Journal of Plant Ecology Advance Access originally published online on October 31, 2008
Journal of Plant Ecology 2008 1(4):227-235; doi:10.1093/jpe/rtn023
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Regulation of the water status in three co-occurring phreatophytes at the southern fringe of the Taklamakan Desert
1 Department of Plant Ecology, Albrecht von Haller Institute for Plant Sciences, Georg-August-Universität Göttingen, Untere Karspüle 2, 37073 Göttingen, Germany
2 Present address: Department of Geobotany, University of Trier, Behringstraße 21, 54296 Trier, Germany
3 Present address: Eidgenössisches Institut für Agrarökologie und Landwirtschaft, Forschungsanstalt Agroscope, Reckenholzstrasse 191, 8046 Zürich, Switzerland
4 Present address: Institute of Geobotany and Botanical Garden, Martin Luther University Halle-Wittenberg, Am Kirchtor 1, 06108 Halle (Saale), Germany
5 Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, 40 South Beijing Road, 830011 Urumqi, China
* Correspondence address. Department of Geobotany, University of Trier, Behringstraße 21, 54296 Trier, Germany. Tel: +49-651-2012393; Fax: +49-651-2013808; E-mail: thomasf{at}uni-trier.de
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Abstract |
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Aims: We investigated the regulation of the water status in three predominant perennial C3 phreatophytes (Alhagi sparsifolia, Populus euphratica, Tamarix ramosissima) at typical sites of their occurrence at the southern fringe of the hyperarid Taklamakan Desert (north-west China).
Methods: In the foreland of the river oasis of Qira (Cele), we determined meteorological variables, plant biomass production, plant water potentials (
L) and the water flux through the plants. We calculated the hydraulic conductance on the flow path from the soil to the leaves (kSL) and tested the effects of kSL, L and the leaf-to-air difference in the partial pressure of water vapour (
w) on stomatal regulation using regression analyses.
Important Findings: Despite high values of plant water potential at the point of turgor loss, all plants sustained L at levels that were high enough to maintain transpiration throughout the growing season. In A. sparsifolia, stomatal resistance (rs; related to leaf area or leaf mass) was most closely correlated with kSL; whereas in P. euphratica, 
70% of the variation in rs was explained by w. In T. ramosissima, leaf area-related rs was significantly correlated with L and kSL. The regulation mechanisms are in accordance with the growth patterns and the occurrence of the species in relation to their distance to the ground water.
Keywords: aboveground growth • extreme aridity • stomatal conductance • transpirational demand • vapour pressure deficit
Received: 1 July 2008 Revised: 27 August 2008 Accepted: 18 September 2008
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INTRODUCTION |
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The Taklamakan Desert, located in the Central-Asian Tarim Basin (Xinjiang province, north-west China), is a hyper-arid desert exhibiting a ratio of mean annual precipitation to mean annual evapotranspiration of <0.03 mm mm–1 (cf. Whitford 2002). In the central part of its southern fringe, mean annual precipitation is
33 mm (Thomas et al. 2000), and potential annual evaporation 2600 mm (Xia et al. 1993). There, river oases exist that are supplied with water by the melting of snow and glaciers in the adjacent Kunlun mountain range. In the transition zone between the oases and the open desert, perennial species form stands of varying density and composition (Bruelheide et al. 2003; Walter and Box 1983). These species bear ecological and economical importance for the human population because they constitute a shelter for the oases from sand drift (Xia et al. 1993) and are used for grazing, forage, construction and fuel (cf. Thomas et al. 2000). Among the perennial plants, the C3 species Alhagi sparsifolia Shap. (Fabaceae), Tamarix ramosissima Ledeb. (Tamaricaceae) and Populus euphratica Oliv. (syn. Populus diversifolia Schrenk; Salicaceae) represent the most abundant life forms around the oases, i.e. clonal herbs/sub-shrubs, shrubs and trees. Detailed information on these species is provided by Gries et al. (2005), Thomas et al. (2000) and Thomas et al. (2006).
Within the framework of a larger research project, evidence was provided that all perennial species in the transition zone between oasis and desert are phreatophytes, i.e. plants that meet their water demand by water uptake from the ground water or its capillary fringe (Gries et al. 2005; Li et al. 2002; Thomas et al. 2006; Zeng et al. 2002). The nitrogen requirements of the plants are also met by uptake from the ground water as has been revealed in a simultaneous study at the same sites (Arndt et al. 2004a). As an outcome of this research project, previous publications on plant water relations have focused on diurnal and seasonal courses of leaf water relations in A. sparsifolia (Li et al. 2002; Zeng et al. 2002), growth and water relations of P. euphratica and T. ramosissima along gradients of the ground water distance (Gries et al. 2003), effects of experimental flooding (Thomas et al. 2006; Zeng et al. 2006) and water use by the vegetation (Thomas et al. 2006). The present study concentrates on comparing regulation mechanisms of the plant water status in three of the predominating C3 phreatophytes (A. sparsifolia, P. euphratica and T. ramosissima), which occur within the same stand or in close-by stands. A strict regulation of water loss by transpiration and/or an effective water supply through the roots and the shoot axes is necessary due to the high transpirational demand that is caused by the large water vapour pressure deficit of the air (VPD) during the greatest part of the growing season. The large distance to the ground water (up to 16 m in level stands, Thomas et al. 2006, and up to 24 m on the top of sand dunes, Gries et al. 2003) renders such regulation mechanisms even more decisive. The water relations of the above-mentioned species were studied during the growing season at sites that are typical for their distribution in the transition zone between oases and desert. It was hypothesized that (i) in A. sparsifolia, which can form monospecific stands at a large distance to the ground water (16 m at the study site), stomatal regulation is mainly governed by the leaf-specific hydraulic conductance on the flow path from ground water to leaf (kSL); (ii) in P. euphratica, which is a typical tree species of the floodplain forests along rivers (Walter and Box 1983), stomatal regulation is mainly determined by the leaf-to-air difference in the partial pressure of water vapour (w) that, at light saturation, is the driving force of transpiration (e.g. Meinzer 2003; Oren et al. 1999) and (iii) in T. ramosissima, the leaf water potential that can assume lower (more negative) values in this salt-tolerant species is significantly correlated with stomatal regulation. In the latter species, a less tight correlation between stomatal regulation and kSL than in A. sparsifolia was expected because the extensive system of roots (Xu and Li 2006; Xu et al. 2007) and buried parts of the shoot (Qong et al. 2002) provide a high potential of tapping into ground water, but, at the same time, increases the length of the flow path from the ground water to the leaves.
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MATERIALS AND METHODS |
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Study sites
The study was conducted at the southern fringe of the Taklamakan Desert, Xinjiang-Uighur Autonomous Region, China, in the foreland of the river oasis of Qira (Cele; N 37° 01', E 80° 48’; 1365 m a.s.l.) in the growing season of 1999. Mean annual temperature is 11.9 ° C and mean annual precipitation 33 mm. At sites that were typical for the distribution of the respective species (cf. Bruelheide et al. 2003), one rectangular, level study plot per species was fenced, which was almost exclusively covered by A. sparsifolia Shap. (maximum height: 1.5 m, maximum leaf area index (LAI): 1.7), P. euphratica Oliv. (syn. P. diversifolia Schrenk) (average height: 6 m; maximum LAI: 2.6) or T. ramosissima Ledeb. (average height: 2 m; maximum LAI: 2.8) (for details, see Foetzki 2003; Thomas et al. 2000, 2006). At the Populus and the Tamarix plots, the distance to ground water was 3.6 and 5.7 m, respectively, as was proven by manual drilling. On the basis of the water table in a nearby well, the ground water distance was estimated to be 16 m at the Alhagi plot. One automatic weather station (Campbell Scientific, Shepshed, UK) each at the Alhagi and the Tamarix site was installed to continuously record climatic variables: air temperature, air humidity, global solar radiation, photosynthetically active radiation (PAR) and wind speed. According to a comparison conducted for 2 years (1999 and 2000), the data of both stations were nearly interchangeable.
Within the framework of a larger study (cf. Thomas et al. 2000), the contribution of natural inundations—which occur, on average, every 2–3 years in summer—to the plants’ water supply was assessed. To this end, the plots were artificially flooded in summer 1999. As the flooding had no measurable effect on the water relations of the investigated species (Thomas et al. 2006), it can be neglected in the context of the present study.
Plant biomass and productivity
The aboveground biomass of the plants was calculated by means of allometric regressions (Gries et al. 2005). In A. sparsifolia, we established correlations between the aboveground biomass, which had been determined through preliminary harvests, and the spherical crown volume. In P. euphratica and T. ramosissima, aboveground biomass was computed from regressions with the basal area diameter. Growth was determined on the basis of these allometric regressions by repeated surveys of the diameters of marked stems (Gries et al. 2005). Leaf mass per unit leaf area (LMA) was calculated through leaf area measurements with a scanner using Delta-T SCAN software (Version 2.04nc; Delta-T Devices Ltd., Burwell, Cambridge, UK).
Plant water relations
In six ramets per species of similar size, diurnal courses (from 7:00 to 19:00 h, including predawn measurements) of the leaf water potential (L) were measured at intervals of 4–6 weeks from early May until the end of September using pressure chambers (Model 1000, PMS Co, Corvallis, OR, USA) with three replicates per ramet and time of measurement. Coinciding with these measurements, diurnal courses (approximately from 8:00 to 19:00 h) of transpiration (E) and stomatal conductance to water vapour (gs) were monitored using a porometer (LI-1600, LI-COR, Lincoln, NE, USA) with equal numbers of ramets and replicates. Measurements of PAR were conducted using a quantum sensor (LI-COR LI-190S-1) that was coupled to the porometer. The water-vapour gradient (leaf-to-air difference in the partial pressure of water vapour, w) was calculated from leaf temperature, air temperature and air humidity measured during porometry and the tabulated value for air pressure at an altitude of 1365 m (86.17 kPa). From April to October, at intervals of 3–5 weeks, water-relation parameters of shoots were measured by establishing pressure–volume curves (pV curves) in three ramets per plot according to Thomas (2000). The osmotic pressure at the turgor loss point (
, which, at this point, equals the tissue water potential) was determined from the pV curves.
In six ramets of A. sparsifolia and T. ramosissima of similar size within each species, xylem sap flow was continually measured during the growing season at the stem base using the heat-balance method (sap-flow meter T693.2, EMS, Brno, Czech Republic; cf. Lindroth et al. 1995). Transpiration and sap flow were related to the area or to the dry matter of the leaves. The hydraulic conductance on the flow path from the soil to the leaves (kSL) was calculated according to
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is the mass of liquid water translocated from the soil to the leaves per unit leaf area (or leaf dry matter) and time, soil is the water potential of the soil and G is the gravitational potential along the distance between ground water level and the leaves of the upper canopy (0.01 MPa m–1). Because the plants were presumed to have contact with the ground water (see Study sites), soil was set to the osmotic potential of the moderately saline ground water (–0.3 MPa, according to the concentrations of inorganic ions analysed in the ground water at the Populus site; Arndt et al. 2004a). In P. euphratica, E was used instead of for calculating kSL. This was possible because the decoupling coefficient 
(calculated according to Jarvis and McNaughton 1986) was below 0.2, indicating a relatively close coupling between the conditions at the leaf surface and those in the free airstream (Herbst 1995; Thomas et al. 2006). Daily maximum kSL values were computed for the time of the day with the greatest quotient of or E, respectively (measured at light saturation of gs) and the difference soil – L – G. To consider the different distances to the water table, kSL x path length was also calculated using ground water depth plus canopy height for the path length.
Data analyses
Mean values and standard errors are given in the presentation of the results. Differences among the species in their diurnal and seasonal courses of water relation parameters were tested using the Friedman test, followed by multiple pairwise comparisons according to the Student–Newman–Keuls method (P < 0.05). Multiple regressions including the predictor variables w, L and kSL, and rs as the response variable, were calculated with SPSS 13.0.1 (SPSS Inc., Chicago, IL, USA). Leaf area-related and leaf mass-related rs (m2 s mol–1 or g s mmol–1)—the inverse value of stomatal conductance (gs)—were chosen because of their linear relationship with the vapour pressure deficit, whereas the relationship between the vapour pressure deficit and gs is curvilinear (cf. Meinzer 2003). Only those values were included that were measured at light-saturating gs and at the same time of the day of measurement. Air temperature and air humidity were not considered as additional predictor variables as they were significantly correlated with w (R2 > 0.5).
For a more detailed analysis of stomatal regulation, we calculated boundary lines for the relationship between stomatal conductance (gs) and w according to Schäfer et al. (2000): within the range between 20 and 65 Pa kPa–1, w data were partitioned to classes with a width of 2.5 Pa kPa–1 (assuring that each class comprised at least five data points); from each class, the outliers were removed using Dixon's test according to Sachs (1984; pp. 279 f); for the remaining data, the mean value and the standard deviation of gs were computed class wise; and data falling above the mean value plus one standard deviation were averaged. From these average values of gs and the corresponding w values, the upper boundary lines were constructed for each species. To avoid effects of rapid growth in spring and of different irradiation, this analysis was performed on time periods when the aboveground biomass of the species had reached at least 80% of peak aboveground biomass, and when light saturation of gs was provided, irrespective of the time of the day (A. sparsifolia: PAR > 980 µmol m–2 s–1; P. euphratica: PAR > 240 µmol m–2 s–1; T. ramosissima: PAR > 510 µmol m–2 s–1; light saturation was determined by plotting stomatal conductance against PAR).
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RESULTS |
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Meteorological variables and plant water status
The PAR reached peak values from mid-May to the beginning of August, whereas the highest temperatures were confined to a period from mid-June to mid-July (Fig. 1). On >90% of the days in the period from mid-April to the end of September, the daily average VPD exceeded 1.5 kPa, and on >80% of the days, the daily maximum exceeded 3 kPa. Peak values of >5.5 kPa were reached in June and July.
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Typical examples of diurnal courses of
L and E in mid-August, when complete data sets could be obtained from all species within 5 days, are shown in Fig. 2. Alhagi sparsifolia exhibited the highest L values and T. ramosissima the lowest. The leaf area-related values of E were highest in P. euphratica. During the growing season, the minimum diurnal water potential of the photosynthetically active plant parts (min) decreased except for T. ramosissima in which, after reaching a minimum in August, it increased slightly towards the end of September (Fig. 3). During the seasonal course, the min values of A. sparsifolia were significantly higher than in P. euphratica and T. ramosissima. Only in T. ramosissima did min fall below –2.5 MPa. The course of the relatively high water potentials at the point of turgor loss 
was parallel to that of min.
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; triangles) in photosynthetically active parts of Alhagi sparsifolia, Populus euphratica and Tamarix ramosissima during the growing season (means ± 1 SE). In the seasonal course, Biomass formation, water translocation and hydraulic conductance
From the first transpiration measurements in May to the last ones in September, LMA increased from 100.3 to 218.4 g m–2 in A. sparsifolia, from 119.0 to 141.7 g m–2 in P. euphratica and from 124.1 to 156.0 g m–2 in T. ramosissima. Alhagi sparsifolia reached its peak aboveground biomass within considerably shorter time than did the other species: starting from 1% of initial aboveground biomass, 100% aboveground biomass were formed within only 56 days; whereas in P. euphratica and T. ramosissima, maximum photosynthetically active biomass was developed after 77 and 79 days, respectively (Fig. 4). In A. sparsifolia, the almost exponential course of aboveground biomass formation coincided with high daily maximum values of
and very high maximum hydraulic conductance on the flow path from the soil to the leaves (kSL), due to high L at that time. Once the peak aboveground biomass had been reached, these values declined drastically (Fig. 4). In P. euphratica and T. ramosissima, the maximum values of kSL and of E or started from a much lower level and exhibited only a slight to moderate decrease (kSL) or no distinct seasonal trend at all (E and ). The maximum values of leaf area-related gs were 
in A. sparsifolia (May), 
in P. euphratica (July and August) and 
in T. ramosissima (May).
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In Fig. 4, the maximum values of
, E and kSL are related to unit leaf area. When normalized by unit leaf dry matter, the decrease in Alhagi‘s 
and kSL was even steeper (from May to September: , 2.42–0.57 
; kSL, 15.28–0.42 
), due to the larger increase in LMA. In the other species, the seasonal patterns of or E and of kSL were similar to those derived from the leaf area-related values.
In May, the differences in kSL among the species were even larger when the path length from the ground water table to the canopy was considered. Corresponding with the high in May, and due to the large estimated distance to the ground water level (16.7 m including the aboveground part of the stem), the leaf area-related kSL x path length was especially high in A. sparsifolia (maximum value: 
), but decreased to the level of T. ramosissima 
after peak aboveground biomass was reached. In P. euphratica, maximum kSL x path length was 
.
Effects of w, L and kSL on stomatal regulation
In A. sparsifolia, kSL was the strongest single predictor variable and, together with w, explained 60% of the total variance in rs (Table 1). By contrast, L and w had only low prediction value when applied as single variables. In P. euphratica, w was the variable with the highest prediction value, whereas kSL and L had much lower predictive power. In T. ramosissima, the regression model yielded significant results only when rs and kSL were related to leaf area instead of dry matter (Table 1). In this case, L and kSL were the strongest predictor variables. There was no significant relationship between rs and w, but together with L or kSL, w contributed significantly to explaining the variance in rs due to a significant correlation between w and L (r2 = 0.666; P < 0.001).
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In P. euphratica, maximum gs was much more responsive to
w than in T. ramosissima (Fig. 5). The different thresholds of light saturation of gs (see Data analyses) did not affect the result because a computation with values of P. euphratica obtained at higher irradiance (
510 µmol m–2 s–1) yielded a slope that did not differ significantly from the one calculated for the lower threshold. In the case of A. sparsifolia, no boundary line with a significant linear or simple non-linear regression could be calculated, probably due to the only small effect of w on gs.
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DISCUSSION |
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Plant water relations under hyperarid conditions
The
min values measured in our study were high (greater than or equal to –3 MPa) compared to typical desert plants. The lowest min values were detected in T. ramosissima that accumulates salt (Arndt et al. 2004a). Desert shrubs of north-western America exhibited minimum stem water potentials between –3 and –6 MPa (Sperry and Hacke 2002), and minimum water potentials of –4.2 MPa were measured in the chamaephyte Anabasis articulata in interdune corridors of the Negev Desert (Veste 2008). In desert plants, minimum values as low as –14 MPa can be found (Schulze et al. 2005). Even in semi-arid environments like in south-eastern Spain, minimum water potentials distinctly below –4 MPa were measured during drought (almost –5 MPa in the shrub Anthyllis cytisoides, Domingo et al. 2003; –8.4 MPa in the perennial tussock grass Stipa tenacissima, Balaguer et al. 2002). However, similar minimum leaf water potentials as in our study were found in woody phreatophytes at a high elevation in arid south-eastern Wyoming (–2.7 MPa in Populus angustifolia, Salix monticola and Salix exigua; Foster and Smith 1991). Owing to the relatively high water potentials at the point of turgor loss , the range of water potentials that allow stomatal opening is rather narrow in the plants of our study. However, our diurnal measurements of E, gs and demonstrate that although min was close to in all but one instances during the daylight period, stomata did not close completely during daylight. Thus, all investigated plant species were capable of maintaining their water potentials high enough to ensure transpiration and, in return, CO2 uptake throughout the growing season.
Especially in A. sparsifolia, high L allowed maximum values of leaf area-related gs (up to 
) that were considerably higher than in typical desert shrubs 
and even desert annuals 
(Körner 1994). High gs in May facilitated high rates of biomass production during that time, before temperature and VPD increased in summer. However, the maximum value of in A. sparsifolia 
is not particularly high when compared to values from other shrub species with small or highly divided leaves in arid regions. In a floodplain in southern Nevada with a distance to the ground water of 0–3 m, average values of 
were measured with sap-flow gauges in T. ramosissima, Pluchea sericea, Prosopis pubescens and S. exigua (Sala et al. 1996). In the small-leaved, drought-deciduous shrub Guiera senegalensis of the Nigerian Sahel, a maximum value as high as 
has been obtained (Allen and Grime 1995). Nevertheless, in A. sparsifolia was high enough to enable a production of aboveground biomass of up to 3.9 Mg ha–1 a–1 (Gries et al. 2005), which is higher than the maximum net primary productivity of typical desert scrub vegetation (up to 2.5 Mg ha–1 a–1; Lieth 1975).
In P. euphratica and T. ramosissima, maximum gs ( 200 mmol m–2 s–1) was similar to that determined in woody species growing in a floodplain of the arid region of the Lower Colorado River (150–250 mmol m–2 s–1 in Populus fremontii, T. ramosissima and Salix gooddingii; Busch and Smith 1995). However, at high elevation (2300 m a.s.l.) with low atmospheric pressure, woody phreatophytes (P. angustifolia, S. monticola, S. exigua) may also exhibit high maximum values of gs (800–900 mmol m–2 s–1; Foster and Smith 1991). In the present study, the leaf area-related average daily sums of transpiration in P. euphratica 
were similar to those determined by measuring xylem sap flow in the mesic tree species Betula pendula that is well known for its high transpirational rates compared to other mesic trees (daily mean value measured in a warm and dry growing season: 
; Backes 1996). The high transpiration rates of P. euphratica were coupled with relatively high rates of annual biomass production (up to 6.5 Mg ha–1 a–1; Gries et al. 2005), which is similar to the lower margin of the productivity of warm temperate mixed forests (6–25 Mg ha–1 a–1; Lieth 1975).
Regulation of the plant water status
A close correlation between stomatal conductance (or stomatal resistance, respectively) and parameters of vapour pressure deficit as was established in P. euphratica in the present investigation has also been detected in other studies on plants in arid environments. Bucci et al. (2005) found a sharp decline of gs with increasing VPD—from the wet to the dry season—in eight isohydric woody species of the Brazilian savannah that exhibited only small differences in the daily min between the seasons. A seasonal change in the sensitivity of stomatal conductance to VPD was also observed in a population of the phreatophytic Prosopis glandulosa that grew at a ground water distance of 4–6 m in the Sonoran desert and was interpreted as an adaptation to avoid drought stress (Nilsen et al. 1983). In a semi-arid region of Southeastern Spain, gs was negatively correlated with VPD in the shrub Retama sphaerocarpa (Brenner and Incoll 1997; Domingo et al. 2003). Populus euphratica, a characteristic species of Central-Asian floodplain forests, is not only restricted to sites with small ground water distances but also forms natural Tamarix–Populus euphratica forests with a ground water distance of >10 m (Wang et al. 1996). A study along a gradient of increasing ground water distance revealed that P. euphratica is able to grow with accumulating sand up to a distance of the canopy to the water table that was as large as 23 m; at this distance, however, aboveground growth as well as stomatal conductance were significantly reduced (Gries et al. 2003). This ability of P. euphratica to tolerate larger ground water distances might be connected with the close correlation between rs and w that was found in the present study: a high responsiveness of the stomatal resistance to changes in w decreases the risk of xylem cavitation under high transpirational demand at a given conducting efficiency (McDowell et al. 2008). In P. euphratica, w was also closely correlated with gs, the inverse value of rs (R2 = 0.817 when related to leaf area, R2 = 0.844 when related to leaf mass; P < 0.001 in both cases). The correlation between w and gs seems to be less tight in Populus species that are more restricted to small ground water distances: in P. fremontii, an obligate riparian tree, linear regression models including VPD explained <29% of the variation in gs (Horton et al. 2001; Pockman and Sperry 2000).
In A. sparsifolia, stomatal regulation is related to the leaf-specific hydraulic conductance on the flow path from ground water to leaf (kSL) more than to the leaf-to-air difference in the partial pressure of water vapour (w). This is true at least for the largest part (beginning of June to September) of the growing season, when the aboveground biomass had reached at least 80% of its peak value. Water transport from the ground water level obviously is achieved through long shoot sections that extend from the soil surface vertically through the soil, with roots formed immediately at the threshold of the capillary fringe as could be observed on vertically eroded river banks (F. M. Thomas, personal observation). Thus, in A. sparsifolia, a high hydraulic conductance is a prerequisite for sustaining high levels of L, gs and growth at large distances to the ground water.
Together with L, kSL also affected stomatal resistance (rs) in T. ramosissima (the lack of significance in the model that used leaf dry matter-related data possibly was due to the smaller range of rs values: minimum and maximum values differed by a factor of 2.4, as opposed to a factor of 3.5 in the case of leaf area-related rs values). The figure of 
(computed as leaf-specific apparent hydraulic conductance from measurements of L and transpiration) in T. ramosissima growing in the Gurbantonggut Desert (north-west China; Xu and Li 2006) fits to the kSL values calculated in our study 
. The correlation between kSL and rs was somewhat surprising because, due to the plant's complex belowground structure (Gries et al. 2003; Qong et al. 2002; Xu and Li 2006), the flow path from the soil to the leaves should be considerably longer and, in addition, more variable than in the other species that have a more direct vertical connection to the ground water. Accordingly, no correlation was found between ground water distance on the one hand and leaf water potential and stomatal conductance on the other (Gries et al. 2003; Mounsif et al. 2002). Possibly, at a moderate distance to the ground water like in our study, the water translocation to the canopy is restricted to some few main paths, in which water flux is immediately controlled by L. Low L that, together with kSL, sensitively control rs might help T. ramosissima to maintain water uptake and growth under high transpirational demand during the entire growing season and might be one reason of this species’ capability of extending farther into the desert than other perennial species (Bruelheide and Jandt 2004; Walter and Box 1983).
In conclusion, our study has revealed that the mechanisms of water status regulation differ among the investigated phreatophytes. In the sub-shrub A. sparsifolia that can grow at ground water distances of as much as 16 m, stomatal conductance is mainly governed by the hydraulic conductance on the flow path from the ground water to the canopy. In T. ramosissima, a salt-accumulating shrub that exhibits the lowest L values, L contributes significantly to stomatal regulation. In P. euphratica, a typical species of floodplain forests, stomatal conductance is most sensitive to w. However, all species are capable of maintaining L within a range that allows transpiration throughout the growing season. Continuous transpiration facilitates CO2 uptake and, hence, sustains carbon assimilation, which may promote the growth of belowground shoots even after the peak of aboveground biomass formation has been reached. Belowground growth forms the basis of an extensive clonal growth, which has been observed in A. sparsifolia and P. euphratica (Bruelheide et al. 2004). This clonal growth obviously is the basis of the occupation of space and of vegetative regeneration once the ground water distance is too large to be bridged from the soil surface.
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FUNDING |
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European Union INCO-DC (Project No. ERBIC18CT980275).
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Acknowledgements |
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The valuable comments of Prof. Dr Michael Runge (Department of Plant Ecology, University of Göttingen) on an earlier draft of the manuscript are gratefully acknowledged. We thank Astrid Rodriguez (Department of Plant Ecology, University of Göttingen) for improving the English. The investigations comply with the current laws of People’s Republic of China where the study was performed.
Conflict of interest statement: None declared.
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References |
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Allen SJ, Grime VL. Measurements of transpiration from savannah shrubs using sap flow gauges. Agric For Meteorol (1995) 75:23–41.[CrossRef]
Arndt SK, Kahmen A, Arampatsis C, et al. Nitrogen fixation and metabolism by groundwater-dependent perennial plants in a hyperarid desert. Oecologia (2004a) 141:385–94.[Web of Science][Medline]
Arndt SK, Arampatsis C, Foetzki A, et al. Contrasting patterns of leaf solute accumulation and salt adaptation in four phreatophytic desert plants in a hyperarid desert with saline groundwater. J Arid Environ (2004b) 59:259–70.[CrossRef]
Backes K. Der Wasserhaushalt vier verschiedener Baumarten der Heide-Wald-Sukzession. Ph.D. Thesis (1996) Germany: Faculties of Mathematics and Natural Sciences, University of Göttingen: Cullvillier.
Balaguer L, Pugnaire FI, Martínez-Ferri E, et al. Ecophysiological significance of chlorophyll loss and reduced photochemical efficiency under extreme aridity in Stipa tenacissima L. Plant Soil (2002) 240:343–52.[CrossRef][Web of Science]
Brenner AJ, Incoll LD. The effect of clumping and stomatal response on evaporation from sparsely vegetated shrublands. Agric For Meteorol (1997) 84:187–205.[CrossRef]
Bruelheide H, Jandt U. Vegetation types in the foreland of the Qira Oasis: present distribution and changes during the last decades. In: Ecophysiology and Habitat Requirements of Perennial Plant Species in the Taklimakan Desert—Runge M, Zhang X, eds. (2004) Aachen, Germany: Shaker. 27–34.
Bruelheide H, Jandt U, Gries D, et al. Vegetation changes of a river oasis at the southern rim of the Taklamakan desert in China between 1956 and 2000. Phytocoenologia (2003) 33:801–18.[CrossRef][Web of Science]
Bruelheide H, Manegold M, Jandt U. The genetical structure of Populus euphratica and Alhagi sparsifolia stands in the Taklimakan Desert. In: Ecophysiology and Habitat Requirements of Perennial Plant Species in the Taklimakan Desert—Runge M, Zhang X, eds. (2004) Aachen, Germany: Shaker. 153–60.
Bucci SJ, Goldstein G, Meinzer FC, et al. Mechanisms contributing to seasonal homeostasis of minimum leaf water potential and predawn disequilibrium between soil and plant water potential in neotropical savanna trees. Trees (2005) 19:296–304.[CrossRef]
Busch DE, Smith SD. Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern U.S. Ecol Monogr (1995) 65:347–70.[CrossRef][Web of Science]
Domingo F, Brenner AJ, Gutiérrez L, et al. Water relations only partly explain the distributions of three perennial plant species in a semi-arid environment. Biol Plant (2003) 46:257–62.[CrossRef]
Foetzki A. Wasserhaushalt und Wassernutzungseffizienz von vier perennierenden Pflanzenarten im Vorland einer zentralasiatischen Flussoase. Berichte des Forschungszentrums Waldökosysteme der Universität Göttingen, Reihe A, 186 (2003) Göttingen, Germany: Forschungszentrum Waldökosysteme.
Foster JR, Smith WK. Stomatal conductance patterns and environment in high elevation phreatophytes of Wyoming. Can J Bot (1991) 69:647–55.[CrossRef]
Gries D, Foetzki A, Arndt SK, et al. Production of perennial vegetation in an oasis-desert transition zone in NW China—allometric estimation, and assessment of flooding and use effects. Plant Ecol (2005) 181:23–43.[CrossRef]
Gries D, Zeng F, Foetzki A, et al. Growth and water relations of Tamarix ramosissima and Populus euphratica on Taklamakan desert dunes in relation to depth to a permanent water table. Plant Cell Environ (2003) 26:725–36.[CrossRef]
Herbst M. Stomatal behaviour in a beech canopy: an analysis of Bowen ratio measurements compared with porometer data. Plant Cell Environ (1995) 18:1010–18.[CrossRef]
Horton JL, Kolb TE, Hart SC. Leaf gas exchange characteristics differ among Sonoran Desert riparian tree species. Tree Physiol (2001) 21:233–41.
Jarvis PG, McNaughton KG. Stomatal control of transpiration: scaling up from leaf to region. In: Advances in Ecological Research—MacFadyen A, Ford ED, eds. (1986) London: Academic Press. 1–49.
Körner C. Leaf diffusive conductances in the major vegetation types of the globe. In: Ecophysiology of Photosynthesis—Schulze E-D, Caldwell MM, eds. (1994) Berlin, Germany: Springer. 463–90.
Li X-Y, Zhang X-M, Zeng F-J, et al. Water relations on Alhagi sparsifolia in the southern fringe of Taklamakan desert. Acta Bot Sin (2002) 44:1219–24.
Lieth H. Primary production of the major vegetation units of the world. In: Primary Productivity of the Biosphere. Ecological Studies 14—Lieth H, Whittaker RH, eds. (1975) Berlin, Germany: Springer. 203–15.
Lindroth A, Cermak J, Kucera J, et al. Sap flow by the heat balance method applied to small size Salix trees in a short-rotation forest. Biomass Bioenergy (1995) 8:7–15.[CrossRef][Web of Science]
McDowell N, Pockman WT, Allen CD, et al. Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought? New Phytologist (2008) 178:719–39.[CrossRef][Web of Science][Medline]
Meinzer FC. Functional convergence in plant responses to the environment. Oecologia (2003) 134:1–11.[CrossRef][Web of Science][Medline]
Mounsif M, Wan CG, Sosebee RE. Effects of top-soil drying on saltceder photosynthesis and stomatal conductance. J Range Manage (2002) 55:88–93.[CrossRef]
Nilsen ET, Sharifi MR, Rundel PW, et al. Diurnal and seasonal water relations of the desert phreatophyte Prosopis glandulosa (honey mesquite) in the Sonoran Desert of California. Ecology (1983) 64:1381–93.[CrossRef][Web of Science]
Oren R, Sperry JS, Katul GG, et al. Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit. Plant Cell Environ (1999) 22:1515–26.[CrossRef]
Pockman WT, Sperry JS. Vulnerability to xylem cavitation and the distribution of Sonoran desert vegetation. Am J Bot (2000) 87:1287–99.
Qong M, Takamura H, Hudaberdi M. Formation and internal structure of Tamarix cones in the Taklimakan Desert. J Arid Environ (2002) 50:81–97.[CrossRef]
Sachs L. Applied Statistics (1984) 2nd edn. New York: Springer.
Sala A, Smith SD, Devitt DA. Water use by Tamarix ramosissima and associated phreatophytes in a Mojave Desert floodplain. Ecol Appl (1996) 6:888–98.[CrossRef]
Schäfer KVR, Oren R, Tenhunen JD. The effect of tree height on crown level stomatal conductance. Plant Cell Environ (2000) 23:365–75.[CrossRef]
Schulze E-D, Beck E, Müller-Hohenstein K. Plant Ecology (2005) Berlin, Germany: Springer.
Sperry JS, Hacke UG. Desert shrub water relations with respect to soil characteristics and plant functional type. Funct Ecol (2002) 16:367–78.[CrossRef]
Thomas FM. Growth and water relations of four deciduous tree species (Fagus sylvatica L. Quercus petraea [Matt.] Liebl. Q. pubescens Willd. Sorbus aria [L.] Cr.) occurring at Central-European tree-line sites on shallow calcareous soils: physiological reactions of seedlings to severe drought. Flora (2000) 195:104–15.
Thomas FM, Arndt SK, Bruelheide H, et al. Ecological basis for a sustainable management of the indigenous vegetation in a Central-Asian desert: presentation and first results. J Appl Bot (2000) 74:212–9.
Thomas FM, Foetzki A, Arndt SK, et al. Water use by perennial plants in the transition zone between river oasis and desert in NW China. Basic Appl Ecol (2006) 7:253–67.[CrossRef]
Veste M. Temporal and spatial variability of plant water status and leaf gas exchange. In: Arid Dune Ecosystems—The Nizzana Sands in the Negev Desert—Breckle S-W, Yair A, Veste M, eds. (2008) Berlin, Germany: Springer. 367–75.
Walter H, Box EO. The deserts of central Asia. In: Ecosystems of the World 5: Temperate Deserts and Semi-deserts—West NE, ed. (1983) Amsterdam, The Netherlands: Elsevier. 193–236.
Wang S, Chen B, Li H. Euphrates Poplar Forest (1996) Beijing, China: China Environmental Science Press.
Whitford WG. Ecology of Desert Systems (2002) San Diego, CA: Academic Press.
Xia X, Li C, Zhou X, et al. Desertification and Control of Blown Sand Disasters in Xinjiang (1993) Beijing, China: Science Press.
Xu H, Li Y. Water-use strategy of three central Asian desert shrubs and their responses to rain pulse events. Plant Soil (2006) 285:5–17.[CrossRef][Web of Science]
Xu H, Li Y, Xu G, et al. Ecophysiological response and morphological adjustment of two Central Asian desert shrubs towards variation in summer precipitation. Plant Cell Environ (2007) 30:399–409.[CrossRef][Medline]
Zeng F, Bleby TM, Landman PA, et al. Water and nutrient dynamics in surface roots and soils are not modified by short-term flooding of phreatophytic plants in a hyperarid desert. Plant Soil (2006) 279:129–39.[CrossRef][Web of Science]
Zeng F, Zhang X, Foetzki A, et al. Water relation characteristics of Alhagi sparsifolia and consequences for a sustainable management. Sci China Ser D Earth Sci (2002) 45(Suppl):125–31.[CrossRef]
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