ORIGINAL PAPER

Hydraulic redistribution in Eucalyptus kochii subsp. borealiswith variable access to fresh groundwater

K. Brooksbank • D. A. White • E. J. Veneklaas •

J. L. Carter

Received: 22 July 2010 / Revised: 11 February 2011 / Accepted: 12 February 2011

� Springer-Verlag 2011

Abstract Salinity caused by land clearing is an important

cause of land degradation in the Western Australian

wheatbelt. Returning a proportion of the cleared land to

higher water use perennial vegetation is one option for

reducing or slowing the salinisation of land. Over the

course of a year patterns of water use by Eucalyptus kochii

subsp borealis (C. Gardner) D. Nicolle, a mallee eucalypt

species, were monitored in three landscape positions with

different water availability. One treatment had groundwater

at 2 m, a second at 4.5 m and a third had groundwater

below a silcrete hardpan thought to be impenetrable to

roots. Hydraulic redistribution was observed in all land-

scape positions, and rates were positively correlated with

the magnitude of soil water potential gradients within the

soil. High rates of hydraulic redistribution, facilitated by

abundant deep water may increase tree water use by wet-

ting surface soils and reducing stomatal closure. This effect

may be countered by increased soil evaporation of water

moved from root to soil following hydraulic redistribution;

the net volumes of redistributed water though lateral roots

was calculated to be the equivalent of up to 27% of

transpiration.

Keywords Sap flow � Salinity � Hydraulic redistribution �Mallee eucalypt � Root architecture

Introduction

In the low rainfall agricultural region of south Western

Australia, replacement of deep-rooted native vegetation

with a farming system based on annual grain crops and

pastures has altered local hydrology, causing water tables

to rise and mobilize salt stored in the soil profile (George

et al. 1999). Salinisation and waterlogging significantly

reduces agricultural productivity (George 1991) and is

predicted to affect more than 25% of Western Australia’s

agricultural land before a new hydrologic equilibrium is

reached (McFarlane et al. 2004).

Re-establishing deep-rooted perennial vegetation, such

as tree crops has potential to at least partially address this

environmental degradation (Stirzaker et al. 1999). It is

widely recognized that commercial options are needed to

facilitate tree planting on a scale sufficient to restore

hydrologic balance in the annual cropping zone of Aus-

tralia (Pate and Verboom 2009). One prospective com-

mercial tree crop is a group of native multi-stemmed or

mallee-form eucalypts collectively known as oil mallees.

Since the early 1990s, over 23 million oil mallees have

been planted in this cropping zone, mostly in narrow

belts separated by wider alleys of annual crops and pastures

(M. Kerkmans, pers comm). Integrating trees into the

existing farming system distributes the hydrological bene-

fits of these plantings across the landscape (Stirzaker et al.

1999).

Communicated by C. Lovelock.

K. Brooksbank (&) � D. A. White � E. J. Veneklaas

School of Plant Biology, University of Western Australia,

35 Stirling Hwy, Crawley, Perth, WA 6009, Australia

e-mail: kbrooksbank@agric.wa.gov.au

K. Brooksbank � D. A. White � E. J. Veneklaas � J. L. Carter

CRC for Future Farm Industries, University of Western

Australia, 35 Stirling Hwy, Crawley,

Perth, WA 6009, Australia

D. A. White � J. L. Carter

CSIRO Sustainable Ecosystems, CSIRO Centre for Environment

and Life Sciences, Private Bag 5, Wembley,

WA 6913, Australia

123

Trees

DOI 10.1007/s00468-011-0551-0

Recent analysis suggests that up to 80% of the landscape

in some catchments will need to be planted to reduce the

area affected by salinity (Bennett and George 2008),

meaning more strategic designs of tree planting will be

required to effect change while minimizing the area

required to be planted. Trees must be planted in locations

where their water use has the effect of reducing ground-

water recharge and where they grow quickly enough to be

an economically viable part of farming systems. This could

be achieved either by planting trees in locations where they

have access to water in addition to current rainfall or by

combining trees with active surface water management

systems. Textural discontinuities, including hardpans and

duplex soils, can cause perched water tables within reach

of the roots of trees. The impact of these water sources on

productivity and the resilience of the farming systems will

depend on the capacity of trees to access and use this water

and to convert it to biomass.

Direct groundwater use has been demonstrated for a

number of species but the amount will vary as a function of

species (White et al. 2002), soil structure (Casper et al.

2003), water quality (Costelloe et al. 2008) and depth to

groundwater (Benyon et al. 2006). The root system archi-

tecture has a significant influence on patterns of soil water

utilization by trees and ultimately canopy transpiration

(Ong et al. 2002). One way that plants can improve access

to limited soil water resources is by hydraulic redistribution

(HR) (Burgess et al. 1998) or transport of water via roots

along water potential gradients from wetter to drier parts of

the soil profile (Richards and Caldwell 1987). This usually

occurs nocturnally when transpiration is low (Hultine et al.

2003), but has also been shown to occur during the day

along water potential gradients due to variation in soil

salinity (Hao et al. 2009). Hydraulic lift is the redistribution

of water from deep soil to surface soil (Horton and Hart

1998) and is the primary focus of this paper.

The ability of a tree to relocate water from one soil

horizon to another or even one zone to another within a

particular soil horizon has been well documented in many

species around the world (Meinzer et al. 2001). A 1998

review found hydraulic redistribution had been confirmed

in 27 species of grasses, herbs, shrubs and trees (Caldwell

et al. 1998), and noted that the phenomenon was likely to

occur wherever plants experienced gradients in soil mois-

ture within their root zone. The adaptive significance of

hydraulic redistribution remains unclear but a number of

possible roles have been proposed: (1) uniform distribution

of water in the soil column after rainfall may promote

water conservation due to lower water potential and soil

conductivity (Ryel et al. 2004); (2) redistribution of water

to the surface may improve availability of nutrients

(Caldwell et al. 1998) (Scholz et al. 2006; Scholz et al.

2008) and (3) improve access to that water for existing

vegetation (Caldwell and Richards 1989; Burgess et al.

2000) and maintain turgor of surface roots (Baker and van

Bavel 1986). In environments that experience long dry

periods, HR has been shown to substantially reduce the rate

of decline of soil water potential over the dry season

(Scholz et al. 2010; Meinzer et al. 2004).

Hydration of surface soils has been identified as poten-

tially playing a role in maintaining stomatal conductance

either through altering the concentration of the hormone

abscisic acid (Wilkinson and Davies 2002), or through direct

hydrostatic signals (Caldwell and Richards 1989; Whitehead

1998). This suggests that rehydration of surface soils by

hydraulic redistribution may help keep stomata open, and

thus improve rates of growth by enabling the trees to main-

tain higher rates of photosynthesis, similar to the stomatal

response to surface irrigation noted by White et al. (1999).

The effects of the presence and depth of groundwater on root

structure and sap flow patterns are less well understood.

Species level information on hydraulic redistribution is

important for understanding ecosystem hydrology and as an

adaptation to resource scarcity (Espeleta et al. 2004).

In this study we investigate patterns of water use and

hydraulic redistribution of Eucalyptus kochii subsp. bore-

alis, an oil mallee, with variable depths to permanent

groundwater and without groundwater by measuring sap

flow in stems, tap roots and lateral roots. The root system of

Eucalyptus kochii subsp. borealis has surface lateral roots

as well as deep tap roots (unpublished data), an architecture

that is well adapted for hydraulic redistribution. We test the

hypothesis that variable access to groundwater will affect

both diurnal and seasonal patterns of water use, and attempt

to quantify net loss of water as a result of efflux following

hydraulic redistribution. The results are used to explore the

potential role of hydraulic redistribution in maintaining

water use during the dry season.

Materials and methods

Site description

All measurements were made between November 2003 and

November 2004 at the Calecono Springs farm on the

Buntine-Marchagee Rd, approximately 30 km south east of

the town of Coorow in Western Australia (30�0202800S,

116�1300000E; Fig. 1). In 1999 the farmer planted Euca-

lyptus kochii subsp. borealis (C. Gardner) D. Nicolle in

north–south oriented belts separated by approximately

110 m, primarily to reduce wind erosion. These tree belts

were comprised of two rows separated by 2 m and within

rows trees were planted every 2 m. The study commenced

when the trees were just over 4 years old. The site was also

used by Carter and White (2009) who also describe details

Trees

123

of the site and experimental infrastructure. The salient

details for this paper are also described below.

The study site has a Mediterranean climate characterized

by a hot dry season (*November to April) and cool wet

season (*May to October). The mean annual rainfall

(1899–2003) was 382 mm, more than 80% of which falls in

the wet season. Annual Priestley–Taylor potential evapo-

transpiration over the same period (Bureau of Meteorology)

was 2,340 mm with a monthly peak of more than 350 mm

in January (Fig. 2). The average annual rainfall since the

trees were planted in 1999 was 380 mm, but only 234 mm

fell in 2004 (measured on site).

Soils consisted of 4–6 m depth of pale yellow sands

consisting mainly of quartz ([80%) with the remainder

goethite and kaolinite. This was overlying a relatively

impermeable granitoid saprolite or silcrete approximately

6 m deep (George 1992). A perched groundwater system

emerged above the silcrete mid-slope and mixed with the

highly saline regional system further downslope. One study

plot was established in each of three landscape positions,

where groundwater was available at 2 m (shallow

groundwater, Shallow GW) and 4.5 m below the surface

(deep groundwater, Deep GW), and where there was no

accessible groundwater (no groundwater, No GW). At the

beginning of the study, the average heights and basal

areas of the trees were 4.31 m, 8.28 9 10-3 m2; 3.18 m,

4.79 9 10-3 m2 and 1.68 m, 1.61 9 10-3 m2 in the

Shallow DW, Deep GW and No GW landscape positions,

respectively. The three plots were approximately 500 m

apart along the same belt of trees running perpendicular to

a 3% slope. Groundwater salinities were relatively uniform

across the site ranging from about 440 mS m-1 under the

crop to 540 mS m-1 under the trees. This is moderately

saline and suitable for most tree species (Department of

Agriculture and Food Western Australia 2005).

Sap flow

Heat pulse sap flow probes (University of Western Aus-

tralia) were installed in three trees per plot using the

approach described by Burgess et al. (2000). To test our

hypothesis regarding hydraulic redistribution and access to

groundwater, sap flow was monitored in stems to estimate

tree water use and in tap and lateral roots to examine

diurnal changes in rates and directions of flow. The heat

ratio method was used to calculate sap velocity and flux

because it can measure low- and reverse-flow and is

therefore ideal for observing hydraulic redistribution in

woody roots (Burgess et al. 2001). The root system was

partially excavated close to the lignotuber using com-

pressed air to minimize mechanical damage to the root

systems. If suitable insertion sites were available, probes

were installed in two tap roots, two lateral roots and one

stem from each tree. We were unable to measure sap flow

in the larger tap roots because they occurred towards the

centre of the root system and access for instrumentation

was obscured by the smaller tap roots. Once the probes

were installed the exposed roots and probe sets were cov-

ered with an insulating blanket.

A total of 15 probe sets were installed across 3 trees in

each plot and connected to an AM16/32 multiplexer and a

CR10X datalogger (Campbell Scientific, Logan UT, USA).

Power was supplied using a sealed lead-acid battery

recharged using a solar panel. Measurements were made

every half hour from late November 2003 to mid April

Fig. 1 Location of the town of Coorow Western Australia where the

study took place

Time (month)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

mm

0

100

200

300

400

Rain Evaporation

Fig. 2 Long-term (100 years) average rainfall and Priestley–Taylor

potential evapotranspiration (mm) at the Coorow town site (source:

Bureau of Meteorology)

Trees

123

2004 using probe sets comprising a central heater probe

and sensor probes 5 mm upstream and downstream of the

heater. The sensor probes had thermocouples 7.5 and

22.5 mm from the end of the probe allowing measurements

at two depths within each sapwood band. Due to the

increase in diameter of the wound surrounding the insertion

point with time and the effect this has on measurements,

the maximum period for reliable sap flow measurements

from the same installation is 6 months (Burgess et al.

2001). Thus, it was necessary to instrument new trees at the

half way point of this 12-month experiment. At the end of

each measurement period the trees were cut down and

measurements made under zero flow conditions for data

calibration (Burgess et al. 2001). In May 2004 the equip-

ment was installed in different trees that were measured

until November 2004 using the same protocol. For the

second set of measurements only one sensor from each

probe set was used for measurement because this allows for

differential wiring which is less prone to earthing problems

(Maherali and DeLucia 2000). Similarities in measured sap

velocities at two sapwood depths in the first set of mea-

surements (average differences were between 1 and 2%)

indicated this allowed estimation of average stem sap flux

density. The distal sensor was placed at the midpoint in the

sapwood band and any portion of the probe outside the

bark insulated with silicon.

Sap flux density (flow rate per cm2 of sapwood) was

calculated from heat pulse velocity data after taking into

account probe misalignment, wounding, wood density and

sapwood area (Burgess et al. 2001). Flow of sap towards the

extremities of the root system will be referred to as acrop-

etal, and flow towards the lignotuber will be referred to as

basipetal (Burgess et al. 2000). Average sapflow was cal-

culated for three seasonal 14-day periods. To represent

each period, sap flux density was calculated for each

point in time, and then the means combined to form

a curve for a single 24-h period. Standard errors for

these averages were typically less than 1.2 cm3 cm-2 h-1

where sap flow rates were above 5 cm3 cm-2 h-1 and less

than 0.5 cm3 cm-2 h-1 when sap flow rates were below

5 cm3 cm-2 h-1. The chosen periods were 11 Dec 2003–24

Dec 2003 (early dry season), 30 Mar 2004–13 Apr 2004

(late dry season) and 8 Aug 2004–22 Aug 2004 (wet sea-

son). Volumetric estimates of sap flow in stems and roots

from the No GW plot were calculated as the product of sap

flux density and the cross sectional area of conducting

wood. Due to the young age of the trees, no heartwood had

formed in the stems or roots so all wood was assumed to be

conducting and the sapwood cross sectional area was cal-

culated from under-bark diameters. This data was then used

for mass balance calculations to examine whether hydraulic

redistribution by the root system was resulting in efflux of

water into the surrounding soil.

Excavations of the root system for the installation of sap

flow monitoring equipment revealed the basic design to be

dimorphic with lateral roots emerging horizontally from the

lignotuber, and tap roots emerging vertically. This architec-

ture is common in trees growing in semi-arid environments

(Pate et al. 1995; Dawson and Pate 1996). Closer examination

of root systems showed that vertical or sinker roots branched

from along the lateral roots. The depth of these roots is

unknown but they provide access to deeper soil water stores at

a distance from the tree. Xylem water flow towards the

extremities of the instrumented root could have originated

from a secondary sinker distal to the sap flow monitoring point

meaning that total nocturnal acropetal flow measured in those

lateral roots may have been underestimated. Due to this

architectural complexity we did not attempt to assess origins

of water or calculate volumes of HR in the landscape positions

where groundwater was present.

Soil water status and environmental conditions

Soil water status was assessed using a neutron moisture meter

(NMM; California Pacific Neutron, Pacheco, California). To

install the access tubes, holes were drilled down to the sil-

crete, and 50 mm PVC pipe, capped at the end, was inserted

into each hole. The holes were drilled slightly smaller than

the pipe and the pipe was forced into the hole to ensure there

were no air spaces between the tube and the soil. The holes

were then backfilled and capped with kaolin–cement slurry

after (Prebble et al. 1981). Access tubes were positioned in a

transect perpendicular to the tree line and spacing was based

on the average tree height at the time of installation which

was 4 m for the Shallow GW plot, 2 m for the Deep GW plot

and 1.5 m for the No GW plot. The first tube was positioned

between the two tree rows, the second on the drip-line of the

belt, then at 1, 2, 3 and 4 tree heights from the belt. A final

access tube was positioned midway between tree belts

approximately 55 m from the plot. Measurements were taken

on individual days that coincided with the sap flow moni-

toring periods in the late dry season, the early dry season and

the wet season. Soil water content measurements were taken

at depths of 0.1 m then every 0.2 m down to 1.5 m and every

0.5 m down to 4.5 m. Neutron count ratios measured using

the probe were converted to a volumetric soil water fraction

by calibrating against measurements of soil cores collected in

summer and winter adjacent to the access tubes at the end of

the project. Neutron data was kriged in order to present a two-

dimensional profile of soil moisture.

Results

Nocturnal flow was observed in lateral and tap roots in all

landscape positions (Fig. 3). This was clearest in the No

Trees

123

GW plot when surface soil was drier than at depth in early

summer. At that time, water moved up the tap roots and out

along the lateral roots. As the dry season progressed and

the deeper soil moisture was depleted, the rate of hydraulic

redistribution progressively decreased (Fig. 4). In the wet

season, when surface soil was wetter than at depth, water

moved acropetally or down tap roots (No GW, Fig. 3).

In the Deep GW plot the average direction of nighttime

flow for all monitored tap roots in the dry season was

acropetal. However, examination of the individual traces

that made up this average showed that flow direction varied

as a function of the basal diameter of the roots measured. In the

smaller diameter tap roots, water tended to move acropetally

or downwards at night, while in the larger diameter tap roots

nocturnal flow was predominantly basipetal or upwards

(Fig. 5). While the monitored tap roots average flow direction

over the dry season in the Deep GW plot and in the late dry

season in the No GW plot appear to be downwards (Fig. 3),

this may have been biased due to the fact that sap flow was not

monitored in the largest tap roots.

Rates of hydraulic redistribution (HR) in the No GW

plot decreased over the early dry season, indicated by

Wet2 stems10 laterals

Early dryS

FD

(cm

3 cm

-2 h

r-1)

-20

-15

-10

-5

0

5

10

15

20

25

30

35 2 stems10 laterals1 tap

Late dry3 stems4 laterals

2 stems10 laterals1 tap

SF

D (

cm3 c

m-2

hr-1

)

-20

-15

-10

-5

0

5

10

15

20

25

30

35 3 stems5 laterals1 tap

3 stems5 laterals1 tap

SF

D (

cm3 c

m-2

hr-1

)

-20

-15

-10

-5

0

5

10

15

20

25

30

35

1 stem3 laterals5 taps

Time (hours)

1 stem3 laterals5 taps

00:00 06:00 12:00 18:00 00:00 00:00 06:00 12:00 18:00 00:00 00:00 06:00 12:00 18:00 00:00

1 stem3 laterals5 taps

A

B

C

Fig. 3 Mean stem, tap root and lateral root sap flux density

(cm3 cm-2 h-1) in Eucalyptus kochii subsp borealis in the Shallow

groundwater plot (a), the Deep groundwater plot (b) and the No

groundwater plot (c) across three seasons. Each trace is an average

over 14 days in each season. Number in legend represents number of

replicates

Trees

123

decreasing rates of nighttime flow in a tap root and a lateral

root shown in Fig. 4. Substantial rates of nighttime flow in

roots in the Deep GW plot indicated hydraulic redistribution

was occurring both early and late in the dry season. Average

nocturnal flow rates in this plot were around 5 cm3 cm-2 h-1,

but peak flows were up to 20 cm3 cm-2 h-1 in some roots

(Fig. 4). Nocturnal flow patterns in the Shallow GW plot were

less consistent, but suggested small amounts of reverse flow in

lateral roots.

In the No GW plot the dominant direction of flow in lateral

roots changed at different times of the year (Fig. 6). In winter,

daytime and nighttime flow was towards the stem, with

nocturnal flows being driven by water redistributed down-

wards by tap roots overnight. As the dry season progressed,

there was an increasing tendency for the balance of flow to be

acropetal (Fig. 6; Table 1) so by the late dry season; more sap

flow in the lateral roots was away from the stem than towards

the stem in a 24 h period. Peak measured net volumes of

acropetal flow in lateral roots in the late dry season were the

equivalent of 0.34 mm rainfall per day. This equates to 27%

of the volume of water transpired daily by each tree in this

landscape position at this time of year.

There appeared to be some reduction is soil moisture as

far as 15 m laterally from in the Shallow GW plot but only

down to 1 m. The reduction in soil moisture under the trees

in the Deep GW plot extends to approximately 6 m while

there appears to be some reduction beyond the extent of

measurement at 6 m in the No GW plot. Including the 2 m

gap between the two lines of trees in the belt, this gives the

tree belt a total zone of influence of 32 m in the shallow

GW plot, 14 m in the Deep GW plot and at least 14 m in

the No GW plot (Fig. 7).

In the Shallow GW plot, the water table is 2 m below

ground and the gradual reduction in soil water content

towards the surface indicates that this whole zone is the

capillary fringe. In the Deep GW plot where the water table

is nearly 5 m below ground, soil water is depleted to this

depth and in the No GW plot to the hardpan directly under

the trees. Reductions in soil moisture under the trees

increases the gradient in soil water content between deep

and shallow soil, most markedly in the Deep GW plot. In

the Deep and No GW plots, soil water under the trees was

not fully replenished in the wet season (data not shown).

The mid alley soil moisture profile is presented separately

within this figure to allow better resolution of the impact of

tree uptake on soil moisture. Field capacity of the soils was

0.16 m3 m-3 and wilting point was 0.05 m3 m-3.

Discussion

Nighttime sap flow was observed in roots in all three

landscape position, confirming that hydraulic redistribution

Time (days)13 Dec 2003 20 Dec 2003 27 Dec 2003 3 Jan 2004

SF

D (

cm3 c

m-2

hr-1

)

-40

-20

0

20

40

60

Tap rootLateral root

Fig. 4 Sap flux density (cm3 cm-2 h-1) of a tap root and a lateral

root of Eucalyptus kochii subsp. borealis in the No groundwater plot

over a 21-day period in the early dry season

Time (hours)00:00 06:00 12:00 18:00 00:00

SF

D (

cm3 c

m-2

hr-1

)

-10

-5

0

5

10

15920 mm2

800 mm2

450 mm2

280 mm2

Fig. 5 Mean diurnal sap flux density (cm3 cm-2 h-1) in four tap

roots of varying sizes of Eucalyptus kochii subsp. borealis in the late

dry season over a 14-day period in the No groundwater plot. Numberin key is sapwood cross sectional area

Time (hours)00:00 06:00 12:00 18:00 00:00

SF

D (

cm3 c

m-2

hr-1

)

-20

-10

0

10

20

30

wetearly drylate dry

Fig. 6 Average diurnal sap flux density (cm3 cm-2 h-1) in lateral

roots (n = 3) of Eucalyptus kochii subsp. borealis trees in the No GW

plot over 10-day periods for three seasons in 2003–2004

Trees

123

is occurring in E. kochii across a range of soil moisture

conditions. Although higher rates of hydraulic redistribu-

tion were measured in the No GW plot, complex root

architecture may have masked the extent of nocturnal flow

in the two plots where a water table was present. Increasing

depth to groundwater had a considerable effect on the

ability of the trees to maintain rates of transpiration as the

dry season progressed. There was evidence to suggest that

hydraulic redistribution results in efflux of water into sur-

face soils during the dry season which is consistent with

HR being a mechanism for maintaining stomatal conduc-

tance, thus facilitating higher rates of water use where

groundwater is present.

Patterns of HR in the No GW plot gave a clear picture of

nighttime flow driven by gradients of soil water potential.

Early in the dry season, the dry surface soil surrounding

lateral roots generated gradients in soil water potential that

provided the driving force for HR within the root system.

Extensive lateral root systems have been reported in belt

planted mallee eucalypts reaching up to 20 m from the

edge of the belt at 10 years of age (Sudmeyer and Flugge

2005; Robinson et al. 2006). Later in the dry season less

water was available to redistribute and as a result the rate

of hydraulic redistribution declined, (e.g. Fig. 4). The small

amount of daytime sapflow in lateral roots in the No GW

plot (Fig. 3 No GW) shows the extent to which trees in this

landscape position are reliant on water deeper in the

unsaturated zone at this time of year. Similar trends have

been noted in other tree species (Scholz et al. 2008).

In the Deep GW plot, tap roots and lateral roots appear

to be making an equal and consistent contribution to water

use throughout the dry season. This may be partially due to

the fact that larger amounts of redistributed water are

available via the lateral roots during the day, as well as

sinker roots emanating from laterals continuing to access

the water table throughout the year. In the presence of a

water table, the soil water potential gradients between

surface soil and deeper soil layers intensify over the dry

season providing an increasingly strong driving force for

water to be redistributed within the root system (Brooks

et al. 2002). Where a water table is present, this gradient

will reach an upper limit (saturated soil at depth, very dry

surface soil) so rates of HR will plateau as was the case in

the Deep GW plot (Fig. 3b). In the Shallow GW plot, HR

appeared to occur at a lower rate, presumably because most

of the soil profile was relatively wet providing less impetus

for nocturnal redistribution; consequently it was more

difficult to relate nocturnal flow to observed patterns of soil

stored water.

The nocturnal direction of water flow in tap roots in the

dry season also appeared to vary with root diameter; the

larger tap roots moved water up at night and smaller tap

roots moved water down at the same time in the dry season

(Fig. 5). This may be because larger diameter tap roots

penetrated deeper in the soil profile where soil water

potentials were high and the gradient for water movement

was upwards, while the smaller tap roots were in shallower,

drier soils (with more negative water potentials) and the

gradient for water movement was downwards, meaning the

same patterns were observed in these smaller tap roots as

the lateral roots. The tap roots instrumented in this study

spanned the mid-size diameters only. These results suggest

that the largest diameter roots in each tree are the dominant

source of redistributed water in the dry season and supply

water at night to both the lateral roots and shallower tap

roots. The movement of hydraulically redistributed water is

not necessarily just between the extremities of the root

system, but will occur among all soil layers where soil

water potentials differ; for example, one simulation model

developed divided the soil profile into 7 vertical layers

between which HR was occurring to varying degrees (Ryel

et al. 2002).

Although the measured rates of HR were highest in the

No GW plot, based on the relative size of the trees in each

plot, we would expect the volumes of water moved through

HR were greatest in the Deep GW plot where potential

gradients between saturated and dry soils were larger

(surface soil moisture 0.00–0.04 m3 m-3 c.f. 0.24–0.28

m3 m-3 at 4.5 m), and where larger volumes of water were

available at depth to be redistributed. Hydraulic redistri-

bution is driven along a potential gradient (Richards and

Caldwell 1987), so larger volumes are likely to be moved

when that potential gradient is larger (e.g., between satu-

rated deep soil associated with groundwater and dry sur-

face soil as found in the groundwater plots) (Meinzer et al.

2004). Given the larger sap wood area of the root systems

Table 1 Xylem sap flow in stems and lateral roots at the no groundwater (No GW) landscape position during the day (sunrise to sunset) and

night (sunset to sunrise) in different seasons

Wet Early dry Late dry

Day Night Day Night Day Night

Lateral roots 1.3 (±0.05) 0.3 (±0.01) 2.1 (±0.2) -2.7 (±0.04) 0.2 (±0.02) -1.7 (±0.02)

Stems 3.9 (±0.3) 7.4 (±0.40) 5.6 (±0.2)

Data shown are mean total daily flows in litres per day per tree (n = 3) (±SE) for 10-day periods in each of the wet season, early dry season and

late dry season over 2003–2004

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123

in the groundwater plots due to the larger size of the trees,

and the sustained rates of nighttime flow throughout the dry

season, there would be a greater capacity to move water

upwards at night in the Deep GW plot than in the No GW

plot. Some indication of the rates of HR achievable was

those measured during the early dry season in the No GW

plot. In mid December when surface soil was dry

(\0.04 m3 m-3), but deeper soil water was relatively

abundant, nocturnal flow rates in some tap roots approa-

ched 20 cm3 cm-2 h-1. It is possible that in the Deep GW

plot, where permanent water was available at depth, similar

flow rates may have been achieved while evaporative

demand caused surface soil moisture to remain low, but

this may have been masked by water moving up sinkers

distal to monitoring points on the lateral roots.

Evidence of efflux

Relative daytime and nighttime volumes of acropetal and

basipetal flow inferred from lateral root sap flux densities

in Fig. 6 and the net volumes of water shown to be moving

acropetally in lateral roots in the No GW plot in Table 1

demonstrate that some of the redistributed water is being

lost through efflux into the surrounding soil. Mass balance

calculations based on sap flow rates in the No GW plot

suggest that substantial volumes of water can move from

root to soil overnight. Earlier work has found similar

amounts of water lost to efflux ranging from 15% of whole

plant daily transpiration in the shallow-rooted xeric shrub

Gutierrezia sarothrae (Wan et al. 1993) to roughly a third

for the shrub Artemesia tridentata (Richards and Caldwell

1987). In a water-limited environment this would appear to

be a high cost unless hydration of surface roots and efflux

was a profitable strategy. As the efflux did not lead to

progressively higher soil moisture, and no other plants

were present over the dry season to take up the exuded

water (Caldwell 1990), the net loss of water from the lat-

eral root system and soil must be due to direct evaporation

from the soil.

The water lost through surface root efflux and evapo-

ration is important to consider in site water balances where

dryland salinity occurs. In order to re-establish the hydro-

logical balance in these landscapes (Stirzaker et al. 1999)

while removing the minimum amount of land from annual

crop production, accurate predictions of the water use

capability of the planted trees will need to be made. The

amount of extra water lost to the system through efflux is

large enough to be incorporated into these calculations.

Conclusions

Hydraulic redistribution (HR) was observed in all land-

scape positions confirming that HR is occurring in this

species across a range of soil moisture pattern scenarios.

Rates of HR increased with increases in soil water moisture

gradients within the rhizosphere, but increasing depth to

groundwater had a considerable effect on the ability of the

trees to maintain rates of transpiration as the dry season

Fig. 7 Soil water content (m3 m-3) for the Shallow (a), Deep (b) and

No groundwater (c) plots late in the dry season at Coorow, Western

Australia. The water table was just below 2 m in the Shallow

groundwater plot and some perched water persisted over the silcrete

layer at 4.5 m in the Deep groundwater plot. There was no water table

in the No groundwater plot and the silcrete was at approximately

5.5 m. The zero point on the x axis marks the position of the tree belt,

and numbers represent distance in metres out into the adjacent field.

The narrow strip associated with each figure shows soil water content

with depth at the midpoint of the alley between tree belts and

represents soil water content in the absence of trees. Soil field

capacity was 0.16 m3 m-3 and wilting point was 0.05 m3 m-3

Trees

123

progressed. High rates of HR facilitated by abundant deep

water may appreciably increase tree water use and tree

growth through its effect on stomatal control. Substantial

amounts of water may be lost to the system through the

evaporation of water moved from root to soil following

HR, and this should be taken into account when calculating

water balances.

Acknowledgments We gratefully acknowledge financial support

for this research from the Cooperative Research Centre for Future

Farm Industries. We thank the Department of Agriculture Western

Australia (DAFWA) for providing administrative and logistical sup-

port, and Stanley Rance, Scott Walker and Shayne Micin for their

assistance in the field and laboratory. Special thanks to the Stacey

family for allowing us access their farm to conduct this work.

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