Australian Journal of Ecology (1981) 6, 459-466

Water stress, transpiration and leaf area index in eucalyptplantations in a bauxite mining area in south-west Australia

B. A. CARBONG. A. BARTLEA. M. MURRAYDivision of Land Resources Management,CSIRO, Private Bag, P.O., Wembley, Australia6014

Abstract

The clearing of native forests for open cut miningfor bauxite is a potential cause for severemodification of the hydrological cycle. In theDarling Range, Western Australia, jarrah(Eucalyptus marginata Sm.) woodlands arebeing cleared for mining and are being replacedwith plantations of other eucalypts.

The plantation trees, which were up to 8 yearsold, showed no greater water stress than those ofthe original forest. The leaf area index of olderplantation trees was usually higher than that oftheoriginal, mature forest. Transpiration rate wasestimated on twigs enclosed briefly in atranspiration chamber. Prior test showed that thismethod was appropriate for comparisons in fieldgrown eucalypts. Field measurements of stratifiedsamples showed little difference in transpirationrate per unit leaf area between plantation forestsand original forests. For the last 3 months ofthedry season, however, transpiration per unit areaof leaves was depressed by 25% in the plantationforest.

Introduction

In the agricultural areas of south-west Australia,water supply is limiting to plant growth duringthe seasonal 6-month drought (Leeper 1960).The original perennial woodlands have beenreplaced by farms growing winter annual plants.This has led to a decrease in annual transpirationand a subsequent increase in recharge of0307-692X/81/120O-O459 $02.00 © 1981 Blackwell Scientific Publ

groundwater. This has caused leaching of saltsstored deep in the soil profile, and resulted insalinization of some agricultural land (Bettenayet al. 1964) and some major rivers (Peck & Hurle1973).

The Darling Range is an ancient peneplainwhich forms the hinterland to Perth. Much oftheRange has been pegged for bauxite mining, andsince 1964 there has been a steady increase in therate of open cut mining. It has been estimated(Thomas 1978) that the total area affected bymining will exceed 600 km by the year 2007. Thebetter grade bauxite ore is mined from theuplands, leaving exposed pits which are severalmetres deep, and have a floor of compacted,kaolinitic subsoil. After replacement of 100 to200 mm of topsoil and deep-ripping of thesubsoil, tree seedlings are planted on a grid of3.75 m (approximately 700 trees per ha).Because of the susceptibility of the originaljarrah (Eucalyptus marginata Sm.) woodland tothe root fungus, Phytophthera cinnamomiRands., eucalypts resistant to the effects of thisfungus have been used for reforestation. Tallowwood (E. mlcrocorys F. Muell.), native to thesummer rainfall zone of northern New SouthWales and southern Queensland, wasextensively planted in the early years ofreforestation. In early days there was also littleeffort to establish undergrowth species. Modernmethods now ensure a rapid establishment andgrowth of these species.

The purpose of our study was to answer threerelated questions. Firstly, did the new plantationtrees suffer excess water stress in the longsummer drought associated with theMediterranean climatic zone? Secondly, did theintroduced trees grow to a similar leaf area indexas the original forest? The third questionconcerned the relative rates of transpiration ofthe new plantation trees and the native forest. Asno technique was available for a directcomparison of total transpiration, we elected to

ications

460 B. A. Carbon, G. A. Bartle and A. M. Murray

measure the transpiration rate of twigs using thetranspiration chamber technique of Grieve &Went (1965). There has been conjecture aboutthe use of transpiration chambers and,therefore, we have tested the applicability of thetechnique for comparisons of transpirationbetween field-grown eucalypts.

The measurement area near Jarrahdale (lat.32°20'S, long. 116°E) has a Mediterranean-typeclimate, with warm dry summers and cool wetwinters. The average annual rainfall is 1207 mm,with 85% of rain falling in the 6 months April toSeptember. Annual pan evaporation is 1530mm, of which 75% occurs in the 6 wannermonths (source Commonwealth Bureau ofMeteorology 1963). The rainfall was 1420 mmand 1280 mm respectively for the 2 years of thestudy.

Methods

Site characteristics

All measurements were taken at the number oneAlcoa bauxite mine area near Jarrahdale on thewestern edge of the Darling Range. Replantedtrees were from 2 to 8 years old, and up to 15 mhigh.

An intensive series of measurements wastaken on native forests and various age-sitecombinations of E. microcorys. Occasionalmeasurements were also taken on replanted E.globulus Labill., and E. muellerana WoWxtt.

Two sites of E. microcorys (3a and 5b) (Table1) were atypical in that topsoil had not beenreturned to the mine floor. Another site (6b) wasunmined, supported E. microcorys, and wasmonitored because it showed obvious waterstress in late summer. At each site a single bore-hole was drilled at the end of summer until eitherbedrock or a water table was encountered. Mostsites had a water table at this time, and the depthof soil above bedrock or water table varied from6 to 14 m. At site 6b, however, there was only2.5 m of soil above bedrock, and there was nopermanent watertable.

Sampling procedures

All measurements for water stress ortranspiration were made on freshly cut twigs with

TABLE 1. Some characteristics and measurements for forest

sites near Jarrahdale

Site

1 a

bcde

2 a

b3 a

b4 a

b5 a

b

6 a

b

Sitetreatment

No mining

ClearedClearedMinedMined

No mining

ClearedMinedMinedClearedClearedMinedMined

No mining

Cleared

Age offorest(years)

Mature

6666

Mature

688Mature626

Mature

6

Eucalyptspecies

marginatacalophyllamicrocorysmicrocorysmicrocorysmeulleranamarginatacalophyllameulleranamicrocorysmicrocoryscalophyllacalophyllamicrocorysmicrocorysmarginatacalophyllamicrocorys

LeafareaIndex

2.6y

i.lX5.0A-3.9 Jf3.8 ;f

2.4 y

4.8^2.3 y4.0A'2.3 yL9y

0.5Z2.5y

2.6y

2.0 y

NoteA'>y>Z.(P < 0.05).

several leaves. Measurements of transpirationwere taken within 1 min of sampling, and thosefor tissue water potential within 3 min, aftertemporary storage in an insulated box. Testscarried out previously had shown that thisprocedure gave: (i) the same transpiration rate asleaves still on the tree (see below) and, (ii) thesame tissue water potential as samples measuredimmediately after cutting from the tree.

Transpiration rate may vary by 30% as afunction of age of leaves, and their degree ofexposure to the sun (Doley 1967). Allmeasurements were therefore taken on twigsbearing fully expanded leaves exposed to directsunlight. This approach provided a consistentbasis for comparison among site-forestcombinations.

To account for diurnal variation in bothtranspiration and moisture stress, readings weretaken between dawn and dusk. Most sites weremonitored five to six times per day, but someonly two or three times. Between two and foursamples per site were measured on eachoccasion, depending on the degree of variabilityencountered. Measurements began in latesummer 1973 and continued monthly for the next2 years.

Tissue water potential

A Scholander bomb (Scholander et al. 1965) wasused to measure xylem pressure of twigs carryingfive to ten leaves. A regression between bombmeasurement and leaf water potential, measur-ed under controlled temperature conditions witha thermocouple psychrometer, was used toconvert bomb readings to leaf water potential.The regression was y = - 1.002JC-232(/• = 0.95), where y = leaf water potential (kPa),A: = bomb reading (kPa). This indicates thatbomb readings were close indicators of leafwater potential.

Leaf area estimation

Leaf area index (LAI) was estimated by themethod of Carbon, Bartle & Murray (1979a). Inthis method, estimations of leaf area were madeon a branch by branch basis. The estimates werethen 'corrected' against measured standards andsummed to provide a corrected estimate of leafarea. This estimate was divided by the groundarea of the measurement plot to give LAL Inplantation forests, with regular tree spacing andrelatively uniform tree size, only six trees wereestimated at each site. In the native forest alltrees in a ground area of 400 m^ were estimated.Six estimators operated at all sites, and theestimations were taken in March 1975.

Transpiration

Transpiration rate was estimated as water lossper unit area of leaves, using the transpirationchamber method described by Grieve & Went(1965). Small twigs of fully expanded leavesexposed to direct sunlight were cut from thetrees, and placed in a sealed, transparentchamber. A small fan promoted air mixing andan electronic hygrometer measured both tem-perature and humidity in the chamber. Astopwatch was used to measure the time takenfor the relative humidity in the chamber toincrease by 2%; this was usually between 2 and10 seconds. The area of the leaves in the samplewas subsequently measured on an electric plani-meter. The results were expressed as mg of watertranspired per cm^ of leaf per minute.

Water relations in eucalypt plantations 461

Transpiration chambers with electric hygro-meters have been tested previously for fieldestimation of transpiration. Grieve & Went(1965) demonstrated that the chambers gaveresults similar to those of the rapid weighingtechnique which measures weight loss of leavesafter excision (Huber 1927; Grieve 1956). Theyalso showed that measures with transpirationchambers coincided well with weight loss ofintact plants in pots. Grieve and Went tested forthe so-called Iwanoff (1928) shock effect, wherethere have been reports of a significant change inthe rate of water vapour loss occurring im-mediately after excision of leaves from theparent plant. They could show this effect forsoft-leaved plants but not for chaparral andsclerophyll plants grown in natural conditions.Previous reports of surges in transpiration weremostly for soft-leaved plants or plants grown inprotected conditions (e.g. Andersson, Hertz &Rufelt 1954; Decker & Wien 1960).

Stark (1967, 1968) again demonstrated theaccuracy and speed of the transpiration chamberfor measurement of hardy plants. In a day Starkmade up to 400 measurements on desert plants.There nonetheless remains some criticism of theuse of the transpiration chamber. Slatyer (1967),in commenting on rapid weighing and trans-piration chamber methods, stated 'any change inthe energy load and wind structure immediatelyinfluences the energy balance of the leaf andhence transpiration'. He further claims that leaftemperature, bulk humidity and windspeedchanges in a closed chamber will cause changesin the rate of transpiration. Kramer (1969) hadsimilar reservations about the effects of theenvironment within the chamber but suggestedno 'serious changes in stomatal opening' forenclosure of less than 1 min.

The validity of the transpiration chambertechnique as an indicator of relative rates oftranspiration for eucalypts is critical to our study.We have made tests, therefore, for the factorssometimes suggested to influence the results.The effect of enclosure within the chamber wastested by comparing the transpiration chamberwith the rapid weighing technique (Huber 1927).Twigs from a group of field grown jarrah treeswere measured in situ with the transpirationchamber. Some 5 min later the twigs were cutand then weighed on a torsion balance. Theywere then suspended near to their originalposition in the tree, where conditions for

462 B. A. Carbon, G. A. Bartle and A. M. Murray

transpiration were similar to those beforeexcision. They were re-weighed 2 min later.Twenty replications were taken over a period of2 days.

Two separate tests were used to measure theeffect on transpiration of severence of twigs fromthe tree. A series of measurements were takenwith leaves still intact on the tree (A) and otherswere measured after severance from the tree(B). An ABBA design was used for the com-parison, with sixteen replications on a warm day,and sixteen on a cool day. More twigs weremeasured on the tree, with each being monitoredevery minute (a reading usually takes less than 10sec). After 10 min they were severed from thetree, but tied to the parent branches.Transpiration chamber readings were continuedevery minute for 15 min.

Data analysis for site-forest combinations

Leaf water potential and transpiration rate bothvary throughout the day. The various site-forestcombinations were compared by fitting appro-priate lines to represent these diurnal changes.This approach was necessary because it was notpossible to measure the different treatmentssimultaneously. Variation around the mean ofthe measures for both leaf water potential andtranspiration rate increased as the absolutereading increased. Therefore the error term inestimation was usually greater for the measurestaken during the middle of the day than for thosenear either dawn or dusk. In order to analysesuch data, the computer program SELFWT(Mclntyre, Division of Mathematics and Statis-tics, CSIRO, Canberra) was used to derive aweighted regression relationship between themeasurement and time of day. SELFWT obtainsthe weights iteratively as a function of deviatesfrom the fitted surface.

Comparisons of water stress and transpirationrates for various site-forest combinations weremade by dividing data into three separate partsof the year. We have referred to the wet season(April to September) as winter. The early part ofthe dry season (October to December), whenstored water should be plentiful, has been separ-ated from the later part (January to March),when soil water may be limiting.

Results

Water stress

A cubic-quartic regression on the self weighteddata for diurnal changes in leaf water potentialshowed a better fit than cubic or quarticregressions fitted separately. It had an averagecorrelation coefficient of 0.85, and an averagestandard error of 140 kPa for the fitted lines.

The fitted lines for leaf water potential showeda decrease from early morning until about themiddle of the day. Thereafter they increaseduntil near sundown when they had returned toclose to the early morning reading. There wereclose similarities in the patterns for trees onbauxite mines, plantation trees on unminedareas, and the native forests. The biggestdifferences in leaf water potential were found inthe period January to March, as shown in Fig. 1.Then there was a consistently higher (lessnegative) water potential for the two year oldplantation of E. microcorys (site 5a) which hadtrees only 1-2 m tall. There was also a trend forthe native forest to show a lower (more negative)potential than the older plantation trees andthere was also a trend for plantations onunmined sites to be lower than those of minedsites. These trends were not statisticallysignificant (p >0.05), but demonstrated that, ingeneral, the plantation trees did not have greaterstress (less potential) than the native forest.

There was a further exception at site 6b(shallow soil and no summer water table). Herethe trees reached water potentials as low as-4800 kPa in March of both years, and severaltrees died, apparently of water stress.

The minimum leaf water potential in winter onthe 2-year-old plantation trees was -720 kPa,compared with -1100 kPa for all other trees.Corresponding figures for October to Decemberwere -1240 kPa compared with -2000 kParespectively.

Leaf area index

An analysis of variance showed that there werethree different groups of results for leaf areaindex (P>0.05) (Table 1). The statisticallydifferent groups in Table 1 are indicated by X, Yand Z (where A'> Y>Z,P< 0.05). The smallest

Water relations in eucalypt plantations 463

0600

-1000

-2000

\

>

TIME OF DAY

1200^ 1 T 1 1 1 1 1 1 r-

^ T AVERAGE STANDARD ERROR^ I OF FITTED LINES

\ /I• V> '^/ /

1—

/

/ /

/

f//

18001 r

/ / y

/ /^ /

//

FIG. 1. Fitted lines for leaf potential. Combineddatafor January to March.

LAI (0.5) was found on the 2-year-old plantationtrees (group Z). The second group (Y) includedall mature (2.5) and regrowth (1.9) native forest,and the three plantations with obviously poorgrowth rates. These were the E, microcorys atthe mined sites 3a (2.3) and 5b (2.5) growingdirectly on clay, and site 6b, the unmined sitewith only shallow soil above bedrock. The thirdgroup (x) contained most of the 6- to 8-year-oldplantations, whose LAI ranged from 3.8 to 5.0.

Transpiration

Transpiration chamber measurements. Thetranspiration chamber gave results for jarrahtwigs which were not statistically different fromthose for the rapid weight technique. The meantranspiration rate per twig was 9.5 mg min withthe chamber, and 9.0 with the rapid weigh, andthe LSDQ 95 was 1.2 mg per min.

There was no significant difference in trans-piration chamber measurements taken before orafter excision from the trees. On the hotter day,mean transpiration rate for twigs on the tree was0.119, and for those severed from the tree it was0.113, and the LSD0.95 was 0.016 mg per cm^ permin. Corresponding figures for the cooler day

were 0.024 and 0.025. with of 0.005 mgper cm^ per min.

The results for three of the twigs measured for10 min on the tree, and then for 15 min afterbeing severed from the tree, are shown in Fig. 2.There was no systematic change in transpirationimmediately after the twig was severed, butsignificant decreases in transpiration rateoccurred after 10 min.

Site-forest combinations, A cubic-quarticregression on the data for diurnal changes intranspiration rate per unit area of leaf again hadthe best fit, had an average correlation co-efficient of 0.80, and an average standard errorof 0.019 mg of water/cm^ of leaf/min. Thisstandard error is equivalent to 13 mg cm~̂ per12-h day. The typical shape of the daily curvesfor transpiration rates, for each of the three'seasons', is shown in Fig. 3. (Data for the waterstressed trees at site 6b were not included). Allleaves showed maximum transpiration rate nearthe middle of the day and close to zerotranspiration at night.

The average daily transpiration rates werecalculated by integrating the area under thefitted lines for the various site-forest com-binations over three measuring periods (Table

464 B. A. Carbon, G. A. BartleandA. M. Murray

••' TWIG 2

SEVERED FROM TREE

12

TIME (mini

FIG. 2. The transpiration rate for three twigs. Measurements were taken each minute before and after severance from the tree.

o< 0.10

AVERAGE STANDARD ERROR

OF FITTED LINES

0600 0800 1000 1200 1400 1600 1800 2000

TIMEOF DAY

FIG. 3. Fitted lines for transpiration. Combined data for all sites except 6b.

Water relations in eucalypt plantations 465

TABLE 2. The average rate of transpiration for selected leaves ineucalypt forests (mg cm~- d~')

Native forestPlantations

On mined sitesOn unmined sites2 years old

Winter

42

494455

Oct-Dec

157

131123139

Jan-Mar

149

113104117

2). The rate for winter was 49 mg cm M ' andthere was no significant difference betweennative and plantation forests. In October toDecember, the transpiration rate of the samplesfrom the native forests was 157 mg cm"'̂ d"',some 17% higher than that from the plantationforests. This difference was significant onlyat a probability <0.10. The 26% difference be-tween native and plantation forests in January toMarch was statistically significant (P<0.05).Throughout the October-December and Janu-ary-March periods the general shape of thecurves for transpiration were similar betweennative and plantation forests, but the maxima forthe plantations were lower. In March, the waterstressed trees at site 6b had a transpiration rate of40 mg cm~^ d"'; less than 25% of otherplantation trees.

Discussion

For most of the year there was little difference inleaf water potential between the native andplantation forests we measured near Jarrahdale.This implies that there was usually littledifference as a consequence of eucalypt species(mainly E. marginata vs E. microcorys) and thatthere has been little consequence of sitetreatment (mined vs unmined) on leaf waterstress.

In the driest part of the year (January toMarch) there was some decrease of leaf waterpotential of the native forest over the plantationtrees. As these differences were not statisticallysignificant it cannot be concluded that the nativeforests were under more stress. Certainly, how-ever, the plantations were not under the greaterwater stress.

Large differences in patterns of moisturestress between various sclerophylls have beendemonstrated by Grieve (1956). We found onlytwo consistent differences which were

statistically significant. Firstly the 2-year-oldtrees of site 5a showed a lesser negative waterpotential than taller trees. The 2-year-old treeswere 2-3 m high, compared with 10 m or morefor most ofthe plantations, and 20 m or more forthe jarrah forest. Height difference in trees, andthe corresponding difference in height of watercolumn in conductive tissues, has been shown toaccount for differences in leaf water potential(Scholander et al. 1965). Secondly, we recordedvery low water potentials at site 6b during latesummer. The E. microcorys at this site alsoappeared susceptible to insect damage. Our datasuggests that E. microcorys was not suitable forplanting at such high density on such unusuallyshallow soil.

Excluding the sites which were speciallyselected for measurement because of theirobvious poor growth conditions (3a, 5b and 6b),and excluding the 2-year-old trees at site 5a, thesix remaining sites of plantation trees had anaverage LAI of 4.4. Unpublished data of theauthors has shown that subsequent plantations,which were given higher applications offertiliser, have attained these high leaf areas infive years or less. The leaf area of plantations wasmuch higher than those for the mature nativeforest areas (average LAI 2.5), which weresimilar to reports for other natural eucalyptforests in south-west Australia (Carbon, Bartle& Murray 1979b). It would appear thereforethat, from the aspect of total transpiring surface,or leaf area index, many of the plantation treesalready had the capacity to use more water thanthe trees of the native forest. Under conditionswhere ground cover, or undergrowth, was com-parable, the plantation forests, unless thinned,would be more likely to deplete soil waterreserves in periods of prolonged drought thanwere the original native forests.

Our tests have shown that the leaf chamberwas an appropriate tool to provide comparativerates of transpiration for our chosen samples.The concern about change in micro-environ-ment shown by Slatyer (1967) was not reflectedin the comparison between rapid weigh andtranspiration chamber techniques. This is notsurprising, when the leaves are usually in thechamber for less than 10 sec; the relativehumidity change is only 2%, and the tempera-ture change is too small to be measured.

We have confirmed Grieve & Went's (1965)contention that there is no measurable 'shock'

466 B. A. Carbon, G. A. BartleandA. M. Murray

effect for field-grown eucalypts. In other tests,however, we found such an effect with thejuvenile leaves of pot-grown eucalypts. We alsofound that the calibration of the electronicsensors changes gradually over years of use.Under such conditions they can provide goodrelative measures between treatments, butconstant checking is needed to ensure theaccuracy of absolute measures.

Doley (1967) has shown that transpiration rateper unit area of leaf is higher for leaves exposedto direct sunlight. It is therefore not possible tomultiply our estimates of leaf area index bytranspiration rate per unit area to obtain anestimate of total transpiration. We can however,imply from our data that the total transpirationfrom the older plantation forests was not lessthan that from the trees of the adjacent nativeforests. Transpiration rate per unit area of leaveswas depressed in plantations, relative to nativeforests, by only 17% in October to December,and by 26% in January to March, with nomeasurable difference in the wet months. Thesedifferences are much less than the differences inleaf area index, where the trees of the betterplantation forests had, on average, 1.75 timesthe LAI of the trees of the native forest.

During the period following clearing andmining, and the subsequent growth of newplantations, recharge of groundwater would behigher than in the pre-mining state. Theplantation forest will grow to the LAI ofthe treesof the original forest in 6 to 8 years. With modernmethods this will be accompanied by a stronggrowth of understorey. After these 6 to 8 years,nett recharge of the groundwater under planta-tions will not be less than under the originalforest.

We have concluded, therefore, that most ofthe eucalypt plantations on bauxite mines insouth-west Australia do not experienceexcessive water stress. The introduced trees soongrow to a leaf area greater than that of theoriginal forest, and subsequently are not likely totranspire less water. Indeed it is likely that someprogramme of tree thinning may be necessary tokeep the leaf area of the plantation forest near tothat of the original forest.

We must emphasize that we have takenmeasurements only on young plantations in

bauxite mines, as no stand has reached matur-ity. We also emphasize that our measurementsnear Jarrahdale provide information relative tothe western edge of the northern jarrah forest,but that our conclusions cannot be extrapolatedinto the presently unmined and drier easternedge.

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Bettenay E., Blackmore A.V. & Hingston F.J. (1964) Aspectsof the hydrologic cycle and related salinity in the BelkaValley, Western Australia. Aust. J. Soil Res. 2,187-210.

Carbon B.A.. Bartle G. A. & Murray A.M. (1979a) A methodfor visual estimation of leaf area. For. Sci. 25,53-8.

Carbon B.A., Bartle G.A. & Murray A.M. (1979b) Leaf areaindex of some eucalypt forests in south-west Australia.Aust. For. Res. 9, 323-6.

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Scholander P.F., Hammell H.T.. Bradstreet E.D. &Hemmingsen E. A. (1965) Sap pressure in vascular plants.Science N.Y. 148,338-45.

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