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TreesStructure and Function ISSN 0931-1890Volume 26Number 6 Trees (2012) 26:1759-1769DOI 10.1007/s00468-012-0745-0

Transpiration in a eucalypt plantation anda savanna in Venezuela

Ana Herrera, Rosa Urich, ElizabethRengifo, Caín Ballestrini, ArmandoGonzález & Williams León

1 23

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ORIGINAL PAPER

Transpiration in a eucalypt plantation and a savanna in Venezuela

Ana Herrera • Rosa Urich • Elizabeth Rengifo •

Caın Ballestrini • Armando Gonzalez •

Williams Leon

Received: 11 January 2012 / Revised: 4 June 2012 / Accepted: 6 June 2012 / Published online: 28 June 2012

� Springer-Verlag 2012

Abstract In Venezuela 30,000 km2 of land is covered by

savannas, of which 410,000 ha have been planted with

several species and hybrids of Eucalyptus for lumber and

pulp production. Popular concern about possible diminu-

tions in water availability of reservoirs near eucalypt

plantations prompted our interest in measuring water use

by these trees. Since these savannas are markedly seasonal,

the response of species to seasonal drought is important.

We aimed to compare the seasonal changes in single-leaf

and whole-plant transpiration in a seasonally dry savanna

with that of trees of E. urophylla in an experimental

plantation. We also examined the seasonal changes in

xylem water potential and stomatal response to air water-

vapour saturation deficit (D). Transpiration in eucalypts

and the dominant savanna species Trachypogon vestitus

and Curatella americana was evaluated using measure-

ments of leaf gas exchange in all three species, sap flux in

eucalypts, microclimatic variables and allometric and

photometric determinations of green area. In E. urophylla

and T. vestitus, but not in C. americana, stomatal

conductance (gs) proved sensitive to D. Integrated values of

daily courses of transpiration rate were scaled to one ha in a

preliminary approach to estimating ecosystem transpiration

(Eha). The Eha of the savanna (the sum of Eha of T. vestitus

and C. americana) was on average 2.4 times that of

eucalypts during the daytime; when nocturnal eucalypt

transpiration was included, the value was 1.9. The evapo-

transpiration calculated by the Penman–Montieth equation

(ETc) of eucalypts was lower than the savanna all year

round. The reference crop ET (ETo) varied little throughout

the seasons, the highest value occurring in March. The ratio

Eha/ETo for the savanna was on average near one during

the dry season and almost two during the rainy season; the

corresponding value for E. urophylla was 0.6 for both

seasons. The ratio Eha/ETc was on average 0.8 for the

species and the savanna. The cumulative Eha for the days of

measurements was higher in the savanna than in the eu-

calypts during the daytime (39.8 and 17.3 mm, respec-

tively), as was the cumulative ETc (37.5 vs. 20.3 mm).

Measured and calculated cumulative ET in eucalypts,

including nocturnal values, were 22.0 and 28.4 mm,

respectively. At the leaf level, both eucalypts and trees of

C. americana apparently may have accessed water from

deep horizons, since their values of W changed less sea-

sonally than in T. vestitus. At the ecosystem level, the

species that presented the largest changes in transpiration

was T. vestitus, which markedly increased savanna tran-

spiration during the rainy season. Our results suggest that,

for the days of this study, and considering the environ-

mental conditions of the ecosystems studied, the type of

measurements and the scaling procedures, stands of

E. urophylla transpire less water than the savanna.

Keywords Curatella � Eucalyptus urophylla �Trachypogon � Savanna � Transpiration

Communicated by M. Adams.

A. Herrera (&) � R. Urich � C. Ballestrini � A. Gonzalez

Instituto de Biologıa Experimental, Universidad Central

de Venezuela, Caracas 1020, Venezuela

e-mail: ana.herrera@ciens.ucv.ve

E. Rengifo

Centro de Ecologıa, Instituto Venezolano de Investigaciones

Cientıficas, Altos de Pipe, Caracas, Venezuela

W. Leon

Facultad de Ciencias Forestales, Universidad de Los Andes,

Merida, Venezuela

123

Trees (2012) 26:1759–1769

DOI 10.1007/s00468-012-0745-0

Author's personal copy

Introduction

Replacement of grasslands with eucalypt plantations may

result in a change in the water balance, mainly through

increased transpiration and interception (Calder 1986). An

analysis of 504 annual catchment observations showed that

afforestation strongly decreased stream flow within a few

years of planting (Jackson et al. 2005).

Concern about possible diminutions in water availability

of reservoirs near eucalypt plantations has motivated

interest in measuring transpiration and related physiologi-

cal and environmental variables (Whitehead and Beadle

2004). Eucalypts have a reputation for being great water

users, but this contention is not justified in all species or

environments (Calder 1986). In Australia, eucalypts have

been planted to lower the water table and thus help reduce

soil salinity (Calder 1986).

To give just one example of great water expenditure by

eucalypts, water use of trees of E. camaldulensis exceeded

rainfall by 62 % (Calder et al. 1997). In contrast, in

Northern Australia, total water loss of the canopy of a

eucalypt open-forest making part of the tropical savanna

mosaic was 870 mm, evapotranspiration of the understory

contributing 64 % of this flow (Hutley et al. 2000). During

the rainy season, this evapotranspiration was due mainly to

evaporation from the bare soil and transpiration by the C4

grass Sorghum spp., and the annual transpiration of the

trees was lower than in the grass. Indiscriminate specula-

tion concerning the water use of eucalypts may be mis-

leading, as wide variations across species can be expected

in, among other variables, transpirational losses in a given

environment (Calder 1986).

The different components of evapotranspiration are

difficult to measure in a natural forest, as is scaling up the

leaf level measurements of gas exchange with IRGAs to

stand-level data (Kallarackal and Somen 2008). This dif-

ficulty can be partly circumvented by measuring sap flux

density (V) and multiplying V by sapwood area of the

plantation. Other appropriate scalars are total leaf area

(difficult to determine) and stem diameter, which can be

easily measured with less error than sapwood or leaf area

(Hatton et al. 1995). Attempts to extrapolate values of

transpiration measured with porometers to canopy tran-

spiration have met with mixed success because of the

poorly quantified influence of boundary layers in limiting

whole-plant transpiration (Wullschleger et al. 1998).

In Venezuela, 30,000 km2 of land is covered by savan-

nas, of which 410,000 ha have been planted with several

species of Eucalyptus. Near the town of Mapire, Estado

Anzoategui, E. urophylla has been planted at a density of

1,111 trees ha-1 (Gonzalez et al. 2005) for lumber and pulp

production. Eucalypt plantations are usually within a few

km of morichales (palm swamps), which not only constitute

important ecosystems and have high recreational value, but

are also the natural water sources for the town’s domestic

use. Afforestation companies have found resistance on the

part of the population in Mapire to eucalypt growing, since

people fear that the plantations could dry out the moric-

hales. We have no evidence that this has happened since the

eucalypts were planted more than 15 years ago.

The vegetation in the understory of the eucalypt

plantations in Mapire is almost nonexistent; plantations

are surrounded by natural savannas composed mostly of

Trachypogon vestitus (syn. montufari, plumosus and spica-

tus; C4 grass) and Curatella americana (C3 evergreen tree),

these being the most important grass and arboreal compo-

nents of the ecosystem, respectively. In sparsely treed sav-

annas of Estado Anzoategui C. americana represents 3.9 %

and T. vestitus 41 % of the importance value index (Dezzeo

et al. 2008). In trees of C. americana, both dawn and midday

xylem water potential (W) remained relatively unchanged

throughout the year at C-0.5 and -1.0 to -1.5 MPa,

respectively (Goldstein et al. 1990; Medina and Francisco

1994). Transpiration rate was as high in the middle of the

dry season as during the rainy season, which, together with a

relative seasonal constancy in W and leaf renewal during the

dry season, has been attributed to root access to water in

deep soil layers (Goldstein et al. 1990).

In T. vestitus, marked daily changes in W and transpi-

ration rate have been reported (Goldstein et al. 1990).

Tussocks of T. vestitus are perennial; leaves dieback under

drought and re-sprout after the first rains. In the grass

stratum of a Venezuelan savanna composed mostly by

T. vestitus, LAI increased with rains from 0.21 to 7.33 (San

Jose and Medina 1975). A decrease in gs of 50 % with an

increase in D of nearly three times was found in plants of

T. vestitus collected in a Venezuelan savanna and subjected

to drought under controlled conditions (Baruch et al. 1985).

From our own observations for over 10 years, eucalypt

plantations in Mapire apparently maintain a constant leaf

area index (LAI) the year round.

Among the mechanisms of drought avoidance in several

species of eucalypt, deep rooting and a high sensitivity of

gs to D have been identified (Dye and Olbrich 1993;

Phillips et al. 2010; White et al. 2000); therefore, exami-

nation of seasonal changes in gs in response to W and D is

important from the viewpoint of the water status of

eucalypts.

Endeavouring to gather data of relatively easy acquisi-

tion to begin to elucidate whether plantations use more

water than the savanna, we aimed to compare, using

measurements of leaf gas exchange, V and microclimatic

variables, the transpiration of eucalypts in an experimental

plantation in Mapire with that in a savanna. Since these

savannas are markedly seasonal, the response of species to

seasonal drought is important. We predicted that (1) in

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T. vestitus and E. urophylla, transpiration would diminish

during the dry season due to water deficit in the former,

higher evaporative demand in the latter and a reduction in

LAI in T. vestitus, and (2) in C. americana, no seasonal

changes in transpiration would be found.

Materials and methods

Field site and plant material

This study was carried from January to October 2008.

Measurements in eucalypts were carried out in five trees of

Eucalyptus urophylla S.T. Blake, with an average height of

15 m and a mean diameter at breast height of 35 cm,

experimentally planted in an area of approximately

100 m2; trees were planted in a rough circle at an

approximate distance of 5 m from each other. The location

was the Ricardo Alfonso Rojas Experimental and Techni-

cal School, Mapire, Estado Anzoategui (Venezuela) at

78420N, 64�460W. Measurements in commercial planta-

tions were not feasible because leaves grew out of reach; in

addition, access to commercial plantations is heavily

restricted by owners, and plantations are not guarded

against vandalism or attacks to the equipment by cattle. In

a savanna approximately 2 km away from the experimental

plantation, measurements were done in tussocks of Trac-

hypogon vestitus Andersson (average height 0.5 m) and

trees of Curatella americana L. (average height 6 m). The

other important arboreal component of this savanna,

Byrsonima crassifolia, is present in very low numbers,

although an IVI of 22.5 has been reported for a sampling

area of 475 km2 (Dezzeo et al. 2008).

Soil water content

Soils are ultisols, of the kandistults group, very poor,

excessively drained and with a predominantly sandy or

sandy loam texture between 40 and 100 cm; they show

deposits of kaolinite clay, are deep, friable and much lix-

iviated (Gonzalez et al. 2005). Gravimetric soil water

content was determined as SWC = 100 (fresh weight-dry

weight)/dry weight in samples taken at depths of 10 and

20 cm (five samples per depth), maintained in sealed glass

vials at 10 �C and transported to the laboratory, where

weights were determined. In the trees at the experimental

site and in tussocks in the savanna, the fine-root density at

20 cm was high and almost nonexistent below; roots of

E. urophylla probably explore deeper horizons, but we

were not allowed to excavate the tree root system. We

found in a few individuals in the commercial plantations

with partly exposed roots a pivoting root that apparently

grew very deep into the soil. In a plantation of E. urophylla

in Brazil, 64 % of fine root biomass was found in the first

20 cm (Witschoreck et al. 2003).

Microclimatic variables

Photosynthetic photon flux density (PPFD) was measured

with a 190-S quantum sensor connected to a LI-185 meter

(LI-COR Inc., Lincoln, NE). In the experimental planta-

tion, air temperature and relative humidity were measured

using two HOBO Pro V2 loggers and data dumped with a

HOBO Waterproof Shuttle (Onset Computer Corporation,

Pocasset, MA). In the savanna, air temperature was mea-

sured with HOBOs or with YSI 405 thermistors connected

to a telethermometer (Yellow Springs Instruments, Yellow

Springs, OH) and relative humidity with HOBOs or a hair

strand hygrometer (Abbeon mod. AB167B, Santa Barbara,

CA). Net radiation, wind velocity, air temperature, relative

humidity and rainfall data for two weather stations near the

study site were obtained from the Instituto Nacional de

Meteorologıa e Hidrologıa (INAMEH, Venezuela).

Xylem sap flux velocity

Xylem sap flux velocity (V) was measured with thermal

dissipation probes, TDPs, designed after Granier (1987).

Two TDPs were inserted at breast height in each one of five

trees, thermally insulated and connected to a DL2 data

logger (Dynamax, Houston, TX). Constant voltage was set

up to yield a maximum temperature difference between the

reference and the heating elements of the probe (dTmax) of

approximately 8 �C. The data logger recorded data every

minute and averaged them every 30 min. Records were

taken for a minimum of 72 h; values shown are those

collected 48–72 h after set-up. In this study, records were

not taken for longer because of safety reasons and based on

the observation that in previous determinations of V in

Ficus obtusifolia, also a latex-containing species, constant

dTmax was obtained after 24 h (Ballestrini et al. 2011).

Values of V were converted to transpiration rate (E) by

multiplying V by sapwood area. Sapwood depth was

determined in oblique sections of the trunk.

Xylem water potential

Xylem water potential was measured at 0530–0630 and

1200–1300 h with a pressure chamber (PMS, Corvallis,

OR) in leaf-bearing branches of five different trees, and in

one leaf of five different tussocks.

Leaf area index and ground cover

Leaf area index of commercial plantation eucalypts and

trees of C. americana was measured in November using a

Trees (2012) 26:1759–1769 1761

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LAI-2000 Plant Canopy Analyzer (LI-COR, Lincoln, NE).

For the determination of LAI in T. vestitus, the living mass

of five tussocks per season was harvested, and whole-plant

green leaf area obtained by multiplying the living mass by

the specific leaf area (SLA) determined in a subsample.

The LAI was calculated as LAI = green leaf area/area of

tussock. Ground cover by T. vestitus and C. americana was

estimated by counting the number of individuals in five

plots (5 9 5 m for T. vestitus and 9 9 9 m for C. ameri-

cana), estimating tussock or tree canopy area, multiplying

these values by the number of individuals in the plot and

extrapolating these values to 1 ha. Area of tussocks

was approximated to a circle; canopy area of trees of

C. americana, to polygons of variable shapes.

Leaf gas exchange

Stomatal conductance and instantaneous transpiration rate

(E) were measured with both a CIRAS 2 IRGA connected to

a PLC (B) chamber (PP Systems, Hoddesdon, UK) and an

LC4 IRGA connected to a PLC (B) chamber (Analytical

Development Co., Hitchin, UK). Previous measurements

determined that values obtained with both systems on the

same leaves at the same time were not significantly different.

Leaves of E. urophylla and C. americana used for mea-

surements were 1.5–3.5 m, and 2 m above ground, respec-

tively. Measurements were made under full sun exposure.

Individual and large-scale transpiration

For individuals of all three species, daily transpiration per unit

leaf area was calculated as the integral of the daily courses of

E. Since plant cover varies widely on a large scale, daily

integrated transpiration was calculated on a one hectare basis

(Eha), in a preliminary approach to estimate ecosystem tran-

spiration, by multiplying Ea by plant cover and LAI (savanna

species) and by LAI (eucalypt commercial plantations). Ref-

erence crop ET (ETo) and species evapotranspiration (ETc)

were calculated, with mean leaf resistances in the case of the

species, using the Penman–Monteith equation. Values of cli-

matic variables were obtained from INAMEH.

Statistics

Values at the individual level are presented as mean ± SE

where applicable, one- or two-way- ANOVA were performed

using the Statistica package.

Results

The rainfall pattern during 2008 (Fig. 1) was very similar

to the average of the previous 21 years (data not shown),

resulting in a dry season from December to April and a

rainy season from May to November, the highest rainfall

occurring in June–July. A yearly total 1,710 and 1,700 mm

was recorded for the Musinacio and the Mapire stations,

respectively. The SWC at both 10- and 20-cm depth near

plants of the three species during the dry season was nearly

half of that during the rainy season (Fig. 2). The SWC was

significantly (p B 0.04) influenced by species, season and

Month

Rai

nfal

l (m

m)

0

100

200

300

400

500

600

Jan Mar May Jul Sep Nov

Mapire

Musinacio

Fig. 1 Annual rainfall for 2008 in the study area measured at two

nearby weather stations (names and symbols indicated). Values are

data points

C. americana

SW

C (

%)

5

10

5

10

1510 cm

20 cmT. vestitus

E. urophylla

Month

0

5

10

OctJunMarJan

Fig. 2 Seasonal changes in soil water content near plants of the

species indicated. Values are mean ± SE (n = 5). Samples were

taken at a 10 cm (empty bars) and 20 cm depth (hatched bars)

1762 Trees (2012) 26:1759–1769

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their interaction at both depths. Near the eucalypts, SWC

was twice as high in March and 50 % higher in June than

near the savanna species, differences disappearing in

October. These differences with the savanna could be due

to the plantation being closer to a morichal than the field

site in the savanna.

The SLA of T. vestitus was significantly (p \ 0.05)

higher in March and June (149 cm2 g-1) than in January

(110 cm2 g-1) and October (129 cm2 g-1). From March to

June, SLA in C. americana changed from 103 to

209 cm2 g-1 and in E. urophylla from 94 to 84 cm2 g-1,

changes indicating that leaves were younger (C. ameri-

cana) or older (eucalypts) in March than in June. Green

leaf area per tussock of T. vestitus increased significantly

with rains; consequently, LAI on a stand scale varied from

2.68 (January) to 1.62 (March), 5.08 (June) and 4.77

(October). The canopy remained apparently unchanged in

trees of E. urophylla and C. americana throughout the

period of study. The LAI was 2.77 ± 0.38 in E. urophylla

and 1.14 ± 0.15 in C. americana. Savanna live plant cover

was estimated to vary in T. vestitus from 26,048 (January)

to 15,754 (March), 49,316 (June) and 46,313 (October)

m2 leaf ha-1, and remained apparently unchanged in

C. americana at 5,862 m2 leaf ha-1.

Both dawn and midday W varied significantly with rains

(Fig. 3). In T. vestitus significant decreases in both vari-

ables occurred during the dry season and, in E. urophylla, a

strong diminution in midday W occurred at the end of the

rainy season.

Seasonal changes in leaf gas exchange and related

parameters of T. vestitus and C. americana are shown in

Fig. 4. Maximum gs was lowest in January (T. vestitus) and

March (C. americana) and highest in October (both spe-

cies); maximum E occurred in October for both species,

whereas minimum E occurred in January, because of lower

gs despite higher D, and in June, because of lower

D despite higher gs. Marked changes in the daily courses of

PPFD were found, due to cloudiness.

Daily values of V in E. urophylla were always highest at

noon, the highest V occurring in October, followed by

January and March, and the lowest in June; a substantial

nocturnal transpiration was found at all seasons, averaging

14 % of daily transpiration (Fig. 5).

Seasonal changes in leaf gas exchange and related

parameters of E. urophylla are shown in Fig. 6. The highest

values of E measured with IRGA and TDPs were similar

among them, occurred at noon and varied little throughout

the year. Pre-noon values of E measured by both methods

were linearly related (r2 = 0.68, p \ 0.05); in the after-

noon, E measured with IRGA was lower than measured

with TDPs. Maximum E occurred at values of D between

1.5 and 2.3 kPa. Daily courses of E roughly followed daily

courses of PPFD at any season; in June, frequent showers

prevented measurements of PPFD.

Changes in maximum gs with dawn and midday W and

D are shown in Fig. 7. In all three species, maximum gs

was unresponsive to either dawn or midday W. In both

T. vestitus and E. urophylla, but not in C. americana, there

was a significant decrease in gs with D.

Seasonal changes in Eha, ETc and ETo are shown in

Fig. 8. The Eha in E. urophylla was always higher than in

C. americana except for October; higher than in T. vestitus

in January, and lower than the sum of Eha of T. vestitus and

C. americana, assumed to represent the Eha of the entire

savanna, throughout the seasons. Transpiration of the

savanna was on an average 2.4 times that of eucalypts

during the daytime; when nocturnal eucalypt transpiration

was included, the value was 1.9. The ETc of eucalypts was

higher than in C. americana throughout the seasons, lower

than in T. vestitus in January and March, and lower than the

savanna all year round. The ETo varied little throughout the

seasons, the highest value occurring in March. The ratio

Eha/ETo for the savanna was on average nearly one during

the dry season and nearly two during the rainy season; the

corresponding value for E. urophylla was 0.6 for both

seasons. The ratio Eha/ETc was on average 0.8 for the

species and the savanna. The cumulative Eha for the days of

measurements was higher in the savanna than in the eu-

calypts during the daytime (39.8 and 17.3 mm, respec-

tively), as was the cumulative ETc (37.5 vs. 20.3 mm).

A-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Month

Dec Feb Apr Jun Aug Oct

ψ (M

Pa

)

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

B

Fig. 3 Seasonal changes in a dawn and b midday xylem water

potential of plants of Trachypogon vestitus (circles), Curatellaamericana (squares) and Eucalyptus urophylla (triangles). Values

are mean ± SE (n = 5)

Trees (2012) 26:1759–1769 1763

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Cumulative transpiration in eucalypts, including nocturnal

values, was Eha, 22.0 mm and ETc, 28.4 mm.

These results are consistent with the observation that in

T. vestitus LAI and E increased with rains 2.3 and 1.3

times, respectively, partly explaining the increase of 2.9

times in Eha. As LAI was measured in both C. americana

and eucalypts only at the end of the rainy season, our

calculations were made with possibly the maximum LAI

for these species and still plants of T. vestitus seemed to

govern changes in ecosystem ET because of the marked

increase in their transpiratory surface.

Discussion

We have found that transpiration in E. urophylla was less

affected by drought than in T. vestitus, in which

E increased with rains; in C. americana, less seasonal

change in E was found, as hypothesized. The response of

E to rainfall was reflected in the seasonal changes of

midday W, these being significant in T. vestitus and

E. urophylla, but not in C. americana. Both T. vestitus and

E. urophylla showed lower Eha at the peak of the rainy

season (June) than at the end of it (October), in spite of

increased gs, probably due to a marked decrease in D.

Our values of dawn W were measured at sunrise, when

daily evaporative demand was beginning to increase;

therefore, they may not accurately reflect the maximum Wattained by plants. In the case of eucalypts, the occurrence

of nocturnal transpiration may have also reduced dawn W.

Dawn W decreased with drought by 0.69 MPa in T. vesti-

tus, while it remained comparatively high all year round in

C. americana, as reported by Goldstein et al. (1990) and

Medina and Francisco (1994). In E. urophylla, dawn W was

also high and changed little throughout the seasons, values

being similar to pre-dawn W in E. camaldulensis and high

as compared to values of pre-dawn W of -3 MPa in

E. leucoxylon and -4 MPa in E. platypus (White et al.

2000). The seasonal decrease in dawn W of E. urophylla

was only 0.37 MPa, which is quite small relative to sea-

sonal differences in pre-dawn W of 2.8 MPa in E. leu-

coxylon and 2.0 MPa in E. platypus (White et al. 2000).

The high dawn W found by us during the dry season

suggests that trees of C. americana and eucalypts draw

Ag s

(mm

ol m

-2 s

-1)

50

100

150

200T. vestitus

B

E(m

mol

m-2

s-1

)

2

4

6

C. americana

Hora del día

January March June October

C

D(k

Pa)

1

2

3

4

D

07 10 13 16

PP

FD

(μm

ol m

-2 s

-1)

0

500

1000

1500

2000

Time (h)

07 10 13 16 07 10 13 16 07 10 13 16

Fig. 4 Seasonal changes in daily courses of a stomatal conductance,

b transpiration rate, c air water vapour pressure deficit, and

d photosynthetic photon flux density for plants of Trachypogon

vestitus (closed symbols) and Curatella americana (open symbols).

Month indicated. Values are mean ± SE (n = 6)

1764 Trees (2012) 26:1759–1769

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water from deep soil layers, as shown for eucalypts by

Calder et al. (1997) and such high dawn W in eucalypts

during the dry season may have been in part the conse-

quence of either a higher SWC, which in turn could have

been due to reduced soil evaporation, hydraulic redistri-

bution (Goldstein et al. 2008), access to the water table or

all three reasons. Many species of eucalypts are known to

produce abundant fine roots in the uppermost horizons of

the soil (Whitehead and Beadle 2004). The higher sensi-

tivity of dawn W in T. vestitus to changes in rainfall was

attributed in a previous study (Goldstein et al. 1990) to the

shallowness of tussock root system, also found by us when

collecting samples for SWC.

Throughout the year of this study, E in E. urophylla was

similar to or even lower than in plants of T. vestitus and

C. americana, and comparable to values reported for this and

other species of Eucalyptus in a wide range of age and growth

conditions, for which mean E was 3.0 ± 0.3 mmol m-2 s-1

(Whitehead and Beadle 2004). Values of Ea measured with

TDPs were higher than in plantations in China (mean

2.3 mm day-1; Morris et al. 2004; Zhou et al. 2004), and in a

natural Australian forest dominated by E. sieberi

(2.2 mm day-1, Roberts et al. 2001), and similar to values in

E. maculata (5.1 mm day-1), E. miniata (3.8 mm day-1),

E. delegatensis (5.0–6.0 mm day-1) and E. tereticornis

(5.5 mm day-1) (data summarized by Whitehead and Beadle

2004).

Values of gs and E in T. vestitus were within the range of

those measured in a Venezuelan seasonal savanna domi-

nated by T. vestitus and Axonopus canescens (Baruch and

Bilbao 1999).

In C. americana, values of gs and E found by us coincide

with those reported for other savannas in Venezuela

(Medina and Francisco 1994; Goldstein et al. 1989). The

lack of response of gs to either dawn or midday W in

C. americana is in agreement with a report that suggests

that plants have a year-round access to soil water (Gold-

stein et al. 1990), as shown before for this species (San Jose

1977).

A strong correlation between gs and D in E. urophylla

indicates a strict control of gs by evaporative demand, as

previously found in several species of eucalypt (Dye and

Olbrich 1993; White et al. 2000). A higher E in E. uro-

phylla during March (high D) relative to June (low D) is in

concordance with the reported increase with drought in

V strongly correlated with D in E. urophylla under field

conditions (Zhou et al. 2004). The increase in V during

drought may seem contradictory with the response of gs to

D; in E. miniata and E. tetrodonta growing in seasonal

Australian savannas, an increase in E between the rainy

and the dry season was associated with an increase in

D and soil water use during drought (O’Grady et al. 1999).

This observation and our own results give further support

to the hypothesis that the trees in the present study count on

a sufficient water supply regardless of season. Low values

of LAI, a high sensitivity of gs to D and deep rooting have

been identified among the mechanisms of drought avoid-

ance in eucalypts (Whitehead and Beadle 2004).

Values of E in E. urophylla measured through sap flux

or with an IRGA were very similar from morning to

midday, when the latter were lower than the former. This

suggests that, in the morning, changes in E were due to

water loss through the leaves, whereas in the afternoon sap

flux refilled vessels and water loss to the atmosphere was

Mar17 %

0.01

0.02

0.03

Oct12 %

Time of day (h)24 12 24 12 24 12

0.00

0.01

0.02

0.03

Jun14 %V

(m

m s

-1)

0.01

0.02

0.03

Jan14 %

0.01

0.02

0.03

0.04

Fig. 5 Daily changes in sap flux of trees of Eucalyptus urophylla for

the months indicated. Values are mean ± SE (n = 6–10). The

proportion of the integrated daily values represented by night time

values is indicated in each panel. The hatched vertical bars indicate

the night period, the arrow the beginning of rains

Trees (2012) 26:1759–1769 1765

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reduced. A considerable night time V was measured; with

our data, we cannot assess whether this flux was due to

canopy evaporation or refilling. In E. parramattensis, night

time sap flux, which was 50–70 % due to refilling rather

than evaporation, was as high as 19 % of the daily flux

(Zeppel et al. 2010). In that study, among the microclimatic

variables, D was the strongest driver of nocturnal flux and

this was not the case in our study, since the highest noc-

turnal V occurred in June when maximum nocturnal D was

40 % that in March, when minimum nocturnal V occurred.

In eight species of eucalypt, nocturnal flux averaged 6 % of

daily flux; when 2-day consecutive records of V with

similar daytime D, but high and low night time D were

compared, it was estimated that 33 % nocturnal flux

reflected recharge of tree storage (Phillips et al. 2010).

Our values of V could be different from the actual sap

flux density because of lack of calibration or of assessment

of wound effect. Calibration of TDPs for a particular spe-

cies seem to be requisite for correct determination of

V (Steppe et al. 2010), despite the previous claims to the

opposite (see Sun et al. 2011). In saplings of six species

differing in xylem anatomy, the use of the coefficients

proposed by Granier (1987) for the calculation of V under-

or overestimated V measured with potometers by as much

as 34 and 55 %, respectively (Sun et al. 2011). Using

uncalibrated TDPs, sap flux of Fagus grandifolia was

underestimated by 60 % (Steppe et al. 2010). We are not

aware of any published procedure for correction for wound

depth, as already noted by Sun et al. (2011). In the present

investigation, the similarity between IRGA and TDP values

of morning E supports the idea that V was correctly

measured.

Eucalypts transpired less water than the savanna during

both seasons. Increases with rains in Eha, not only in eu-

calypts but also in T. vestitus, may have been due to an

increase in both gs and LAI. The LAI in T. vestitus was

almost the same as in eucalypts during the dry season but

twice that in eucalypts during the rainy season, which

partly helps explain why ET of the savanna is much higher

than in eucalypts during the rainy season. An increase with

rains in LAI of T. vestitus in a savanna in the Venezuelan

Llanos from to 0.6 to 4.1 corresponded to a change in

C

D(K

Pa)

1

2

3

4

B

2

4

6 fluxIRGA

D

PP

FD

(μm

ol m

-2 s

-1)

0

500

1000

1500

2000

Ag s

(mm

ol m

-2 s

-1)

100

200

300

400January

March June

October

Time (h)

E(m

mol

m-2 s

-1)

07 10 13 1607 10 13 16 07 10 13 16 07 10 13 16

March June

Fig. 6 Seasonal changes in daily courses in trees of Eucalyptusurophylla of a stomatal conductance, b transpiration rate (closedsymbols measurements made with IRGA, open symbols measure-

ments made with thermal dissipation probes), c air water vapour

pressure deficit, and d photosynthetic photon flux density. Month

indicated. Values are mean ± SE (n = 6). Lack of data in daily

courses is due to rains preventing measurement. The hatched barsindicate a period with rains

1766 Trees (2012) 26:1759–1769

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transpiration from 0.7 to 6.5 mm (San Jose and Medina

1975). In a savanna dominated by T. vestitus with scant tree

cover in Estado Monagas, Venezuela, ET decreased with

drought from approximately 3 to 1 mm day-1 (San Jose

et al. 2008). In the present study, drought caused a 34 %

decrease in Eha, suggesting that the savanna in Mapire has a

better access to water than in Estado Monagas.

The ratio Eha/ETo was lower than one for T. vestitus,

C. americana, eucalypts and the savanna throughout the

seasons, except for the savanna in October when the ratio

reached 1.53. In contrast to our results in eucalypts, values

of ETc/ETo as high as 1.96 and 2.81 were found in tropical

plantations of E. tereticornis and E. grandis, respectively

(Kallarackal and Somen 2008).

The Eha of the savanna could increase if nocturnal

transpiration was measured in T. vestitus or C. americana.

In C. americana, nocturnal stomatal aperture was demon-

strated by an infiltration method (Labouriau 1964), and

significant values of night-time transpiration were deter-

mined by Goldstein et al. (1989) by both heat pulse and

IRGA measurements (18 and 5 % of daytime transpiration,

respectively). We have found no reports of night-time

transpiration in T. vestitus, although its occurrence cannot

be discarded, since nocturnal transpiration has been found

in a number of C4 species (Caird et al. 2007).

Our values of Eha in eucalypts could be lower, since they

were calculated on a stand basis but measured in individual

trees planted at a much lower density. A rough calculation

of stand transpiration, where V was multiplied by sapwood

area of the experimental trees and the number of trees per

ha in a plantation, produced a figure 1.2 times as high as

mean Eha. Determination of the decoupling coefficient hcould modify our figures; in a 9-year-old plantation of

E. grandis h was as low as 0.25 (Mielke et al. 1999).

There were substantial differences between Eha and ETc.

One possible explanation for differences is the occurrence

of high error propagation due to the scalars used for cal-

culating ecosystem transpiration. Important sources of

error when scaling up eucalypt individual to stand values

are the incorrect determination of LAI, sapwood area

variations in the stand and errors inherent to the measure-

ment of V (Hatton et al. 1995). As for the savanna species,

incorrect measurement of LAI and lack of knowledge of hcould lead to erroneous values of ecosystem transpiration.

E. urophylla

ψ (MPa)

0

100

200

300

T. vestitus

100

200

300

400

C. americana

g s (

mm

ol m

-2 s

-1)

100

200

300

400

r2=0.12

r2=0.09

r2=0.00

r2=0.38

r2=0.11

D (kPa)

-3 -2 -1 1 2 3 4

r2=0.54

A

B

C

D

E

F

Fig. 7 Responses of stomatal

conductance of plants of

Trachypogon vestitus, Curatellaamericana and Eucalyptusurophylla to a–c dawn (filledsymbols) and midday (opensymbols) xylem water potential,

and d–f air water vapour

pressure deficit. Values are

mean of the maximum ± SE in

a, b and c and data points in d,

e and f. Species, regression lines

and determination coefficients

are indicated

Trees (2012) 26:1759–1769 1767

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Conclusions

Eucalypts and savanna species differed in their response to

water availability. At the leaf level, both eucalypts and

trees of C. americana apparently used water available from

deep horizons, since their values of W changed less than in

T. vestitus. At the ecosystem level, the species that pre-

sented the largest changes in transpiration was T. vestitus,

which markedly increased transpiration during the rainy

season. Our results suggest that, for the days of this study,

and considering the environmental conditions of the eco-

systems studied and the type of measurements and scaling

procedures, stands of E. urophylla transpire less water than

the savanna.

Acknowledgments We appreciate permission to carry out part of

this study at the ‘‘Ricardo Alfonso Rojas’’ Experimental and Tech-

nical School in Mapire. This research was funded by grant CDCH

03.00.6524.2006 (Venezuela) and Posgrado en Botanica, Fac. Cien-

cias, UCV.

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