Botanical Studies (2008) 49: 351-361.
*
Corresponding author: E-mail: anshq@nju.edu.cn
(S.Q. An), alex_xuzhen@163.com (Z. Xu); Tel: +86 25
83594560; Fax: +86 25 83594560.
INTRODUCTION
Evapotranspiration (ET) is one of the important climatic
factors controlling energy and mass exchange between
terrestrial ecosystems and the atmosphere (Chen et al.,
2006) and plays a specially important role in semiarid
landscapes (Huxman et al., 2005). During the growing
season in many arid and semiarid environments, energy
and mass fluxes are temporally and spatially heterogeneous
due to monsoonal precipitation which is episodic and
localized (Yepez et al., 2003; Williams et al., 2004).
Hereinto, ET usually accounts for 90% of precipitation
inputs in these ecosystems (Wilcox et al., 2003), and shifts
precipitation inputs rapidly in mass and energy cycles
between ecosystem and ambient components (Yepez et al.,
2003). The change of ET, respectively its two components
during the dynamic wetting and drying cycles (Ehleringer
et al., 1991, 1999; Jackson et al., 1998; Yepez et al., 2003),
provide a detailed insight how biotic and abiotic factors
change vegetation and eco-hydrological processes, such
as ecosystem productivity (Huxman et al., 2005; Yepez et
al., 2005) and vegetation influences on water and energy
exchange (Moreira et al., 1997; Wang and Yakir, 2000;
Yakir and Sternberg, 2000).
Effective methods, such as lysimetric method (Wangati
and Blackie, 1971), sap flow measurement techniques
(Jackson et al., 2000), models and remote sensing (Kairu,
1991; Nagler et al., 2007), and micrometeorological
techniques (Lenschow, 1995; Moncrieff et al., 2000), are
used to measure or estimate ET. But, there are several
limitations in using these methods: lysimetric data are
point data and cannot be used for verifying regional
ET estimates (Kairu, 1991); the application of sap flow
is limited to individual plants, particularly large trees
(Kairu, 1991; Ehleringer and Field, 1993); models and
remote sensing approaches need a lot of soil, plant and
atmospheric input data and field validation to refine at
appropriate scales (Kairu, 1991); and micrometeorological
methods are unable to distinguish different components of
ET (Wang and Yakir, 2000). With these limitations, spatial
Partitioning evapotranspiration flux components
in a subalpine shrubland based on stable isotopic
measurements
Zhen XU
1,2
, Haibo YANG
1
, Fude LIU
1,
*, Shuqing AN
1,
*, Jun CUI
1
, Zhongsheng WANG
1
, and
Shirong LIU
3
1
Laboratory of Forest Ecology and Global Changes, School of Life Science, Nanjing University, Nanjing 210093, China
2
State Power Environmental Protection Research Institute, Nanjing 210031, China
3
The Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, China
(Received June 1, 2007; Accepted May 27, 2008)
ABSTRACT.
Soil, vegetation and atmospheric water vapor (0.1~3 m above ground) were sampled in a
subalpine shrubland covered with Quercus aquifolioides during three days in the early monsoon period in
Wolong Nature Reserve, China. In June 2006, the average LAI of Q. aquifolioides was 2.05 m
2
m
-2
and the
average community coverage was more than 90%. Isotope turbulent mixing relationships, isotopic values
of transpired water from plants and that of evaporating water vapor from soil surface were used to estimate
fractions of transpiration and evaporation contributing to the total evapotranspiration (ET). The method
worked well for £_D, but it was imprecise for £_
18
O because the minute isotopic differences between transpired
water and soil water. The results from £_D showed that fractional contributions of plant transpiration to ET
were 74.5¡Ó9.9%, 65.6¡Ó8.3% and 96.9¡Ó2.0% on 21st, 24th and 25th June, 2006, respectively, implying that ET
is mostly generated by plant transpiration. Notably, the transpiration from herbage layer for ET was likewise
important as that from shrub layer. Our approach is useful for partitioning ET in semiarid subalpine shrubland
at an ecosystem scale on short time steps. This approach improves the understanding of water exchange in
semiarid ecosystems, and offers an opportunity to measure and validate component fluxes with accurate spatial
representation at a common scale.
Keywords: Evapotranspiration; Flux partitioning; Quercus aquifolioides; Semiarid shrubland; Stable isotopes.
PhySIOlOgy
pg_0002
352
Botanical Studies, Vol. 49, 2008
representation is especially difficult to overcome (Wilson
and Meyers, 2001).
Stable isotopic tracer methods offer a new opportunity
to study the components of ET at the field-scale, from
the leaf level to ecosystem (Kao, 1997; Kao and Chang,
1998; Wang and Yakir, 1995; Harwood et al., 1998; Kao
et al., 2000; Wang and Yeh, 2003; Kao et al., 2002), and
can partition the ET from different compartments of the
ecosystem incorporating measurement of water vapor
(Gat, 1996; Yakir and Wang, 1996; Brunel et al., 1997;
Moreira et al., 1997; Wang and Yakir, 2000; Yepez et al.,
2003, 2005; Williams et al., 2004). The basis for using
stable isotopes to study ET is the differences between
soil evaporation and leaf transpiration; and ET, the sum
of evaporation and transpiration, also has a different
isotopic characteristic than with the background moisture
in most cases (Gat and Matsui, 1991; Wang and Yakir,
2000). These isotopic distinctions and the concentration of
moisture around vegetation can be used to estimating ET
by generating Keeling plots, the isotope turbulent mixing
relationships (Williams et al., 2004). Then, the contributing
fractions of evaporation and transpiration to total ET, net
ecosystem discrimination and soil disequilibrium effects,
can be identified (Yakir and Sternberg, 2000; Yepez et
al., 2003, 2005). Combinations of stable isotope analyses
and other measurements, such as sap flow (Cramer et al.,
1999; Williams et al., 2004) and micrometeorological
methods (Yepez et al., 2003; Griffis et al., 2004), also
offer unique information about ecosystem functioning
and dynamics (Cable and Huxman, 2004). Using stable
isotopes to estimate ET flux components is now a widely
used approach in terrestrial ecosystem studies (Helliker et
al., 2002). However, this method requires high precision,
in isotopic sampling and analysis which is currently at the
limit of detection (Yakir and Sternberg, 2000).
In this study, we partition ET from a semiarid subalpine
shrubland in Wolong Nature Reserve, Western China,
with the use of stable isotopes. We generated Keeling
plots with data from five layers inside the vegetation to
assess ecosystem isotopic flux. We attempt to determine
the relative contributions of soil evaporation, shrub
and herbage transpiration to ecosystem-scale ET in the
semiarid subalpine shrubland.
MATERIAlS AND METhODS
Study site
The study site was located on Balang Mountain in
Wolong Nature Reserve, China (30¢X51.437¡¦ N, 102¢X
58.308¡¦ E, altitude 2,743 m). Around the study site, there
is a relatively flat terrain (slope < 5¢X, extent about 200 m
¡Ñ 800 m). Vegetation at the site was a subalpine shrubland
composed of alpine oak (Quercus aquifolioides Rehd. Et
Wils.), which dominates the slope facing south of Balang
Mountain between 2,700 m and 3,600 m altitude. The
vegetation composed of alpine oak has obviously xeric
characteristics (WNRAB, 1987). Under the canopy, the
dominant herbage species is Cystopteris montana (Lam.)
Bernh. ex Desv., which belongs to geophytes. Diameter at
base height (DbH) and height of Q. aguifolioides varied
from 2 cm to 5 cm and from 1.1 m to 2.5 m, respectively;
the height of herbage varied from 0.01 m to 0.5 m. In June
2006, the average LAI of Q. aquifolioides was 2.05 m
2
m
-2
(LAI-2000, Li-Cor Inc., Lincoln, NE, USA). The average
community coverage was more than 90% in the June.
The soil type is similar to Cambisols and the soil depth is
generally about 50 cm (WNRAB, 1987; Liu et al., 2006).
East Asian Monsoon and Indian Monsoon can
influence Wolong from April to October. But on Balang
Mountain, the influence of plateau climate is stronger.
Hence, this region is dry and cold, and the diurnal range
in temperature is large (WNRAB, 1987). Mean annual
precipitation amount is 710 mm, and about 62¡Ó7% of
precipitation occurs from July to September; mean annual
evaporation amount, mean annual temperature and mean
annual relative humidity are about 800 mm, 3¢XC and 79%,
respectively (Zheng et al., 2006). Precipitation in this
region has a high spatial and temporal variability except
for July, August and September.
There is a meteorological observation field of Wolong
Ecological Station in the east of study site, about 150 m
away. In this field, precipitation amount was recorded
by CR2-06 pluviometer (Songtao digital technology,
Chengdu, China) with an automatic recorder. Evaporation
outside vegetation was measured every day by evaporation
pan. Wind speeds at 10 m were measured every hour by
a ZL wind velocity indicator (Shanghai Meteorological
Instrument Factory Co., Ltd., Shanghai, China).
At the study site, actual evaporation from vegetation
was estimated every day by another evaporation pan
inside vegetation. Wind speeds at 3 m were measured
by DEM6 cup anemometer with wind vane (Tianjin
Meteorological Instrument Factory, Tianjin, China).
Photon flux density (PFD), transpiration rate of alpine oak
leaves, and atmospheric pressure were recorded every 15
minutes using a LI-190 Quantum Sensor in combination
with a standard chamber of a LI 6400 gas analyzer (Li-Cor
Inc., Lincoln, NE, USA). Soil temperature (5 cm depth)
was measured by angle pipe geothermometers at three
randomly chosen locations. Air temperature and relative
humidity were measured every 5 minutes by PT1000
and HIH3610 probes of HT501-II (Hartech Co., Ltd.,
Hangzhou, China), respectively. Vapor pressure deficit
(VPD) and water vapor concentration were calculated
from data recorded by HT501-II.
Theory of flux partitioning
Naturally, there are several kinds of stable isotopes
in water molecules:
2
H (D),
1
H,
18
O,
17
O an d
16
O .
Thereof, D,
1
H,
18
O and
16
O were measured in this study.
Concentrations of these isotopes are expressed as deviation
from an international standard (V-SMOW) and using £_
notation in per mil (.):
£_ (.) = [(R
s
/R
st
) - 1] ¡Ñ 1000
(1)
pg_0003
XU et al. ¡X Partitioning ET flux components in a subalpine shrubland
353
where R
s
and R
st
are the molar ratio of the heavy to light
isotopes in the sample and the standard, respectively.
Soil water and leaf water are the sources of
evapotranspiration. In the processes of transfer, isotopic
composition of water is modified due to the fractionations
of equilibrium isotope effects, kinetic and vital effects, or
transport effects (Gat, 1996). Craig and Gordon (1965)
described a model to calculate the isotopic ratios of
evaporating water vapor from soil surface (£_
E
) as:
£_
E
= [£\
*
£_
L
- h£_
ab
- £`
*
- (1 - h) £`
k
]/[ (1 - h) + (1 - h)
£`
k
/1000]
(2)
where £_
L
is the isotopic composition of liquid water at
the evaporating surface; £_
ab
is the isotopic composition
of the background atmospheric water vapor; £\
*
is the
temperature-dependent equilibrium fractionation factor
for
18
O o r D; £`
*
= (1 - £\
*
) ¡Ñ 1000.; £`
k
is the kinetic
fractionation factor, about 18.9. for oxygen and 17.0.
for hydrogen in a turbulent boundary layer (Wang and
Yakir, 2000); h is the relative humidity (range 0 to 1)
normalized to the temperature of the soil surface.
In this paper, £\
*
< 1 and £\
*
= 1/£\
+
(Gat, 1996); and £\
+
can be calculated by the equation provided by Majoube
(1971):
18
O£\
+
= [1.137(10
6
/T
2
) - 0.4156(10
3
/T) - 2.0667]/1000
+ 1
(3)
D£\
+
= [24.844(10
6
/T
2
) - 76.248(10
3
/T) + 52.612]/1000
+ 1
(4)
where T is soil temperature recorded at 5 cm depth in
degrees Kelvin.
From Eq. 2, we see that the evaporating water vapor
from soil surface is more depleted in both
18
O and D with
respect to liquid water (i.e. £_
E
<£_
L
). It also shows that this
depletion is a function of isotopic compositions of liquid
water at the evaporating surface and the atmospheric
vapor, the relative humidity, and fractionations associated
with the diffusivity of water molecules across the boundary
layer (Yakir and Sternberg, 2000). Many experimental
results for soil water undergoing evaporation have shown
that the predictions, which were generated by Craig-
Gordon model, are reliable (Walker and Brunel, 1990;
Mathieu and Bariac, 1996a, 1996b).
If isotopic steady state (ISS) of plants lasts sufficiently
long and the isotopic enrichment by leaves can be ignored,
the transpiration water from leaves will have the same
isotopic signature as the source water used by plants
(Dawson, 1993; Moreira et al., 1997), which means we
can use the isotopic compositions of water from stem or
xylem or sap to replace that of transpiration water from
leaves (Yakir and Sternberg, 2000). Flanagan et al. (1991),
Wang and Yakir (1995) reported that broadleaved species
would gradual approach to ISS within 1~3 h after drastic
changes in ambient conditions in the laboratory. Notably,
because the time required to approach ISS is dependent on
the humidity surrounding the leaf and the turnover time of
leaf water (Wang and Yakir, 1995), isotopic compositions
of transpiration vapor and stem water will not always be
the same during observation periods. This limitation could
be overcome by averaging all the values measured over
the long-term (Flanagan et al., 1991; Wang and Yakir,
1995; Harwood et al., 1999). In this paper, we assumed
the transpiring vegetation to be under isotopic steady state,
and then isotopic values of transpiration vapor (£_
T
) were
determined by analyzing isotopic values of stem water
(Moreira et al., 1997; Wang and Yakir, 2000).
To partition the total evapotranspiration flux, we also
need to know the isotopic compositions of ET (£_
ET
). In
the ecosystem, the combination of some background
amount of a substance and some amount of the substance
from sources is usually considered as the atmospheric
concentration of this substance (Yakir and Sternberg,
2000). Hence, based on mass balance, £_
ET
can be
estimated by the Keeling plots (isotope turbulent mixing
relationships) described by Keeling (1961):
£_
ebl
= C
a
(£_
a
- £_
ET
)/C
ebl
+ £_
ET
(5)
where £_
ebl
is the isotopic signature of the substance in the
ecosystem which can be collected from the ecosystem
boundary layer; C
a
, C
ebl
are the concentrations of the
substance in the atmosphere and the ecosystem boundary
layer, respectively; £_
a
is the isotopic signature of the
substance in the atmosphere.
This is a linear relationship, and when used with water
vapor the y-intercept reflects the isotopic values of ET
(Moreira et al., 1997; Yepez et al., 2003). When using
this model to estimate £_
ET
, Yakir and Sternberg (2000)
suggested two assumptions: (1) there is no loss of water
vapor from the ecosystem excluding turbulent mixing with
the atmosphere; (2) sources of the combined atmospheric
water vapor come from evaporated and transpired water
vapor and the background atmospheric vapor. Though
these assumptions would not be met perfectly in field
works, Gat (1996) and Wang and Yakir (2000) considered
that isotopic variations caused by these imperfect
conditions would not generate significant influence on
partitioning ET.
The fractional contributions by transpiration and
evaporation to ET are calculated by software Isoerror
(version 1.04; Phillips and Gregg, 2001) and IsoSource
(version 1.3.1; Phillips and Gregg, 2003) provided by
Health and Environmental Effects Research Laboratory
of US Environmental Protection Agency (http://www.epa.
gov/wed/pages/models.htm). When there are only two
sources (e.g. combined shrub and herbage transpiration
as one source), Isoerror calculates the mean and the
standard error of the fractional contributions based on the
uncertainty generated by the variability of both sources
and the intercept in a linear regression on Keeling plots
(Phillips and Gregg, 2001). When there is three or more
sources (e.g. if shrub and herbage transpiration are
independent processes), IsoSource examines all possible
combinations of each source contribution (0~100%) in
small increments, then all feasible solutions are statistic to
generate the mean and the standard error of the fractional
contributions (Phillips and Gregg, 2003).
pg_0004
354
Botanical Studies, Vol. 49, 2008
The sampling heights of water vapor have significant
influence on the spatial resolution of Keeling plots
(Flanagan and Ehleringer, 1998; Dawson et al., 2002;
Yepez et al., 2003). Therefore, Wang and Yakir (2000),
Yepez et al. (2003) suggested collecting samples in
collection profiles from lower to upper heights to improve
spatial representation.
Soil water, plant water and vapor collection
In the growing season, samples were collected before
the period of precipitation sets in frequent are heavy (21st,
24th, and 25th June, 2006). Using a hand-auger, soil was
sampled from the surface to 10 cm. Sampled branches of
alpine oak were 0.5~1.0 cm in diameter, 2~3 cm in length
and from each of them the bark was removed. Stems of
C. montana were collected from the basal portions. Every
plant sample was composed of 2~3 stems from different
individuals. The sampling period on each day was from
10:00 to 12:00 h and from 13:00 to 15:00 h; and in every
hour three soil samples, three branch samples of alpine oak
and three stem samples of C. montana were collected. Soil
and plant samples were placed into screw-cap glass vials
(5 ml) and sealed with Parafilm, then stored at about 2¢XC.
Soil and plant water was extracted by cryogenic vacuum
distillation (Ehleringer et al., 2000).
The average isotopic signature of water from soil cores
(£_
S
) was used for £_
L
in Eq. 2. Soil temperature (T, 0.05
m depth) was recorded at the same time soil cores were
collected. Because £_
T
was the isotopic combination of
shrub transpiration (£_
Ts
) and herbage transpiration (£_
Th
),
it could be determined by the weighted average of the
isotopic value of bulk vegetation (Yepez et al., 2003).
Based on the average canopy cover of the alpine oak of
about 60% in the June, the weighted value was calculated
as £_
T
= 0.6 £_
Ts
+ 0.4 £_
Th
, assuming equal transpiration rates
per unit leaf area of both functional groups.
Water vapor was collected from 5 heights at a time
(0.1 m, 0.5 m, 1.5 m, 2.0 m and 3.0 m). During the
collection period mentioned above, sampling was started
at 10:00, 11:00, 13:00 and 14:00 h. For each group vapor
was collected during 30 min with a flow rate of 250 ml
min
-1
using a PM850.5 vacuum pump (voltage 6 V, Ruiyi
Mechanical Design Center, Chengdu, China). The air was
circulated through a set of 45 cm long glass traps (modified
from Helliker et al., 2002) which were immersed in a
mixture of ethanol and liquid nitrogen (about -80¢XC).
Traps were made of 9 mm (o.d.) Pyrex glass attached to
6~9 mm (o.d.) Cajon Ultra-Torr adapters which framed
in 9 mm (o.d.) Swagelok Union Tee. After sampling the
traps were sealed with Parafilm and stored at about 2¢XC.
Samples were transferred to 9 mm (o.d.) Pyrex tubes by
cryogenic vacuum distillation (Ehleringer et al., 2000).
Near the vapor sampling inlets, probes of HT501-
II recorded air temperature (T
a
, in Kelvin) and relative
humidity (h, as a fraction between o and 1) every 5 min.
Using T
a
, h and atmospheric pressure (P
a
, in hPa), water
vapor concentration was calculated by (McRae, 1980):
[H
2
O] (mmol mol
-1
) = 10h[P
A
exp(13.3185t - 1.9760t
2
-
0.6445t
3
- 0.1299t
4
)]/P
a
(6)
where P
A
is standard atmosphere pressure (about 1013.25
hPa) and t = 1 - (373.15/T
a
).
Using the inverse of average vapor concentration
(1/[H
2
O]) during sampling period of each height as
independent variables, and isotopic values of water
vapor (£_
18
O or £_D) collected at the corresponding height
as dependent variables, Keeling plots were generated.
The isotopic ratio of ET (£_
ET
) was obtained from the
intercept of the Keeling plots (described in Eq. 6). Using
£_
T
(including £_
Ts
and £_
Th
) and £_
S
as sources, the fractional
contributions of transpiration and evaporation to ET (£_
ET
)
were calculated by Isoerror and IsoSource.
Stable isotope and data analysis
The stable isotope ratio analysis for
18
O was processed
as follows: at a constant temperature of 25¢XC, water
samples are mixed with 0.1% CO
2
for isotopic equilibrium
over 18 h, and then the gas mixture was measured by mass
spectrometry. This automated continuous flow analysis
was performed by Gas Bench II and MAT-253 (Finnigan
MAT, Bremen, Germany).
To measure isotopic concentrations of D, water samples
were dropped into the reaction furnace with carbon column
(> 1400¢XC), and then the produced gas was separated and
H
2
was transferred to mass spectrometer using helium as
carrier gas. This automated continuous flow analysis was
performed by TC/EA (Finnigan MAT, Bremen, Germany)
and MAT-253.
The water samples were isotopically analyzed at Key
Laboratory of Nuclear Resources and Environment,
Ministry of Education, East China Institute of Technology.
The standard deviation for repeated analysis of laboratory
standards with above methods was . 0.2. for
18
O and .
2. for D. In this study, the sample analyses for
18
O and D
were repeated five times independently.
In this paper, the parameters were calculated by SPSS
(Statistical Package for the Social Sciences, Version 13.0)
using standard type I linear regressions. Keeling plots
were pictured by Excel 2007.
RESUlTS
Environmental conditions during sampling days
The last precipitation event occurred six days before the
first sampling day (21st June). And after two precipitation
events of 19.2 mm and 5.3 mm, the second and third
sampling days (24th and 25th June) were chosen (Figure
1). Amount of outside and inside vegetation evaporation
pan were more than 2.1 mm d
-1
and 0.9 mm d
-1
,
respectively (Figure 1).
Influenced by cloud on 21st June, the PFD was no more
than 1000 £gmol m
-2
s
-1
. 24th and 25th June were sunny
and in most of sampling time the PFD was close to 2000
£gmol m
-2
s
-1
(Figure 2). The average VPD from 9:00 to
pg_0005
XU et al. ¡X Partitioning ET flux components in a subalpine shrubland
355
16:00 hours in these three days were 1.40¡Ó0.69 kPa, 1.23
¡Ó0.52 kPa and 1.21¡Ó0.28 kPa, respectively, and the peak
of VPD on each day was occurring between 9:00 and
10:00 h (Figure 2). From 9:00 to 16:00 h, wind speeds
at 10 m ranged from 0.2 to 10.0 m s
-1
and the daily peak
wind speed was occurred around 14:00 h (Figure 2); wind
speeds at 3 m respectively were 0.97¡Ó0.51 m s
-1
, 0.41¡Ó0.18
m s
-1
and 0.83¡Ó0.20 m s
-1
in the three days (Figure 2). The
transpiration rates of alpine oak respectively were 0.50¡Ó
0.17 mmol m
-2
s
-1
, 0.93¡Ó0.50 mmol m
-2
s
-1
and 1.61¡Ó0.53
mmol m
-2
s
-1
on the three days (Figure 2).
Isotopic ratios of evaporation and transpiration
water
Isotopic compositions of soil water (£_
S
) ranged from
-8.5. to -4.3. for £_
18
O, and from -71.9. to -20.1. for
£_D, respectively. Isotopic ratios of stem water from Q.
aquifolioides (£_
Ts
) ranged from -9.3. to -4.7. for £_
18
O,
and from -66.7. to -22.0. for £_D, respectively. Isotopic
ratios of stem water from C. montana (£_
Th
) ranged from
-8.0. to -4.4. for £_
18
O, and from -57.5. to -11.6.
for £_D, respectively. Isotopic compositions of vapor (£_
a
)
ranged from -15.9. to -6.4. for £_
18
O, and from -102.4.
to -55.4. for £_ D, respectively. These results indicated
that isotopic values of evaporating water vapor from
soil surface (£_
E
, soil evaporation, Table 1) were more
isotopically depleted relative to vapor generated by plant
transpiration (£_
T
, Table 2) during the three sampling days.
Figure 1. Daily precipitation, pan evaporation outside and inside
vegetation from 15th to 30th June, 2006.
Figure 2. The environmental conditions of sampling days (21st,
24th and 25th June, 2006). (a) Photon flux density (£gmol m
-2
s
-1
)
from 9:00 to 16:00 h; (b) wind speeds (m s
-1
) recorded at 3 m
and 10 m from 9:00 to 16:00 h; (c) vapor pressure deficit (VPD,
kPa) from 9:00 to 16:00 h; (d) transpiration rate of alpine oak
(mmol m
-2
s
-1
) from 10:00 to 15:00 h.
Table 1. Parameters used to estimate the isotopic values of evaporating water vapor from soil surface (£_
E
) with Craig-Gordon model
(Craig and Gordon, 1965). Soil temperature (T) was the average of three randomly chosen locations. Relative humidity (h) was
the average at 0.1 m height. The average isotopic values of water at the soil surface (£_s, 0~10 cm) was used for the isotopic value
of liquid water at the evaporating surface (£_
L
). £_
ab
was the average value for vapor collected at 0.1m above the ground. £\* is the
temperature-dependent equilibrium fractionation factor; £`* = (1 - £\*) ¡Ñ 1000.; £`
k
is the kinetic fractionation factor for molecular
diffusion.
Date
T (¡ÓSD, K) h (¡ÓSD, %)
£_
S
(¡ÓSD, .) £_
ab
(¡ÓSD, .) £\* £`* (.) £`
k
(.) £_
E
(¡ÓSD, .)
21st June Morning 289.60¡Ó0.07 40.8¡Ó4.2 £_
18
O -7.0¡Ó1.0 -11.4¡Ó0.6 0.990045 10.0 18.9 -38.9¡Ó1.0
£_D -33.2¡Ó8.0 -71.7¡Ó2.8 0.921191 78.8 17.0 -150.0¡Ó6.0
Afternoon
289.30¡Ó0.07 53.7¡Ó2.9 £_
18
O -7.3¡Ó0.7 -11.5¡Ó1.8 0.990019 10.0 18.9 -41.9¡Ó2.6
£_D -33.5¡Ó12.9 -77.5¡Ó0.4 0.920901 79.1 17.0 -161.9¡Ó4.9
24th June Morning 287.32¡Ó0.96 46.4¡Ó25.2 £_
18
O -7.1¡Ó0.5 -14.3¡Ó1.1 0.989844 10.2 18.9 -39.8¡Ó6.0
£_D -25.5¡Ó2.7 -89.9¡Ó6.7 0.918960 81.0 17.0 -140.8¡Ó23.2
Afternoon
292.87¡Ó0.18 35.3¡Ó3.4 £_
18
O -7.1¡Ó0.6 -10.6¡Ó0.8 0.990324 9.7 18.9 -38.1¡Ó0.8
£_D -29.8¡Ó6.1 -75.2¡Ó1.7 0.924289 75.7 17.0 -133.1¡Ó5.1
25th June Morning 286.03¡Ó0.69 37.6¡Ó8.9 £_
18
O -6.4¡Ó1.2 -12.5¡Ó1.0 0.989729 10.3 18.9 -37.3¡Ó1.2
£_D -45.7¡Ó8.9 -77.1¡Ó2.6 0.917679 82.3 17.0 -167.1¡Ó2.7
Afternoon
293.07¡Ó0.66 26.9¡Ó3.0 £_
18
O -7.0¡Ó0.9 -10.2¡Ó2.4 0.990341 9.7 18.9 -37.1¡Ó1.1
£_D -60.3¡Ó11.0 -59.8¡Ó6.2 0.924474 75.5 17.0 -169.8¡Ó10.2
pg_0006
356
Botanical Studies, Vol. 49, 2008
Keeling plot analyses and fractional
contributions of sources
Significant regression lines were found in Keeling
plots pictured by £_D, but those plotted by £_
18
O were not
significant (significance level 0.05, Table 3). Table 3 also
shows the slope and intercept of Keeling plots. Only
significant regression lines are shown in Figure 3. All
intercepts of Keeling plots were close to symbols which
reflected isotopic values of plant transpiration relative to
that reflected isotopic compositions of soil evaporation.
This meant plant transpiration contributes more to ET than
soil evaporation.
Considering shrub and herbage transpiration as one
source and soil evaporation as another one, the fractional
contributions of plant transpiration to total ET (T/ET)
were 96.9¡Ó1.6%, 97.7¡Ó2.0% and 95.2¡Ó1.3% for £_
18
O, 74.5
¡Ó9.9%, 65.6¡Ó8.3% and 96.9¡Ó2.0% for £_D on 21st, 24th
and 25th June, respectively. The estimated contributions
of soil evaporation given by £_
18
O were obviously less
than that obtained by £_D on 21st and 24th June (Figure 4).
Because Keeling plots for £_
18
O were not significant, £_
ET
for
18
O might be imprecise. Consequently, we partitioned
ET flux into different ecosystem components using £_ D
results. Fractional contributions of transpiration from
Q. aquifolioides and C. montana were also calculated
independently (Figure 4). Results showed that the fractions
of shrub and herbage transpiration were similar, and it
Table 3. Slope and intercept of the regression lines between £_
18
O or £_D values of water vapor collected at different heights
(0.1~3 m above ground) and the inverse of the corresponding vapor concentration. The intercept indicates the isotopic values of
evapotranspiration (£_
ET
). C.I. = confidence interval.
Data
Keeling plots
C.I. (95%) for intercept
Slope (¡ÓSD) Intercept (¡ÓSD) R
2
P
n
Lower Upper
21st June £_
18
O
-50.68¡Ó33.50 -7.71¡Ó1.96 0.139 0.159
16
-11.95
-3.56
£_D
-336.70¡Ó56.47 -58.20¡Ó3.30 0.718 0.00004* 16
-65.29
-51.14
24th June £_
18
O
-71.01¡Ó34.33 -8.37¡Ó2.40 0.211 0.055
18
-13.46
-3.28
£_D -322.50¡Ó128.80 -64.72¡Ó9.01 0.281 0.023*
18
-83.83
-45.63
25th June £_
18
O
-40.66¡Ó26.37 -8.64¡Ó1.58 0.130 0.146
18
-12.01
-5.32
£_D -309.60¡Ó101.78 -53.06¡Ó6.08 0.366 0.008*
18
-65.98
-40.18
*The significance level is 0.05.
Table 2. Average isotopic values of stem water from Quercus
aquifolioides (£_
Ts
) and from Cystopteris montana (£_
Th
), and the
weighted average isotopic values for plant transpiration (£_
T
=
0.6 £_
Ts
+ 0.4 £_
Th
).
Date
£_
Ts
(¡ÓSD, .) £_
Th
(¡ÓSD, .) £_
T
(¡ÓSD, .)
21st June £_
18
O -7.0¡Ó0.9 -6.1¡Ó0.9 -6.7¡Ó0.7
£_ D -28.8¡Ó5.1 -17.6¡Ó4.1 -24.8¡Ó3.3
24th June £_
18
O -8.0¡Ó1.1 -7.2¡Ó0.4 -7.6¡Ó0.7
£_ D -30.9¡Ó5.5 -21.3¡Ó8.7 -26.8¡Ó6.0
25th June £_
18
O -7.9¡Ó0.5 -6.2¡Ó0.6 -7.2¡Ó0.3
£_ D -56.2¡Ó7.4 -43.2¡Ó10.2 -49.4¡Ó6.3
Figu re 3. Keeling plots for £_D of water vapor c ollected at
different heights (0.1~3 m above ground) on 21st, 24th and 25th
June, 2006, respectively. Keeling plots for £_
18
O of water vapor
in these days are not present because the significances of these
regression lines were p > 0.05. The samples of water vapor were
started to collect at 10:00, 11:00, 13:00 and 14:00 h, and vapor
was collected for 30 min with a flow rate of 250 ml min
-1
for
each group. The slope and intercept of the regression lines are
shown in Table 3.
pg_0007
XU et al. ¡X Partitioning ET flux components in a subalpine shrubland
357
implicated that herbage layer also have important function
in local water exchange.
DISCUSSION
The dynamics of woody plants in semiarid and arid
landscapes have important implications for hydrology,
ecology, and society due to woody plants potential of
changing water cycles in these regions (Huxman et al.,
2005). In these arid and semiarid ecosystems, ET is
the most important component of the local water cycle
(Wilcox et al., 2003). Thus, describing the relations of
ET flux and different ecosystem components will tell
us more information about ecosystem function in these
regions, such as presenting the factors that controlling
ecosystem production (Jackson et al., 1998; Huxman et al.,
2005) or underlying ecosystem-level water-use efficiency
(Yepez et al., 2005). Stable isotopes and Keeling plots
provided unique information about the ecosystem ET flux
in a subalpine shrubland dominated by Q. aquifolioides.
Our results demonstrate that distinguishing different
components of ET by stable isotopes is feasible in a
semiarid subalpine shrubland.
In this study, we assumed that the plants were approach
to isotopic steady state during the sampling periods
on each day. Laboratory experiments told us ISS of
broadleaved species would gradual approach in 1~3 h after
drastic changes in ambient conditions (Flanagan et al.,
1991; Wang and Yakir, 1995). Flanagan et al. (1991) and
Yepez et al. (2003) suggested that small leaves, relatively
constant radiation and VPD, and high transpiration rates
would help plants promote a rapid progression to ISS
due to a fast turnover time of leaf water. At the study
site, alpine oak has evergreen leaves which were 2.5~5
cm in length and 1.5~2.5 cm in width in June, 2006. We
collected samples from 10:00 to 12:00 h and from 13:00
to 15:00 h, and the start time was 2.5~3 h after sunrise.
From 10:00 to 16:00 h, radiation (represented by photon
flux density) and VPD was relative stable (Figure 2) on
each day. In these sampling periods, pan evaporation rates
were high especially on the 24th and 25th June (Figure 2);
evaporation rates on 21st June were lower than on other
days but also were close to those reported by Yepez et al.
(2003). Therefore, we believe that we collected samples
under the ambient conditions that likely promoted a rapid
approach to ISS.
Many works indicated that the potential deviations
from ISS of transpired vapor (e.g. 1~3. for £_
18
O) should
have minimal influence on partitioning the ET into its
components (Flanagan et al., 1991; Wang and Yakir,
1995; Harwood et al., 1998; Yepez et al., 2003, 2005;
Williams et al., 2004) because the magnitude of isotopic
variation of transpired vapor is too small compared to
the highly isotopically depleted soil evaporation (Table
1, Table 2; Gat,1996; Wang and Yakir, 2000; Yepez et
al., 2003). In this work, the maximal variations of total
transpired vapor from ISS were 0.7. for £_
18
O and 6.3.
for £_D, respectively (Table 2). It would represent 1.8%
and 3.7% changes in the ratio of plant transpiration to
soil evaporation for £_
18
O and £_D, respectively, and these
variations fell in current 95% confidence intervals.
£_
Ts
differed from £_
Th
(Table 2). We considered that it
may be caused by shrub using soil water correspondingly
deeper than herbs. The source water in the soil became
surprisingly negative on 25th June compared to 24th
June. Using £_ D as a sample, (1) £_
Ts
on 24th June was
-30.9¡Ó5.5. and detailed instances were -26.2¡Ó2.9. on
10:00 h, -28.9¡Ó1.8. on 11:00 h, -31.5¡Ó6.2. on 13:00
h and -36.8¡Ó0.8. on 14:00 h; (2) £_
Ts
on 25th June was
-56.2¡Ó7.4. and detailed instances were -56.6¡Ó8.7. on
10:00 and 11:00 h, and -55.9¡Ó7.2. on 13:00 and 14:00
h. Forasmuch, we suggested that: following the upper
soil horizons went short of soil water, shrubs and herbs
became to use deeper soil water which was recharged by
precipitation on 22nd and 23rd June that might be rather
negative (Xu et al., 2008).
The regression lines using £_
18
O of water vapor
at different heights above ground and the inverse of
corresponding water concentration were not significant in
this study. Two conditions may account for these results:
(1) our method depends on the isotopic differences
Figure 4. Total ET partitioned into shrub transpiration, herbage
transpiration and soil evaporation in the ecosystem dominated by
Quercus aquifolioides in shrub layer and Cystopteris montana in
herbage layer. Numbers indicate the estimated fraction of each
ecosystem component (¡ÓSD, %) using IsoSource (Phillips and
Gregg, 2003) and Isoerror (Phillips and Gregg, 2001).
pg_0008
358
Botanical Studies, Vol. 49, 2008
between £_
T
(£_
Ts
or £_
Th
or both) and £_
S
values (Wang and
Yakir, 2000). In this work, £_
18
O values of Q. aquifolioides
and C. montana were similar to that of shallow soil water
(Table 1 and Table 2). Wang and Yakir (2000) pointed
out that the difference between £_
T
and £_
S
values could
diminish if the soil surface is drying up. Besides this
aspect, alpine oak preferring to utilize shallow soil water
(Xu et al., unpublished) also might diminish the difference
between £_
T
and £_
S
values. The equilibrium enrichment
factor (£`
*
) of £_D is much greater than that of £_
18
O (Table
1), therefore in this study the isotopic differences between
plant and soil water for £_D might be less influenced by
the water uptake mode and dry shallow soil horizons; (2)
the small number of observations (Table 3) leaves us with
a relatively high degree of statistical uncertainty. Though
Yepez et al. (2003) only used 11 observations to generate a
significant regression line of Keeling plots for partitioning
understory ET, more observations of different heights and
long-playing collection may help to improve the spatial
resolution (Flanagan and Ehleringer, 1998; Dawson et al.,
2002), the precision and the reliability (Harwood et al.,
1999) of Keeling plot analyses.
Results of T/ET showed that plant transpiration was
the dominant source for total ET in June, 2006, the
early monsoon period. These results also reflected the
vegetation which is dominated by alpine oak that used the
available moisture efficiently (Yepez et al., 2003). These
results consisted with other studies conducted in tropical
rain forest ecosystems (Shukla et al., 1990; Nobre et al.,
1991; Moreira et al., 1997), arid and semiarid ecosystems
(Wang and Yakir, 2000; Yepez et al., 2003, 2005; Williams
et al., 2004), but disagreed with a study in fallow bush
land of woody shrubs (Brunel et al., 1997). Brunel et al.
(1997) reported that plant transpiration only contributed
20% of the total ET. This disagreement may be caused
by low vegetation coverage (only 20%) in the study of
Brunel et al. (1997). We observed the fractions of soil
evaporation for £_D decreased obviously on 25th June
relative to 24th June (Figure 4). There occurred 19.2 mm
and 5.3 mm precipitation on 22nd and 23rd June (Figure
1), respectively, but 24th and 25th June were sunny and
pan evaporation rates were high (Figure 2). Therefore,
we conclude that soil horizons close to ground surface
were drying up along with the plant water uptake, and
water for soil surface evaporation in these horizons was
absent. This trend of fractions of soil evaporation in ET
is in agreement with the findings by Wang and Yakir
(2000): the contribution of soil evaporation to total ET
is small or negligible in arid and semiarid environments
except for short periods after precipitation events. Another
comparison in this study, the fraction of soil evaporation
to ET based on £_D was similar on 21st June to that of
24th June. We mentioned that the transpiration rate was
not high due to low PFD on 21st June (Figure 2). Thus,
the surface soil horizons did not dry up too much by
plant water uptake and evaporation. In contrast with soil
evaporation, T/ET as determined via £_D was increased
from 24th to 25th June. This is consisted with Yepez et al.
(2005) who found T/ET increased over several days after
irrigation in semiarid grassland.
CONClUSIONS
The analyses of stable isotopes and Keeling plots
allowed us to partition ET into different flux components
in a subalpine shrubland covered with Q. aquifolioides
in the Wolong Nature Reserve. Our findings indicate that
ET is mostly generated by plant transpiration, more than
65% on three sampling days during the early monsoon
period. Transpiration from the herbage layer appears to
be as important as that from the shrub layer. In this work,
using £_
18
O to partition ET is unreliable mainly because
the isotopic differences between transpired water and
soil water were minute. Our findings help to improve
our understanding of the water fluxes and functioning
of the alpine oak shrubland ecosystem. With further
improvements of the method it could possibly be used to
study regional energy and water exchange.
Acknowledgements. This study was supported by
NKBRSF, PR China (No.2002CB111504), by science and
technology supporting program of State Forestry Bureau
of China, and by Open Foundation of the Key Laboratory
of Nuclear Resources and Environment of Ministry of
Education, ECIT (060602). We gratefully acknowledge Dr.
YH Xie, Dr. ST Zhang, Dr. XL Liu, Dr. YH Liu, and Mr.
NJ Fan for their help in field work. We thank Prof. JY Pan
and Mr. ZB Yan for their works in the mass spectrometry
analysis. We also thank Prof. Werner Eugster for his
valuable scientific comments and precious time.
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