Selection of species and provenances for low-rainfall areas: physiological responses of Eucalyptus cloeziana and Eucalyptus argophloia to seasonal conditions in subtropical Queensland

Selection of species and provenances for low-rainfall areas:physiological responses of Eucalyptus cloeziana and Eucalyptus

argophloia to seasonal conditions in subtropical Queensland

Michael R. Ngugia,*, Mark A. Huntb, David Doleyc, Paul Ryanb, Peter Darta

aSchool of Land and Food Sciences, University of Queensland, St. Lucia, Qld 4072, AustraliabCooperative Research Centre for Sustainable Production Forestry, Queensland Forestry Research Institute,

Locked Mail Bag 16, Gympie, Qld 4570, AustraliacDepartment of Botany, University of Queensland, St. Lucia, Qld 4072, Australia

Abstract

Responses of stomatal conductance (gs) and net photosynthesis (A) to changes in soil water availability, photosynthetic photon

flux density (Q), air temperature (T) and leaf-to-air vapour pressure deficit (D) were investigated in 4-year-old trees of a dry

inland provenance of Eucalyptus argophloia Blakely, and two dry inland provenances (Coominglah and Hungry Hills) and a

humid coastal provenance (Wolvi) of Eucalyptus cloeziana F. Muell. between April 2001 and April 2002 in southeast

Queensland, Australia. There were minimal differences in A, gs and water relations variables among the coastal and inland

provenances of E. cloeziana but large differences between E. argophloia and E. cloeziana. E. argophloia and to a lesser extent

the Hungry Hills (inland) provenance of E. cloeziana maintained relatively higher pre-dawn water potential (cpd) during the dry

season suggesting possible access to water at depth. Simple phenomenological models of stomatal conductance as a function of

Q, T and D explained 60% of variation in gs in E. cloeziana and more than 75% in E. argophloia, when seasonal effect was

incorporated in the model. A Ball–Berry model for net photosynthesis explained between 70 and 80% of observed variation in A

in both species. These results have implications in matching the dry and humid provenances of E. cloeziana and E. argophloia to

suitable sites in subtropical environments.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Boundary line analysis; Drought tolerance; Forest productivity; Net photosynthetic rates; Phenomenological model; Stomatal

conductance

1. Introduction

The forest industry in Queensland, Australia, is

committed to the expansion of its plantation resource

in accordance with the target for nation-wide plantings

of 80 000 ha per year as envisaged in Plantations of

Australia, The 2020 Vision (The 2020 Vision, 1997).

Planted trees will contribute to sustainable supplies of

wood products (The 2020 Vision, 1997), sequestration

of atmospheric CO2 (Rossiter and Lambert, 1998) and

help to lower water tables on lands subject to degrada-

tion by rising water tables and salinity (Sands et al.,

1999; National Land and Water Resources Audit,

2000).

Forest Ecology and Management 193 (2004) 141–156

* Corresponding author. Present address: Conservation Services,

Environmental Protection Agency, Level 8, 160 Ann Street,

Brisbane 4000, Queensland Australia. Tel.: þ61-7-3224-7100;

fax: þ61-7-3227-6386.

E-mail address: [email protected] (M.R. Ngugi).

0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2004.01.027

The current forest plantation resource in Queens-

land is about 189 000 ha (Wood et al., 2001), and is

concentrated in the humid coastal region with annual

rainfall >1000 mm. However, with increasing scarcity

and cost of land suitable for plantation forestry, expan-

sion will mainly occur in low rainfall areas with

annual rainfall between 600 and 1000 mm, and high

summertime vapour pressure deficit (Loxton and For-

ster, 2000). Eucalyptus argophloia Blakely and Euca-

lyptus cloeziana F. Muell. are among the priority

species for hardwood forest plantings in subtropical

regions of Queensland and New South Wales (Sewell,

1997; Keenan et al., 1998) but little is known of the

responses of these species to water stress. Conse-

quently, an understanding of the relationship between

growth and water use, and physiological responses to

changes in environmental variables is required for a

more insightful selection of species for planting in low

rainfall zones.

The growth and productivity of trees is determined by

their genetic potential and plant–environment interac-

tions that influence the rates of physiological processes

(Teskey et al., 1987; Beadle and Turnbull, 1992).

Availability of water, nutrients and suitable tempera-

tures are among the most important factors that limit

plant productivity and yield across sites (Turner and

Kramer, 1980; Kramer, 1983; Beadle and Turnbull,

1992; Landsberg, 1997). Studies of water relations of

various eucalypts growing in their natural environments

during times of severe drought have reported either

avoidance or tolerance, or both mechanisms of adapta-

tion, to water stress (Doley, 1967; Sinclair, 1980; Atti-

will and Clayton-Greene, 1984; Davidson and Reid,

1989; Prior et al., 1997b; Prior and Eamus, 1999).

Species that are profligate in their water use are reported

to avoid tissue water deficit by maintaining access to a

water source through deep root systems (Doley, 1967;

Sinclair, 1980; White et al., 2000) enabling them to

maintain high pre-dawn water potentials, relatively

high stomatal conductance and presumably continued

transpiration even under moderate water stress (David-

son and Reid, 1989; Abrams, 1990). Restriction of

water loss through stomatal closure and increase in

water uptake by increased root growth are important

drought avoidance mechanisms for maintaining posi-

tive turgor, growth and survival of plants. However

these strategies can be associated with decreases in the

rate of photosynthesis and the amount of carbohydrate

available for above-ground biomass production, respec-

tively (Kramer, 1983; Jones, 1993; Nilsen and Orcutt,

1996).

E. cloeziana has a wide natural distribution in

coastal (annual rainfall > 1400 mm) and inland

(annual rainfall < 700 mm) Queensland (Boland

et al., 1984). The large differences in rainfall, tem-

perature, geology and soils between habitats suggest

that the species has a high level of genetic variability.

Nevertheless, a physiological basis of intra-specific

variation in growth and survival under drought con-

ditions has not been established for this species. E.

argophloia has a restricted natural distribution near

Chinchilla in southern inland Queensland, where it

occurs under warm subhumid (annual rainfall around

700 mm) conditions (Boland et al., 1984). There is

very limited information on the silviculture of this

species.

In a glasshouse environment E. argophloia main-

tained higher relative water content at turgor loss

point, bulk modulus of elasticity and apoplastic water

content, and developed lower xylem pressure potential

(Ngugi et al., 2003a). These results suggest that E.

argophloia is more drought tolerant and can extract

more water from drying soil than E. cloeziana and

consistent with anatomical characteristics showing

that E. argophloia and to a lesser extent a dry inland

provenance of E. cloeziana have a greater total leaf,

palisade, epidermal layer and cuticle layer thickness

than a humid coastal provenance of E. cloeziana

(Ngugi et al., 2003a). Under well-watered and

water-limited conditions E. argophloia maintained

higher gas exchange and greater biomass production

than E. cloeziana (Ngugi et al., 2003b). Thus there

appear to be limited differences between E. cloeziana

provenances and distinct differences between E.

cloeziana and E. argophloia in responses to drought.

This study investigated growth, and diurnal and

seasonal gas exchange characteristics of a single

dry inland provenance of E. argophloia and a humid

coastal and two dry inland provenances of E. cloezi-

ana in response to the prevailing environmental con-

ditions in southeast Queensland over an annual

weather cycle. We determined: (1) key differences

in drought resistance among E. argophloia and inland

and coastal provenances of E. cloeziana; and (2)

responses of stomatal conductance and photosynthesis

to changes in environmental variables.

142 M.R. Ngugi et al. / Forest Ecology and Management 193 (2004) 141–156

2. Materials and methods

2.1. Site description

Measurements were made at two forest sites: Beer-

burrum State Forest (humid site) and Benarkin State

Forest (subhumid site) in southeast Queensland, Aus-

tralia. The climate of both sites is subtropical, char-

acterised by hot humid summers and cool, dry winters.

December and January are the hottest months with

mean maxima over 29 8C, whilst July is the only

month in which mean daily temperature falls below

10 8C (Linacre and Hobbs, 1977). Trees on these two

sites were established by Queensland Forestry

Research Institute (QFRI) on 8 April 1998 as a part

of a silvicultural experiment investigating growth

differences among species and provenances of several

potential commercial hardwood species.

At Beerburrum SF (589), the experimental plots

were located in Compartment 7, Basin Logging Area

(278000S, 1528500E) at approximately 60 m altitude.

The soil was predominantly yellow kandosol (yellow

earth), with a dark-brown sandy clay loam A horizon

(30 cm) and a light yellow-brown sandy clay B hor-

izon (90 cm). The mean soil depth was >0.9 m and

there were no mottles at this depth. The average slope

was 48 and the aspect easterly. Annual rainfall ranges

between 1000 and 1600 mm and mean minimum and

maximum temperatures of the coldest and hottest

months are 7.4 and 29.1 8C, respectively. Two prove-

nances of E. cloeziana; Coominglah (inland) from

Monto and Wolvi (coastal) from Toolara Forest

(Table 1), were investigated on this site. The ground

was cultivated using a chopper roller, weeds were

controlled with an application of simazine (6 l ha�1)

and the seedlings were fertilised using a complete

fertiliser (N, P, K, Cu, Zn, S and B) at the rate of 440 g

per tree.

At Benarkin SF (283), measurements were made in

Compartment 202, Cherry Logging Area (268550S,

1528080E) at approximately 440 m altitude. The site

was flat and the soil type was krasnozem (red ferrosol),

with a dark red-brown clay loam (10 cm) and light

clay (25 cm) A horizon and a red-brown light clay B

horizon (90 cm). The mean soil depth was >0.9 m and

there were no mottles at this depth. The annual rainfall

is approximately 900 mm and mean monthly mini-

mum and maximum temperatures of the coldest and

hottest months are 3.8 and 29.0 8C, respectively.

Ground preparation, weed control, and fertiliser appli-

cation methods and rates were similar to those used

at Beerburrum SF. Three provenances of E. cloeziana;

Coominglah, Wolvi and Hungry Hills, and a single

provenance of E. argophloia were investigated

(Table 1).

At each site, the selected provenances were

planted in adjacent plots (0.01 ha) of three rows

by seven trees at an initial spacing of 4 m between,

and 2.5 m within rows: half the stems were thinned at

age 2–3 years. Three healthy growing trees selected

from each plot and provenance at each site were

marked and used for gas exchange and leaf water

potential measurements throughout the study. The

mean height and diameter at breast height (1.3 m) of

the selected trees were measured in April 2002 at age

4 years. There were no significant (P < 0:05) differ-

ences in diameter at breast height among E. cloezi-

ana provenances at Beerburrum and Benarkin and

small differences in height growth (Table 2). The

height and diameter values for E. argophloia

were significantly lower than those of E. cloeziana

provenances (Table 2). Rainfall distributions at

Table 1

Seed source information for the four Eucalyptus provenances investigated at Beerburrum and Benarkin forest sites (experiment was

established by Queensland Department of Primary Industries—Forestry)

Provenance Seedlot

no.

No. of

parent trees

Origin

Locality Latitude Longitude Altitude (m) Mean annual

rainfall (mm)

E. cloeziana humid 4367 10 Toolara SF (627), Wolvi 268070S 1528500E 120 1200

E. cloeziana dry 7 10þ Coominglah SF (28), Monto 258000S 1518000E 400 730

E. cloeziana dry 12195 10 Hungry Hills, Eidsvold 258180S 1518220E 310 780

E. argophloia dry 5520 18 Ballon SF (28), Chinchilla 268200S 1508200E 300 685

M.R. Ngugi et al. / Forest Ecology and Management 193 (2004) 141–156 143

Beerburrum and Benarkin over the measurement

period (April 2001–April 2002) were generally simi-

lar (Fig. 1), but the total rainfall recorded at Benarkin

(760 mm) for the period was greater than that at

Beerburrum (720 mm).

2.2. Leaf gas exchange and water potential

Physiological data were collected over 2 days at 3-

monthly intervals, coinciding with the mid-month of

the four seasons: autumn, winter, spring and summer

in eastern Queensland. The measurements were made

on 16 and 17 May, 17 and 18 July, and 22 and 23

October 2001, and 21 and 22 January, and 17 and 18

April 2002 at Beerburrum and Benarkin sites, respec-

tively. Leaf gas exchange and leaf water potential were

measured on at least three young and fully formed

leaves from terminal twigs on the upper crown of each

sample tree using an LI-6400 portable photosynthesis

system (Li-Cor, Inc., Lincoln, NE) and a Scholander-

type pressure chamber (PMS Instruments, Corvallis,

OR), respectively. The tree crowns were accessed

using a hydraulic Cherry Picker mounted at the back

of a light truck.

Table 2

Average height and diameter at breast height (dbh, 1.3 m) of 4-year-old trees investigated for gas exchange and water relations as measured on

17 and 18 April 2002

Site Provenance Height (m) dbh (cm)

Beerburrum (humid site) E. cloeziana (Coominglah) 12.1 � 0.8 ab 14.0 � 0.9 a

E. cloeziana (Wolvi) 13.3 � 0.7 a 15.7 � 0.9 a

Benarkin (subhumid site) E. argophloia 6.6 � 0.3 c 8.5 � 0.3 b

E. cloeziana (Coominglah) 10.5 � 0.2 b 13.0 � 0.9 a

E. cloeziana (Hungry Hills) 11.7 � 0.4 ab 14.3 � 1.4 a

E. cloeziana (Wolvi) 10.9 � 0.7 b 14.1 � 1.4 a

For each variable, mean � standard errors having the same letters are not significantly different at P < 0:05.

Fig. 1. Total monthly rainfall for Beerburrum and Benarkin sites between April 2001 and April 2002. Leaf water relations and gas exchange

data were collected in May, July, October 2001, and January, April 2002.


Measurements of leaf water potential were taken

pre-dawn (0500 h) and at two hourly intervals between

0800 and 1600 h at Beerburrum, and at pre-dawn,

mid-morning, mid-day and mid-afternoon at Benar-

kin. For the measurement of leaf water potential, a

twig was excised from the crown and immediately

sealed in a plastic bag until leaves were cut from the

twig and measured within 2–5 min.

Gas-exchange measurements were taken concur-

rently with leaf water potential at both sites between

0800 and 1600 h. Although clouds and rain prevented

full sets of measurements from being obtained on

several days, all were restricted to fully sunlit portions

of the crown at the ambient temperature. The cuvette

was kept open and in the shade when not in use to

minimise heating and to ensure that the temperature

of the chamber remained close to ambient. The humid-

ity within the chamber was maintained as close as

possible to ambient during measurement. Most of

the gas-exchange measurements were made on light

saturated leaves (photosynthetic photon flux density,

Q > 800 mmol m�2 s�1). On two occasions, the LI-

6400 used was equipped with a 6400-02B LED light

source installed above the leaf chamber. Incident Q was

maintained at 800 mmol m�2 s�1 on the first occasion

and on the second, at 1500 mmol m�2 s�1.

Data on air temperature (T), leaf-to-air vapour

pressure deficit (D) and Q were measured concurrently

with leaf net CO2 and water vapour exchange by

sensors located within the cuvette of LI-6400 system.

2.3. Data analysis

Analysis of variance was performed by PROC GLM

(SAS Institute, Cary, NC) to compare physiological

variables between and among provenances at each

site. When differences were significant at P < 0:05,

means were compared using Duncan’s multiple range

test. The potential influence of environmental and

physiological factors on gas exchange was elucidated

using ‘boundary line analysis’ (Webb, 1972; Cham-

bers et al., 1985). The upper limit of the scatter of

points was used to delineate the response of the

dependent variable to the particular independent vari-

able when other variables were not limiting (Jarvis,

1976). Boundary lines and threshold values of these

relationships were visually estimated in a similar

manner as in other field ecophysiological studies of

stomatal conductance and photosynthesis (e.g. White-

head et al., 1981; Foster and Smith, 1991; Foster,

1992; Grossnickle and Arnott, 1992; White et al.,

1999; Kolb and Stone, 2000; Horton et al., 2001).

However, the validity of estimates obtained with this

analysis depends on having sufficient data to describe

the upper limit and the assumption of there being no

measurement errors for points on the upper boundary

(Chambers et al., 1985).

A phenomenological model (Jarvis, 1976) of sto-

matal conductance was developed from the data set

using a combination of boundary line analysis and

non-linear and linear regression analysis methods

(SAS Institute, Cary, NC) as described by Dye and

Olbrich (1993) and White et al. (1999). Stomatal

conductance was plotted against simultaneously mea-

sured values of photosynthetic photon flux density, air

temperature and vapour pressure deficit. A Ball–Berry

model (Collatz et al., 1991) that predicts assimilation

using stomatal conductance and relative humidity

(Katul et al., 2000) was fitted to the assimilation

dataset using linear regression.

3. Results

3.1. Water potential, gas exchange and

microclimate

The driest period of the year was between June and

September. However, the total amount of rainfall

recorded over a period of 15 days preceding the dates

of field data collection at the two sites was highly

variable, with no rain event recorded in July at Benar-

kin and only a small amount (2.4 mm) at Beerburrum.

The lowest values of pre-dawn leaf water potentials

were observed in July (winter) whilst the lowest mid-

day leaf water potentials were obtained in January

(summer).

Diurnal trends of water potential, gas exchange,

temperature and leaf-to-air vapour pressure deficit for

July (cold and dry) and January (warm and wet) are

shown in Fig. 2. Leaf water potential (c) was highest

at pre-dawn followed by a decrease to a minimum at

mid-day and recovery in the late afternoon but not to

the pre-dawn level. The reduction in c at mid-day was

greater in January than July. This was associated with

higher daily maximum air temperatures and leaf-to-air


Fig. 2. Daily time course of, air temperature (T, closed triangles) and leaf-to-air vapour pressure deficit (D, closed squares), and leaf water

potential (c), stomatal conductance (gs) and net photosynthesis (A) in July (winter) and January (summer) for (a) inland (open squares) and

coastal (diamonds) provenances of E. cloeziana at the humid site (Beerburrum) and (b) for E. argophloia (open triangles), and coastal

(diamonds) and inland (Coominglah, open squares and Hungry Hills, closed circles) provenances of E. cloeziana at the subhumid site

(Benarkin).


vapour pressure deficits (D). Air temperature (T) and

D increased until mid-day and then remained gener-

ally high during the afternoons. At both sites there

were limited differences in the daily course of cbetween humid coastal and dry inland provenances

of E. cloeziana but throughout the year, E. argophloia

at Benarkin had higher pre-dawn water potential (cpd)

than the E. cloeziana provenances. During July a dry

inland provenance (Hungry Hills) of E. cloeziana

showed higher cpd than the humid coastal (Wolvi)

and another dry inland (Coominglah) provenance of

the same species (Fig. 2).

Fig. 2. (Continued ).


The highest values of cpd (>�0.3 MPa) at both

sites were observed in autumn (May 2001, data

not shown). The lowest values of cpd (�0.95 and

�1.65 MP) for inland and coastal provenances

of E. cloeziana at Beerburrum and Benarkin

were observed in July: c decreased to �2.01 and

�2.73 MPa, respectively, at mid-day but recovered

in the late afternoon. Similarly the lowest value of

cpd for E. argophloia (�0.56 MPa) was observed in

July and decreased to a c of �1.96 MPa at mid-day.

The lowest values of c at mid-day in July were

greater than those obtained at mid-day in January

(Fig. 2a and b).

Stomatal conductance (gs) and net photosynthetic

rates (A) of E. cloeziana provenances and that of E.

argophloia were greatest between 0800 and 1000 h,

but decreased with increasing T and D (Fig. 2). The

pattern of leaf gas exchange was similar for both

provenances of E. cloeziana at Beerburrum except

for January when the Coominglah provenance had

greater mid-day gs and A. Although, there was a mid-

day decrease in gs for E. cloeziana and E. argophloia

at Benarkin, E. argophloia maintained greater gs

throughout all measurement dates.

3.2. Stomatal conductance models

The relationship between stomatal conductance of

upper canopy leaves and environmental variables

determined using boundary line analysis was utilised

to develop phenomenological models of the form

proposed by Jarvis (1976) to predict stomatal con-

ductance. Since large physiological differences

between E. cloeziana and E. argophloia and limited

Fig. 3. The relationship between stomatal conductance (gs) and photosynthetic photon flux density (Q) for (a) E. cloeziana and (b) E.

argophloia measured from upper canopy foliage. Eq. (1) was fitted to the boundary line scaled to a maximum gs of 435 mmol m�2 s�1 for E.

cloeziana and 460 mmol m�2 s�1 for E. argophloia. The data at 800 and 1500 mmol m�2 s�1 were those collected using LICOR-6400

equipped with a light source.


differences among E. cloeziana provenances were

found in this and previous studies (Ngugi et al.,

2003a,b), separate models were fitted to each species.

Seasonal variation was incorporated by determining

parameters for a particular season only when this

improved the fit of the overall model.

3.2.1. Photosynthetic photon flux density (Q)

A non-rectangular hyperbolic (Thornley and John-

son, 1990) function (ƒ(Q)) was fitted to normalised

data of stomatal conductance (gs) plotted against Q.

Normalised values of stomatal conductance were

obtained by dividing measured values of gs by the

maximum value (gs,max) obtained from the upper limit

of the boundary lines in Fig. 3

f ðQÞ ¼ 1

2yaQ þ 1 �

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðaQ þ 1Þ2 � 4yaQ

q� �(1)

The maximum stomatal conductance (gs,max), slope

(a) and shape (y) parameters for E. cloeziana were

435 mmol m�2 s�1, 0.006 and 0.71 and for E. argoph-

loia were 460 mmol m�2 s�1, 0.004 and 0.86, respec-

tively.

3.2.2. Air temperature (T)

A function f(T) described by Jarvis (1976) was fitted

to the upper boundary line curve of a plot of normal-

ised gs against air temperature

f ðTÞ ¼ bðT � T1ÞðTh � TÞc ð0 � f ðTÞ � 1Þ (2)

where

c ¼ Th � T0

T0 � T1

and b ¼ 1

ðT0 � T1ÞðTh � T0Þc (3)

The normalised values for f(T) vary from 1 at T0 (the

temperature at maximum conductance) and zero at Tl

and Th, the lower and the upper end of the temperature

range, respectively, as shown in the boundary line fit

(Fig. 4). The assigned values for the two species were:

T1 ¼ 0 8C, T0 ¼ 28 8C and Th ¼ 50 8C.

Fig. 4. A boundary line showing the relationship between stomatal conductance (gs) and air temperature (T) for (a) E. cloeziana and (b) E.

argophloia. The fitted model was scaled to a maximum of 435 mmol m�2 s�1 for E. cloeziana and 460 mmol m�2 s�1 (b) for E. argophloia.

T0 ¼ 0 8C, Tm ¼ 28 8C and Th ¼ 50 8C for both species.


3.2.3. Leaf-to-air vapour pressure deficit (D)

To obtain f(D), all measured stomatal conductance

values were first adjusted to remove the influence of Q

and T (White et al., 1999), and the adjusted stomatal

conductance (gs0) was plotted against D

gs0 ¼ gs

f ðQÞf ðTÞ (4)

f ðDÞ ¼ a expðbDÞ (5)

where a and b are regression coefficients. An expo-

nential decay function (f(D)) was fitted to the plot of

adjusted gs0 and D. The fit of the equation was

improved by identifying seasons with marked differ-

ences from the others. In E. cloeziana, separate curves

were fitted for data collected during summer, and

those collected during winter, spring and autumn

(Fig. 5a) whereas in E. argophloia separate curves

were fitted to data collected during winter and those

collected during summer, autumn and spring (Fig. 5b).

3.2.4. Stomatal conductance model performance

The final formulation of the equation for predicting

gs was

gs ¼ gs;maxf ðQÞf ðTÞf ðDÞ (6)

In E. cloeziana, Eq. (5) explained 60% of the observed

gs during summer and 58% of observed gs during

autumn, winter and spring whilst for E. argophloia,

Eq. (5) explained 76% of the observed gs during

spring, summer and autumn, and 79% of the observed

gs during winter. To evaluate the performance of

Eq. (5) in predicting gs, a linear regression line fitted

to the plot of observed against the predicted values

was compared to a 1:1 line (Fig. 6). The equation

overestimated lower and underestimated higher values

of gs for the two species but more so in E. cloeziana

with intercept of 44 mmol m�2 s�1 and slope of

0.6 than in E. argophloia with intercept of

19 mmol m�2 s�1 and slope of 0.89.

Fig. 5. The relationship between stomatal conductance (adjusted to maximum irradiance and air temperature) (gs0) and vapour pressure deficit

(D) for (a) E. cloeziana during summer (closed symbols, y ¼ 1226:3 e�0:68x, r2 ¼ 0:57), and winter, spring and autumn (open symbols,

y ¼ 553:84 e�0:71x, r2 ¼ 0:71) and (b) E. argophloia during spring, summer and autumn (open symbols, y ¼ 584 e�0:47x, r2 ¼ 0:84), and

winter (closed symbols, y ¼ 416 e�0:78x, r2 ¼ 0:40).


3.3. Net photosynthesis model

The Ball–Berry model (Collatz et al., 1991) is an

empirical relationship that describes the correlation

between A and gs and includes the effects of relative

humidity (RH) and CO2 concentration (Cs) at the leaf

surface

gs ¼ mA RH

Cs

þ b (7)

This equation can be re-arranged as (Katul et al., 2000)

A ¼ b þ Csgs

m RH(8)

The term m is the slope and b is the intercept obtained

by linear regression analysis of data obtained from

leaf gas-exchange measurements. However these

equations do not apply when A tends to zero,

Q < 50 mmol m2 s�1 and Cs < 100 mmol m�2 s�1

(Collatz et al., 1991). The regression coefficients

for E. cloeziana were m ¼ 0:088 and b ¼ 3:978,

and for E. argophloia m ¼ 0:093 and b ¼ 22:689.

In E. cloeziana, Eq. (7) explained 72% of the observed

A during summer and 81% of observed A during

autumn, winter and spring whilst for E. argophloia,

Eq. (7) explained 72% of the observed A during spring,

summer and autumn, and 77% of the observed A

during winter. The regression line fitted to a plot of

observed values of net photosynthesis against the

predicted values and relative to a 1:1 line (Fig. 7)

showed that Eq. (7) slightly overestimated lower and

underestimated higher values of A in E. cloeziana with

Fig. 6. A comparison between observed and predicted stomatal conductance (gs) for (a) E. cloeziana with a regression line y ¼ 44 þ 0:59x,

r2 ¼ 0:61 and (b) E. argophloia with a regression line y ¼ 19:4 þ 0:89x, r2 ¼ 0:81, in relation to a 1:1 line (dashed).


intercept of 0.648 mmol m�2 s�1 and slope of 0.94

whereas in E. argophloia lower values were over-

estimated and higher values were underestimated as

shown by the fitted regression line with intercept of

5.3 mmol m�2 s�1 and slope of 0.60.

4. Discussion

There were large differences in gas exchange and

water relations between E. argophloia and E. cloezi-

ana. On the humid Beerburrum site, the coastal and

inland provenances of E. cloeziana differed little in

gas exchange except in summer when the coastal

provenance reduced gas exchange more in response

to increased vapour pressure deficit. On the subhumid

Benarkin site, E. argophloia showed higher pre-dawn

water potential during summer and winter compared

to the E. cloeziana provenances, and maintained

higher stomatal conductance (gs) and net photosynth-

esis (A) during periods of high leaf-to air vapour

pressure deficit (>3 kPa). Provenances of E. cloeziana

differed little at Benarkin site.

The diurnal trend for leaf water potential (c) for E.

cloeziana and E. argophloia was typically of a sinu-

soidal nature (Sellin, 1999) characterised by a high

pre-dawn value, a mid-day depression and a slight

recovery in the late afternoon. Similar sinusoidal

patterns have been reported for field grown trees of

Eucalyptus fasciculosa (Sinclair, 1980), E. globulus

(Pereira et al., 1986), E. macrocarpa (Attiwill and

Clayton-Greene, 1984) and for Quercus species

Fig. 7. Measured net photosynthesis compared with those predicted by the Ball–Berry model for (a) E. cloeziana and (b) E. argophloia. A

regression line for the E. cloeziana data was y ¼ 0:94x þ 0:65, r2 ¼ 0:70 and y ¼ 0:59x þ 5:51, r2 ¼ 0:50 for E. argophloia. The dashed line

is 1:1.


(Abrams et al., 1990) during periods when soil water

was not severely limiting. A marked seasonal variation

existed, with a greater mid-day depression during the

warm and humid summer than during the cold and dry

winter as well as a decreasing tendency for recovery in

the afternoon (Fig. 2). However, the range of water

potential values (�0.56 to �2.73 MPa) obtained at

both Beerburrum and Benarkin did not indicate severe

water stress at any time of the year.

Unimodal curves with mid-morning (0800–1000 h)

maxima in gas exchange and limited recovery in the

late afternoon obtained for the coastal and inland

provenances in this study were associated with vapour

pressure deficits that increased from mid-morning and

remained high in the afternoon. This pattern of

response is similar to that reported for Callitris colu-

mellaris and E. macrocarpa in subhumid New South

Wales (Attiwill and Clayton-Greene, 1984), E. tetro-

donta in northern Australia (Prior et al., 1997a), Pinus

ponderosa in northern Arizona (Kolb and Stone,

2000), Prunus amygdalus in northeast Portugal

(Matos et al., 1998), and Quercus and Castanea

species in North America (Abrams et al., 1990).

The highest values of A obtained for E. cloeziana

provenances and for E. argophloia were within the

range of 18.3–22.2 mmol m�2 s�1 reported for E.

nitens in Tasmania (Battaglia et al., 1996) and higher

than those reported for field grown E. globulus

(12 mmol m�2 s�1) trees in Portugal (Pereira et al.,

1986).

During critical periods of high evaporative demand

in summer, the dry inland provenance of E. cloeziana

at Beerburrum and E. argophloia at Benarkin devel-

oped lower water potential while maintaining signifi-

cantly higher gs and A than the humid coastal

provenances of E. cloeziana (Fig. 2). A similar low-

ering of leaf water potential reported for Eucalyptus

wandoo (Colquhoun et al., 1984) and for Eucalyptus

pulchella (Davidson and Reid, 1989) was associated

with the ability to extract more water from the soil—a

characteristic reported for eucalypts growing on

drought prone sites (Doley, 1967; Ladiges, 1975;

Clayton-Greene, 1983). The tendency to maintain

relatively high gas exchange indicates little regulation

of water loss as has been reported for E. marginata

(Doley, 1967; Colquhoun et al., 1984), E. calophylla

(Colquhoun et al., 1984), E. microcarpa (Attiwill and

Clayton-Greene, 1984) and E. leucoxylon (Sinclair,

1980). In contrast, stomatal closure of the humid

coastal provenances of E. cloeziana at mid-day illus-

trates the ability of the provenance to effectively

control water loss when evaporative demand is great-

est. A similar response to that of the coastal prove-

nance has been reported for E. maculata and E.

saligna (Colquhoun et al., 1984).

The significantly higher and more seasonally con-

stant pre-dawn water potential cpd for E. argophloia

provided strong evidence that this species is more

effective in avoiding soil water deficit than E. cloezi-

ana. This finding was consistent with glasshouse

results showing that E. argophloia was more drought

resistant than E. cloeziana (Ngugi et al., 2003a,b). The

Hungry Hills (inland) provenance of E. cloeziana also

maintained significantly higher and more constant cpd

during winter and summer demonstrating greater

drought avoidance than the Wolvi (humid) and Coo-

minglah (inland) provenances of E. cloeziana

(Fig. 2b). A similar tendency to maintain relatively

small changes in cpd during wet and dry seasons

reported for E. camaldulensis and E. saligna in Med-

iterranean southwest Western Australia has been taken

to indicate that the species avoided severe stress by

accessing water in the capillary fringe of the water

table (White et al., 2000). It is likely that E. argophloia

also avoided severe water deficits through the devel-

opment of an effective root system. This contention is

consistent with the observation of more extensive root

development in E. argophloia than in E. cloeziana

under water deficits in glasshouse experiments (Ngugi

et al., 2003c). The larger range of cpd values of the

coastal and inland provenances of E. cloeziana

obtained during the wet and dry season (data not

provided) possibly indicate access to water from parts

of the soil profile that are subject to changes in soil

water content as reported for E. leucoxylon and E.

platypus (White et al., 2000).

The simple phenomenological models developed

for E. cloeziana and E. argophloia in this study

provide a basis for examining the effect of environ-

mental variables on stomatal conductance. These

models use maximum stomatal conductance (gs,max)

as a single plant parameter in a similar manner to

models developed by Jarvis (1976) and applied to E.

globulus and E. nitens (White et al., 1999), E. grandis

(Dye and Olbrich, 1993), Quercus rubra (Ogink-Hen-

driks, 1995) and several conifer species(Livingston


and Black, 1987). The gs,max for E. argophloia

(460 mmol m�2 s�1) and E. cloeziana (435 mmol

m�2 s�1) were observed between 0800 and 1000 h

in summer and autumn (wet seasons) and are greater

than 387 mmol m�2 s�1 reported for irrigated E. glo-

bulus and E. nitens (White et al., 1999) and

320 mmol m�2 s�1 for rainfed trees of E. grandis

(Dye and Olbrich, 1993).

The use of the three explanatory variables light

(Q), temperature (T) and leaf-to-air vapour pressure

deficit (D) and incorporating the seasonal effect in the

vapour pressure deficit function to describe the adap-

tation of gs to environment explained 60% of the

variation in gs in E. cloeziana and up to 80% in E.

argophloia. This is consistent with the suggestion of

Jarvis (1976) that parameters in the phenomenologi-

cal model depend on the physiological condition of

the plants, previous weather and season. The response

of gs to light was typical of that found in woody

species (Livingston and Black, 1987; Dye and

Olbrich, 1993; White et al., 1999). However, the

distribution of data points under the upper boundary

line was partly limited by the use of a complementary

light source on several occasions in order to maintain

light saturation at the leaf surface (Fig. 3). The shape

and slope parameters obtained for E. argophloia are

similar to those reported for E. globulus and E. nitens

(White et al., 1999) and resulted in a steeper rise in gs

with increasing photon flux density than in E. cloezi-

ana (Fig. 5).

Most of the measurements in this study were made

on light saturated leaves (Q > 800 mmol m�2 s�1) and

air temperatures ranging from 16 to 38 8C, hence the

predicted gs was more responsive to D than Q. While

most plants exhibit a decline in gs with D, there is

considerable variation between species in the sensi-

tivity of the response (Whitehead et al., 1981). In the

f(D) function, the slope parameters for E. cloeziana

during summer (�0.68), and autumn, winter and

spring (�0.71) were greater than those for E. argoph-

loia during spring, summer and autumn (�0.47), E.

globulus and E. nitens (�0.63) (White et al., 1999),

and E. grandis during summer (�0.30) and winter

(�0.61) (Dye and Olbrich, 1993). The greater slope

(more negative) for E. cloeziana indicates that their

stomata are more sensitive to D leading to a steeper

decline in gs with increasing D. The winter data for E.

argophloia had the highest slope but the r2 for the

curve was small (0.40), reflecting the limited distribu-

tion of data (Fig. 3).

The gs model for E. cloeziana consistently under-

estimated gs > 300 mmol m�2 s�1. These data were

obtained mainly between 0800 and 0900 h during

winter and autumn when D was <2 kPa and during

summer when D was 2–3 kPa. Since day-time tem-

peratures were lowest during these hours, stomatal

response was mainly attributed to light. This under-

estimation was larger in E. cloeziana than in E.

argophloia. The simple conductance model provided

here appears suitable for predicting gs of both species,

however an independent data set is required to validate

these predictions.

A model that estimates net photosynthesis (A) from

measured stomatal conductance and environmental

variables based on the Ball–Berry model (Collatz

et al., 1991) gave satisfactory predictions of A for

E. argophloia and E. cloeziana. For both species the

model explained more than 70% of the measured

variation in A compared to 60% reported for Pinus

taeda (Katul et al., 2000). An alternative to the use of

relative humidity in the Ball–Berry model that

involves the use of a vapour pressure deficit correction

function, has been suggested as a possible improve-

ment (Leuning, 1995; Monteith, 1995). Further work

should examine whether these functions could

improve the prediction of the fitted model.

Minimal differences in the responses of the coastal

and inland provenances of E. cloeziana to environ-

mental factors at Beerburrum and Benarkin are indi-

cated by a lack of significant difference (P < 0:05) in

seasonal cpd values, maximum A and gs values, and

height and diameter growth at age 4 years. These

results are consistent with the limited differences in

rainfall at the two field sites (Fig. 1) and suggest that

traits conferring drought tolerance in the inland pro-

venance, such as thicker leaves (Ngugi et al., 2003a)

do not limit productivity significantly when the pro-

venance is grown on a humid site. However when E.

argophloia and E. cloeziana provenances were grown

adjacent to each other at Benarkin (long-term annual

rainfall < 1000 mm), differences in gas exchange

and water relations variables were observed indica-

ting physiological differences in adaptation to envir-

onmental variables. The smaller height and diameter

growth of E. argophloia at Benarkin relative to

E. cloeziana provenances (Table 2) may probably


indicate greater allocation to below-ground biomass, a

common genotypic adaptation to water-limited envir-

onments. During the course of this study, differences

in rainfall between Beerburrum and Benarkin were

small, and the totals were below the long-term mean

annual rainfall for each site. However, physiological

variables were more correlated to rainfall in the pre-

ceding 15 days than to the monthly totals. Conse-

quently, differences between species and among

provenances were more pronounced during the winter

and summer and periods that were characterised by

high evaporative demand.

Acknowledgements

M. Ngugi was supported by a University of Queens-

land Graduate School postgraduate scholarship, a

School of Land and Food Sciences scholarship and

Wilf Crane Memorial Award (Institute of Foresters of

Australia). We thank Dr. Mark Lewty and Mr. Geoff

Dickinson (Queensland Forest Research Institute

(QFRI)) for allowing us to use the two experimental

sites, Messrs Jaimie Cook and Mark MacDonald

(QFRI) for assistance with field measurements and

Dr. Christopher Lambrides, CSIRO Plant Industry,

Brisbane, for the loan of LI-6400 portable photosynth-

esis system.

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Selection of species and provenances for low-rainfall areas: physiological responses of Eucalyptus cloeziana and Eucalyptus argophloia to seasonal conditions in subtropical Queensland

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