Evaluation of water use of Caragana microphylla with the stem heat-balance method in Horqin Sandy Land, Inner Mongolia, China

agricultural and forest meteorology 148 (2008) 1668–1678

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Evaluation of water use of Caragana microphylla with the stem heat-balance method in Horqin Sandy Land, Inner Mongolia, China Guangyang Yue a,*, Halin Zhao a, Tonghui Zhang a, Xueyong Zhao a, Li Niu a, Sam Drake b a Naiman Desertification Research Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, 320 Donggang West Road, Lanzhou 730000, PR China b Office of Arid Lands Studies, University of Arizona, 1955 E. 6th Street, Tucson, AZ 85719, USA

article info

abstract

Article history:

Transpiration rates of a 15-year-old sand-fixation plantation of the leguminous shrub

Received 15 July 2007

Caragana microphylla in the Horqin Sandy Land, northeast China, were measured by sap

Received in revised form

flow gauges throughout the summer of 2006. We extrapolated the measurements of water

16 May 2008

use by individual plants to determine the area-averaged transpiration (Ts) of the shrubland.

Accepted 22 May 2008

The method used for the extrapolation assumes that the transpiration of a shrub was proportional to its leaf area. Similar results were found when comparing transpiration estimated with sap flow measurements to the actual evapotranspiration measured by the

Keywords:

weighed lysimeter method, indicating that the scaling procedure can be used to provide

Caragana microphylla

reliable estimates of shrub transpiration. Daily sap flow rates and Ts were mainly controlled

Sap flow

by photosynthetically active radiation (Q) and vapor pressure deficit (D), whereas soil

Water use

moisture had more influence on monthly change in Ts. Maximum stand-level transpiration

Leaf area

rates for C. microphylla ranged from 1.64 to 1.95 mm day1, with an average value of

Scaling-up

1.19 mm day1. The seasonal (1 June to 31 August) total transpiration amounted to 109 mm, representing 66% of the incoming precipitation over this period. These results suggest that C. microphylla is highly effective at utilizing scarce water resources in semi-arid environments. # 2008 Elsevier B.V. All rights reserved.

1.

Introduction

The Horqin Sandy Land (commonly called ‘Horqin Sandy Grassland’), located in the semiarid agro-pastoral transition zone of northeastern China, is one of the most severely desertified areas in China (Zhu and Chen, 1994). To help rehabilitate desertified lands, vegetation for windbreaks and sand-fixation has been widely planted since the mid 1970s (Cao et al., 2004). Caragana microphylla, a native leguminous

shrub, is the dominant species on mobile and semi-mobile dunes in the Horqin Sandy Land, and is widely used in plantations for phytoremediation because of its adaptation to aridity and infertile sandy conditions (Cao et al., 2000). However, the disparity between water supply and demand is becoming particularly acute as the initially simple, cultivated vegetation system comprised of C. microphylla has developed toward a more complex, cultivated and natural ecosystem capable of reversing desertification (Su and Zhao,

* Corresponding author. Tel.: +86 931 496 7222; fax: +86 931 496 7201. E-mail address: [email protected] (G. Yue). 0168-1923/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2008.05.019

agricultural and forest meteorology 148 (2008) 1668–1678

2003). This situation has prompted a need to determine the water requirement of C. microphylla shrubs, so as to assess the effects of their cultivation on local hydrological resources. To our knowledge, little is currently known about the actual water use of this shrub or its response to environmental conditions. Several methods have been developed to determine the quantity of water transpired by plants, including the Penman– Monteith method (Allen et al., 1998), the plant chamber method (Goulden and Field, 1994), chemical and isotopic tracers methods (Calder et al., 1992), weighed lysimeters, the Bowen ratio method (Prueger et al., 1997; Heilman and Brittin, 1989), the eddy correlation method (Tanner, 1987) and the aerodynamic combined method (Perrier and Tuzet, 1991). These measurements can be carried out in different systems and at various spatial and temporal scales (Wilson et al., 2001), offering a range of options for study. However, some measurement methods are limited in their application to measuring transpiration rate in situ (Smith and Allen, 1996; Chabot et al., 2005). Methods using sap flow measurement through intact plant stems have become a routine component of investigations of vegetation water use, particularly since commercial instrument systems became available in the 1990s (Swanson, 1994; Smith and Allen, 1996; Grime and Sinclair, 1999). The theoretical basis of each sap flow method was reviewed by Smith and Allen (1996), who advocated selecting the method most appropriate to a particular practical application. Accordingly, the method we tested and used in this paper is the heatbalance method developed by Sakuratani (1981,1984), Baker and Van Bavel (1987) and Steinberg et al. (1989), which is well adapted to small diameter stems (Senock and Ham, 1993; Chabot et al., 2005) and enables water use from a single component of mixed vegetation to be continuously measured in situ under widely varying environmental conditions. The accuracy of such a non-intrusive technique was confirmed by subsequent studies (Dugas, 1990; Dugas et al., 1993; Steinberg et al., 1990a,b; Devitt et al., 1993; Lascano et al., 1992; Hall et al., 1998). Since sap flow is generally measured at the shrub stem scale and our field conditions are different from those in previous studies, it is necessary to develop and test an appropriate sampling strategy and scaling method to scale up stem-level transpiration measurements to the whole shrubland stand. In the present paper, we have attempted to extrapolate in situ sap flow measurements of 12 C. microphylla stems in an even-aged 15-year-old C. microphylla shrub plantation to determine the aggregate transpiration of the nearly 175 shrubs in the stand. The method used for this extrapolation assumed that the transpiration of a shrub was proportional to its leaf area. Specifically, our objectives were to: (1) develop an appropriate scaling procedure for scaling up the sap flow from individual stems to the whole shrub and to the plot; (2) assess the utility of using sap flow gauges to determine the stand-level transpiration of shrub plantations in a semiarid sandy environment; (3) analyze daily transpiration rates; (4) quantify the key drivers, such as photosynthetically active radiation (Q) and vapor pressure deficit (D), of transpiration in this sandy shrub system.

2.

Materials and methods

2.1.

Site description

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The study was conducted at the Naiman Desertification Research Station (NDRS) of the Chinese Academy of Sciences, located in the south-central part of the Horqin Sandy Land, Inner Mongolia, China (428550 N, 1208420 E, 345 m a.s.l). In this area, climate is semiarid temperate continental monsoon, with a windy and dry winter and spring, and warm and comparatively wet summer followed by a short and cool autumn. The 30-year mean annual precipitation is 362 mm with 70% occurring between July and September, the mean annual potential evaporation is about 1935 mm, and the 40year mean annual temperature is 6.5 8C (range from 5.1 to 7.7 8C). The annual frost-free period is about 130–150 days. The average annual wind speed varies between 3.4 and 4.1 m s1 with the frequent occurrence of gales (wind speed >20 m s1) (Li et al., 2000). The soils are light yellow, and highly susceptible to wind erosion due to their characteristics of coarse texture and loose structure with a high proportion of sand (85–95%) and low organic matter content (0.15–0.5 organic C) (Su and Zhao, 2003). Dunes, alternating with gently undulating interdunal lowland and grassland areas, characterize the landscape in this region, with 20–120 m thickness of sandy deposits (Su et al., 2005). Land uses in the study area include cropland, grazed grassland, artificial sand-fixed shrubland and woodland. The study site used was a 100 ha sand-fixation shrubland with a gently undulating landform of 5 m relief, located about 500 m west of NDRS. The dominant plant species in the whole area was even-aged 15-year-old C. microphylla, accompanied by some trees (e.g. Pinus luestris L., Populus simonii Carr.), shrubs (e.g. Salix gordejevii Chang et Skv.), subshrubs (e.g. Artemisia halodendron Turcz., Artemisia scoparia Waldst.), and grasses and forbs (e.g. Ephedra sinica Stapf., Chloris virgata Swartz, Corispermum elongatum Bge., Setaria viridis L., Portulaca oleracea L., Tribulus terrestris L., Digitaria ciliaris Rotz.). The C. microphylla shrubs occupied roughly 27% of the land surface area, and the average spacing between them was 1–3 m.

2.2.

Vegetation measurements

The experiments were carried out in the C. microphylla shrub plantation from June to August 2006, during one complete summer season. A representative sample site on the top of a fixed dune, 50 m  50 m in area (0.25 ha), was selected. All vegetation and sap flow measurements were conducted inside this plot, and used for scaling up the transpiration estimate from individual stems to the entire shrubland. The basal diameter, d (mm), of 1335 randomly selected stems in the 0.25 ha sample plot was recorded in July 2005. Diameters were measured 5 cm above ground level. All the branches of six sampled C. microphylla shrubs (Samples A–F, Fig. 3) were harvested and defoliated after completion of the study and the area of the leaves was measured. The area of all stripped leaves for each stem, Li (cm2), was measured using an optical scanner and computer image manipulation techniques (Liu et al., 2004). The total basal stem diameter of each sampled shrub was also recorded, so that a linear regression between Li

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and basal stem cross-sectional area, A (cm2), could be calculated for each sampled shrub. The leaf area index (LAI) of each isolated C. microphylla shrub in the study plot was estimated using an LAI-2000 Plant Canopy Analyzer (LI-COR, USA) in the middle days of each month (15 June, 16 July and 16 August). That is, 175  3 LAI measurements were carried out. The measurement of individual shrubs has been described and justified by LICOR (1992) and Brenner et al. (1995). To measure the foliage density of a shrub, we used the DISTS vector (LI-COR, 1992), and set each of the five values to the magnitude of the longest measured stem length Lmax (m). The LAI value was thus expressed as leaf area density (LAD, m2 m3) of a shrub shaped like a perfect hemisphere, with radius = Lmax. Thus, the total leaf area (Ai, m2) of the measured shrub was calculated by the following expression: Ai ¼

2.3.

2pL3max LAD 3

2.4.

In the study site, a sparse and open shrubland, spacing between shrubs is not uniform, but the majority of leaves are not shaded. Sap flow measured in sampled individual stems has been found most strongly correlated with foliage area for C. microphylla in our previous study (Yue et al., 2006). So it is assumed that the amount of transpiration of all stems was proportional to leaf area, which has been confirmed for fallow savannah shrubs by Allen and Grime (1995). Accordingly, the extrapolation method we used is based on the foregoing assumption, and Ts (mm day1), the mean daily stand transpiration for the plot (0.25 ha) was estimated from the sap flow in gauged stem i as

Ts ¼

A total of 12 stems, out of the 6 sampled C. microphylla shrubs, were fitted and measured with sap flow gauges (Dynamax Inc., Houston, TX, USA, Models SGA5, SGB9 or SGB10) during the measuring period. The gauge-equipped stems, ranging in basal diameter from 5 to 12 mm, were selected on the basis of a statistical analysis (Fig. 2) within the representative sampling plot (0.25 ha) for determining the ‘‘mean stem’’ (Rana et al., 2005) before the gauges were set up. The stems selected were in good condition and well developed to support the weight of the sensor and survive throughout the measurement period (Chabot et al., 2005). The theoretical basis and methodology of sap flow gauging are well described in the cited references, so only a brief summary and details pertinent to our application follows. It has been demonstrated that the magnitude and relative importance of different errors is highly dependent on operating conditions (Grime and Sinclair, 1999). Therefore, gauges were strictly installed following the instructions recommended by the manufacturer. To minimize the asymmetric influence of thermal gradients induced by the environment, gauges were fixed at least 40 cm above the soil surface. Stems were prepared by minimal sanding, and coated with a layer of silicone-based dielectric grease, to ensure good thermal contact between the stem surface and the thermocouples and to prevent sensor corrosion and normal stem respiration. Each gauge was protected from the weather using a shield wrapping of several layers of aluminum foil, to reduce solar heating and extraneous thermal gradients across the heated section of the stem to a negligible level. Additionally, conical shelters were attached to the stem just above the gauges and joints were sealed with grafting wax, to prevent water flowing down the stems into the gauges. The signals from the gauges were recorded at 10-s intervals and stored as 30-min averages with an automatic data logger. Gauges were checked and removed to different stems every 2 weeks to avoid plant damage due to sensor heating (Kjelgaard et al., 1997). No data were recorded for several days because of power deficits, especially during continuously cloudy or rainy periods.

n X 1000AFi =rAs Li i¼1

(1)

Sap flow measurements

Calculation of area average transpiration

n

(2)

where A is the total leaf area in the plot (m2), Li the leaf area of stem i (m2), Fi the sap flow measured in stem i (kg day1), r the density of water (kg m3), As the land surface area of the plot (m2), and n is the number of gauged stems. Two sources of uncertainty, not quantified in this study, are related to the scaling process: (1) direct uncertainty due to sap flow and leaf area measurements; (2) uncertainty linked to the extrapolation of sap flow from a sample of stems to the transpiration of the shrub stand.

2.5. Meteorological and actual evapotranspiration measurements An automated weather station was situated about 500 m away from the experimental field. All climatic data were measured once a minute, averaged hourly, and recorded by a data logger throughout the measuring season. Vapor pressure deficit (D) was determined using relative humidity (RH) and air temperature (TA) measurements based on equations adapted from Campbell and Norman (1998). Daily average D (DZ) was normalized by the number of light hours to account for seasonal differences in day length. The transducers were located 1.5 m above the ground surface. Rainfall was measured in a clear area with a ground level tipping-bucket rain gauge. Water table depth data were collected every 5 days at a well located in the center of the study field. In order to test the accuracy of the stem heat-balance method for evaluating the transpiration rate of C. microphylla in the field, a whole month of comparative measurements between the stem heat-balance method and the lysimeter method were performed in June 2005 on shrubs growing in a weighed lysimeter at NDRS. Sap flow gauges were attached to the stems of shrubs being weighed. The experimental shrubs were transplanted into the lysimeter container 5 years ago, and their growing conditions were managed to be similar to those in study field.

2.6.

Data analyses

Stepwise multiple regression analyses were conducted to describe the relationships between diurnal changes in sap flow rates and significant environmental factors over a

agricultural and forest meteorology 148 (2008) 1668–1678

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The regressions of stem leaf area on basal stem crosssectional area for shrub Samples A–F are shown in Fig. 3. The regressions fit the data fairly well, showing that a strong relationship exists between stem cross-sectional area and leaf area. However, the slope of the regression differed among the samples (R2 values ranging from 0.70 to 0.97), suggesting that individual shrubs differed allometrically, possibly due to different local growing conditions in the plot. Furthermore, the highly significant R2 values (0.9677 and 0.9688) for the regression equations of Samples D and E with 16 and 10 stems per shrub, respectively, indicate that the linear relationship between basal cross-sectional area and leaf area per stem can be reliably determined in relatively small shrubs in our study.

3.3. Fig. 1 – Comparison between shrub transpiration values (Ts) determined by the stem heat-balance method and those (El) calculated by the weighed lysimeter method throughout June 2005. The solid line represents the 1/1 relationship.

monthly measurement period. All statistical analyses were performed with the SPSS software package (version 13.0 for windows, SPSS Inc., USA), with a = 0.05 as the threshold for statistical significance. Linear and nonlinear curve fits were performed in Origin (Version 7.0, OriginLab Corp., USA).

3.

Results

3.1. Comparison between shrub transpiration and total evaporation The comparison between transpiration (Ts) evaluated by the extrapolated sap flow method and evapotranspiration (El) measured directly by the weighed lysimeter method during June of 2005 is presented in Fig. 1. Daily variation in Ts showed a trend similar to that for El, and the slope of the regression line between the values was close to unity (0.91), and the coefficient of determination was 0.82. The cumulative reference evapotranspiration reached the value of 132 mm in 30 days, when extrapolated transpiration was 113 mm, showing that the sap flow method underestimated transpiration by 14.3% in comparison with direct measurement.

3.2.

Sap flow measurements

Fig. 4 shows the diurnal trend in hourly means of Q and D, and in half-hourly means of sap flow in three stems from different C. microphylla shrubs from 23 to 25 June. The daily course of sap flow was strongly influenced by Q and D, clearly shown by the coincident peaks and troughs in Figs. 4(a) and (b). Partially adjusted determinant coefficients of environmental factors (Q and D) with sap flow rate are given in Table 1. During the summer season, diurnal changes in sap flow rates were mainly determined by Q (R2 = 0.748–0.840) and D (R2 = 0.244– 0.756), which explained 83.4–92.7% of the variation in sap flow. No significant time lag between sap flow and Q and D was observed, indicating the sensitivity of C. microphylla to these driving climatic factors. However, sap flow velocity of C. microphylla is controlled by multiple meteorological factors (Yue et al., 2006). There is a good correspondence between the consistent difference in sap flow rates observed in the three different diameter stems (10.8, 11.2 and 5.5 mm) and the ranking of their leaf areas (0.2862, 0.2016 and 0.0514 m2). In Fig. 4(c), the sap flows in the three stems have been normalized by their leaf areas. The three time series exactly overlie one another, which showed that sap flow per unit leaf area was much less variable than sap flow per stem. Since we observed that sap flow rates were clearly linked to leaf area and that greater sap flows were

Vegetation measurements

Stem diameters measured in the sample plot ranged from 0.5 to 24.8 mm, and a frequency distribution of stem diameter classes in relation to leaf area is presented in Fig. 2. It shows that stems of diameter 4–12 mm, representing 89% of the total stems and contributing 66% of total leaf area, must have been responsible for the major part of the transpiration of C. microphylla concerned. The stems equipped with sap flow gauges were selected in this intermediate size range, so despite the restricted number of gauges deployed (6–8, sampling approximately 0.1% of stems), the sap flow measurements were of the most significant proportion of the stem population.

Fig. 2 – Frequency distribution of stem diameter and leaf area in 2 mm classes for a total of 1335 randomly chosen stems in the 0.25 ha sample plot. Total leaf areas in the classes were estimated by the regression equations in Fig. 3; error bars represent the S.D. about the mean.

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Fig. 3 – Regressions between basal stem cross-sectional area and leaf area per stem for C. microphylla at the end of sap flow measuring period. Samples A–F are different shrubs.

Fig. 4 – (a) Hourly patterns of photosynthetically active radiation (Qo) and vapour pressure deficit (D) measurements from 23 to 25 June 2006; (b) half-hourly patterns of sap flow (g hS1) in three C. microphylla stems with different leaf areas from 23 to 25 June 2006; (c) sap flows normalized by leaf area (mg mS2 sS1) in the same stems as (b) over the same time period. The basal diameter and leaf area of stem S1, S2, S3 were 10.8, 11.2, 5.5 mm and 0.2862, 0.2016, 0.0514 m2, respectively.

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Table 1 – Partially adjusted coefficients of determination between sap flow velocity and significant environmental factors in June, July and August 2006 Significant environmental factors

Partial coefficient and significance (two-tailed)

Month 2006 June

July

Photosynthetically active radiation, Q

R P

2

August

0.840 <0.001

0.748 <0.001

0.812 <0.001

Vapor pressure deficit, D

R2 P

0.393 <0.001

0.244 <0.001

0.756 <0.001

Fig. 5 – Relationship between mean hourly sap flows and leaf area and stem diameter from 23 to 25 June 2006.

obtained on larger leaf areas, a more detailed analysis showed that significant differences appeared within a diameter class (Fig. 5). For example, sap flow measured on two stems of basal diameters 10.8 and 11.2 mm produced maximum values of 58

and 40 g h1, respectively (Fig. 4(b)), indicating that there was no relationship between stem diameter and sap flow. This suggested that leaf area was more reliable than stem diameter for the extrapolation.

Fig. 6 – Daily values for (a) area-average transpiration (Ts) determined by scaling sap flow rates of sampled individual stems for the whole experiment period (1 June to 31 August 2006). No data were recorded on some days, because of power or gauge failure. (b) Maximum, minimum, and mean temperature (TA). (c) Mean vapor pressure deficit normalized by light hours (DZ), and sums of photosynthetically active radiation (Qo). (d) Mean wind speed (Ws) and precipitation.

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Table 2 – Monthly climatic data, total leaf area, plot LAI, and shrub transpiration for the study site during the period of sap flow measurement Month June 2006 July 2006 August 2006

3.4.

Total rainfall (mm)

Mean maximum air temperature (8C)

Mean minimum air temperature (8C)

Total leaf area (m2)

Leaf area index (LAI)

62.4 41.3 62.8

27.6 30 30.1

14.8 17.4 16.4

1205.76 1458.31 1374.64

0.48 0.58 0.55

Daily transpiration variation

Fig. 6 shows daily climatic changes and stand-level values for water use by C. microphylla evaluated using the extrapolated sap flow method over the whole experiment period. The summer of 2006 was hot and relatively rainy (Table 2, Fig. 6(b)). The rainfall for June, July and August at the site was 167 mm (Fig. 6(d)), comprising 65% of the total annual rainfall (258 mm). Shrub stand transpiration calculated during the experiment period had an average of 1.19 mm day1, ranging between 0.07 and 1.95 mm day1. Table 2 shows there was only slight variation in total leaf area over the 3 months, with transpiration rates remaining fairly stable due to leaf area being almost fully developed throughout the summer season. Maximum stand-level transpiration rates for C. microphylla ranged from 1.64 to 1.95 mm day1, while mean monthly stand transpiration rates were 1.18, 1.16 and 1.22 mm day1 in June, July and August, respectively.

Mean transpiration (mm day1) 1.18 1.16 1.22

Higher Ts coincided with higher values of daily photosynthetically active radiation (Qo) and mean daily vapor pressure deficit normalized by daylight hours (DZ) (Fig. 6). This indicates that variation in shrub stand transpiration is mainly due to weather changes. Evaporative demand was low as a result of cloudy weather; e.g. on 4 July and 28 July, transpiration rates were 0.71 and 0.75 mm day1, respectively. The effect of rainfall can be seen clearly in Fig. 6. On days when rain occurred, transpiration rates were greatly reduced. For example, the sudden drop in C. microphylla transpiration on 13 June (0.34 mm day1) was caused by a continuous rainfall event with associated humid and cloudy weather conditions; similarly, the succession of rainy days on 29 June and 26 August suppressed transpiration rates. In contrast, on some days when rainfall duration was short, followed by sunshine, e.g. on 12 July, 22 July and 7 August, relatively high transpiration values of 1.04–1.2 mm day1 were observed.

Fig. 7 – Relationship between daily area-average transpiration (Ts) and mean daily vapor pressure deficit normalized by light hours (DZ) and daily sums of photosynthetically active radiation (Qo) in June, July and August 2006. Regression equations are: Qo, June: y = S0.0004x2 + 0.0505x S 0.1604; July: y = S0.0007x2 + 0.078x S 0.7547; August: y = S0.0003x2 + 0.0448x + 0.0016; DZ, June: y = S0.1389x2 + 0.8361x + 0.2689; July: y = S0.0956x2 + 0.7207x + 0.2555; August: y = S0.1722x2 + 1.1401x S 0.0817.

agricultural and forest meteorology 148 (2008) 1668–1678

The distinct rise of Ts after rainfall (13 June 2006) suggests that available water had become limiting in the root zone. Daily patterns of Ts showed a reduction from 1.91 to 0.76 mm day1 under sunny weather conditions in mid-July, associated with a 14-day dry period, although Qo remained constant (Fig. 6). The 10-day cumulative Ts after the last rainfall in July (27 July 2006) amounted to 12.93 mm, which was 53.92% higher than that before. The regression analysis in Fig. 7 shows that low significant R2 values were found between Ts and Qo (R2 = 0.2997, P = 0.017) and DZ (R2 = 0.3687, P = 0.005) in July, indicating that Ts was not only related to DZ and Qo. Considering the low rainfall of 41.3 mm, soil moisture was expected to be one of the most important drivers of Ts in July. So it was concluded that the most significant driver of summer Ts can vary among months. DZ and Qo significantly explained the variability of Ts in June and August, whereas soil water content was more important than DZ and Qo in driving Ts in July 2006 (Fig. 7).

4.

Discussion

4.1. Comparison between shrub transpiration and total evaporation Our results showing an underestimation of transpiration by the sap flow method, compared with the weighing lysimeter, are in contradiction to those of authors who conclude that the extrapolation of stem-level flows to the transpiration of a canopy often produces a systematic overestimation of transpiration (Ham et al., 1990; Groot and King, 1992; Shackel et al., 1992; Chabot et al., 2002). The steady-state assumption of a constant proportionality between sap flow rates and leaf area for all stems is generally advanced to explain this overestimation (Hall et al., 1998; Chabot et al., 2005). The small discrepancy between Ts and El might be due to two different factors in our case. First, the calculated shrub transpiration rate Ts was assumed to be nearly equal to the sum of the transpiration from all sunlit leaves (Hall et al., 1998), and the majority of leaves were assumed to be sunlit. However, a more accurate value of total leaf area of a shrub might be slightly greater than that obtained from Eq. (1), though there were no experimental data to confirm this in the present study. The second possible source of error may be that the contribution of evaporation from the soil surface in the weighed lysimeter is not taken into account by the sap flow method (Allen, 1990). Subtracting soil evaporation from El, Fig. 1 could give acceptable results with daily differences of less than 14.3% between the sap flow method extrapolated to whole shrubs and a weighing lysimeter reference, suggesting that our scaling procedure can be used to provide reliable estimates of seasonal shrub transpiration in a semiarid environment.

4.2. Scaling up from single stem to area average transpiration To obtain transpiration rate per unit ground area through the period of sap flow measurements, sap flow rates measured by the stem heat-balance method need to be extrapolated by an appropriate scaling factor, so that the total transpiration can

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be assessed in relation to the area’s hydrological budget (Hall et al., 1998). Scaling-up procedures may vary according to the characteristics of the stand (Smith and Allen, 1996). In general, stand transpiration could be estimated most simply by extrapolating water use on the basis of plant density (Ham et al., 1990; Dugas and Mayeux, 1991), or the ground area occupied by individual trees (Hatton and Vertessy, 1990), or the relationships determined between sap flow rates and stem diameter, stem basal area, sapwood area or leaf area index (Allen and Grime, 1995; Soegaard and Boegh, 1995; Vertessy et al., 1995). Leaf area, the biometric parameter we used for the extrapolation, has been considered a more accurate scaling procedure (Vertessy et al., 1995; Hatton and Wu, 1995; Granier et al., 2000; Oren et al., 1999a,b). However, leaf area measurements are difficult, and integration of leaf area actually measured in all shrub branches is prohibitive, particularly for C. microphylla with its rather small ovate leaves (3–6 mm wide). Moreover, destructive sampling of shrubs is not usually acceptable. It is therefore necessary to find an expedient experimental procedure for large-scale leaf area measurement. Highly linear relationships between basal stem cross-sectional area and foliage area have been reported for several plant species (Whitehead et al., 1984; Allen and Grime, 1995; Chabot et al., 2005), and these proportions have been used to estimate leaf area for all plants by measuring each stem diameter instead of direct leaf area measurement. To verify whether this approach is feasible for C. microphylla, the regressions between basal stem cross-sectional area and leaf area per stem for shrub Samples A–F were calculated (Fig. 3). In our case, the different slopes of the regressions indicate that shrubs in the plot cannot be assumed to have a particular linear relationship between stem basal cross-sectional area and foliage area. Somewhat differently from any of the scaling methods suggested by Smith and Allen (1996), we took advantage of the sparse distribution of C. microphylla and the ease of directly measuring LAD for individual shrubs to create a new scaling method based on these direct LAD measurements for all shrubs in the study plot. This avoids the need to measure leaf area for many individual stems or estimate stand-level leaf area index values. This approach is probably most similar to one suggested by Hatton and Vertessy (1990) for estimating stand transpiration by scaling-up water use on the basis of the ground area occupied by individual trees (Smith and Allen, 1996), but is more direct.

4.3.

Daily transpiration variation

In previous studies where sap flow gauges were used to provide stand-level estimates of transpiration in similar environments, Allen and Grime (1995) obtained values of 1.5–2 mm day1 for savannah shrubs. These values agree closely with those obtained for C. microphylla in the present study. Seasonal (1 June to 31 August) total transpiration from C. microphylla amounted to 109 mm (Fig. 8(a)). Thus the shrubs, which occupied 27% of the land surface, transpired 66% of the rainfall over this period. Furthermore, the depth of the water table presents an evident downward trend, as shown in Fig. 8, suggesting that a great increase in the amount of groundwater

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small to supply the entire soil profile, so hydraulic lift, which is a very important phenomenon in arid ecosystems (Caldwell and Richards, 1989), was thought to be occurring below the shrubs. Water is absorbed by C. microphylla roots in deeper moister soil layers and transported to the drier upper soil layers at night. This water redistribution process increased moisture availability in the densely rooted soil, resulting in a higher effectiveness of water uptake by deep roots (Xu et al., 2007).

5.

Fig. 8 – (a) Evolution of cumulative values of area average transpiration (Ts) and rainfall. For shrub stand transpiration during the whole experiment period, missing data from 11 days were interpolated with the average value (1.19 mm dayS1). (b) Trend in groundwatertable profile.

use occurred between June and August 2006. As previously described, land use in the study area consisted mainly of largearea croplands distributed around the site. The disparity between water supply and water demand is particularly acute under the good growing conditions of June to August in this area, and cropland irrigation by groundwater pumping was considered the dominant cause of the water table decline. In addition, vegetation is typically sparse, so the soil surface is often considered as important a source of water vapor as plant leaves (Allen, 1990; Wallace et al., 1993). It was therefore concluded that, during the experiment period, soil water replenishment by rainfall cannot meet the demand for water use in this area. The transpiration response of C. microphylla to greater evaporative demand was shown to exhibit nonlinear relationships with increasing DZ or Qo, or even a decline in transpiration at high soil water deficit (Fig. 7), indicating stomatal closure to regulate transpiration to prevent runaway cavitation, with the rate of this closure related to soil moisture. These results agree well with the findings of Ewers et al. (2002) and Pataki et al. (2000). Although transpiration from some shrub species is frequently restricted by soil water availability in the upper soil profile, the shrub C. microphylla could still meet its evaporative demand, remaining mostly between 1.09 and 1.95 mm day1, despite high temperature and intense radiation on representative clear days (Fig. 6). This resulted presumably from their deeper, more extensive rooting systems. Root distribution patterns of C. microphylla in this plot were similar to those reported by Cao et al. (2004) in the same area. The roots of C. microphylla can reach as deep as 2.5 m though the majority of absorbing roots were within the upper 0.9 m of soil and decreased with depth. In the even-aged C. microphylla fixed sandy land, the soil layer is very deep and porous (Cao et al., 2004), and summer precipitation was too

Conclusions

C. microphylla is a low-transpiring shrub species with low daily water consumption. Our study reported reliable data on the stand transpiration of planted C. microphylla shrubland, namely 109 mm for the summer of 2006, with a daily average of 1.19 mm and a maximum of 1.95 mm. The daily sap flow rates and Ts mainly depended on climatic factors such as Q and D, whereas soil moisture had more influence on monthly variation in Ts. Moreover, our measurements showed that the shrub species growing in an even-aged plantation appeared to be resistant to environmental drought, implying that C. microphylla is highly effective at utilizing scarce water resources in semi-arid environments. On the basis of these results, we recommend C. microphylla as ideally suited to applications of sand-fixation in the study area. Considering the satisfying results in the experimental plot (0.25 ha), the question arises as to whether the extrapolation based on the stem heat-balance method can be applied to a larger area where the age and developmental stage of the shrubs, as well as soil moisture and radiant energy, are more variable. Further research is needed to answer this question.

Acknowledgements The authors sincerely wish to thank Xiaoyong Yi and Xinping Liu for their critical review and comments on the manuscript. We are also grateful to Wei Zhao, Qingtao Meng, and Shaokun Wang for their help with field measurements. This work was funded by the National Key Technology R&D Project (2006BAD26B0201) and the Chinese Academy of Sciences Knowledge Innovation Project (KZCX2-YW-431).

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