全 文 ：Received 23 May 2003 Accepted 27 Aug. 2003
Supported by the Knowledge Innovation Program of The Chinese Academy of Sciences (KSCX2-SW-120), the State Key Basic Research and
Development Plan of China (2002CB111503), Overseas Talent Chinese Foundation from The Chinese Academy of Sciences and the Natural
Science Foundation of Guangdong Province (010567).
* Author for correspondence. Tel: +86 (0)20 37252708; Fax: +86 (0)20 37252615; E-mail: .
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Acta Botanica Sinica
植 物 学 报 2004, 46 (2): 202-210
Measured Sap Flow and Estimated Evapotranspiration of Tropical
Eucalyptus urophylla Plantations in South China
ZHOU Guo-Yi1*, YIN Guang-Cai1, Jim MORRIS2, BAI Jia-Yu3, CHEN Shao-Xiong4,
CHU Guo-Wei1, ZHANG Ning-Nan3
(1. South China Institute of Botany, The Chinese Academy of Sciences, Guangzhou 510650, China;
2. Center for Forest Tree Technology, P.O. Box 137, Heidelberg, Victoria 3084, Australia;
3. Research Institute of Tropical Forestry, Chinese Academy of Forestry, Guangzhou, 510520, China;
4. China Eucalyptus Research and Development Center, Chinese Academy of Forestry, Zhanjiang 524022, China)
Abstract: During the period of September 12, 1999 to September 24, 2000, we measured sap flow of
eucalyptus (Eucalyptus urophylla S.T. Blake) plantations using heat pulse technique, and the relevant
environmental vaiables, such as soil evaporation and canopy interception, etc, at Hetou and Jijia sites,
Leizhou Peninsula, Guangdong Province. Based on the measurements of sap flow and estimates of
evapotranspiration, the following can be concluded: (1) the maximum of diurnal xylem sap flux density (SFD)
at Hetou, where covered with coarse-textured soils formed on Quaternary sediments, was almost twice of
that at Jijia, where located on clay-rich soils derived from basalt; (2) SFD was highly correlated to water
vapor pressure deficit (VPD) of ambient air near the canopy layer; (3) the correlation between SFD and air
temperature also depends on soil properties and soil water potential; (4) the relative differences between
measured and modeled evapotranspiration were small, being 5.26% at Hetou and 6.14% at Jijia; (5) the
plantation transpiration accounted for 62.2% and 51.3% of the evapotranspiration at Hetou and Jijia,
respectively; and (6) the averaged SFD per unit leaf area (ASPULA) was a good index to estimate the
amount of water consumption of tree species.
Key words: Eucalyptus urophylla plantation; water consumption; sap flux density (SFD); vapor pressure
deficit (VPD); transpiration; evapotranspiration
Concerns over excessive water use by exotic eucalyp-
tus species have been raised in several countries where
commercial plantations have been established (Calder et
al., 1997; Kallarackal et al., 1997), but water use character-
istics of many species planted in various climates and physi-
ographic settings have not yet been fully understood. There
is still much to learn about how the physiological and envi-
ronmental factors impact on the water use by eucalyptus
plantations (Hatton et al., 1998).
To quantify the hydrological influences of eucalyptus
plantations established over 200 000 hm2 on the Leizhou
Peninsula, Guangdong Province, South China, many re-
searchers carried out some catchment studies for more than
20 years (Zhou et al., 1995), including continuous mea-
surement of microclimatic variables, soil and underground
water dynamics, runoff and soil erosion mechanism, etc.
Because of lacking of accurate quantitative estimation of
plantation transpiration and the related studies, we have
not yet reached any comprehensive conclusions about the
hydrological influences of eucalyptus plantations on the
water balance at catchment scale.
Direct measurement of sap flow is an accurate method
to determine the transpiration of a single tree (Steinberg et
al., 1990; Grime et al., 1995). Sap flow measurements had
rarely been used to estimate transpiration of an entire stand
before the 1960s (Ladefoged, 1963; Doley et al., 1966). Such
extrapolation requires not only accurate measurements of
sap velocity, but also detailed inventories of stand-level
properties including spacings between tree stems or crowns
(Hatton et al., 1990), basal stem area, leaf area or sapwood
area (Hatton et al., 1995). Although the scaling-up from
individual trees to a stand based on sapwood area are con-
ceptually simple, estimations of an individual tree or forest
water use by this way are subject to large uncertainties
(Smith et al., 1996; Kostner et al., 1998), among which the
most important is the actual proportion of water transport
passing through the sapwood (Wullschleger et al., 2000).
Previous studies have shown that radial variation in sap
ZHOU Guo-Yi et al.: Measured Sap Flow and Estimated Evapotranspiration of Tropical Eucalyptus urophylla Plantations in South China 203
velocity is large in many woody species (Dye et al., 1991;
Becker, 1996; Phillips et al., 1996; Oren et al., 1999), with
sap velocities being the greatest in the outer sapwood and
progressively lower toward heartwood. Zhou et al. (2002)
reported that the radial variation in sap flux density (SFD)
was a function of sapwood thickness for two eucalyptus
plantations at both sites, which has provided a way to es-
timate the average SFD throughout the sapwood of a tree
based on several monitoring points, resulting in a more
accurate method of scaling-up of transpiration from indi-
vidual trees to a stand.
Heat-balance method and heat-pulse method are two
main methods for continuous measurement of SFD without
disturbing the natural environments, compared with weight-
ing method. Heat-balance method uses sap flow gauges,
each consisting of a flexible heating element and two
thermocouples, pressed firmly against the outside of the
stem above and below the heating element. Sap flow is
calculated by the temperature difference between thermo-
couples above and below the heated stem section after
subtracting heat loss due to conduction by stemwood
(Sakuratani, 1981; Baker et al., 1987; Granier, 1987; Steinberg
et al., 1989). Heat-pulse (compensation) method uses heat-
pulse sensor, each composing of a heater and an unheated
thermocouple pair, and being connected in two sides of the
heater in vertical direction for measuring the temperature
difference (Granier, 1987; Barrett et al., 1995). The latter
provides a practical means to estimate the water use of
individual trees and is often a reasonably accurate alterna-
tive for measuring forest and woodland transpiration in a
complex heterogeneous terrain (Hatton et al., 1995).
A heat-pulse system developed by Edwards Industries
of New Zealand was used to quantify the water use of
Eucalyptus urophylla at the two different sites on Leizhou
Peninsula. The system offers superior accuracy and more
reliable than all other alternative equipment. Development
and customization software for convenient SFD data col-
lection and analysis were done by CFTT (Center for Forest
Tree Technology, Victoria 3084, Australia).
This paper is to deal with SFD dynamics, and the corre-
lations between SFD and environmental factors of two
Eucalyptus urophylla plantations. Based on this, the laws
of transpiration and water consumption were then
discussed, and evapotranspiration was calculated by a theo-
retical model (Zhou et al., 1988).
1 Materials and Methods
1.1 Site description
The plantations are located at two sites (Hetou and Jijia)
in the Nandu River watershed on Leizhou Peninsula,
Guangdong Province, China. The Hetou site (21°05 N,
109°54 E) is on sandy soil of sedimentary origin, while the
Jijia site (20°54 N, 109°52 E) is on basalt-derived clay soil,
approximately 40 km away. The climate is tropical, with long
term monthly mean air temperature of around 28 ℃ for July
and 16 ℃ for January. Annual rainfall varies from 1 300 mm
in the south to 2 500 mm in the north of the peninsula with
high monthly variations. Over 80% of the rain falls between
April and September, up to half of this is due to typhoon
that occurs up to seven times per year. At the study sites,
the E. urophylla plantations were planted in mid-1996 (3 m
× 2 m spacing at the Hetou site, 2.0 m × 1.5 m spacing at
the Jijia site). A 40 m × 40 m plot was taken at each site in
Sept., 1999, and a set of standing trees within the plot were
selected for monitoring according to the diameter distribu-
tion of the stand.
1.2 Environmental monitoring
Instruments for measuring air temperature, relative
humidity, solar radiation and wind speed were installed at
both sites in September 1999, and half-hourly measurements
were recorded using a data logger. Rainfall above the
canopy was measured using a tipping bucket rain gauge.
Soil water contents were also measured at four depths (50,
150, 250 and 350 cm) using soil moisture sensors (Theta
Probes, Delta T Devices, UK).
Soil particle size composition was measured using
samples taken every 30 cm to a depth of 4 m at two loca-
tions within the sampling area at each site, and cores were
excavated from 4 spots to a depth of 0.8 m for bulk density
determination. Points on the soil moisture characteristic
curve describing the relationship between soil water con-
tent in mass fraction to matrix potential was obtained by
the filter paper method.
1.3 Measurement of soil evaporation and canopy inter-
ception
Soil evaporation was measured by several small lysim-
eters placed in different locations at both sites. The lysim-
eters can allow the soil water to exchange freely through
the bottom of the equipment. Soil evaporation over a given
period was estimated as the net reduction in weight after
accounting for rainfall input.
Rainfall was measured by tipping bucket rain gauges,
and throughfall by a series of 4 troughs placed beneath the
canopy with a total area of 1.06 m2. The troughs drained
into a large tipping bucket recorder (1.0 mm). Stemflow was
measured using PVC hosepipe cut in half laterally and nailed
tightly around the trunks of four trees and also drained
each to a tipping bucket recorder (0.5 mm). Daily interception
Acta Botanica Sinica 植物学报 Vol.46 No.2 2004204
was the difference of rainfall and the sum of throughfall
and stemflow.
1.4 Sapwood area and other supplementary measure-
ments
The sapwood area was calculated using empirical equa-
tions relating the measured sapwood area (y, cm2) to stem
diameter (x) at breast height (1.3 m) (DBH, x, cm): y =
0.281 5 x2 + 1.141 1 x (R2 = 0.90) for Hetou, and y = 0.302 7 x2
+ 1.718 9 x (R2 = 0.92) for Jijia. Supplementary measurements,
such as probe separation (i.e., the accurate distance be-
tween the heater and the two thermocouples), wound size,
volumetric water and wood contents of the selected stand-
ing trees were accurately measured regularly to correct SFD.
Probe separation was accurately determined by spacing
blank probes outside the cambium at a distance equal to
the depth of the sensor implanted. Wound size, wood and
water contents were measured by the method described in
Hatton et al. (1990). Individual tree leaf area (LA) (m2) was
the leaf area index (LAI-estimated with the Canopy Area
Analyzer, LAI2000, Li-Cor, Inc., Lincoln, NE) multiplied by
the vertical projection area of the canopy. Six U6 (a clone of
E. urophylla) trees were randomly selected at both sites to
obtain the relationship between LA (m2) and DBH (cm)
(LA=1.521 8 DBH1.082 1 (R2 = 0.90, n = 6, P = 0.01)). The
relationship was used to estimate leaf areas of other trees
in the stand.
1.5 Sap flux density monitoring
During the observed period, heat-pulse sensors were
cycled through a representative sample of trees (20 at
Hetou and 18 at Jijia) for about 4-6 weeks per tree at both
sites. Four heat-pulse probes were positioned at the breast
height in each tree in four different directions (North,
South, East and West) and inserted into the sapwood in
different depths according to the tree diameter. The con-
trolled module/data logger was programmed to provide a
heat-pulse. Measurements were recorded every 30 min.
Zhou et al. (2002) reported that the radial variation in SFD
was a function of sapwood thickness at both sites: y =
5.006 2 x3-9.116 1 x2 + 4.454 4 x + 0.463 4 (R2 = 0.81, n = 72,
P = 0.01) for Hetou, and y = 3.667 5 x3-7.295 5 x2 + 3.682 6
x + 0.567 4 (R2 = 0.94, n = 80, P = 0.01) for Jijia (where, y is
the ratio of SFD of one sensor to the average of four
sensors placed at different depths, x is the ratio of the
depth of sensor to the radial sapwood thickness). The
relationships were used to calculate the average SFD of
the whole sapwood of a tree. The transpiration of a single
tree and a stand can be calculated from the average SFD
using the methodology of Edwards et al. (1984) and Olbrich
(1991).
1.6 Measured and calculated evapotranspiration
Total evapotranspiration Et (mm) is the sum of canopy
transpiration Ec (mm), soil evaporation Es (mm), and evapo-
ration from canopy interception and dew Ei (mm), that is:
Et = Ec + Es + Ei (1)
Total evapotranspiration Et (mm) can be also calculated
using the following theoretical model (Zhou et al., 1988;
1995):
Et = Ep {1+ - [1+ ( ) ] } (2)
where, Ep is the potential evapotranspiration of the whole
plantation in a day (mm), and is calculated using the amount
of radiation energy entering the plantation and other me-
teorological variables; s is the daily change of water stor-
age in the ecosystem after considering the rainfall amount
in the same day (mm). Here the daily change of available
soil water is taken as a substitute for s, because the amount
of water in vegetation biomass of both plantations is much
smaller than that in the soil, and is therefore not included in
s. h is relative humidity in fraction. n is a dimensionless
constant (0-∞), representing the water holding capacity
of an ecosystem. For example, n is equal to ∞ for an eco-
system with water outflow being zero, and is equal to zero
for an ecosystem with all water flowing away. Here, we
assumed n to be 0.75 at Jijia and 1.25 at Hetou.
Ep was estimated using the Penman-Monteith equation:
Ep=
g ea+ R0D
(3) g + D
where g is the psychrometric constant, 0.61; R0 is the net
radiation expressed in the same unit as Ep (mm); D is the
slope of the saturated water vapor pressure with tempera-
ture curve at air temperature t (℃); ea is given by equation
(4):
ea = (0.27+0.002 43 v)(es-ed) (4)
where es and ed are saturated water vapor pressure at air
temperature t (℃) and actual water vapor pressure at a
reference height, respectively, and v is the wind speed at
the same reference height.
2 Results
2.1 Diurnal patterns of SFD
All SFD values for E. urophylla plantation on four dates,
Spring (March 16, 2000), Summer (Jun. 11, 2000), Autumn
(Sept. 14, 1999) and Winter (Dec. 8, 1999), showed strong
diurnal variations (Fig.1). Half hourly SFD increased in the
morning, peaked just after 12:00, and decreased towards
early evening.
S
Ep S
Ep
1
nh
h2-h-1
+n+1 nh
h2-h-1
+h+1
ZHOU Guo-Yi et al.: Measured Sap Flow and Estimated Evapotranspiration of Tropical Eucalyptus urophylla Plantations in South China 205
ANOVA test showed that SFD at both sites were signifi-
cantly different (P < 0.001). On average, SFD of E.
urophylla plantation at Jijia was about half of that at Hetou,
despite of that the distance between both sites is less than
40 km and their topography is similar. This probably re-
sulted from soil physical properties.
2.2 Annual dynamics of soil water content
Figure 2 shows that the daily available soil water con-
tent differed significantly between both sites, but annual
average values are not significantly different, being equal
to 336 ± 36 mm at Hetou and 350 ± 81 mm at Jijia. Compared
with the basalt-derived clay soil at Jijia, the Hetou sandy
SFD and available soil water is not statistically significant
for Hetou (R2 = 0.04), but is significant for Jijia (R2 = 0.13).
The results suggest that SFD is limited by available soil
water at Jijia but not at Hetou, even though the annual
average available soil water content at both sites was similar.
The results reported here also demonstrated that the “avail-
ability” of soil water is strongly affected by some soil physi-
cal properties.
Because of the higher water conductivity of sandy soil
at Hetou, water supply for transpiration was sufficient, SFD
is not limited by available soil water and the forest stand
would consume much more water than that of Jijia, despite
Fig.1. Diurnal variation of sap flux density (SFD) of Eucalyptus urophylla stands on four dates in Spring, Summer, Autumn and Winter
at both sites. Each SFD value represents the mean of four sensors in four directions and depths.
Fig.3. The relationship between daily sap flux density (SFD) and available soil water content.
Fig.2. Available soil water content of the top 4 m soil at both sites during
the studying period (Sept.12, 1999-Sept.24, 2000). Available soil water
was calculated as the difference between field moisture content and moisture
content at the matric potential –1 500 kPa.
soil with a sedimentary origin has a looser structure,
and its lateral soil water exchange and infiltration
should be much quicker. As a result, for Hetou site,
the decreased soil water near the roots resulted from
water consumption by transpiration and soil evapo-
ration can be easily compensated in time with water
or rainfall from the neighboring area, leading to a
milder change in available soil water content near
the root. However, it was opposite for Jijia soil.
2.3 Impact of soil water content on SFD
Figure 3 shows the relationship between daily
SFD and available soil water content during Sept.
12, 1999 to Sept. 24, 2000. Correlation between daily
Acta Botanica Sinica 植物学报 Vol.46 No.2 2004206
of same tree species at both sites.
2.4 SFD and leaf area
Average LAI was 1.98 for Hetou and 1.51 for Jijia,
respectively. The relationship between measured SFD and
calculated leaf area of individual trees is shown in Fig.4.
The amount of average leaf areas of whole trees at Hetou
and Jijia sites was 21.2 m2 and 14.9 m2, respectively. SFD
tends to increase with the leaf area at both sites, even
though the correlation was not statistically significant (Fig.
4). Because of the correlation among leaf area, sapwood
area, crown size and basal area of an individual tree, it is
very difficult to separate the effect of leaf area from those
of the other factors on SFD.
The averaged SFD per unit leaf area (ASPULA) at Hetou
and Jijia were 1.083 ± 0.520 and 0.838 ± 0.306 mL·cm-2·
h-1·m-2, respectively. Since the plantations at both sites
were of the same variety, the difference in ASPULA be-
tween them resulted mainly from difference in the environ-
mental factors, among which the available soil water con-
tent played a crucial role as described above.
2.5 SFD and environmental factors
Figure 5 shows the variations of SFD with air
temperature, relative humidity and saturated water vapor
Fig.4. Variation of sap flux density (SFD) with leaf area for the
sampled trees.
Fig.5. Relationship between sap flux density (SFD) and environmental factors at Hetou and Jijia sites. VPD, vapor pressure deficit.
pressure deficit for one year at Hetou and Jijia sites.
Regressions were fitted to the measurements of SFD
and each of the environmental variables. The results are
shown in Table 1 together with the statistics of the
regressions.
Figure 5 and Table 1 indicate that air temperature, rela-
tive humidity and VPD had statistically significant influ-
ences on the SFD of E. urophylla plantations at both sites,
except for air temperature at Jijia. Relative humidity and air
temperature are two important environmental variables that
affect SFD, but VPD influences SFD by increasing the driv-
ing force of transpiration directly, and is the most signifi-
cant environmental variables (Oren et al., 2001).
2.6 Evapotranspiration
The evapotranspiration (Et) as shown in Fig.6 was cal-
culated using equation (2). It can also be obtained by sum-
ming Ec, Es and Ei. Table 2 shows the annual values trans-
formed from the available data days in Fig.6.
Relative difference in the estimated Et by two methods
is 5.26% at Hetou, 6.14% at Jijia. The amount of transpira-
tion estimated from the measured SFD accounts for 62.2%
of evapotranspiration at Hetou, and 51.3% at Jijia. Consid-
ering of the low LAI values at both plantations and their
difference in soil properties, we consider that the transpira-
tion fractions of total evapotranspiration are reasonable.
Although the maximum of the diurnal SFD variations at
Hetou are almost twice as large as those at Jijia during four
seasons with a few exceptions, average daily Et and Ec are
2.88 ± 1.19 mm and 1.43 ± 0.55 mm at Hetou, 2.94 ± 1.74 mm
and 1.47 ± 0.67 mm at Jijia, respectively, and are not signifi-
cantly different between two sites. The standard devia-
tions of daily Et and Ec at Hetou are smaller than those at
Jijia, because soil moisture at Hetou varied less than that at
Jijia, retesting the great impact of available soil water
ZHOU Guo-Yi et al.: Measured Sap Flow and Estimated Evapotranspiration of Tropical Eucalyptus urophylla Plantations in South China 207
content on Et and Ec.
3 Discussion
Two causes might result in the low correlation between
SFD and leaf area. One is the measurement errors. Hatton et
al. (1990) reported that errors in SFD measurements were
approximately 13%, and there was the additional potential
error in the flux estimates for individual stem when strati-
fied sampling of SFD with depth and bole quadrant based
on four sensors of 25%. Another possibility is that it is
very difficult to separate the effect of leaf area from those
of the other factors on SFD, because leaf area is an inte-
grated factor related to sapwood area, crown size and basal
area of an individual tree. Previous studies also found a
poor linear relationship between leaf area and transpiration
flux during water stress (Greenwood et al., 1982; Hatton et
al., 1995). However, impact of leaf area on SFD should not
be neglected, Oren et al. (1999) reported that an approxi-
mately 40% reduction in LAI by a hurricane resulted in a
decreases of about 18% in SFD and stand transpiration.
Average SFD per unit leaf area (ASPULA) is an impor-
tant index for estimating water use of various species, as
proposed by Phillips et al. (2002). It can also be used to
estimate the practical water consumption of a species, su-
perior to the traditional index— water use efficiency (water
use per unit increase of biomass), which is mainly deter-
mined by species’ genetic characteristics and other envi-
ronmental conditions. The ASPULA value of U6 trees,
1.083 2 mL·cm-2·h-1·m-2 at Hetou and 0.8376 mL·cm-2·h-1·
m-2 at Jijia, were much larger than those of mountain ash
(E. regnans) trees (0.11 mL·cm-2·h-1·m-2) and hazel
(Pomaderris aspera ) trees (0.36 mL·cm-2·h-1·m-2) (Vertessy
et al., 2001). According to the data from Roberts et al. (2001),
Table 1 Relationship between sap flux density (SFD) and each of environmental variables at Hetou and Jijia sites
Sites Factors Relationships
Hetou SFD (y) and air temperature (x) y = 127 x - 80 (R2 = 0.39, n = 220, P = 0.001)
SFD (y) and relative humidity (x) y= 1 570 x-1.956 (R2 = 0.34, n = 205, P = 0.001)
SFD (y) and VPD (x) y= 3 638 x0.489 (R2 = 0.75, n = 235, P = 0.001)
Jijia SFD (y) and air temperature (x) y = 57 x + 442 (R2 = 0.13, n = 195)
SFD (y) and relative humidity (x) y= 1 023 x-1.995 (R2 = 0.34, n = 185, P = 0.001)
SFD (y) and VPD (x) y= 2 367 x0.435 (R2 = 0.71, n = 225, P = 0.001)
VPD, vapor pressure deficit.
Table 2 Et , Ec , Es , and Ei during the available data days and the whole year
Sites Available days
Ec E s Ei Et (equation (1)) Et (equation (2)) Relative difference
(mm) (mm) (mm) (mm) (mm) (%)
Hetou 207 291.0 103.9 73.3 468.2 444.2 5.26
365 513.2 183.2 129.2 825.6 783.3
Jijia 225 361.2 287.8 54.5 703.5 661.5 6.14
365 585.9 466.9 88.4 1 141.2 1 073.2
Abbreviations are the same as in Fig.6.
Fig.6. Et and Ec during the studying period (Sept.12, 1999-Sept.24, 2000). Some data are missing as a result of lighting-induced
equipment failure. Ec, canopy transpiration; Ei, evaporation from canopy interception and dew; Es, soil evaporation; Et, total
evapotranspiration.
Day of year (d)
Acta Botanica Sinica 植物学报 Vol.46 No.2 2004208
the ASPULA values were 0.08, 0.36, and 0.47 mL·cm-2·h-1· m-2
for Eucalyptus sieberi L. Johnson forests in 160, 45, and 14
years old. It demonstrated that the index ASPULA could
also reflect the influences of plantation age on its water
use.
Sap flux along the tree trunk is driven by the water po-
tential difference between two different heights of a tree,
which are influenced by both environmental variables near
the canopy and soil water potential near the roots. Differ-
ent soil types at Hetou (sandy soil) and Jijia (clay soil) have
led to different soil water potentials at both sites. As com-
pared with Jijia site, pressure deficit between soil water
potential and leaf water potential at Hetou is greater, and
therefore water flux trough the stem at Hetou is also greater,
even though the climatic conditions are very similar at both
sites. The daily and diurnal maximum SFD at Hetou were
almost twice of those at Jijia (Fig.1). In the following we
shall discuss the possible causes for the differences.
Firstly, the differences in tree physiological
characteristics, such as sapwood areas, leaf areas, and
diameters, etc., at both sites, have significant influences on
SFD, as have been shown in previous studies (Dunn et al.,
1993; Roberts et al., 2001). Secondly, the crown stomatal
conductance and leaf hydraulic conductance may vary with
tree size and tree height at both sites. Phillips et al. (2002)
found that the crown stomatal conductance, leaf specific
hydraulic conductance and the sapwood conductivity in
the overstory of Pseudotsuga menziesii trees differed
greatly for trees with different heights. Thirdly, there may
be much active lateral soil water flow at Hetou, and the
water loss from evapotranspiration can be more promptly
replenished after rainfall because of the loose structure of
sandy soil. Finally, the microclimatic factors, VPD or rela-
tive humidity, air temperature, radiation, etc., though only
differed a little between both sites, also contribute partly to
the difference in SFD. There was an obvious nonlinear rela-
tionship between hourly VPD and SFD at both sites, the
study of Vose et al. (2000) on cottonwoods Populus
deltoides showed similar result. The regression relation-
ship between SFD and air temperature is less significant at
Jijia than at Hetou, which indicated that the influence of air
temperature on water pressure deficit between canopy and
roots (as mentioned above) was more indirect compared
with VPD or relative humidity. Both canopy transpiration
and soil evaporation will increase with air temperature, pres-
sure difference in xylem will not therefore increase quickly.
This mechanism is more important for clay soil than for
sandy soil because of their different soil water conductivity.
Sap flow through the stem is also driven by negative
pressure that relies on the cohesion of sap water. Different
species (or varieties) have their own maximum SFD values
under a certain negative pressure. Environmental variables
and physiological characteristics of species can influence
SFD only within the range of 0 to maximum SFD. Maximum
SFD may be affected by the hydraulically or chemically
mediated feedbacks during stomatal closure under condi-
tions when too much water was lost from transpiration.
Therefore, maximum SFD, can be used as another valuable
index for evaluating water consumption of a plantation.
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