had an equal basal area (that of the transpiration. Other effects of plant capaci- actual mixed stand), but a different LAI due tance apart from the time shift were neglected to a different foliage mass of pine and spruce in the analyses. canopies (see below). Most of the analyses were performed on diurnal courses (time step Canopy conductance was then calculated of 15 min) for respective tree species. Since from the inverse of the Penman-Monteith equa- scaling the tree sap flow rates to stand tran- tion with known transpiration fluxes estimated spiration is conveniently performed on a daily form the measured sap flow that was corrected basis, the diurnal courses (15 min or hourly for its time lag behind transpiration. The stor- resolution) of sap flow representative for age term was assumed negligible and aerody- monospecific pine and spruce stands in abso- namic conductance (g was calculated from ) a lute units (mm/h) were obtained as follows: a wind profile equation valid for near stable the respective daily totals were interpolated to conditions. The fraction of the net radiation the average diurnal courses of sap flow from all that is absorbed by the canopy (R was esti- ) n measuring points for the respective species. mated from net radiation above canopy (R ) no This way, the diurnal dynamics of sap flow for according to the Beer’s law: species was retained, representing an average for a stand and the fluxes were expressed in correct absolute units. Some missing values in the sap-flow mea- where k is the extinction coefficient (set to 0.5 daily basis were interpolated here) and LAI is the projected leaf area index surements on
where p p are parameters to be fitted, Rg is LAI for thehypothetical pine or spruce (-). 12 , the short-wave radiation (W m and VPD is ) -2 stands was calculated from the monospecific vapour pressure deficit (kPa). The fitting was total stand LAI (4.6) and the proportion of the performed with the weight given by the actual current foliage mass of the species. The foliage value of g This minimizes the influence of mass was calculated using the biomass func- . c night values, when g approaches zero and vari- c tions of Marklund [26]. The species foliage ations in g have practically no importance for mass represented 14 and 86 % of the total c calculation of transpiration fluxes. For weighted actual stand foliage mass for pine and spruce, least square fitting, weights are included in the respectively. Using the current basal area for sum of squares to be minimized. To avoid stand and species, and the amount of leaf adding a sub-function of air temperature (T ) a biomass, it was estimated that a monoculture of into equation (2), g was set to zero for the pine with the basal area of the present stand c days with average daily T less than 5 °C. The a (29.3 m would have LAI of 2.0, whereas a ) 2 criteria for the goodness of fit were standard pure spruce stand would reach LAI of 5.8. error of the estimate and coefficient of deter- These LAI values are similar as published else- ). 2 mination (r where for actual monospecific stands of pine (e.g. Lindroth [23]) and of spruce [I] in Swe- den. The schematic distribution of tree foliage mass and the measured and approximated green 2.5. Effect of soil drought on water crown height for the individual species is and carbon fluxes - quantification shown in figure 1. The effect of drought on transpiration was applied for parameterization of The equation as a difference between potential quantified simplified form of Lohammar [24] was a c g and actual fluxes, which is herewith called tran- equation. In that equation, we linearized the spiration deficit. The fluxes were represented radiation term giving the final form of by the calculated drought-free transpiration (E) and the transpiration estimated from sap flow (E these analyses were per- ); Q measurements
formed separately for pine and spruce, which 1980s. The difference between the average were normalized into corresponding monospe- annual precipitation was about 160 mm cific stands as outlined above. and the 1990s were evidently drier. The The effect of decreased canopy conduc- a annual precipitation in the region varied production was assessed for a period tance on from about 450 to 970 mm between dry of three summer months (1 July to 30 Septem- and wet years in the period 1981-1995. ber) using the photosynthesis module of FOR- EST-BGC [30, 31]. In the model, the equation There was also a large variability in the from Lohammar et at. [24] combines meso- distribution of precipitation within a year. phyll and stomatal conductance to calculate For the two studied measurement years of gross photosynthesis. Mesophyll conductance, 1994 and 1995, there was a difference in which represents the leaf biochemistry pro- cumulative precipitation - over 170 mm - cesses, was calculated for the actual mixed stand using FOREST-BGC with parameteri- during a large part of the growth season zation from Cienciala et al. [10] and it was (figure 2), though the annual sums dif- assumed to be equal for the hypothetical mono- fered only by about 100 mm. cultures of individual tree species. Stom- atal/canopy conductance to CO was obtained 2 Apart from precipitation, the climatic from g to water vapour as described above, c which was corrected by the factor 1.56 to conditions were similar for the two studied account for differences in diffusivity between years, 1994 and 1995. The mean daily water vapour and CO . 2 evapotranspiration for the growth seasons of 1994 and 1995 calculated according to the Turc [37] equation reached 2.53 and 3. RESULTS 2.23 mm, and the seasonal sum of 450 and 424 mm, respectively. The length of the growth season was 178 and 190 days for 3.1. Climatic conditions 1994 and 1995, respectively, using the threshold of 5 °C for daily mean air tem- The annual precipitation decreased in the 1990s (91-95) as compared to the perature.
parameterization period for pine and 3-d 3.2. Parameterized canopy spruce, respectively. For the actual mixed conductance and drought-free stand, the simulated drought-free transpi- transpiration ration (E) was similar for the two growth seasons of 1994 and 1995: the daily mean For the 3 days (9-11 July 1995) values reached 1.34 and 1.17 mm and the selected for parameterization of canopy seasonal sum was 238 and 223 mm, conductance (g function, the fitted func- ) c respectively. E for a hypothetical pine tion explained 88 and 80 % of the variation stand was similar to a spruce stand when in actual hourly data of g for pine and c fluxes were low. On the other hand, E of spruce, respectively, with standard error spruce is up to about 25 % larger for sum- of the estimate 0.0001m·s in both cases -1 mer months, when evaporative conditions (figure 3). The fitted parameters (coeffi- were high (figure 4). For 1994, the sea- cient of variation) p and p were 2.06E- 2 1 sonal daily average (seasonal total) of 5 m·s (15.3 %) and 0.68 kPa (35.5 %) -1 potential E reached 1.21 (215) and 1.42 for pine, and 1.60E-5 m·s (16.4 %) and -1 (253) mm for pine and spruce monospe- 0.21 kPa (82.5 %) for spruce, respec- cific stands, respectively. For 1995, which tively. The fitted g functions for species c was more moist, the corresponding val- had similar magnitudes on canopy level. ues of E were slightly lower and reached However, canopy conductance expressed 1.07 (203) and 1.24 (235) mm for pine per unit leaf area would be higher for pine as compared to spruce approximately in the ratio of 2 to 1. The calculated drought-free (i.e. poten- calculated tial) transpiration, that was using the parameterized g functions, c explained 94 and 91 % of the variations of the time-shifted sap-flow rate for the
and spruce similar magnitudes for both species even monospecific stands, respec- tively. during high evaporative conditions in the summer months, showing low or non-exis- tent limitations to transpiration by drought. 3.3. Transpiration deficit However, a small transpiration deficit developed for spruce, e.g. in August and The actual transpiration (E was lower ) Q June. For pine, there was no detectable than the drought-free transpiration (E) for reduction in transpiration by drought for most of the growth season in 1994, most of the growth period. However, the whereas these two fluxes matched each fluxes of E were considerably lower as Q other for most of the following growth compared to E during spring. This was season in 1995 (figure 4). The reduction in obvious mostly in1995 when more mea- transpiration (transpiration deficit) was sured data were available to build a con- largest in July 1994 for both species. In tinuous record for the spring period (fig- 1994, transpiration deficit was small or ures4 and 5). For both species, there was non-existing during the second half of September and October for both pine and a 10-d period at the end of July 1995, spruce, and also during June for spruce. when E was lower than E In this period, . Q 1995, the fluxes of E and E reached In Q the temperature was unusually low and
The diurnal courses of E and E were sim- radiation was highly variable due Q to pass- ilar in shape and magnitude for the periods ing scattered clouds. of sufficient water supply. The time lag supply for the The differences in water between E and E was small - about Q growth seasons changed both the mag- two 0-30 min - during summer period and suf- nitude of the measured fluxes and the ficient water supply: under these condi- shape of the diurnal sap flow curve (fig- tions the rates of E and E practically Q ure 5). The example period of two simi- matched (figures 6 and 7). A diurnal sap lar days in July documents the large dif- flow curve under deficit conditions was ferences in measured sap flow between typically less correlated to the predicted the two years of1994 and 1995 (figure 6).
rate of E and the time lag between E and after the rains, E reached Q trees: Q spruce E increased. The time lag also increased higher magnitudes than during the previ- during the autumn, when air temperature ous warm period, despite the much lower decreased. vapour pressure deficit at that time. The progression of transpiration deficit during the pronounced dry spell 1-15 July 3.4. Limitations to carbon 1994 was different for different species assimilation (figure 4). Pine was able to balance a part of the evaporative demand: the fluxes of E and E correlated reasonably well and E A comparison of the drought-induced Q Q was reaching about 60 % of E during that limitations to water and carbon cycle for period. On the contrary, transpiration in the summer period showed large differ- spruce gradually declined and E reached ences between the dry year, 1994, and the Q only about a third of E at the end of this more moist year, 1995. In 1994, the dry spell. However, the recovery after rain drought-reduced canopy conductance for was usually more rapid for spruce trees the 3-month period July-September as compared to pines. An example of this reduced transpiration by 41 and 46 % in can be seen on the period of 14 to 18 July pine and spruce, respectively. The assess- 1994 (Figures4 and 7). Here, 14 and 15 ment by the carbon module of the FOR- July are the last days of the previous warm EST-BGC model showed that this affected and dry period that resulted in consider- the tree carbon cycle by limiting gross able transpiration deficit - about 40 % in photosynthesis by 20 and 22 %, respec- pine and 70 % in spruce. Both species tively (figure 8). In 1995, the effect of reacted strongly to precipitation events on drought for the period July-September 15 and 17 July and largely increased their was small and beyond the accuracy of the water uptake relative to evaporative con- applied estimation for pine trees. A tem- ditions. This increase stronger in porary reduction in fluxes occurred in was
poses, however, most authors use a con- August for spruce (figure 5), but these stant value of time lag between transpira- quantities were small relative to seasonal tion and sap flow; they use it not only for water or carbon budget. a diurnal course of a particular day, but also across a season. For example, Granier and Loustau [13] used a constant time lag 4. DISCUSSION of 1 h for different sites and years and noted that this could be the reason for some of the differences between their 4.1. Methodical aspects of modelled and observed transpiration rates. transpiration deficit analysis Köstner et al. [18] noted that a correction for a sap flow time lag of increased max- The parameterized g and the Penman- c imum g values calculated via sap flux by c Monteith equation is a robust, but sensitive a factor of ca 1.6. Our data indicate that detection tool for quantification of tran- the time lag increased both under water spiration deficit. The previous work from deficit conditions and towards the end of the site suggested that analysis of transpi- the season (figure 5), probably correlated ration deficit performed with a daily time with a decreasing air temperature. Appar- step was uncertain and a finer resolution ently, there is a need to adapt some of the was recommended [9]. The hourly reso- capacitance models to assist with the anal- lution permits us to utilize the informa- yses of g providing a more realistic esti- c tion given by the shape of the diurnal sap mate of time lag and transpiration using flow curve as a sensitive indicator of sap flow data. The problem discussed drought conditions on a tree/stand level. above is associated mostly with low-speed The resolution also permits parameteriza- conducting systems such as that of tion to be performed over a much shorter conifers. It is also obvious that the capac- time period. Selecting another period of itance effects increase with tree size and the moist year 1995 for parameterization with decreasing flux rate and water ten- indicated differences in transpiration cal- sion inside trees. The correlation of sap culations of about 10 %. This is an accept- flow and transpiration rates can be very able accuracy with respect to the mini- high for more rapidly transpiring and/or mum parameterization involved. smaller trees, where the problems of heat balance technique discussed above dimin- The problem associated with hourly/ ish [8]. The above estimation of the time minute analyses is the asymmetrical shape lag using the product of radiation and of the sap flow curve with respect to tran- vapour pressure deficit that is cross-cor- spiration. These two fluxes usually corre- related with the sap flow curve may be a late well under high flow conditions in the good approximate solution when actual middle of the day, whereas similarity transpiration is not available. decreases at the onset and the end of the day, when the effect of tree capacitance Another problematic area in parame- increases and the accuracy of the sap flow terization of conductance data is the treat- measurements decreases. The capacitance ment of night values. These are sometimes effects in trees have been described else- included (e.g. Gärdenäs and Jansson, [12]) where (e.g. Schulze et al. [33], Carlson and sometimes omitted [4] in calculations and Lynn [5], Machado and Tyree [25] of mean daily g and for fitting analyses. c and Köstner et al. [19]). A rigorous treat- To emphasise the importance of high day- ment of time lag requires a model that light conductance values relative to low includes plant capacitance and resistance, g during nights, we fitted the conductance c such as, e.g. SPAC [20]. For practical pur-
Kellner, unpublished data). At that time, function equation (2) with larger weight the surface becomes for large g This decreased the standard soil temperature near . c higher relative to that in deeper layers, error of the estimations in equation (2) for whereas the opposite gradient is observed both species. for winter period. Therefore, we rejected The conductance function with lin- the hypothesis of soil temperature gradient earized radiation member has the advan- and low temperatures in the deeper lay- tage of minimum parameters for fitting. ers as the reason for decreased pine tran- It is obvious, that a fully exposed leaf spiration. We speculate that the low pine would require a curvilinear radiation transpiration rates in spring are associated response function to modulate saturation. with species-specific fine-root growth. However, this is not required for dense coniferous canopies of northern latitudes, For moist conditions, simulated tran- where canopy light saturation effect is spiration and the measured fluxes mostly unlikely. matched with differences of about 10 %, which is an expected accuracy with respect to the used tree sample density [7]. For the second half of July 1995 the rates of 4.2. Transpiration deficit for pine measured E temporarily exceeded the and spruce Q rates of modelled E, which is also pro- nounced on the average monthly values The data demonstrate that trees respond (figure 5). At that time, temperature was sensitively to fluctuations of moisture. unusually low and radiation was very scat- Transpiration is limited by both annual tered by high clouds. We tested the param- variation in precipitation and its seasonal eterized FOREST-BGC model that uses distribution. The results for the summer the Penman-Monteith equation on a daily period of 1994 show that the quantitative basis and that was previously parameter- differences in transpiration deficit between ized for the site on a stand level [10]. The pine and spruce were small in the studied model showed similarly low fluxes of tran- mixed stand. However, it would be too spiration that were also exceeded by the early to conclude that species composi- actual measurements of water uptake for tion has little importance for modelling that period. We suspect that the specific purposes on a regional scale. In the stud- combination of diffuse radiation and other ied mixed stand, trees were most proba- climatic variables made the Penman-Mon- bly fully adapted to coexistence. A pure teith-based calculations to under-predict monospecific stand may develop a differ- the actual transpiration rates at that time. ent strategy of water use and this should be verified by independent measurements. analyses above, however, do not The explain the action of the regulatory mech- We could not explain the decreased anisms available to trees. For example, water uptake for pine during the spring little is known about fine-root growth period (figure 4). In spring of 1995, water too of the full-grown and old trees. For smaller was abundant at the site and also spruce trees, Santantonio [32] noted that this pro- transpiration matched the predicted rates of cess may be very dynamic and compen- E at that time. A similar transpiration pat- tern for species was also observed in the sates for uptake in drought spells within a season. Some experiments indicate a following spring periods of 1996 and 1997 (data not shown). The measurements of decreased allocation of carbon to roots under drought [3, 13], reduced surface root soil temperature in different depths growth [2] and reallocation of root growth showed a small or non-existent tempera- to deeper layers [29], whereas other stud- ture gradient during the spring period (Erik
than the above estimated decrease in pho- an increased root growth under ies show tosynthesis. There is not much literature on drought [11]. Our data show a good cor- the magnitude of the effect of drought on respondence of E and E for several moist Q production for adult trees and stands, periods across the two measurement years because most of the studies were per- (figure 5). This is indirect evidence that formed on seedlings or young trees. root growth dynamics was not large Swedish studies performed on a ca 25- enough to alter the parameterized rates of year-old Norway spruce indicated an transpiration during the studied period. above-ground accumulation of carbon in Another uncertainty is the VPD irrigated trees of about 30-40 % relative to response in the conductance model. This the drought-treated ones [27, 28]. or a similar negative response is frequently Drought limitation to photosynthesis used in conductance models, though it quantitatively similar for the species very likely only reflects the limitations was during the summer period of 1994. The that are caused by other factors (water ten- observed differences were not significant sions in plant and soil and total resistance with respect to the simplifications involved of plant to flow). Therefore, the estimated in the tree carbon balance assessment rates of potential transpiration during very (assumed equal photosynthetic capacity high evaporative conditions (e.g. July for the two species). However, similarly as 1994) remain uncertain and should be val- for transpiration, larger differences idated, e.g. by an irrigation experiment in between species may be expected if real the field. monospecific stands were compared, where trees may establish a site-specific water balance. For the mixed stands in the 4.3. Drought limitations to region, a competition for water will prob- photosynthesis ably be less important as compared to the light requirements, which are of major Carbon and water cycles in a tree are importance for pine trees. tightly linked by sharing stomata as a com- mon pathway for water and carbon fluxes. This way, a drought-induced decrease of 5. CONCLUSIONS stomatal conductance limits both transpi- ration and the rate of carbon assimilation. Pine and spruce have specific responses Since carbon assimilation involves addi- may affect water and drought and this to tional resistances in the mesophyll cells, carbon fluxes on larger spatial scales. the relationship between water and car- However, for the studied summer period, bon fluxes is not linear. Our assessment the effects of drought were quantitatively of the drought effects on carbon assimi- similar for the two species. lation was only approximate. Neverthe- Parameterized canopy conductance and less, the estimated decrease in gross pho- the Penman-Monteith equation are easy tosynthesis of about 20 % for the summer tools for analysis of the measured fluxes period of 1994 indicates that the magni- and quantification of transpiration deficit tude of production loss in the region is especially with respect to the minimal considerable. In the complete tree carbon - amount of used parameters. An hourly budget, respiration fluxes are rather con- time step largely reduces the length of the servative as they are mostly related to the period required for parameterization of amount of standing biomass. This sug- conductance functions; it increases the gests that the relative losses of mer- sensitivity of transpiration deficit analy- chantable production will still be higher
sis ric surface temperature, by using the shape of diurnal flux For. Meteo- Agric. rol. 57 (1991) 171-186. to identify water deficit conditions. curves Cermak J., Deml M., Penka M., A new [6] However, it is important to understand method of sap flow rate determination in that the approach offers practically no trees, Biol. Plant (Praha) 15 (3) (1973) explanation of the mechanisms that make 171-178. trees limit their water uptake. Such a task Cermak J., Cienciala E., Kucera J., Lindroth [7] A., Bednarova E., Individual variation of the would require a physically sound non- rate in large pine and spruce trees sap-flow steady state simulation of water tensions in and stand transpiration: A pilot study at the soil and trees with a fine time resolution. central NOPEX site, J. Hydrol. 168 (1995) This will be the topic of our next study. 17-28. Cienciala E., Lindroth A., Gas-exchange and [8] sap flow measurements of Salix viminalis trees in short-rotation forest. I. Transpiration ACKNOWLEDGEMENTS and sap flow, Trees Struct. Funct. 9 (5) (1995) 289-294. We thank Jan Seibert for precipitation data Cienciala E., Kucera J., Lindroth A., Cermak [9] J., Grelle A., Halldin S., Canopy transpira- and Erik Kellner for access to soil temperature tion from a boreal forest during a dry year, measurements. We are indebted to Ann-Sofie Agric. For. Meteorol. 86 (1997) 157-167. Morén for assistance with sap flow instru- Cienciala E., Running S.W., Lindroth A., mentation and data retrieval. Some of the cli- [10] Grelle A., Ryan M.G., Analysis of carbon matic variables were retrieved from SINOP and water fluxes from the NOPEX boreal for- database of the NOPEX project. Funding for est: Comparison of measurements with FOR- this project was obtained from the Swedish EST-BGC simulations, J. Hydrol. (1998) in Natural Science Research Council (NFR) and press. from the Swedish Council for Forestry and Clemensson L.A., Asp H., Fine-root mor- [11] Agricultural Research. The senior author also phology and uptake of 32P and 35S in a Nor- acknowledges the support from NFR in his way spruce (Picea abies (L.) Karst.) stand postdoctoral stay at the Rocky Mountain Exper- subjected to various nutrient and water sup- iment Station, USDA, Fort Collins, Colorado, plies, Plant Soil 173 (1995) 147-155. USA. Gärdenäs A.I., Jansson P.E., Simulated water [12] balance of scots pine stands in Sweden for different climate change scenarios, J. Hydrol. 166 (1995) 107-125. REFERENCES Gerant D., Podor M., Grieu P., Afif D., Cornu [13] S., Morabito D., Banvoy J., Robin C., Dizen- Alavi G., Radial stem growth of Picea abies [1] gremel P., Carbon metabolism enzyme activ- in relation to spatial variation in soil mois- ities and carbon partitioning in Pinus halepen- ture conditions, Scand. J. For. Res. 11 (1996) sis Mill. exposed to mild drought and ozone, 209-219. J. Plant Physiol. 148 (1996) 142-147. Beier C., Gundersen P., Hansen K., Ras- [2] Granier A., Loustau D., Measuring and mod- [14] mussen L., Experimental manipulation of the transpiration of a maritime pine elling water and nutrient input to a Norway spruce canopy from sap-flow data, Agric. For. Mete- plantation at Klosterhede, Denmark. II. orol. 71 (1994) 61-81. Effects on tree growth and nutrition, Plant Halldin S., Gottschalk L., van de Griend A.A., [15] Soil 168-169 (1995) 613-622. Gryning S.-E., Heikinheimo M., Högström Blanck K., Lamersdorf N., Dohrenbusch A., [3] U., Jochum A., Lundin L.-C., Science plan Murach D., Response of a Norway spruce for NOPEX, Technical Report No. 12, Upp- forest ecosystem to drought/rewetting exper- sala University, Uppsala, 1995. iments at Solling, Germany, Water Air Soil Hinckley T.M., Dougherty P.M., Lassoie J.P., [ 16] Pollut. 85 (1995) 1251-1256. Roberts J.E., Teskey R.O., A severe drought: Bringfelt B., Lindroth A., Synoptic evapo- [4] impact on tree Quercus alba growth, phenol- transpiration model applied to two northern ogy, net photosynthetic rate and water rela- forests of different density, J. Hydrol. 95 tions, Am. Midl. Nat. 102 (2) (1979) (1987) 185-201. 307-316. Carlson T.N., Lynn B., The effects of plant [5] Karl T.R., Knight R.W., Plummer N., Trends [17] water storage on transpiration and radiomet- in high frequency climate variability in the
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