Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:

Tree Physiology 17, 195--203
© 1997 Heron Publishing----Victoria, Canada

Water use by Eucalyptus tereticornis stands of differing density in
southern India
JOSE KALLARACKAL and C. K. SOMEN
Plant Physiology Division, Kerala Forest Research Institute, Peechi 680653, Kerala, India

Received July 14, 1995

Summary We studied water use by Eucalyptus tereticornis
Sm. in two plantations, differing in tree density (1800 stems ha −1
at Site I and 1090 stems ha −1 at Site II), in different years. At
both sites, stomatal conductance, predawn and midday water
potentials and microclimate were measured and used to estimate hourly transpiration by the Penman-Monteith equation.
Growth in girth was also measured.
Stomatal conductance was closely correlated with atmospheric vapor pressure deficit (D); however, stomata did not
close completely even at high D (≈ 5.0 kPa). Midday leaf water
potentials did not fall below − 2.0 MPa during any part of the
year at either site. Predawn leaf water potentials were
greater than − 0.25 MPa during the postmonsoon period, but

declined to − 0.7 MPa at Site I during the premonsoon period.
Transpiration estimates ranged from 0.6 to 1.2 mm h −1 at Site I
and from 0.2 to 0.6 mm h −1 at Site II. The extrapolated
transpiration values for the rain-free days of the year were
1563 mm and 853 mm for Sites I and II, respectively. Growth
in girth was negligible during the premonsoon period. Photosynthesis was not affected by the minor water stress that
developed during the premonsoon period.
Keywords: growth, photosynthesis, stand density, stomatal
conductance, transpiration, water relations.

Introduction
The use of eucalypts as plantation trees in the tropics has been
criticized on the grounds that they consume water in excessive
amounts. During the past decade, therefore, several studies
have been undertaken to investigate water use by eucalypts
(see reviews by Davidson 1985, Poore and Fries 1985, FAO
1988, Calder 1992, Calder et al. 1992, Calder 1994). In their
review, Poore and Fries (1985) concluded that there is no
general answer to the question of whether eucalypts consume
water in excessive quantities because site conditions, climate

and species all influence water use. Thus, there is a need to
study the hydrological impact of each eucalypt species under
the conditions of interest.
There is little information on water use by eucalypt trees in
the humid tropics where annual rainfall is around 3000 mm.
There have been several studies on the water relations and
water use of eucalypt seedlings in the humid tropics of India

(e.g., Dabral 1970, Rawat et al. 1984, 1985), but the validity of
extrapolating the results of these studies to plantation trees has
been questioned by Calder et al. (1992).
To gain a better understanding of water use by eucalypt trees
in the humid tropics where they have been introduced on a
massive scale, we investigated water use by two Eucalyptus
tereticornis Sm. stands differing in tree density, which were
located in the Kerala State Forest. Eucalyptus tereticornis was
chosen for study because it is the major eucalypt species
planted in Kerala State, occupying more than 25,000 ha of the
total 40,000 ha under eucalypts.

Materials and methods
Site description
Two plantations of E. tereticornis were monitored in different
years (1990/91 and 1991/92 for Sites I and II, respectively).
Both sites, which are about 250 km apart, are in the Kerala
State Forest, in the southwestern part of India. The climate is
tropical with annual rainfall ranging from 2500 to 3000 mm,
80% of which is contributed by two monsoons. Site I at Varavoor in the Trichur Forest Division (10°41′ N, 76°12′ E) had
4-year-old one-stem coppices regenerated from 10-year-old
root-stock. Hence the root-stock was 14 years old at the time
of the study. Trees at Site II, at Palode in the Trivandrum Forest
Division (8°45′ N, 76°59′ E), also had 4-year-old one-stem
coppices, however, the coppices had been regenerated after
two 10-year rotations. Thus, the root-stocks at Site II were 24
years old. Both sites were situated about 100 m above sea level.
Soils at both sites were deep red and lateritic with nearly 1 m
of deep loamy soil at the top. At Site I, stand density was 1800
stems ha −1 (original number planted was 2500), whereas at
Site II, stand density was 1090 stems ha −1. At both sites,
average tree height was 10 m, and average tree diameter varied

from 8.5 to 9.0 cm. The canopy in both plantations had not
closed at the time of measurement. At both sites, there was a
ground cover consisting of several weed species, predominantly Eupatorium sp. and some grasses, most of which dried
up during the premonsoon period. This ground cover, along
with eucalypt leaf litter, covered the soil in the dry season.
There was occasional grazing of the sites by stray cattle.

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KALLARACKAL AND SOMEN

Weather data
Measurements of maximum and minimum temperatures, rainfall and relative humidity were recorded at weather stations
located nearly 5 km from Site I and about 3 km from Site II.
The minimum and maximum temperatures recorded at Site I
ranged from 20.8 (January 1991) to 38.2 °C (March 1991).
Those recorded at Site II ranged from 18.6 (January 1992) to
33.7 °C (March 1992). Annual rainfall is 3210 and 2465 mm
at Sites I and II, respectively. At Site I, the South-West Monsoon predominates and the wettest months are June to August.
The North-East Monsoon occurs during September and October (Figure 1a). Rainfall is negligible from November to May.

Site II is also affected by the monsoons in May to August and
in October and November. The year may be divided into three
periods----the premonsoon period (January--May), the monsoon period (June--August), and the postmonsoon period (September--December). Less water is available to plants during the
premonsoon period than during the other periods. The pan
evapotranspiration rate (PET) recorded for Sites I and II is
shown in Figures 1a and 1b.
Microclimate
To monitor the microclimate 2 m above the canopy, a 12-mhigh, steel scaffold tower was installed at each site to mount
the meteorological equipment. Air temperature (Ta), and relative humidity (RH) were measured with a shielded thermistor
and a carbon-electrode RH chip (Model 207 Temperature and
RH probe, Campbell Scientific Inc., Logan, UT). Wind speed
(u) was measured with a cup counter anemometer (Model
014A, Met One, Sunnyvale, CA) equipped with a switch
closure mechanism. Net radiation (Rn) was measured with a
Fritschen type net radiometer (REBS Inc., Washington, DC).
Shortwave radiation (S) was measured with a pyranometer

sensor (Model LI-200S, Li-Cor, Inc., Lincoln, NE). All sensors
were connected to a data logger (Model 21X, Campbell Scientific Inc.) that recorded measurements at 5-s intervals and
stored the hourly averages.

Stomatal conductance (gs)
A steady-state porometer (Model LI-1600, Li-Cor, Inc.) was
used to measure stomatal conductance of leaves at ambient
relative humidity. The canopy was treated as a two-layered
structure, and the vertical depth of each layer was approximately 2 m. At each site, a total of eight well-exposed sample
leaves, one from each canopy layer of four trees accessible
from the scaffold tower, were sampled hourly from sunrise to
sunset. The porometer measurements were made on both the
upper and lower sides of the amphistomatous leaves. Stomatal
frequency on both sides of the leaf was determined microscopically from epidermal imprints of several sample leaves,
which were made with transparent adhesive tape. Daily patterns of gs were followed on the sampling days shown in
Table 1.
Leaf water potentials (Ψ)
Predawn and midday leaf Ψ were measured on a total of five
fully exposed leaves collected from the upper layers of different trees on the sampling days shown in Table 1. The sample
leaves were enclosed in a polyethylene bag just before detaching from the plant. We adopted the precautions for sampling
outlined by Turner (1988). A Scholander-type pressure chamber (Soil Moisture Equipment Corp., Santa Barbara, CA) was
used to determine the balance pressure, which was assumed to
equal leaf Ψ. Pressure-volume curve analyses were performed

according to the method of Turner (1988), to determine the

Figure 1. Monthly rainfall (solid
bars) and PET (stippled bars) totals during the study. Data were
collected near Site I (a) and
Site II (b).

WATER USE OF EUCALYPTUS TERETICORNIS

197

Table 1. Measurement dates at the two experimental sites.

Canopy transpiration (Et)

Site I

Site II

Canopy transpiration, Et, was estimated hourly using the Penman-Monteith equation (Monteith 1965):

October 9, 1990
November 16, 1990
December 6, 1990
January 23, 1991
February 20, 1991
March 31, 1991
April 25, 1991
May 21, 1991

September 14, 1991
December 1, 1991
January 11, 1992
February 19, 1992
April 21, 1992
June 5, 1992
August 25, 1992

water potential at the turgor loss point on five days during the
premonsoon period.

Canopy conductance (gc )
Canopy conductance was calculated as:
g c = Σg sL ,

(1)

Et =

∆(Rn − G ) + ρcpDg a
,
λ [∆ + γ(1 + g a /g c)]

where Et = evapotranspiration rate (mm h −1), λ = latent heat of
vaporization of water (kJ g −1), Rn = net radiation (W m −2), G =
soil heat flux (which can be ignored for daily calculations), ρ =
density of air (kg m −3), cp = the specific heat of air at constant
pressure (J kg −2 °C −1), ∆ = the slope of the saturation vapor
pressure curve for water (kPa °C −1), D = vapor pressure deficit
(kPa), ga = aerodynamic conductance (m s −1), γ = psychrometric constant (kPa °C −1), and gc = canopy conductance (m s −1).
Canopy transpiration was calculated hourly assuming a single-layer model, because the canopy was relatively open and

there was little difference in stomatal conductance among
leaves within the canopy. We assumed that the leaves were
always at ambient temperature, and used atmospheric D instead of leaf to air D.

where L = leaf area index of the conducting surface, and gs is
the mean stomatal conductance. Mean gs was obtained by
sampling eight leaves distributed in different zones of the
canopy each hour. As far as possible, the same leaves were
measured every hour.

Growth measurements

Leaf area index (L)

Results

Leaf area index was measured with an LAI-2000 canopy
analyzer (Li-Cor, Inc.). The values given by the instrument
were usually modified by masking one or two horizontal
angles scanned by the instrument using the C-2000 software

program provided by the manufacturer. Sampling dates are
shown in Table 1.
Values of L obtained with the LAI-2000 were similar to
values estimated from stem girth by means of an allometric
equation derived from destructive measurements made in a
eucalypt plot close to Site I:
lnL = −5.3938 + 2.1444 lngbh , (R2 = 0.89)

(2)

where gbh is girth at breast height.

Diameter growth of 40 eucalypt trees located around each
measurement site was followed by monthly measurements of
gbh during the study period.

Microclimate
The highest temperature at Site I was over 38 °C, which was
reached in March 1991. At Site II, the highest temperature was
33.5 °C, which was reached in April 1992. Values of D remained relatively high during January to March at both sites in
their respective years (> 2.0 kPa) and D reached 5.0 kPa at
Site I. From December 1990 to February 1991, u was high at
Site I because of a strong easterly wind. Values of ga, calculated
from the wind speed measurements at both sites, during the
premonsoon and postmonsoon months are shown in Figures 2a
and 2b. Both S and Rn remained relatively high during the
premonsoon period.
Stomatal conductance (gs)

Aerodynamic conductance (ga )
Aerodynamic conductance (ga) was calculated as the reciprocal of aerodynamic resistance (ra) following the equation of
Monteith (1965):
2

 (z − d )  2 −1
ku ,
ra = ln
zo 


(4)

(3)

where u = mean wind speed (m s −1), z = anemometer reference
height (m), d = zero plane displacement, calculated as 0.64h
(where h = crop height (m)), z o = the roughness length (= 0.13h),
and k = von Karman’s constant (= 0.41).

The leaves of E. tereticornis are amphistomatous. Stomatal
frequency for samples of fully expanded leaves from both sites
was 598 ± 18 mm −2 on the abaxial side and 420 ± 18 mm −2 on
the adaxial side. Figure 3a shows consecutive gs measurements
made on the adaxial and abaxial surfaces of the same leaves
during the course of a day. Values of gs obtained by sampling
two layers of the canopy were not significantly different (Figure 3b). Hence gc was calculated from gs using Equation 1 and
assuming a single-layer model for the canopy.
Diurnal variations in gs followed a consistent pattern, increasing during the morning until noon and then decreasing in
the afternoon. At both sites, gs was lower during the premonsoon months compared with the other periods. We found a

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KALLARACKAL AND SOMEN

Figure 2. Aerodynamic conductance calculated from windspeed measurements 2 m above
the canopy for the premonsoon
(closed symbols) and postmonsoon (open symbols) months at
Site I (a) and Site II (b).

Figure 3. Stomatal conductance at Site I on October 9,
1990. (a) Diurnal variations in
stomatal conductance on the
adaxial (closed symbols) and
abaxial (open symbols) sides
of the same leaf. Data are
shown for two leaves ( , h
and , n). (b) Stomatal conductance from eight sample
leaves, four from each of the
upper (--------) and lower (- - -)
layers of the same canopy.
Bars indicate the SE of measurements.

higher range of gs values during the pre- and postmonsoon
months at Site II than at Site I (Figures 4a and 4b).
Multiple regression analysis indicated that Rn and D were
the most important factors controlling gs. We plotted the gs data
obtained at the two sites against the corresponding D values,
ignoring the gs values measured when Rn was less than
300 W m−2, and identified the pre- and postmonsoon values
(Figures 5a and 5b). In general, the postmonsoon values of gs
were higher than the premonsoon values at Site I (Figure 5a).

The low gs values during the premonsoon period at low atmospheric demand indicate that Ψ could be an important parameter controlling gs. Although there is some scatter at low D in
Figure 5a, the general trend is for gs to remain more or less
unchanged (200 ± 50 mmol m −2 s −1) irrespective of D. This
trend was less evident at Site II than at Site I (Figure 5b). In
contrast, there is a trend for stomatal closure at high values of
D (Figure 5). Predawn Ψ did not decline as much at Site II as
at Site I, which probably explains why gs did not differ signifi-

WATER USE OF EUCALYPTUS TERETICORNIS

199

Figure 4. Diurnal variation in
stomatal conductance at Site
I (a) and Site II (b). Postmonsoon (open symbols) and premonsoon (closed symbols)
values are shown. Each point
is the mean of individual
measurements on a total of
eight leaves, four from each
layer of the canopy. Lines
represent measurements on
the dates shown in Table 1.

Figure 5. Relationship between stomatal conductance
(gs) and vapor pressure deficit (D) at Site I (a) and Site II
(b). Only gs values taken at a
net radiation ≥ 300 W m −2
are shown. The open symbols
represent postmonsoon values, and closed symbols premonsoon. Values include
measurements on all dates
shown in Table 1.

cantly between the pre- and postmonsoon periods. At Site II,
D rarely went above 3.0 kPa.
Leaf water potentials ( Ψ)
At Site I, predawn Ψ reached its lowest value of − 0.71 MPa in
May 1991, whereas the lowest midday Ψ values (−1.68 MPa)
were reached in March and May 1991 (Figures 6a and 6b). The
high predawn and midday Ψ values noted in April 1991 were

probably the result of a few summer showers (see Figure 1). At
Site II, minimum predawn and midday Ψ values occurred in
January 1992 and were − 0.34 MPa and −1.40 MPa, respectively. Thus, trees at Site II experienced less water stress than
trees at Site I. Examination of the seasonal variation in Ψ
showed that values were highest during the monsoon and the
postmonsoon periods. Analysis of the pressure-volume curves
(data not presented) showed that Ψ at turgor loss point was

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KALLARACKAL AND SOMEN

Figure 6. Seasonal variations in predawn (open symbols) and midday
(closed symbols) leaf water potentials at Site I (a) and Site II (b). Each
data point is the mean of five measurements. Bars indicate ± SE.

−1.75 MPa, indicating that, at both sites, the trees probably do
not reach the turgor loss point at any time during the year. The
water tables at both sites, which were monitored at nearby
wells, remained between 10 and 15 m deep during the dry
season. During the monsoon and postmonsoon periods, the
water tables decreased to 2--3 m deep as a result of soil
recharge and saturation by the prolonged and heavy monsoon
rains.
Leaf area index (L)
Leaf area indices at Sites I and II were 2.17 and 0.60, respectively. Seasonal variation in L (data not shown) was negligible
at both sites during the study period.

Transpiration
Maximum hourly transpiration, which was observed between
1000 and 1600 h, ranged between 0.4 and 1.2 mm h −1 at Site I,
and 0.2 and 0.6 mm h −1 at Site II (Figure 7). At Site I, transpiration rates during the pre- and postmonsoon periods were
similar, whereas at Site II transpiration rates were generally
higher during the premonsoon period than during the postmonsoon period. When the hourly values were accumulated to give
daily rates, we obtained values of 3.5 to 7.7 mm day −1 at Site I
compared with values of 2.0 to 4.9 mm day −1 at Site II. We
extrapolated the daily values to obtain seasonal transpiration
values of 729 and 834 mm for the pre- and postmonsoon

Figure 7. Hourly variations in
transpiration estimated by the
Penman-Monteith equation at
Site I (a) and Site II (b) for the
postmonsoon (open symbols)
and premonsoon (closed symbols) months. Values are for
measurements on all dates
shown in Table 1.

WATER USE OF EUCALYPTUS TERETICORNIS

201

Figure 8. Mean growth in girth of 40
trees measured at Site I (a) and at
Site II (b). The bars indicate the SE.

periods, respectively, giving a total of 1563 mm at Site I. The
corresponding values at Site II were 248 mm and 605 mm,
giving a total of 853 mm.
Growth
At both sites, water stress during the premonsoon period resulted in a reduction in diameter growth (Figures 8a and 8b).
The reduction was less pronounced at Site II than at Site I,
probably because of the relatively higher water potentials. We
observed the production of new leaves during the premonsoon
months, indicating that not all growth was inhibited.

Discussion
Both study sites are located in the foothills of the Western
Ghats range of mountains. Rainfall was about 30 percent
greater at Site I than at Site II during the periods studied. We
also found microclimatic differences in temperature, RH, and
vapor pressure deficit between the sites. In addition to different
sampling years between sites, these differences are the result
of the effects on Site I of the Palghat Gap: a gap in the Western
Ghats range of mountains through which the dry winds of the
Deccan blow during certain seasons.
Diurnal gs values showed seasonal variation. At both sites,
gs was lower during the premonsoon season than during the
other periods. The lack of significant variations in gs between
leaves of the upper and lower canopy layers, which was also
observed by Roberts et al. (1992), could be the result of the
open nature of the canopy as well as the pendulous leaves of
this species. In E. grandis W. Hill ex Maiden., gs varies among
leaves depending on their position in the canopy (Kallarackal
1997).
The poor correlation between gs and D indicates that some
factor(s) beside Rn interacted with D. The importance of plant
water status was demonstrated when the pre- and postmonsoon

values were separated. If leaf water potential was not involved
in regulating gs, the gs values observed during the premonsoon
period would have been as high as those observed during the
postmonsoon period when atmospheric demand was low. Although we did not observe predawn leaf water potentials indicative of severe water stress, the sensitivity of gs to Ψ was
demonstrated. For example, a predawn Ψ of less than − 0.25 MPa
altered gs considerably. Davies and Zhang (1991) reported that
drying out of part of the root system can affect gs, even though
predawn Ψ remains unchanged. The role of Ψ at Site II was
less clear than at Site I, probably because of the milder water
stress experienced by trees at Site II (cf. Figure 6b). At both
sites, stomatal closure in response to increasing D was apparent. In several plant species, gs is well correlated with D (Lange
et al. 1971, Schulze et al. 1972, Calder 1978, Roberts 1978,
Colquhoun et al. 1984, Kallarackal 1993). Pereira et al. (1986,
1987) found a close correlation between gs and D in E. globulus Labill.; however, the correlation also depended on leaf
water potential. Based on our own results and those of other
studies, we conclude that, in eucalypts, stomatal conductance
is regulated by atmospheric vapor pressure deficit as well as by
leaf water potential.
Predawn leaf water potential is a good indicator of the water
potential of soil accessed by the roots (Crombie et al. 1988).
At both sites, trees had predawn Ψ values greater than
− 0.25 MPa during the postmonsoon period. From the midday
Ψ values, it seems that the turgor loss point was never reached
at either site. The Ψ values also showed that trees at Site I were
more water stressed than trees at Site II. The relatively high Ψ
values of trees at Site II may be associated with the rootstocks
being ten years older at Site II than at Site I. It is also possible
that the low transpiration rates at Site II, as a result of low L,
could have led to higher water potentials. Compared with
eucalypts grown in other parts of the world, the Ψ values
reported in our study are high (Pereira et al. 1986, Zimmer-

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KALLARACKAL AND SOMEN

mann and Milburn 1982). Relatively high leaf water potentials
are characteristic of several tropical trees (Robichaux et al.
1984). However, Roberts et al. (1992) reported midday water
potentials as low as --3.6 MPa in E. tereticornis growing in the
neighboring state of Karnataka where the annual rainfall is
approximately 800 mm. The relatively high water potentials
that we observed during the premonsoon suggest that the tree
roots have access to the water table. The water table during the
premonsoon period was between 10 and 15 m deep. Several
studies in Australia have shown that eucalypt roots sometimes
access deep phreatic aquifers (Carbon et al. 1982, Greenwood
et al. 1985).
Hourly transpiration at Site I was twice that at Site II. The
increase in transpiration rates during the premonsoon period
compared to the postmonsoon period at Site II reflects the
increased atmospheric demand during this period. Although a
much higher atmospheric demand was also observed at Site I
during the premonsoon period than during the postmonsoon
period, it was not accompanied by an increase in transpiration
rate, perhaps because the low leaf water potentials enhanced
stomatal closure at Site I, resulting in a reduction in stomatal
conductance (Figures 4a and 4b). However, the diurnal pattern
of transpiration rate at Site I (Figure 7a) did not follow the
diurnal pattern of stomatal conductance (Figure 4a). Likewise,
at Site II, transpiration rates were greater during the premonsoon period than during the postmonsoon period, whereas gs
was higher during the postmonsoon period.
Transpiration at Site I was 95% of PET, whereas at Site II it
was nearly 60% of PET. If we assume that transpiration during
the monsoon months is negligible because of low vapor pressure deficit and overcast conditions, the transpiration figures
reported here represent nearly 50% of the annual rainfall at
both sites. Whitehead and Kelliher (1991) have reported a
similar loss in a Pinus radiata D. Don catchment in New
Zealand. In Karnataka, India, Calder et al. (1992) and Harding
et al. (1992) have reported transpiration losses equal to or
slightly less than the annual rainfall. However, annual rainfall
reported at these sites was much less than at our study sites.
Greenwood et al. (1985) reported that eucalypt evapotranspiration can exceed annual rainfall by four times.
Although direct comparison between our experimental sites
is not possible because observations were made in different
years, it is likely that the lower density of trees at Site II, which
resulted in lower L, was mainly responsible for the lower
transpiration rates at this site compared with those at Site I.
Nevertheless, it is difficult to conclude that spacing is the sole
reason for the difference in transpiration rates between the
sites, because rainfall and atmospheric demand were much
higher at Site I than at Site II. However, some recent studies in
eucalypt catchments have shown that increased spacing and
thinning can increase water yield by reducing transpiration
(ICAR 1987, Stoneman and Schofield 1989, Cornish 1993).
In conclusion, we observed differences in water use characteristics of E. tereticornis at two sites with different climatic
and silvicultural conditions. Although direct comparisons between the sites was not possible, we obtained circumstantial
evidence of lower water use in the more widely spaced planta-

tion. Stomatal conductance was regulated by atmopheric vapor
pressure deficit as well as by leaf water potential. Even when
roots were subjected to only minor water stress, stomatal
conductance was considerably affected. Although transpiration by the more widely spaced plantation was only half that
of the more closely spaced plantation, growth was affected
during the premonsoon period depending on the magnitude of
water stress. These results have implications for forest management programs in the tropics where water conservation is
an important issue during afforestation.

Acknowledgments
We are grateful to Dr. Ian Calder, Institute of Hydrology, Wallingford,
UK who advised us on methodology during the early stages of this
work. We thank the Ministry of Environment and Forests, Government
of India for financial assistance.

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