Climatic controls on sap flow dynamics and used water sources of Salix psammophila in a semi-arid environment in northwest China

ORIGINAL ARTICLE

Climatic controls on sap flow dynamics and used watersources of Salix psammophila in a semi-arid environmentin northwest China

Jinting Huang • Yangxiao Zhou • Lihe Yin •

J. Wenninger • Jing Zhang • Guangcai Hou •

Eryong Zhang • S. Uhlenbrook

Received: 23 June 2013 / Accepted: 7 June 2014 / Published online: 18 July 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Systematic field measurements were conducted

to investigate the relationships between the sap flow of

Salix bushes (Salix psammophila) and climatic variables

and the sources of water use in the semi-arid Hailiutu River

catchment in northwest China. The statistical analyses

identified the controls of climatic factors on sap flow and

the water use strategy of Salix bushes in periods charac-

terized by different wetness conditions. Correlation anal-

yses indicate strong positive correlations between diurnal

sap flow, net radiation, air temperature and wind speed but

a negative correlation with relative humidity. The peak

values of sap flow occured at the same time as the peak

values of net radiation but a few hours ahead of the peak

values of temperature and the relative humidity minimum.

Multiple regression equations between diurnal sap flow and

climatic factors (i.e., net radiation, temperature, relative

humidity and wind speed) can account for more than 80 %

of the variations in sap flow. Correlation and multiple

linear regression analyses between sap flow, soil water

contents and groundwater level were used to quantify the

water sources of S. psammophila. The results indicate that

Salix bushes can use both soil water and groundwater for

transpiration. In the dry periods, the cumulative transpira-

tion of Salix caused a continuous decline of soil water

contents and shallow groundwater levels. During rain

events, sap flow was very low, but immediately after the

rain, the peak value of sap flow increased significantly due

to better soil water availability. The correlation and

regression analysis identified a strong relationship between

sap flow and groundwater levels that fluctuated at depths of

145.5–180.7 cm. Therefore, Salix bushes can be consid-

ered as groundwater-dependent plants in the Hailiutu River

catchment.

Keywords Salix psammophila � Sap flow � Climatic

factors � Plant water use � Soil water �Groundwater-dependent ecosystems

Introduction

A large-scale land use and land cover change has been

taking place in the Mu Us Desert that is part of the Ordos

Plateau in northwest China (Wu and Zou 2010). This area

suffers from frequent sandstorms, and farmland is threa-

tened by moving sands. In the beginning of the 1980s, the

Chinese government initiated a reforestation project called

the ‘‘Three North Forest Shelterbelts’’ (Zhang and Song

2003). Starting in 2000, the ‘‘Return Farmland to Forest

and Grassland’’ project was initiated (Wang et al. 2009).

Water resources are very precious in this area because of

the area’s semi-arid climate with scant rainfall and strong

evaporation. Allocating scarce water resources for socio-

economic development while maintaining the ecosystem in

J. Huang (&) � L. Yin � J. Zhang � G. Hou

Xi’an Institute of Geology and Mineral Resources, No. 438,

Youyidong Road, Xi’an 710054, People’s Republic of China

e-mail: [email protected]; [email protected]

Y. Zhou � J. Wenninger � S. Uhlenbrook

Department of Water Science and Engineering, UNESCO-IHE

Institute for Water Education, P.O. Box 3015, 2601 DA Delft,

The Netherlands

J. Wenninger � S. Uhlenbrook

Faculty of Civil Engineering and Applied Geosciences, Water

Resources Section, Delft University of Technology,

P.O. Box 5048, 2600 GA Delft, The Netherlands

E. Zhang

China Geological Survey, No. 45, Fuwai Street, Beijing 100037,

People’s Republic of China

123

Environ Earth Sci (2015) 73:289–301

DOI 10.1007/s12665-014-3505-1

a healthy state is a primary challenge (Hultine et al. 2010),

especially in arid areas (Yang et al. 2012). Proper species

selection for vegetating sand dunes might reduce water use

to acceptable levels (van Dijk and Keenan 2007) and

minimize the adverse effects on water resources, based on

lessons from ecological restoration projects (Cao et al.

2011). In addition, the large-scale development of natural

resources (coal and gas) is taking place in the area (Yin

et al. 2011), facilitating the possible loss of large amounts

of precious water from the overlying loose water-bearing

sand to a large extent during the mining process. Further-

more, mining may have adverse impact on plants (Wang

et al. 2013). In the Mu Us Desert, one of the dominant

desert bushes is Salix psammophila (C. Wang et Chang Y.

Yang) (S. psammophila) (Xiao and Zhou 2001). S. psam-

mophila grows in desert sandy soil and can tolerate moving

sand dunes that can periodically cover part of the plant.

The plant is used in soil and water conservation, wind

sheltering and sand fixation. S. psammophila is very

important for preventing desertification and sandstorms

(Huang et al. 2006). Thus, one of the scientific challenges

for this area is to quantify the water use of S. psammophila

and its response to changing environmental conditions.

Evaluating the ecosystem response to environmental

fluctuations is crucial, among other things, to the man-

agement and restoration of plant communities (Wright and

Chambers 2002; Ridolfi et al. 2006). These processes

depend on sap flow, which drives the physiological

responses of desert plants, and sap flow depends on the

plant’s water balance and on subsurface hydrology (Lambs

and Berthelot 2002). Therefore, accurately estimating the

response of sap flow to changes in environmental variables

is the basis for comprehending the physiological response

of plants to their habitats. Sap flow changes in response to

the variations of environmental variables, such as radiation

intensity, relative humidity, air temperature, wind speed,

rainfall and soil moisture availability (McDowell et al.

2008). The sap flow of some typical desert plants, such as

Caragana korshinskii, Tamarix elongate Ledeb, Populus

euphratica, Elaeagnus angastifolia, Artemisia ordosica

and Nitraria sphaerocarpa in relation to climatic variables

has been previously analyzed (Zhang et al. 2003; Qu et al.

2007; Xia et al. 2008; Huang et al. 2010; Liu et al. 2011),

but little information is available on the transpiration of

S. psammophila. More work in desert plants in relation to

changing environmental variables is needed to improve the

understanding of the dominating processes and controls for

different plant species.

Potential water sources for plant survival include soil

water and groundwater. In water-limited environments,

high vapor pressure deficit (VPD) may lead to the

development of water stress. To maintain open stomata,

plants tend to extract water from the deep soil layers or

groundwater via deep roots under conditions of water

unavailability in shallow soil layers (Naumburg et al.

2005; West et al. 2008). In arid desert areas, the deep

roots of trees can reach ten meters to tap groundwater

(Lubczynski 2009). Phreatophytic vegetation will respond

to groundwater level decreases caused by groundwater

pumping in the short term (Shatfroth et al. 2000; Lite and

Stromberg 2005) and long term (Copper et al. 2006).

Large-scale afforestation will induce a decline of

groundwater table and then affect the phreatophytic

vegetation (Zheng et al. 2012). A previous study indi-

cates that water and salt conditions have a significant

influence on S. psammophila growth, based on using the

influence of water and salt on vegetation (IWSV) model

in the Mu Us Desert (Chen et al. 2011); however, further

proof is needed under different soil moisture wetness

conditions.

In this study, systematic field measurements were

conducted to investigate the relationships between the

sap flow of S. psammophila, climatic factors and dif-

ferent water sources in the semi-arid Hailiutu River

catchment in northwest China. The objectives of the

study are (1) to establish an empirical relationship

between sap flow and climatic factors, such as net

radiation, air temperature, relative humidity and wind

speed; (2) to assess the water use of S. psammophila

under different soil moisture wetness conditions; and (3)

to evaluate S. psammophila suitability for preventing

desertification in the Mu Us Desert. The results of the

study provide further insights into the water use strategy

of S. psammophila and provide scientific information for

the selection of appropriate vegetation species for pre-

venting desertification.

Research site and field measurements

Research site

The experimental site is located in the Hailiutu River

catchment (between 38�060 and 38�500N, 108�370 and

109�150E), on the Ordos Plateau, northwest China (Fig. 1).

The Hailiutu River is one of the branches of the Wuding

River, which is the major tributary of the Yellow River.

The total area of the Hailiutu River catchment is approxi-

mately 2,645 km2. The surface elevation is 1,250 m above

sea level, and the climate is semi-arid. Based on the

meteorological data for a 50-year period (1957–2007) from

a metrological station *40 km northwest of the study site,

the long-term average annual air temperature is 8.1 �C, and

the lowest and highest monthly average air temperature is

-8.6 �C in January and 23.9 �C in July. The average

annual precipitation is 340 mm, and approximately 70 %

290 Environ Earth Sci (2015) 73:289–301

123

occurs between July and September. The average annual

potential evaporation is estimated to be 2,180 mm. The

lowest monthly average wind speed is approximately

1.5 m/s in December and January, and the highest is

approximately 3.2 m/s in April (Yin et al. 2013). The

landscape is characterized by undulating vegetated sand

dunes. The research site consists of an area of

25 m 9 25 m. At and around the research site, the domi-

nant natural vegetation is S. psammophila, along with

sporadically distributed A. ordosica herbs, Korshinsk pea-

shrub herbs and Hedysarum laeve Maxim herbs. The

height of the S. psammophila varies from 1.5 to 3.0 m, and

the average height is approximately 2.0 m. It has a 1–2 m

crown but developed root systems that are 1.35 m in soil

depth and 4 m in the horizontal direction surveyed from a

selected cluster. The soil is uniform Aeolian sand, and the

dry bulk density is 1,450 kg/m3. The depth to water table is

approximately 1.5 m, and the range of variation during the

experimental period was 35.2 cm. A set of instruments was

installed to measure meteorological variables, the sap flow

of S. psammophila, soil water contents, soil matrix poten-

tials and groundwater levels.

Measurement of climatic variables

A Bowen ratio energy balance system (Campbell Scientific

Inc., USA) was installed at the research plot. Wind speed

(U, m/s) and direction were measured by a 05130-5 RM

Yong wind monitor (R.M. Young Co., Michigan, USA) at

5.25 m above the ground surface. Net radiation (Rn, W/m2)

was measured by a net radiometer, NR-LITE sensor (Kipp

& Zonen, Delft, The Netherlands) mounted at 1.0 m above

the Salix canopy (3.75 m above ground surface). Air

temperature (T, �C) and relative humidity (RH, %) were

measured using two integrated temperature–humidity sen-

sors inside radiation shields (model HMP45C, Vaisala Co.,

Helsinki, Finland). The heights of the two fixed measure-

ments were 3.25 and 4.25 m above the ground surface. Soil

heat flux (F, W/m2) was measured at two locations 1 m

distant from each other by heat flux plates (model HFP01)

installed 80 mm below the soil surface. Stored heat in the

soil (S, W/m2) was calculated using soil heat capacity, and

temperatures were measured at depths of 20 and 60 mm

(TCAV thermocouple, Campbell Scientific, Logan, Utah,

USA). The total soil heat flux was calculated as the sum of

Fig. 1 Location of the Hailiutu

River catchment and the

research site

Environ Earth Sci (2015) 73:289–301 291

123

soil heat flux and heat storage. Precipitation (P, mm/h) was

measured using a rain gauge (Model 52203 RM Young rain

gauge, R.M. Young Co., Michigan, USA) at 2 m above the

ground surface and away from the Salix canopy. All data

were collected by a data-logger (CR3000, Campbell Sci-

entific, Logan, Utah, USA) every 60 s, and 1-h averages

were calculated and stored. The instruments used in the

study are listed in Table 1.

Measurement of sap flow

The height of the selected S. psammophila for sap flow

measurements is 2.2 m, which is close to the average

height of S. psammophila around the research site. A sap

flow system (Flow 32-1K, Dynamax Inc., Houston, TX,

USA) was used to measure sap flow in the S. psammophila

stems from 29 May to 13 July 2011. The theory and

methodology of using sap flow gauges have been described

previously in detail by Yue et al. (2008). Eight branches

with a stem circumference of 8 cm from the S. psammo-

phila in different directions, i.e., eastern, south, west, north,

southeast, southwest, northeast and northwest, were

equipped with sap flow sensors (SGB9-WS; Dynamax Inc.,

Houston, TX, USA) at a mean height of 70 cm above the

ground surface. The data were recorded at 10-s intervals

and stored as 1-h averages using a CR1000 data-logger

(Campbell Scientific, Logan, Utah, USA).

Measurement of soil water contents

At the research site, the unsaturated zone has an

approximately 5–15 cm dry layer in the upper soil profile

caused by strong evaporation. Thus, two types of sensors

with different resolution were used to measure the soil

water contents (SWC, cm3/cm3) at 8 different depths,

both based on the time-domain reflectometry (TDR)

technology. All sensors were inserted horizontally into the

soil. The sensors installed at 6 and 15 cm depths are

CS616 (Campbell Scientific, USA, the resolution is 1 %).

The data were recorded at 10-s intervals and stored as 1-h

averages using a CR1000 data-logger (Campbell Scien-

tific, USA). The sensors installed at 20, 40, 70, 100, 120

and 140 cm depths are 6005CL2 (Wintrase SEC Co. Ltd,

USA; 2 % resolution). The data were recorded every 10 s

and stored every 4 h with a data-logger (Wintrase SEC

Co. Ltd, USA).

Measurement of groundwater level

A MiniDiver (DI501, Eijkelkamp, The Netherlands) was

installed in a piezometer (at a 160-cm depth below the

ground surface) to monitor the groundwater level and

temperature. The piezometer was covered by a ventilated

PVC cap for protection. The data were recorded at 10-s

intervals and stored as 1-h average values.

Table 1 Instruments used for

measurement of meteorological

variables at the research site

Variable Height (m) Horizontal

distance

to the bush

stem (m)

Instrument or sensor type

Wind speed (W/m2) 5.25 3.0 05130-5 RM Yong wind

monitor

Net radiation (W/m2) 3.75 3.0 NR-LITE sensor

Rainfall (mm/h) 3.0 52203 RM Young rain gauge

Air temperature

(K) and

relative humidity

(%)

3.25 3.0 HMP45C temperature–

humidity

sensors4.25 3.0

Soil heat flux (W/m2) -0.08 3.0 HFP01 heat flow plates

Soil temperature (�C) -0.06 3.0 TCAV thermocouple

Sap flow 0.7 Flow 32-1K

Soil water content -0.06, -0.15 1.5 CS616

-0.2, -0.4, -0.7

-1.0, -1.2, -1.4

1.5 6005CL2

Groundwater level -1.6 1.5 DI501

Root auger -0.15, -0.3, -0.45,

-0.6, -0.75, -0.9,

-1.05, -1.2, -1.35,

-1.5

– –


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Roots survey

Root distribution somewhat reflects used water sources of

plants. Considering the root distribution that varies with

depth and radial distance, soil cores were taken around

the S. psammophila using a Root Auger (Eijkelkamp, The

Netherlands) after defoliation. Undisturbed, uniform soil

samples of 10 cm diameter and 15 cm height were

obtained, and the position of each sample was recorded

including the horizontal radial distance from the stem and

depth to the point of each sample. Four symmetrical

transects, centered with the stem and having a radius of

4 m, were investigated. The total area of the root distri-

bution was determined to be 50.24 m2. Roots were sieved

out of the core sample and washed at the sampling day.

The cleaned roots of each core sample were weighed

using an electric balance (readout 0.01 g; resolution limit

is 1 %), and photos were taken on scaled paper (resolu-

tion is 1 mm). The total root length of fine roots (diameter

\2 mm) was measured with GIS free source software

(www.gvsig.org). The root length density (cm/cm3) of

each sample was determined by dividing the total root

length by the core volume. The vertical distribution of

fine roots was determined from the average of the root

length density at the same sample depth and radial dis-

tance within the root zone of the S. psammophila (Fig. 2).

Two maxima of the root length density can be observed,

one between 0 and 30 cm, with a second one being

observed between 60 and 120 cm (the peak at approxi-

mately 80–90 cm) soil layer.

Results of measurements

Climatic variables

During the study period (29 May 2011–13 July 2011), the

total precipitation was 71.8 mm and heavy rainfall events

occurred on July 1 and July 2 with a total amount of

42.8 mm. The temperature varied between 6.6 �C during

the night and 32.9 �C during the daytime with a daily

average of 21.7 �C. Relative humidity ranged from 20 to

100 % with a daily average of 45 %. The net radiation

reached the maximum value of 897 W/m2 approximately

12:00 and displayed negative values during the night. The

maximum of wind speed was 6.8 m/s, whereas the daily

average was approximately 2 m/s (Fig. 3). Net radiation

and temperature display clear diurnal fluctuations, but the

peaks do not occur at the same time. Relative humidity and

wind speed have more complex variations and change in

opposite directions. It is interesting to note that during the

rain on July 1 and July 2, net radiation was nearly zero,

temperature was low, and relative humidity was very high.

The climatic variables’ changes over the long-term period

have similar characteristics compared to that from a me-

trological station *40 km northwest of the study site. The

pattern of rainfall is similar to that in other arid regions

(Loik et al. 2004; Zhao and Liu 2010).

Sap flow of S. psammophila

The sap flow (g/h) of S. psammophila indicates diurnal

fluctuation patterns in all 8 branches. As sap flow has

positively skewed distribution, the harmonic mean of sap

flow of 8 branches is used as a representative average. The

total plant water use was estimated from the harmonic

mean of the measured branches multiplied by the total

amount of branches (n = 179). The average of the sap flow

of the Salix bush during the measurement period is

3,186 cm3/h or 0.076 m3/d. The total water use was divi-

ded by the root distribution area, 50.24 m2, determined by

the root survey to obtain the transpiration rate (SF, mm/h).

The calculated transpiration rate and cumulative transpi-

ration rate are shown in Fig. 4. The cumulative transpira-

tion from May 29 to July 13 was 68 mm with an average of

1.5 mm/d, which is in accordance with previous monitor-

ing results (Ding et al. 2011).

Soil water content at various depths

The measurements of soil water contents are shown in

Fig. 5. During the dry period, the volumetric soil water

contents (SWC, cm3/cm3) decreased with time and were

increased with depth. The small amount of rainfall on June

only caused an increase of soil water content in the top

Fig. 2 Vertical root length density of the experimental S.

psammophila


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http://www.gvsig.org

layer. The reason is that the soil was too dry and strong

evaporation. The heavy rainfall started at 0:00 on July 1,

and the soil water contents increased after 2 h at a depth of

6 cm. The process of infiltration continued along with the

rainfall. The infiltration front reached 100 cm when the

rainfall stopped at 12:00 on July 2. The rapid movement of

soil water indicated that the vadose zone has good

infiltration ability. At the 140 cm depth, soil water content

remained high during the whole period, indicating that the

capillary height is very close to 140 cm depth.

Based on the variations of soil water contents and root

distribution, soil water contents can be grouped into three

layers. Layer 1 represents the soil water contents from 6 to

40 cm (SWC1). Layer 1 is very dry and responds to rain

Fig. 3 Measured climatic

variables at the research site

during the investigated period

Fig. 4 Diurnal fluctuations of

transpiration rate (SF, mm/h)

and cumulative transpiration

(SFc, mm) of S. psammophila at

the research site

Fig. 5 Precipitation, soil water

content (SWC) and depth to

water table (GWT) at the

research site during the research

period


123

very fast. Layer 2 represents soil water content from 70 to

120 cm. The average soil water content (SWC2) continu-

ously decreases until the rainfall event in July 1 and July 2.

The response to this heavy rain event in layer 2 is delayed

compared with layer 1. Layer 3 represents the soil water

content at 140 cm (SWC3). In addition, the soil water

content is very close to saturation and also responds to the

heavy rain with a time delay.

Water table fluctuations

The groundwater level continuously decreased until the

heavy rain on July 1 and July 2 (Fig. 5). The groundwater

level did not respond to the rain events in June and only

increased after the heavy rainfall in early July.

Analysis of sap flow in relation to climatic factors

The influence of climatic factors on the sap flow of S.

psammophila was analyzed for three distinct periods: per-

iod 1 was from May 29 to June 12, representing a long dry

period; period 2 lasted from June 30 to July 7, representing

a wet period, and period 3 was defined from July 8 to 13,

representing a relative dry period after a wet period.

Relationship between sap flow and climatic factors

The correlation coefficients between the hourly sap flow and

climatic factors for the three periods are shown in Table 2.

The sap flow is positively dependent on the net radiation,

temperature and wind speed, but negatively dependent on

the relative humidity in all of the three periods. It can further

be seen that the correlation coefficient between the sap flow

and net radiation is much higher than those of temperature,

relative humidity and wind speed.

Lag time between sap flow and climatic factors

Figure 6 shows the diurnal patterns of sap flow and climatic

factors. The diurnal fluctuation of sap flow closely follows

the net radiation in all three periods, and the peak of sap flow

occurs at the same time as the peak of net radiation. The

peak of temperature and the minimum relative humidity

occur some hours later than the sap flow peak.

A cross-correlation analysis was used to identify the time

lag between sap flow and climatic variables. There was no

time lag between sap flow and net radiation (Table 2).

However, the peak of the temperature and the minimum

value of relative humidity lagged behind the peak of sap flow

by 3 h in period 1, 1 h in period 2 and 2 h in period 3. The lag

time between the peak of sap flow and the peak of the wind

speed was 2 h in periods 1 and 3 and 1 h in period 2.

Regression analysis between sap flow and climatic

factors

Multiple linear regression analysis was applied for the

three periods. The multiple linear regression equation was

defined as follows:

Table 2 Correlation, cross-correlation coefficients, R, and lag time between sap flow and climatic factors

Correlation Cross-correlation

SF Rn T RH U SF Rn T RH U Lag

Period 1 (May 5 June 12) SF 1 1.00

Rn 0.87 1 0.87 1.00 0

T 0.56 0.41 1 0.83 0.74 1.00 3

RH -0.43 -0.31 -0.7 1 -0.70 -0.60 -0.71 1.00 3

U 0.56 0.5 0.61 -0.54 1 0.66 0.65 0.63 -0.56 1.00 2

Period 2 (June 30 July 7) SF 1 1.00

Rn 0.92 1 0.92 1.00 0

T 0.69 0.61 1 0.82 0.74 1.00 1

RH -0.66 -0.52 -0.64 1 -0.78 -0.67 -0.90 1.00 1

U 0.45 0.48 0.43 -0.4 1 0.44 0.50 0.40 -0.24 1.00 1

Period 3 (July 8 July 12) SF 1 1.00

Rn 0.92 1 0.93 1.00 0

T 0.64 0.56 1 0.77 0.70 1.00 2

RH -0.56 -0.5 -0.9 1 -0.72 -0.64 -0.80 1.00 2

U 0.44 0.46 0.61 -0.61 1 0.46 0.48 0.46 -0.65 1.00 2

SF sap flow, Rn net radiation, T temperature, RH relative humidity, U wind speed, lag Lag time (h)


123

SFðtÞ ¼ b0 þ b1RnðtÞ þ b2TðtÞ þ b3RH(tÞ þ b4UðtÞ ð1Þ

The derived regression coefficients are listed in Table 3

together with the coefficient of determination (R2). The

multiple linear regressions can simulate sap flow very well

as shown by the scatter plots (Fig. 7). The coefficients of

determination indicate that 81, 90 and 87 % of the varia-

tions in sap flow can be explained with the combined

variations of the net radiation, temperature, relative

humidity and wind speed in periods 1, 2 and 3, respectively.

The correlation coefficients between the hourly sap flow

of two clusters adjacent to S. psammophila and meteoro-

logical variables measured from May to September 2012 and

from July to August 2013 indicate that hourly sap flow is

significantly correlated with climatic variables. The average

Fig. 6 Diurnal patterns of sap

flow, SF, and different climatic

factors on a May 31 in period 1,

b July 4 in period 2 and c July

10 in period 3

Table 3 Regression

coefficients of Eq. 1 and

coefficients of determination for

the investigated periods

b0 b1 b2 b3 b4 R2

Period 1 -0.00569 0.00019 0.00219 -0.00013 0.00239 0.81

Period 2 0.03415 0.00021 0.00146 -0.00055 -0.00320 0.90

Period 3 -0.03849 0.00021 0.00344 0.00022 -0.00623 0.87

Fig. 7 Scatter plots of the

calculated sap flows based on

multiple linear regressions and

observed sap flows for the three

periods


123

correlation coefficient between the sap flow and net radiation,

air temperature, relative humidity and wind speed is 0.57,

0.48, -0.36 and 0.21, respectively, in 2012. In addition, the

average correlation coefficient between sap flow and net

radiation, air temperature, relative humidity and wind speed

is 0.88, 0.66, -0.63 and 0.42, respectively, in 2013. This

indicates that the net radiation affects the dynamics of sap

flow significantly in hourly scale, which is consistent with the

correlation analysis data collected in 2011.

Analysis of sap flow in relation to water sources

The wetness conditions of soil have a significant effect on

water use strategy of plants. To quantify the water source

of experimental S. psammophila, three periods were ana-

lyzed during the research period according to soil water

content (SWC) and the depth to groundwater table (GWT)

changes effected by rainfall input, i.e., the dry period

character as SWC decrease and GWT increase with no

rainfall input, the wet period character as SWC increase

and GWT decrease after rainfall input and the relative dry

period character as SWC decrease and GWT increase after

a few days of rainfall input.

Dry period

In this dry period, the soil water contents and water table

(Fig. 8) decrease with the increase of the cumulative

transpiration rate of S. psammophila. The correlation

coefficients between the cumulative transpiration rate (SFc)

and soil water contents in all layers display very strong

negative correlations (Table 4), whereas the correlation

between the cumulative transpiration rate and the depth to

groundwater table is positive. All this evidence supports

the conclusion that S. psammophila, which is the dominant

vegetation also in the site surrounding the environment,

uses soil water and groundwater for transpiration during

this dry period.

Fig. 8 Relation between

cumulative transpiration rate,

soil water contents at different

layers (SWCa), and depth to

groundwater table in dry period

Table 4 Correlation coefficients between cumulative transpiration

rate (SFc) and soil water contents at different depths and water table

depth in the three periods

SFc SWC1 SWC2 SWC3 GWT

Period 1 (May 5

June 12)

SFc 1

SWC1 -0.97 1

SWC2 -0.99 0.96 1

SWC3 -0.89 0.92 0.91 1

GWT 0.99 -0.96 -0.99 -0.9 1

Period 2 (June 30

July 7)

SFc 1

SWC1 -0.21 1

SWC2 0.76 0.17 1

SWC3 0.62 0.08 0.91 1

GWT -0.78 -0.13 -0.99 -0.93 1

Period 3 (July 8

July 12)

SFc 1

SWC1 -0.95 1

SWC2 -0.7 0.7 1

SWC3 -0.91 0.92 0.77 1

GWT 0.78 -0.73 -0.75 -0.75 1


123

As the transpiration of S. psammophila causes the

decrease of the groundwater level, a regression equation

between the depth to groundwater table (GWT) and

cumulative transpiration rate (SFc) in this period was as

follows:

GWT ¼ 160:36þ 0:3103SFc ð2Þ

The regression equation yields a coefficient of determi-

nation of 0.99, indicating that 99 % of the variation in the

depth to groundwater table can be explained by the variation

of the cumulative transpiration during this dry period.

Wet period

The heavy rainfall on July 1 and July 2 reversed the trend

of the decrease of soil water contents and water table depth.

The soil water contents and groundwater levels increased

(recharge event) despite the increase of the cumulative

transpiration of S. psammophila in wet period (Fig. 9).

Only soil water contents in the shallow soil layer 1

decreased after July 2. The cumulative transpiration rate

remained constant during July 1 and July 2 as there was

almost no transpiration during the rainy days. The corre-

lation coefficients between the cumulative transpiration

rate and soil water contents at the shallow soil layers are

negative but are positive at the middle and deep soil layers

(Table 4). All this evidence indicates that the amount of

rainfall infiltrated into the soil was sufficient to supply

water for transpiration, whereas excess water increased the

soil water contents and groundwater level. However,

immediately after the rainfall, evaporation and

transpiration caused the decrease of soil water content in

the shallow soil layer (SWC1).

Relative dry period

Figure 10 shows that soil water contents in all layers and

groundwater levels gradually decreased with the increase

of the cumulative transpiration rate 5 days after the rain-

fall. The correlation coefficients between the cumulative

transpiration rate and soil water contents in all layers are

negative, whereas a positive correlation can be seen with

depth to the groundwater table (Table 4).

The regression equation between the depth to water

table (GWT) and cumulative transpiration rate in the rel-

ative dry period was as follows:

GWT ¼ 146:976þ 0:2852SFc ð3Þ

The coefficient of determination is 0.78, indicating that

78 % of the variation in the depth to water table can be

explained by the variation of the cumulative transpiration.

Groundwater level changes are not only caused by the

transpiration of S. psammophila but also by delayed

groundwater recharge. Therefore, the decrease of the

groundwater level is slower than that predicted by tran-

spiration of S. psammophila.

Discussion

Previous study results indicated that sap flow of plants has

a relationship to climatic variables, such as radiation and


cumulative transpiration rate

and, soil water contents at

different layers (SWCa) and

depth to groundwater table

(GWT) in wet period


123

VPD in arid and semi-arid areas. The empirical relation-

ships found between the sap flow of plants and radiation

have been identified as hyperbolic (Guan et al. 2012),

quadratic (Ortuno et al. 2006) or other non-linear functions

(Oguntunde 2005). In their study, a threshold value of

radiation was detected. Sap flow increases rapidly with the

increase of radiation below the threshold value but remains

at a constant level beyond the threshold. However, in our

results, the relationship between the sap flow and net

radiation was found linear in all investigated dry and wet

periods. Many researchers (e.g., Hogg and Hurdle 1997;

Liu et al. 2011) combined the temperature and relative

humidity into VPD to analyze the water demand of the

atmosphere and the ability of plants to satisfy this demand.

They also found a threshold of VPD in relation to the

increase of sap flow. This threshold value was not found in

our study. The multiple linear regression equations deter-

mined between hourly sap flow and climatic factors were

established with very high coefficients of determination

and represent the combined effects of climatic factors on

the control of sap flow (Eq. 1; Fig. 7).

Rainfall pulses can rapidly increase sap velocity and

alter the water-use efficiency of vegetation (Schwinning

and Sala 2004; Zeppel et al. 2008). In our results, sap flow

after rainfall was larger than that before rainfall for all

rainfall events (Fig. 4) regardless of the increase of soil

water contents triggered by different amounts of rainfall

(Fig. 5). The ratio between the first day before a rainfall

event to the time when maximum sap flow occurred after

the rainfall did not increase linearly with the amount of the

rainfall. For example, the sap flow after rainfall of 13.8 mm

(on Jun 13) was lower than that 2.4 mm (on Jun 22). This

finding agrees with other desert studies (e.g., Reynolds

et al. 2004; Zhao and Liu 2010). Under the strong evapo-

ration in the semi-arid climates and given the low water-

holding capacity of the soil at our experimental site (sandy

soil), we would expect S. psammophila to be subjected to

water stress in the experimental period. However, an

exception is that sap flow of the June 7 in the dry period

was larger than that after small rain events (except for July

1–2 rainfall events). This phenomenon indicated that even

during the first dry period S. psammophila did not experi-

ence obvious water stress.

The correlation and regression analyses indicate that the

experimental S. psammophila depends on groundwater for

transpiration. The role of groundwater supply in the

maintenance of the transpiration during drought for dif-

ferent plants has been reported elsewhere, through different

approaches (Miller et al. 2010; Chen et al. 2013). To sus-

tain high transpiration rates given the low soil water con-

tents in the upper soil (Fig. 5), S. psammophila needs to tap

groundwater. Therefore, the root distribution has strong

influence on the water use strategy (Canadell et al. 1996).

At our site, the root distribution was characterized as

dimorphic root system (Fig. 2). Shallow-rooted plants will

depend more dynamically on shallow moisture, whereas

deep-rooted plants are more coupled to deep soil water.

This finding is also in line with the water sources of

S. psammophila as evaluated by model and isotope meth-

ods on the Ordos Plateau (Chen et al. 2011; Wang et al.


cumulative transpiration rate,

soil water contents at different

layers (SWCa), and depth to

groundwater table (GWT) in

relative dry period


123

2013). Therefore, S. psammophila can be classified as

groundwater-dependent vegetation in this semi-arid envi-

ronment, which has effects on groundwater recharge and

discharge and consequences on groundwater resources

management. The examined S. psammophila used on

average 3,186 cm3/h or 0.076 m3/d of water in the growing

period from May 29 to July 13. Ding et al. (2011), using a

portable LI-1600 stoma meter, found monthly water con-

sumption values in the same order of magnitude of indi-

vidual plants such as S. psammophila, A. ordosica, A.

ordosica and Caragana intermedia. Chen et al. (2013)

found that S. psammophila consumes more water than A.

ordosica based on diurnal water table fluctuation. More-

over, Zhou et al. (2013) confirmed that S. psammophila

consumes more water than Salix matsudana at the Hailiutu

river catchment. Therefore, S. psammophila might not be a

suitable species for stabilizing sand dunes in water-scarce

environments.

Conclusions

Systematic field measurements from May 29 to July 13

identified the relationships between sap flow of S. psam-

mophila, climatic factors and water sources in the semi-arid

Hailiutu River catchment in northwest China. The diurnal

fluctuations of sap flow of S. psammophila were mainly

driven by net radiation, but sap flow also depends on the

temperature, relative humidity and wind speed. The daily

peak of sap flow occurs at the same time as that of net

radiation but a few hours ahead of the peak of temperature

and the minimum of relative humidity. The sap flow of S.

psammophila increased almost linearly with net radiation,

temperature and wind speed but the decrease of relative

humidity. Multiple regression of sap flow of S. psammo-

phila and climatic variables can account for more than

80 % of the variations in diurnal fluctuations of sap flow.

The studied S. psammophila can use both soil water and

groundwater for transpiration. The cumulative transpiration

of S. psammophila caused the continuous decline of soil

water contents at all depths and groundwater levels during

the dry period from May 29 to June 12. The heavy rainfall

on July 1 and 2 provided sufficient water to restore the

depleted soil water and groundwater while maintaining

water supply for transpiration. The peak of sap flow after

the rain increased significantly on July 3, 4 and 5. In the

relative dry period, after the rain from July 8 to 12, tran-

spiration of S. psammophila caused a decrease of soil water

contents and groundwater level but at a slower rate.

Regression and correlation analyses demonstrate a strong

relation between sap flow and groundwater levels. There-

fore, S. psammophila can be considered as groundwater-

dependent plant in the semi-arid Hailiutu River catchment.

Finally, comparing these results to other drought-resis-

tant plants used to combat desertification (such as A.

ordosica), it is remarkable that S. psammophila appears to

consume much water for transpiration. Therefore, S.

psammophila might not be a water-efficient plant for

desertification prevention in water-scarce environments.

Further investigations are required to compare water uses

of dominant plants in the area.

Acknowledgments This study was supported by the Dutch gov-

ernment’s Asia Facility for China project ‘‘Partnership for research

and education in water and ecosystem interactions,’’ the Honor Power

Foundation, the project ‘‘Study on the transpiration dynamics of

phreatophytic vegetation in response to groundwater fluctuation in the

semiarid area (41102160)’’ by the National Natural Science Foun-

dation of China, the project ‘‘The construction of a scientific platform

for the investigation of the relationship between water and vegetation

in the Ordos Basin (201311076)’’ by the Ministry of Land and

Resources of the People’s Republic of China and Shaanxi Science and

Technology Research and Development Program (2011KJXX56). We

would like to thank the three anonymous reviewers for their helpful

comments and suggestions that greatly improved the earlier version of

the manuscript.

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Climatic controls on sap flow dynamics and used water sources of Salix psammophila in a semi-arid environment in northwest China

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