JXB Advance Access originally published online on November 8, 2004
Journal of Experimental Botany 2005 56(409):155-165; doi:10.1093/jxb/eri013
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RESEARCH PAPER |
Annual and seasonal variation of sap flow and conductance of pine trees grown in elevated carbon dioxide and temperature
1Chengdu Institute of Biology, the Chinese Academy of Sciences, PO Box 406, Chengdu 610041, PR China
2The Centre of Excellence for Forest Ecosystem and Management, University of Joensuu, PO Box 111, FIN-80101 Joensuu, Finland
* To whom correspondence should be addressed. Fax: +358 13 251 4444. E-mail: kaiyun{at}joensuu.fi
Received 27 April 2004; Accepted 17 August 2004
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Abstract |
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Measurements of sap flow, crown structure, and microclimate were used to estimate the transpiration of individual 30-year-old Pinus sylvestris L. trees grown in elevated temperature and CO2. The trees were enclosed in closed-top chambers and exposed either to current ambient conditions (CON), or elevated CO2 (+350 µmol mol–1; EC), or elevated temperature (+2 to +6 °C; ET) or a combination of EC and ET (ECT) since 1996, and the measurements were made from 1999 to 2001. EC significantly increased annual sap flow per tree (Ft.m) by 14% in 1999, but reduced it by 13% in 2000 and 16% in 2001. The CO2-induced increase in Ft.m in 1999 was due to a large increase in foliage area of trees, which more than compensated for a small decrease in crown conductance (Gc). The CO2-induced decreases in Ft.m in 2000 and 2001 resulted from a pronounced decline in Gc, which was much greater than the increase in foliage area. The CO2-induced increase in sensitivity of Gc at high vapour pressure deficit (VPD) did not alter the general response of sap flow to CO2 enrichment, but it did affect the diurnal courses of sap flow on some days during the main growing season (days 150–240). ET increased Ft.m by 53%, 45%, and 57% in 1999, 2000, and 2001, respectively, attributable to the combined effects of greater foliage area and maximum crown conductance, lower stomatal sensitivity to high VPD, and higher transpiration demand relative to the control treatments. There was no significant interaction between CO2 and temperature on sap flow, because ECT entailed approximately similar patterns of sap flow to ET, suggesting that the temperature played a dominate role in the case of ECT under boreal climate conditions.
Key words: Closed-top chamber, CO2 and temperature elevation, crown conductance, Pinus sylvestris, sap flow
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussion and conclusionsReferences |
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General Circulation Models (GCMs) predict that high latitudes will experience the greatest warming as a consequence of the increase in atmospheric concentrations of carbon dioxide (CO2) and other greenhouse gases (IPCC, 1996
). The rising temperature is expected to reduce soil moisture due to enhanced evapotranspiration (Mitchell et al., 1990), an effect that will be especially pronounced on the southern edge of the boreal zone, where the supply of water could be further limited by reduced accumulation of snow and shortening of the period of snow cover (Pastor and Post, 1988, Kellomäki and Väisänen, 1996). Since water availability and use in transpiration are correlated with the productivity of the forest ecosystem, there is an urgent need to be aware of how transpiration will be modified by future climate changes.
Transpiration depends on biologically mediated responses to aerial and soil environments, which can, in turn, exercise feedback effects on the energy balance, turbulence, and humidity in the forest stand, thereby determining the state of the environment itself. Therefore, an understanding of transpiration under changing climatic conditions involves the analysis of the interactions between the tree and environmental variables (Gutierrez and Meinzer, 1994). Stomatal conductance decreases under elevated CO2 (Mousseau and Saugier, 1992; Eamus, 1996; Curtis, 1996), which reduces transpiration per unit leaf area (Morison, 1987; Field et al., 1995; Kimball et al., 1993; Curtis, 1996), but this response is not always true, particularly at the canopy level (Berryman et al., 1994; Saxe et al., 1998; Ceulemans et al., 1999). Samarakoon and Gifford (1995), for example, compared cotton, wheat, and maize grown in glasshouses with controlled temperature and relative humidity, and found that water loss per pot of cotton increased as a consequence of a large increase in leaf area and a small decrease in conductance at elevated CO2, while maize showed very little leaf-area response, resulting in significant water conservation. Jones et al. (1984, 1985) showed a reduction in soybean transpiration per unit leaf area at elevated CO2, but little decrease in transpiration per ground area, because the reduced bulk canopy conductance at elevated CO2 was offset by increased leaf area and water pressure deficits. Furthermore, a review of some recently published data concerning different functional groups of plants has suggested that stomatal responses to CO2 appear to be smaller in trees than in other life forms (Bunce, 1992; Knapp et al., 1996; Saxe et al., 1998), whereas the leaf area of trees usually increases under conditions of elevated CO2 (Saxe et al., 1998; Ceulemans et al., 1999), as the result of the impact of elevated CO2 on whole-tree transpiration may be more variable (Dufrene et al., 1993; Heath and Kerstiens, 1997; Centritto et al., 1999; Ellsworth, 1999) than on single leaf transpiration. In addition, adjustments of both stomatal conductance and leaf area may also be closely related to root restriction, for example, a change in leaf area ratio or soil water availability rather than to elevated CO2 (Gifford, 1988; Eamus, 1996; Heath and Kersteiens, 1997). It is clear that the variable responses of canopy transpiration to elevated CO2 may result from multiple mechanisms and factors, and any interpretation of the effects of CO2 on transpiration must consider the spatial and temporal scales on which the observations are made.
An experiment was set up in a typical Scots pine stand in eastern Finland using closed-top chambers to evaluate the acclimation of growth and water use to elevated CO2 and temperature. In recent years, responses of transpiration and stomatal conductance at the individual leaf level have been reported (Wang and Kellomäki, 1997) and results from short-term measurements for sap flow (one growing season; Kellomäki and Wang 1998, 2000). The aims of this paper are to report the results from three years of continual sap flux measurements, and analysis of the responses of transpiration and conductance at the whole-tree scale to elevated CO2 and temperature.
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Materials and methods |
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Site and treatment description
The experiment was established in a naturally seeded stand of Scots pine located near the Mekrijärvi Research Station (62° 47' N, 30° 58' E, 145 m asl), University of Joensuu, in eastern Finland. Sixteen trees of approximately the same crown size and height were chosen and enclosed individually in closed-top chambers in the summer of 1997. The chamber is a cylindrical structure with eight walls, an internal volume of approximate 19.3 m3 and a ground area of 5.2 m2. The four walls facing south and west are constructed of special heating glass (5640 W for each chamber; Eglars Oy, Imatra, Finland) and the four north and east-facing walls of dual-layer acrylic cell glass. The computer-controlled heating and cooling system (CAJ-4511YHR, L'Unite-Hermetiqe, Barentin, France), together with a set of magnetoelectric valves (controlling the pure CO2 supply), enabled the two variables of temperature and CO2 concentration to be adjusted automatically inside the chambers to conform to the ambient conditions (CON), or to achieve a specified enrichment in CO2 (EC), a rise in temperature (ET), and a combined elevation of CO2 and temperature (ECT). Each treatment was applied to four replicates.
The CO2 concentration was enriched each day throughout the year. The warming treatments were designed to correspond to the climatic scenario predicted for the site after doubling of the atmospheric concentration of CO2 (Hänninen, 1995; Kellomäki and Väisänen, 1997). The CO2 concentration was monitored with a CO2 sensor (GMP111, Vaisala Inc., Helsinki, Finland) located in the middle of the crown of each tree. The relative humidity and temperature within the crown were recorded separately using an RH&T probe (HMP131Y, Vaisala Inc., Helsinki, Finland), in which the relative humidity was measured by a Vaisala ‘Humicap’ sensor. In addition, global solar radiation (Model SKS1110 silicon pyranometer, Skye Instruments, UK) was measured in two layers of the crown (top and middle) and the volumes of soil water content at depths of 5 cm and 15 cm were measured with four soil moisture probes (ThetaProbe ML1, Delta-T Devices Ltd. UK). All the sensors were connected to a data logger. Measurements were taken at 15 s intervals. During the growing season, soil water was supplied to the chambers to correspond to the water content in the ambient soil. The CO2 concentration was 670–730 µmol mol–1 for 86% of the time in the elevated CO2 chambers, and the hourly mean air temperature was 2.0–6.0 °C higher than the true ambient temperature for 80% of the time in the elevated temperature chambers (Fig. 1). Even if, there were still some significant chamber effects such as the solar radiation in the chambers was reduced by 35–47% for 82% of the time in the growing seasons compared with the outside chambers, the relative humidity of the air was increased by 5–10% for 72% of the time, and some physiological parameters, for example, the light-saturated photosynthesis rate, maximum photochemical efficiency of photosystem II, and chlorophyll content, changed significantly (Kellomäki et al., 2000). By contrast with those, the crown architecture and main phenophase of the sample trees were not modified significantly by the enclosure in the chambers (Kellomäki et al., 2000).
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Measurements of sap flow and crown parameters
Sap flow was monitored simultaneously over three growth seasons (1999, 2000, and 2001) in a total of 16 trees, four for each treatment, by the constant-power heat balance method (Sakuratani, 1981
; Baker and van Bavel, 1987). Sap flow gauges designed for trunks of 6–7 cm in diameter (model; SGB50 and SGA70; Dynamax, Inc., Houston, TX, USA) were attached about 0.3 m above the soil surface. Rough bark and dead tissue were removed carefully, and a thin layer of silicone grease-based electrical insulating compound was spread on the trunk at the point where the gauge was to be installed. Gauges were covered with clear plastic cling film for waterproofing and aluminium foil was wrapped around both the gauges and sections of stem above and below them to minimize externally-induced temperature gradients. In view of the diameter growth of sample trees, the gauge was reinstalled every 10 d and the stem area parameter in the software was updated every day whereas the sap data were recomputed later. The power supply to the gauges, about 1.5 W for the SGB50 model and 2.0 W for the SGA70 model, was switched on from 2 h before sunrise until 2 h after sunset. The thermal conductivity of the stem was assumed to be 0.42 W m–1 K–1 (Van Bavel, 1994) for all trees. The first gauge conductance value (Ksh) was determined from the minimum pre-dawn values, after which Ksh was checked and adjusted every 10 d. The gauge signals were scanned every 30 s with datalogger-multiplexer units (models 21 x/AM416, Campbell Scientific, Logan, UT, USA) and stored as 30 min averages. Sap flow was calculated automatically according to equation (2) of Van Bavel (1994). A software filter was set to eliminate spurious flow calculations at times of low or high flow conditions. As the ‘nightly power down mode’ was used in the measurements, all the analyses of daily sap flow in this paper are based only on measurements made between 06.00 h and 21.00 h LST (local standard time).
The foliage area of each tree in the experiment was approximated by three methods: (i) the area of current-year needles was calculated by multiplying the sampled needle expansion rate and the number of current-year needles, the expansion rate being measured on the sample shoot after bud bursting (Kellomäki and Wang, 1998; Zha et al., 2001); (ii) seasonal variation in the crown foliage area was also estimated by means of a hemispherical photograph taken from the base of the crown (Hemi View v.2.1) assuming a semi-ellipsoidal crown for calculation of the gap fractions (Campbell, 1986); and (iii) two independent measurements were made separately in May and August of the year using the sample-branch method (Wang, 1996; Kellomäki and Wang, 1997). The purposes of the independent measurements were to determine the proportions of the different age classes in the total foliage area and the distribution pattern of foliage within the crown, and to calibrate the estimated foliage area from the hemispherical photograph by means of a clumping factor (fc in Kellomäki and Wang, 2000).
Crown conductance on the basis of needle area (Gc; mm s–1) was calculated from the sap flow rate per tree (Fs, g stem–1 s–1) using a simplified form of the Penman–Monteith equation:
 | (1) |
is the psychrometric constant (kPa K–1), 
is the latent heat of vaporization (J kg–1), 
is the density of moist air (kg m–3), cp is the volumetric heat capacity of moist air at constant pressure (J kg–1 K–1), and VPD is the vapour pressure deficit (kPa) (Monteith and Unsworth, 1990). This simplification requires the following conditions: (i) boundary layer conductance is high; (ii) there is no vertical gradient in VPD through the crown; and (iii) there is negligible water stored above the Fs measurement position. In this case, the VPD was recorded at the middle of the crown, and the wind speed in the chamber was almost constant, with a mean of 0.8 m s–1 in the growth season and 0.6 m s–1 in the ‘off season’ (Kellomäki et al., 2000). These conditions correspond to a very large boundary layer conductance relative to stomatal conductance (Kellomäki and Wang, 2000), thus allowing reasonable estimates of Gc to be achieved from simple relationships.
Statistical analyses
It was assumed that any significant responses to treatments were due to the growth temperatures and CO2 concentrations and not to unknown chamber effects. Repeated-measures analysis of variance (Moser et al., 1990) was used to test the effects of the growth conditions (CON, EC, ET, and ECT) and date of measurement on sap flow. Two-way ANOVA was used to test the difference in daily total sap flow between treatments on specific dates. To account for differences in needle area and branch growth caused by treatments, sap flow was expressed on the basis of projected needle area, the individual tree and projected area of the crown, respectively.
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Results |
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Crown profile
Regardless of the year of the experiment, the factor-enriched treatments (EC, ET, and ECT) significantly increased the total needle area per tree (Fig. 2). Since the treatments induced a change in tree phenology and the length of the growing season, the magnitude of the enhancement depended on the season, year, and treatments applied. EC increased the maximum foliage area by 5.7% (P=0.068), 2.8% (P=0.122), and 2.2% (P=0.175) in 1999, 2000, and 2001, respectively, ET increased it by 3.5% (P=0.051), 6.3% (P=0.041), and 7.4% (P=0.046), and ECT increased it by 3.3% (P=0.059), 5.1% (P=0.047), and 6.7% (P=0.048). The greater foliage area resulted from an increase in the length of the current-year needles in the case of both ET and ECT, but more older needles in the lower layers of the crown in EC. In addition, elevated temperature resulted in greater growth of current-year shoots than in CON, whereas the mean angle of the branches of the whorl with respect to the main stem (
s) did not change. Consequently, the projected ground area (Ag) of the crown (m2 m–2 ground) increased by 3.7%, 16.3%, and 10.2% for ET in 1999, 2000, and 2001, respectively, compared with CON.
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Sap flow
The diurnal sap flow totals (Ft) changed in each of the three years (Fig. 3) with annual mean from 0.15 to 3.77 kg tree–1 d–1.
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The trees grown in warmer chambers (ET and ECT) had much greater Ft than those in the control chambers on almost all days throughout the three years (right panel in Fig. 3), whereas the trees in the CO2-enriched chambers (EC) had lower Ft in 2000 and 2001 but higher Ft on most days in 1999. Furthermore, warming increased Ft in the early spring and late autumn more than in the main growing season (days 150–240), whereas the CO2-induced decrease in Ft was evident only in the main growing season. In view of the difference in the measurements in the course of a year, it is significant that both increase in Ft for ET and ECT, and decrease in Ft for EC was greater with the longer time the trees were in the chambers, while ET and ECT showed the similar pattern among different years.
Considering the differences in total needle area and crown size between the treatments (Fig. 2), the sap flow data in Fig. 3 was also calculated per unit needle area and per unit crown projected area on the ground. The annual mean diurnal sap flow totals (Ft.m) for the treatments (Table 1) indicate that the needle-area-based Ft.m did not change significantly in EC compared with CON in 1999, but was reduced in 2000 and 2001, and that the trend in Ft.m induced by the treatment was similar regardless of whether Ft.m was expressed on the basis of the ground area or the whole tree, although the values were lower on the basis of ground area. EC reduced the whole-tree-based Ft.m over the main growing season by 14% (P <0.05) and 16% (P <0.05) relative to CON in 2000 and 2001, respectively, corresponding to needle-area-based decreases of 17% and 20%, respectively, whereas ET increased the value by 28% (P <0.01), 27% (P <0.01), and 34% (P <0.01) in 1999, 2000, and 2001, respectively, corresponding with needle-area-based increases of 15%, 17%, and 25%, respectively.
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Responses of sap flow to VPDm and Rs
Since growth of stem diameter of each experimental tree was monitored in this study, it is easy to obtain a continual and corresponding relationship between the sap flow rate determined on the basis of stem area (Fs, g m–2 s–1) and vapour pressure deficits (VPDm). Since the preliminary plotting over all three periods of measurement gave a wide scatter in the results, the plotting was limited to data obtained when solar radiation (Rs) was >5 W m–2 (on a daily basis) during the main growing season. The results then give a significant correlation (R2 >0.72 and P <0.05) between mean daily Fs and mean daily VPDm for the four treatments (Fig. 4).
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The relationship between Fs and VPDm was strongly linear at small VPDm, but reached a plateau (Fig. 4). As the linear part showed a clear response to low light and the curved part to high light, the data can be clearly divided into two parts at about a value for Rs of 100 W m–2. To quantify the change in Fs with increasing VPDm, a linear fitting to the data when Rs was less than 100 W m–2 showed no significant differences in the slopes (P=0.14–0.43) and intercepts (P=0.09–0.26) between the treatments (Table 2), i.e. the treatments did not significantly change the relationship of Fs to lower level of VPDm. Whereas a second-order polynomial fitting to the curvilinear part showed significant differences between the treatments (Table 2), i.e. Fs had a different degree of acclimation to higher VPDm under different growing conditions. To quantify this acclimation, the linear regression was extrapolated to higher values of VPDm to represent the potential Fs (Fp), (Fp was defined here as the maximum flux that would occur for a given set of environmental conditions (Pataki et al., 1998
)), and it was assumed that the values for the predicted Fs from the second-order regression (Fr) represented actual Fs data, then the difference (
Fs) between Fp and Fr was calculated for each treatment. The response of Fs between treatments to higher VPDm gave an evaluation of the acclimation of Fs to growing conditions (lower panels in Fig. 4), i.e. compared with the case in CON, there was larger increase in Fs with increasing VPDm in EC, and a greater decrease in ET and ECT than in CON. Differences in Fs induced by treatments were enhanced with increasing exposure time, and the critical value of VPDm corresponded with the critical transition from a linear to a curvilinear and tended to be smaller with exposure time.
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Crown conductance
The crown conductance per unit area (Gc) was calculated for each chamber using equation (1). In view of the time lags between sap flow and VPD records, the average VPD during the h before each sap flow measurement was used in the calculation of Gc (Kellomäki and Wang, 2000
). For a particular treatment, the seasonal dependence of Gc appeared, in general, to be approximately coupled to that of sap flow or of VPD (left panels in Fig. 1). To compare the differences in the acclimation of Gc to the environmental conditions, the monthly mean maximum Gc (Gc.max) was estimated from an exponential fit [Gc=Gc.max EXP(–bVPD)] of the relationship between Gc and the daytime hourly mean of VPD during the hour before sap flow measurement for each month of the three years in question (Figs 5, 6). Regardless of the treatments, the estimated Gc.max showed a clear dependence on the month of measurement, with the maximum value of Gc.max in July (Fig. 6). It is interesting that the increase in Gc.max caused by elevated temperature got greater with the duration of growing in chambers in the main growing season, but smaller in the other time. Similarly, CO2-induced decreases in Gc.max were enhanced with exposure year independently of the season. EC reduced Gc.max by 1%, 6%, and 10% relative to CON in 1999, 2000, and 2001, respectively, while ET increased Gc.max by 4%, 8% and 12% during the main growing season of 1999, 2000, and 2001 and by 23%, 20%, and 14% at other times. ECT increased Gc.max by 2%, 6%, and 10% during the main growing season of 1999, 2000, and 2001 and by 20%, 17%, and 11% at other times, respectively.
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Discussion and conclusions |
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Elevated CO2 enhanced sap flow in general in the first measuring year (1999), but reduced it significantly in the other two years (2000 and 2001). Real measurements of transpiration on large, whole trees grown in elevated CO2 at their natural site over several growth seasons have been lacking. Numerous studies on crop and herb species have shown that canopy transpiration under elevated CO2 may increase (Chaudhuri et al., 1990
; Kimball et al., 1994; Samarakoon and Gifford, 1995; Fredeen et al., 1998), decrease (Kimball et al., 1999; Ham et al., 1995; Drake et al., 1997) or not change (Dugas et al,. 1994; Jones et al., 1995; Kimball et al., 1994). Although these variable responses may result from multiple mechanisms and factors (Drake et al., 1997; Saxe et al., 1998; Ceulemans et al., 1999), relative adjustment in total leaf area and leaf stomatal conductance are important (Drake et al., 1997; Saxe et al., 1998; Ceulemans et al., 1999). The increased sap flow of trees in the first year in this case could be attributed to a large increase in their foliage area in response to elevated CO2, with no change, or at least no proportional reduction, in crown conductance, whereas the reduced the sap flow of trees in the other two years could have resulted from a substantial reduction in conductance if the impact of soil water stress is ignored (Eamus, 1996; Will and Tesky, 1977; Dufrene et al., 1993; Ellsworth et al., 1995). The above interpretation was reasonable for most data during the measuring periods, but not all. In the case of the sap flow results in the first year, for example, if it is assumed that the CO2-induced stimulation of foliage area is the only cause of the increase in sap flow, a higher daily sap flow rate should always be recorded throughout the measurement period, showing a pattern of enhancement that is coupled with foliage development (Fig. 2). However, the annual pattern of sap flow was not of this kind. A reduced diurnal sap flow total occurred on some days in the main growing season of the first year (right panels in Fig. 3), which was also found in the sap flow patterns of Sorghum bicolor (L.) Moench and Glycine max (L.) Merr. growing under elevated CO2 (Dugas et al., 1997). The explanation of this variation in sap flow with time may be a complex process, and vary with species and site conditions (Dugas et al., 1994; Eamus, 1996; Senock et al., 1996). Sap flow was connected with solar radiation and VPD, however, it was found that increased sap flow caused by CO2 was largely associated with lower solar radiation and VPD, whereas a CO2-induced decrease, on some days, was associated with a high demand for transpiration because the largest decrease often occurred in the afternoon. This was further confirmed by plot of sap flow against VPD (Fig. 4), in which case the linear part of the relationship between the mean daily sap flow and VPD was not altered significantly but the curved part decreased greatly. This implies that the CO2-induced increase in sap flow in the first year is largely attributable to the CO2-induced increase in the sensitivity of crown conductance to high VPD. This is consistent with early findings reported for this tree species (Garcia et al., 1993; Wang and Kellomäki, 1997), but contrasts with a two-year study of beech seedlings showing stomatal conductance was reduced by 22% in elevated CO2 only on cloudy days and not in bright sunshine (Heath and Kerstiens, 1997). Similarly, the reduced sap flow of trees under elevated CO2 in the other two years could be a result of crown conductance being reduced relatively more than foliage area being stimulated. This is supported by smaller CO2-induced increases in total foliage area per tree both 2000 (2.7% of the peak value) and 2001 (2.2%) than in 1999 (5.8%; Fig. 2). The yearly mean crown conductance decreased progressively from 1% in 1999, to 6% in 2000 and 10% in 2001 (Fig. 6), indicating conifers with a smaller stomata response to CO2 enrichment than other species (Bunce, 1992; Knapp et al., 1996; Saxe et al., 1998).
Under boreal conditions, low temperature is a key limiting factor for the growth and development of trees during most of the year. Thus, the dominant role of temperature in controlling growth, development, and water use in this experiment is not surprising. In agreement with the early findings (Kellomäki and Wang, 1998, 2001), the warmer temperature not only increased the surface area of individual needles, but also advanced foliage development in the early spring and extended the retention of foliage area per tree during the late growth season. As a result, sap flow was significantly greater in elevated temperature than in the control throughout the three years (Fig. 3; Table 1). This increase in sap flow rates is related to two other factors, in addition to the larger needle area: higher VPD as a consequence of increased air temperature during monitoring (Fig. 1), and the acclimation of transpiration processes to growth temperatures.
When hourly sap flow measurements in the main growing season in 2000 were plotted against hourly mean air temperature, the relationship was linear, but the relationship was not significant (P=0.44 for ET and P=0.55 for CON). The plotting of sap flow per unit stem area against VPD showed that trees growing at elevated temperature maintained a constant transpiration rate when VPD was above a critical value (about 1.0 kPa in 1999 and 2000, and 1.2 kPa in 2001; see Fig. 4). This relative insensitivity of sap flow to high VPD could involve many changes related to transpiration such as temperature (Idso et al., 1987) and the water potential of foliage (Kellomäki and Wang, 1996), xylem hydraulic resistance (Sperry and Pockman, 1993), and water uptake of root system (Kellomäki and Wang, 2001). Early measurements on the foliage of Scots pine indicated that temperature elevation significantly reduced the sensitivity of stomatal conductance to decreasing foliage water potential under short-term water-stress conditions (Kellomäki and Wang, 1996), and to increasing VPD (Wang and Kellomäki, 1997). It is possible that temperature-induced adjustments in foliage stomatal conductance, as a consequence of modifications in the foliage water potential of Scots pine, is important in keeping high sap flow rates under short-term stress conditions.
As discussed above, acclimation of transpiration to either CO2 or temperature was attributed largely to differences in foliage area, crown conductance, and VPD among the chambers. Mechanisms of acclimation to combined elevated CO2 and temperature may be more complex, however. In an experiment conducted in a catchment area with boreal vegetation, Beerling (1999) found that increased CO2 and elevated temperature (+5 °C in winter, +3 °C in summer) together reduced foliage stomatal conductance, which was attributed to the acclimation of the stomata to CO2, but increased total transpiration rate, even if there was a reduction in soil moisture, was attributable to a greater increase in VPD associated with the warmer air temperatures. Stomata show a significant acclimation to elevated CO2 but not to the smaller increase in air temperature in Beerling's study (1999). In this experiment, the responses of foliage area, sap flow rate, and crown conductance to combined elevated CO2 and temperature were almost the same as those brought about by elevated temperature alone. The disparity in the results could be related to differences in tree ages, experimental conditions, and exposure time. In particular, the temperature elevation used in this experiment was markedly high in winter (a mean increase of 6 °C), which may have important consequences for processes other than water loss, for example, heat damage, increased rate of development, stimulation of senescence etc.
In summary, (i) elevated CO2 significantly enhanced whole-tree transpiration rate during the first measuring year due to a large increase in whole-tree foliage area, 1998, but reduced it in the subsequent years of 1999 and 2000 as a consequence of a greater decrease in crown conductance which off-set the increase in foliage area per tree; (ii) trees growing in elevated temperature always had higher sap flow rates throughout three measuring years; and (iii) the response of sap flow to the combination of elevated temperature and CO2 was similar to that of elevated temperature alone, indicating a dominant role for temperature and a lack of interaction between elevated CO2 and temperature.
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Acknowledgements |
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This work was undertaken as part of the China–Finland co-operation project ‘Responses of the Ecosystem Processes of High-Frigid Coniferous Forest to Climate Change’ funded by the Academy of Finland and the National Science Foundation of China (NSFC) (no. 90202010; 30211130504) and the ‘Programme of 100 Distinguished Experts’ funded by the Chinese Academy of Sciences and the Finnish Centre of Excellence Programme (Project no. 64308) funded by the Academy of Finland, the National Technology Agency (Tekes) and the University of Joensuu. Thanks also to Matti Lemettinen, Alpo Hassinen, and Risto Ikonen for developing and maintaining the experimental infrastructure.
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References |
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Baker JM, van Bavel CHM. 1987. Measurement of mass flow of water in stems of herbaceous plants. Plant, Cell, and Environment 10, 777–782.
Berryman CA, Eamus D, Duff GA. 1994. Stomatal responses to a range of variables in two tropical tree species grown with CO2 enrichment. Journal of Experimental Botany 45, 539–546.
Bunce JA. 1992. Stomatal conductance, photosynthesis and respiration of temperate deciduous tree seedlings grown outdoors at elevated concentration of carbon dioxide. Plant, Cell and Environment 15, 541–549.[CrossRef]
Beerling DJ. 1999. Long-term responses of boreal vegetation to global change: an experimental and modelling investigation. Global Change Biology 5, 55–74.
Campbell GS. 1986. Extinction coefficients for radiation in plant canopies calculated using an ellipsoidal inclination angle distribution. Agricultural and Forest Meteorology 36, 317–321.[CrossRef][Web of Science]
Centritto M, Lee HSJ, Jarvis PG. 1999. Interactive effects of elevated CO2 and drought on cherry (Prunus avium) seedlings I. Growth, whole-plant water use efficiency and water loss. New Phytologist 141, 129–140.[CrossRef][Web of Science]
Ceulemans R, Janssens IA, Jach ME. 1999. Effects of CO2 enrichment on trees and forests: lessons to be learned in view of future ecosystem studies. Annals of Botany 84, 577–590.
Chaudhuri UN, Kirkham MB, Kanemasu ET. 1990. Carbon dioxide and water level effects on yield and water use of winter wheat. Agronomy Journal 82, 637–641.
Curtis PS. 1996. A meta-analysis of leaf gas exchange and nitrogen in trees under elevated carbon dioxide. Plant, Cell and Environment 19, 127–137.
Drake BG, Gonzales-Meler M, Long SP. 1997. More efficient plants, a consequence of rising atmospheric CO2? Annual Review of Plant Physiology and Plant Molecular Biology 48, 609–639.[CrossRef][Web of Science]
Dufrene E, Pontailler JY, Saugier B. 1993. A branch bag technique for simultaneous CO2 enrichment and assimilation measurements on beech (Fagus sylvatica L.). Plant, Cell and Environment 16, 1131–1138.[CrossRef]
Dugas WA, Heuer ML, Hunsaker D, Kimball BA, Lewin KF, Nagy J, Johnson M. 1994. Sap flow measurements of transpiration from cotton grown under ambient and enriched CO2 concentrations. Agricultural and Forest Meteorology 70, 231–245.[CrossRef][Web of Science]
Dugas WA, Prior SA, Rogers HH. 1997. Transpiration from sorghum and soybean growing under ambient and elevated CO2 concentrations. Agricultural and Forest Meteorology 83, 37–48.[CrossRef][Web of Science]
Eamus D. 1996. Responses of field grown trees to CO2 enrichment. Commonwealth Forestry Review 75, 39–47.
Ellsworth DS. 1999. CO2 enrichment in a maturing pine forest are CO2 exchange and water status in the canopy affected? Plant, Cell and Environment 22, 461–472.
Ellsworth DS, Oren R, Huang C, Phillips N, Hendrey GR. 1995. Leaf and canopy response to elevated CO2 in a pine forest under free-air CO2 enrichment. Oecologia 104, 139–146.[CrossRef][Web of Science]
Field CB, Jackson RB, Mooney HA. 1995. Stomatal responses to increased CO2: implications from the plant to the global scale. Plant, Cell and Environment 18, 1214–1225.[CrossRef]
Fredeen AL, George WK, Field CB. 1998. Influence of fertilization and atmospheric CO2 enrichment on ecosystem CO2 and H2O exchanges in single- and multiple-species grassland microcosms. Environmental and Experimental Botany 40, 147–157.[CrossRef][Web of Science]
Garcia RL, Long SP, Wall GW, Kimball BA, Pinter PJ. 1993. Gas exchange of spring wheat leaves and canopies in response to full-season CO2 enrichment and differential irrigation. America Society of Agronomy Poster Session (abstract). Cincinnati, Ohio.
Gifford RM. 1988. Direct effects of higher carbon dioxide concentrations on vegetation. In: Pearman GI, ed. Greenhouse: plannning for climate change. Australia: CSIRO, 506–519.
Gutierrez MV, Meinzer FC. 1994. Energy balance and latent heat flux partitioning in coffee hedgerows at different stages of canopy development. Agricultural and Forest Meteorology 68, 173–186.[CrossRef][Web of Science]
Ham JM, Owensby CE, Coyne PI, Bremer DJ. 1995. Flues of CO2 and water vapor from a prairie ecosystem exposed to ambient CO2 and elevated atmospheric CO2. Agricultural and Forest Meteorology 77, 73–93.
Hänninen H. 1995. Effects of climatic change on trees from cool and temperature regions: an ecophysiological approach to modelling of bud burst phenology. Canadian Journal of Botany 73, 183–199.[CrossRef][Web of Science]
Heath J, Kerstiens G. 1997. Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech. Plant, Cell and Environment 20, 57–67.
Idso SB, Kimball BA, Mauney JR. 1987. Atmospheric carbon dioxide enrichment effects on cotton midday foliage temperature: implications for plant water use and crop yield. Agronomy Journal 79, 667–672.
IPCC [Intergovernmental Panel on Climate Change]. 1996. Climate change 1995. In: Houghton JT, Filho LGM, Callander BN, Harris N, Kattenberg A, Maskell K, eds. The science of climate change. Cambridge: Cambridge University Press.
Jones HG. 1992. Plants and microclimate, a quantitative approach to environmental plant physiology, 2nd edn. Cambridge, UK: Cambridge University Press.
Jones P, Allen Jr LH, Jones JW, Boote KJ, Campbell WJ. 1984. Soybean canopy growth, photosynthesis, and transpiration responses to whole-season carbon dioxide enrichment. Agronomy Journal 76, 633–637.
Jones P, Allen Jr LH, Jones JW, Valle R. 1985. Photosynthesis and transpiration responses. to short- and long-term CO2 treatments. Agronomy Journal 77, 119–126.
Kellomäki S, Väisänen V. 1996. Model computations on the effect of rising temperature on soil moisture and water availability in forest ecosystems dominated by Scots pine in the boreal zone in Finland. Climate Change 32, 423–445.[CrossRef]
Kellomäki S, Väisänen H. 1997. Modelling the dynamics of the forest ecosystem for climate change studies in the boreal conditions. Ecological Modelling 97, 121–140.[CrossRef]
Kellomäki S, Wang KY. 1996. Photosynthetic responses to needle water potentials in Scots pine after a four-year exposure to elevated CO2 and temperature. Tree Physiology 16, 765–772.[Web of Science][Medline]
Kellomäki S, Wang KY. 1997. Effects of long-term CO2 and temperature elevation on crown nitrogen distribution and daily photosynthetic performance of Scots pine. Forest Ecology and Management 450, 309–327.[CrossRef]
Kellomäki S, Wang KY. 1998. Growth, respiration and nitrogen content in needles of Scots pine exposed elevated ozone and carbon dioxide in the field. Environmental Pollution 101, 263–274.[CrossRef][Medline]
Kellomäki S, Wang WK. 2000. Modelling and measuring transpiration from Scots pine with increased temperature and carbon dioxide enrichment. Annals of Botany 85, 263–278.
Kellomäki S, Wang KY. 2001. Growth and resource use of birch seedlings under elevated carbon dioxide and temperature. Annals of Botany 87, 669–682.
Kellomäki S, Wang KY, Lamittene M. 2000. Controlled environment chambers for investigating tree response to elevated CO2 and temperature under boreal conditions. Photosynthetica 38, 69–81.[CrossRef][Web of Science]
Kimball BA, LaMorte RL, Seay RS. 1994. Free-air CO2 enrichment on energy balance and evapotranspiration of cotton. Agricultural and Forest Meteorology 70, 259–278.[CrossRef][Web of Science]
Kimball BA, Mauney JR, Nakayama FS, Idso SB. 1993. Effects of increasing atmospheric CO2 on vegetation. Vegetatio 104/105, 65–75.[CrossRef]
Kimball JS, Running SW, Saatchi SS. 1999. Sensitivity of boreal forest regional water flux and net primary production simulations to sub-grid scale landcover complexity. Journal of Geophysical Research 104, 27789–27801.[CrossRef]
Knapp AK, Hamerlynck EP, Ham JM, Owensby CE. 1996. Responses in stomatal conductance to elevated CO2 in 12 grassland species that differ in growth form. Vegetatio 125, 31–41.[CrossRef][Web of Science]
Mitchell JFB, Manabe S, Mlesho V, Tokioka T. 1990. Equilibrium climate change – and its implications for future. In: Houghton JT, Jenkins GT, Ephaums JJ, eds. Climate change. Cambridge: Cambridge University Press, 131–175.
Monteith JL, Unsworth MH. 1990. Principles of environmental physics. London, UK: Edward Arnold.
Morison JIL. 1987. Intercellular CO2 concentration and stomatal responses to CO2. In Zeiger E, Farquhar GD, Cowan IR, eds, Stomatal function. Stanford University Press, 229–251.
Moser EB, Saxton AM, Pezeshki SR. 1990. Repeated measures analysis of variance: application to tree research. Canadian Journal of Forest Research 20, 524–535.[CrossRef]
Mousseau M, Saugier B. 1992. The direct effect of increased CO2 on gas exchange and growth of forest tree species. Journal of the Air and Water Management Association 41, 798–807.
Pastor J, Post MW. 1988. Response of northern forests to CO2-induced climate change. Nature 334, 55–58.[CrossRef]
Pataki DE, Oren R, Katul G, Sigmon J. 1998. Canopy conductance of Pinus taeda, Liquidambar styraciflua and Quercus phellos under varying atmospheric and soil water conditions. Tree Physiology 18, 307–315.
Sakuratani T. 1981. A heat balance method for measuring water flux in the stem of intact plants. Journal of Agriculture Meteorology 37, 9–17.
Samarakoon AB, Gifford RM. 1995. Soil water content under plants at high CO2 concentration and interaction with the direct CO2 effects: a species comparison. Journal of Biogeography 22, 193–203.[CrossRef][Web of Science]
Saxe H, Ellsworth DS, Heath J. 1998. Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139, 395–435.[CrossRef][Web of Science]
Senock RS, Ham JM, Loughin TM, Kimball BA, Hunsaker DJ, Pinter PJ, Wall GW, Garcia RL, LaMorte RL. 1996. Sap flow in wheat under free-air CO2 enrichment. Plant, Cell and Environment 19, 147–158.
Sperry JS, Pockman WT. 1993. Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis. Plant, Cell and Environment 16, 279–287.[CrossRef]
Wang KY. 1996. Canopy CO2 exchange of Scots pine and its seasonal variation after four-year exposure to elevated CO2 and temperature. Agricultural and Forest Meteorology 82, 1–27.
Wang KY, Kellomaki S. 1997. Stomatal conductance and transpiration in shoots of Scots pine after 4-year exposure to elevated CO2 and temperature. Canadian Journal of Botany 75, 552–561.[Web of Science]
Will RE, Teskey RO. 1997. Effect of elevated carbon dioxide concentration and root restriction on net photosynthesis, water relations and foliar carbohydrate status of loblolly pine seedlings. Tree Physiology 17, 655–661.
Van Bavel MG. 1994. Dynagauge installation and operation manual. Houston, TX: Dynamax, Inc.
Zha TS, Ryyppö A, Wang KY, Kellomäki S. 2001. Effects of elevated carbon dioxide concentration and temperature on needle growth, respiration and carbohydrate status in field-grown Scots pines during the needle expansion period. Tree Physiology 21, 1279–1287.
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