IMPACT OF TREE CLEARING ON SOIL ATTRIBUTES FOR A PASTORAL PROPERTY IN CENTRAL QUEENSLAND, AUSTRALIA.

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IMPACT OF TREE CLEARING ON SOIL ATTRIBUTES FOR APASTORAL PROPERTY IN CENTRAL QUEENSLAND, AUSTRALIA

Kamaljit Kaur1,2, Rajesh K. Jalota3, and David J. Midmore1

Impacts of tree clearing on soil properties in grazing systems ofQueensland largely remain undetermined. In the past, most of the landwas cleared to develop pastures to enhance pasture production. Thepresent research highlights how tree clearing affects soil attributes overtime on a pastoral property in the semiarid zone of central Queensland,Australia. Paired cleared and uncleared pasture systems were selected torepresent three different times since clearing (5, 11–13, and 33 years).Three woodland plant communities, which are common in the region,were studied: poplar box (Eucalyptus populnea), ironbark (Eucalyptusmelanophloia), and brigalow (Acacia harpophyllla). Soil macronutrientsand micronutrients and exchangeable cations were studied down theprofile for 0–5, 5–10, 10–20, 20–30, and 30–60 cm depth. The resultssuggest that tree clearing initially increased concentration of somenutrients (such as total N in the topsoil) that consequently augmentedpasture production. However, this scenario changes with increased timesince clearing: concentrations of some soil nutrients increased, whereassome decreased, thus disturbing the equilibrium in soil functions thatfurther impacts upon pasture yield. The impacts of clearing on soilproperties were more apparent at deeper layers of soils. Increase inconcentrations of Na at 30–60 cm in cleared pastures of E. populnea andE. melanophloia raises concern for sodicity. Results from this studysupport other findings in the region that changing native woodlands tointroduced grass pastures is likely to have consequences on soil nutrients,which can affect cattle production gains and the sustainability of pasturesystems. (Soil Science 2007;172:516–533)

Key words: Macronutrients, micronutrients, pasture systems, soilproperties, tree clearing.

IN Australia, large areas of land were clearedfor agriculture and pastoral development

during the 1950s, with a greater emphasis tomaximize rather than to optimize production(Conacher and Conacher, 1996). The NationalLand and Water Resources Audit (2002) stated

Bthe inherent potential of Australian soils that areolder as well as poorer compared to the otherproductive soils found elsewhere in the world,has been largely misinterpreted in the past.^

In Queensland, most land clearing happenedafter the 1950s (e.g., an average of 471,000 ha ofland per year has been cleared for the last 15 years,during 1988–2003; Department of NaturalResources and Mines, 2005) and in 1999–2001,577,000 ha of land were cleared per year(Department of Natural Resources and Mines,2003). Approximately 90–95% of total clearedland was developed into pastures. Until 1985,government policies favored clearing to developland for pastoral and agricultural systems (Boulteret al., 2000), but thereafter, permission wasmandatory from the Queensland Department of

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0038-075X/07/17207-516–533 July 2007

Soil Science Vol. 172, No. 7

Copyright * 2007 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A.

1Plant Sciences Group, Primary Industries Research Centre, Central Queensland

University, Rockhampton, Queensland 4702, Australia.

2School of Marine and Tropical Biology, James Cook University, Townville,

Queensland, Australia 4811. Dr. Kaur is corresponding author. E-mail: kamaljit.

[email protected]

3Centre for Environment Management, Central Queensland University, Emerald

Campus, Queensland 4720, Australia.

Received Jul 24, 2006; accepted Feb 13, 2007.

DOI: 10.1097/SS.0b013e31804fa286


Natural Resources to clear land, even for clearingof nonremnant vegetation. From September2004, a change in government policy stoppedland clearing. This was predicated mainly on thelosses of biodiversity and not based upon access toinformation on the long-term effects of clearingon soil properties or, equally important for alandholder, on productivity of pasture systems.Without substantial data on the long-term effectsof clearing on soil properties and pasture pro-ductivity, it is difficult for government to imposepolicies on clearing and for a landholder tounderstand and adapt to them.

For the development of pasture systems,cleared land was commonly sown to introduced

grass species (such as Cenchrus ciliaris L., com-monly known as buffel grass) that exploit the fluxof nutrients released upon clearing (Lawrenceet al., 1994). This initially leads to greaterpasture production, whereas later, pasture pro-duction declines as the pastures age (Lawrenceet al., 1993, 1994; Graham, 1996; Kaur et al.,2005), and this may accompany changes inecosystem functions such as soil mineralization.However, the economic assessments of treeclearing for pasture development often ignorethe associated risks on changes in soil properties,salinity, sodicity, and in soil structural degrada-tion. The longer-term losses/benefits of treeclearing in pasture development are difficult to

Fig. 1. Location of the property ‘‘Avocet,’’ associated regional ecosystems, and location of various cleared (graycolor) and uncleared sites (colored). DCDB = Digital Cadastral Database. Source: Department of Natural Resourcesand Mines, 2003).

VOL. 172 ~ NO. 7 IMPACTS OF TREE CLEARING ON SOILS PROPERTIES 517


quantify until these associated risks are ac-counted for over time since clearing.

The impact of clearing could vary accordingto soil and vegetation type. In the present study,various chemical properties were comparedbetween cleared and uncleared woodland com-munities where clearing occurred at 5, 11–13,and 33 years before the study. The nutrientstatus was studied down the soil profile, con-trasting with previous studies (Graham et al.,1985; Lawrence et al., 1993) where only the top(0–10 cm mostly) depth was sampled. It wasimportant to assess the variation in soil proper-ties with depth as changes in deep layers mayhave a greater potential to alter soil functions(Brown and Lugo, 1990). The main objectivesfor the present research were the following:

& Compare soil status of cleared and uncleared(native woodland) pastures to quantify thelong-term consequences of clearing in devel-oped pasture systems.

& Examine the trends in soil properties overtime since clearing.

& Study variation in soil properties with depth.

Different time since clearing treatments willprovide data on long-term changes in soilproperties to inform future policy decisions ontree clearing. The inferences for changes in soilproperties after clearing will support futuredecisions on tree clearing. The study will alsohelp to estimate whether the gains fromincreased pasture productivity upon clearingare offset by losses of soil properties or not.

MATERIALS AND METHODS

Study Design and Area

A total of 18 sites, nine pairs for cleared anduncleared treatments of each of three selectedtree community across three age groups ofclearing, were identified within a 5000-hasemiarid grazing property BAvocet^ in centralQueensland (NW 148.13-E, 23.79-S, NE148.16-E, 23.80-S, SE 148.21-E, 23.85-S, andSW 148.12-E, 23.86-S; Fig. 1). The propertyhas belonged to the same landholder for the last55 years, with full records of all land clearance.These sites were selected with the guidance ofresearch staff at the Department of NaturalResources, Mines, and Water and at Environ-mental Protection Agency, Emerald, to be therepresentative vegetation types of the region.

Average annual rainfall in the region is600 mm, with sporadic summer storms duringOctober-February (Commonwealth of AustraliaBureau of Meteorology 2003). Average (1865–2001) minimum and maximum temperatures are6–8 -C and 23–25 -C during winter ( June–August) and 22–24 -C and 33–36 -C duringsummer (December–February).

Sites were selected in pairs for clearedand uncleared woodlands of Eucalyptus populneaF. Muell. (poplar box), Eucalyptus melanophloiaF. Muell. (silver-leaved ironbark), and Acaciaharpophylla F. Muell. ex. Benth. (brigalow)communities across three age groups of clearing(i.e., recent, 5 years; medium, 11–13 years; old,33 years). Thus, the study represents a 3 (typesof tree communities) � 3 (time since clearing)

TABLE 1

Details of date of clearing (all sites were chain-pulled), postclearing treatments, and annual stocking rate at cleared and

uncleared sites for E. populnea, E. melanophloia, and A. harpophylla communities

Tree community

Cleared treatments Uncleared treatments

Time of clearing

(years)

Postclearing

treatment

Stocking rate

(adult cattle haj1)

Stocking rate

(adult cattle haj1)

E. populnea

Recent clearing May 1996 Burnt in Oct 1999 1/5 1/5

Medium clearing Dec 1987 Blade ploughed in 1994 1/3 1/4.8

Old clearing July 1967 Blade ploughed in 1987 1/6 1/4.8

E. melanophloia

Recent clearing May 1996 Burnt in Oct 1999 1/5 1/5

Medium clearing Oct 1990 Blade ploughed in 1994 1/3 1/3

Old clearing July 1967 Blade ploughed in 1987 1/6 1/6

A. harpophylla

Recent clearing May 1996 Burnt Oct 1999 1/5 1/11

Medium clearing Dec 1987 Blade ploughed in 1994 1/3 1/4.8

Old clearing July 1967 Blade ploughed in 1987 1/6 1/4.8

518 KAUR, ET AL. SOIL SCIENCE


� 2 (cleared versus uncleared) factorial design(Fig. 1). As such, the uncleared sites werereplicated three times for each tree community

on this property, but the periods since clearingwere not. None of the uncleared woodland siteshad been cleared in the past 55 years. The paired

Fig. 2. Concentration of SOC (mg kgj1) at various cleared and uncleared treatments for (A) E. populnea, (B) E.melanophloia, and (C) A. harpophylla communities. Different letters at any depth within each community denote asignificant difference (P G 0.05) in means between any cleared and uncleared treatments.



Fig. 3. Concentration of total P (mg kgj1) in soil at various cleared and uncleared treatments for (A) E. populnea,(B) E. melanophloia, and (C) A. harpophylla communities. Different letters at any depth within each communitydenote a significant difference (P G 0.05) in means between any cleared and uncleared treatments.



cleared and uncleared sites were selected in closeproximity (i.e., 50 m across the fence) with theassumption that they had similar biophysicalcharacteristics (soil type, slope, and vegetation)before clearing (according to information pro-vided by the landholder) and, to some extent, tominimize variation in grazing management forthe same cattle grazed the cleared and theirpaired uncleared sites. The cleared sites werechain-pulled and then sown to buffel grass(C. ciliaris L.) within a year of clearing. Detailson time and methods of clearing and the rate ofstocking over the last 20 years are presented inTable 1. All recently cleared plots were burnt inOctober 1999, those 11–13 years since clearingwere blade-ploughed in 1994, and those cleared33 years ago were blade-ploughed in 1987.There was no application of fertilizers to any ofthe sites. Further details on climate and siteselection are presented by Sangha et al. (2005a).

Each tree community grows on a particularsoil type, E. populnea on Sodosol (sandy-loamtexture contrast, nongravely soils), E. melano-phloia on Kandosol and Dermosol (sandy or clayloam, moderate, brown, nongravely, or slightlygravely soils), and A. harpophylla on Sodosol(clay, gray, and deep soils) (using the AustralianSoil Classification by Isbell, 2002). These soils arerelatively poor in nutrient content and organicmatter, with maximum depth to 60 cm, atwhich sandstones and rocks are reached.

At each site, a representative area of 1 ha ofthe total area (minimum 9 20 ha) at each sitewas marked for data collection.

Soil Sampling and Processing

Eight soil cores were taken randomly with ahydraulic soil corer at different distances anddirections from a central point in each 1-haarea. Each core was segmented into 0–5, 5–10,10–20, 20–30, and 30–60 cm. All eight sampleswere bulked for each depth. Visible roots andpebbles were removed during processing. Thedetails of sampling procedure and processing arepresented by Sangha (2003). The samples wereanalyzed for macronutrients such as available N(NO3

j) and P (Colwell) (using segmented flowanalyzer), total N and P (using CHN analyzerand inductively coupled plasma–atomic emis-sion spectrometer [ICP-AES], respectively),organic carbon (Walkley and Black method).Samples were also analyzed for micronutrients(Cu, Zn, Fe, and Mn; using ICP-AES) and forexchangeable cations (Ca, K, Mg, and Na usingICP-AES to assess cation exchange capacity

[CEC]). These analyses were performed at soillaboratories of Incitec (Brisbane, Queensland,Australia). Data on soil pH, bulk density, andelectrical conductivity are presented by Sanghaet al. (2005a).

Statistical Analysis

The main effects (fixed terms) of treecommunity (E. populnea, E. melanophloia, andA. harpophylla), cleared (recent, medium, andold age of clearing) and uncleared treatments,and soil depth (each for 0–5, 5–10, 10–20,20–30, and 30–60 cm depth) plus their inter-actions were analyzed for each variable applyingthe repeated-measures residual maximum like-lihood (REML) model using Genstat version 6.0(Genstat Committee, 2002). The correlationwithin a treatment across different depths wasanalyzed for all the main effects by applying themost suitable variance model AD2 (Antede-pendence order 2).

REML analysis showed significant (P G0.05) interactions among tree community andcleared and uncleared treatments across depthfor the main effects in all the soil variables. TheF approximation is based upon 66 df of error forthree-way interactions. For each variable, leastsignificant differences of means were calculatedfrom an standard error of differences of meansmatrix (obtained from REML analysis) for allthe treatment effects and were used to comparecleared (recent, medium, old) and unclearedtreatments for each specific depth in a particulartree community. The means from REMLanalysis were used to present the results.

RESULTS

Macronutrients

Concentration of soil organic carbon (SOC)declined sharply with depth at all sites (Fig. 2).Tree clearing had a significant effect on SOConly at 30–60 cm depth: in E. populnea, recentclearing had a greater concentration thanuncleared (Fig. 2A); in E. melanophloia, therewas no difference among sites (Fig. 2B); and inA. harpophylla, recent clearing had a greaterconcentration than medium clearing (Fig. 2C).The average SOC for 0–60 cm depth suggestedno impact of age of clearing (average values Tstandard error for uncleared, recent, medium,and old cleared sites in E. populnea: 7333 T 1622,12,000 T 2529, 8600 T 1939, and 8400 T 1469mg kgj1, respectively; in E. melanophloia: 7533 T1771, 7800 T 1428, 7400 T 1503, and 7200 T



Fig. 4. Soil available P (mg kgj1) at various cleared and uncleared treatments for (A) E. populnea, (B) E.melanophloia, and (C) A. harpophylla communities. Different letters at any depth within each community denote asignificant difference (P G 0.05) in means between any cleared and uncleared treatments.



Fig. 5. Concentration of total N (mg kgj1) at various cleared and uncleared treatments for (A) E. populnea, (B) E.melanophloia, and (C) A. harpophylla communities. Different letters at any depth within each community denote asignificant difference (P G 0.05) in means between any cleared and uncleared treatments.



1067 mg kgj1, respectively; for uncleared,recent, medium, and old cleared sites in A.harpophylla were 10,866 T 2217, 10,400 T 2014,9200 T 1933, and 10,200 T 2059 mg kgj1,respectively). The paired t test for the averageSOC of 0–60 cm depth also did not suggest anysignificant difference between cleared (irrespec-tive of age group) and uncleared treatments,across and within tree communities (averagevalues for cleared and uncleared treatments were9666 T 1170 and 7333 T 1622 mg kgj1 in E.populnea, 7466 T 722 and 7533 T 1771 mg kgj1

inE. melanophloia, and 9933 T 1079 and 10,866 T2217 mg kgj1 in A. harpophylla). Clearing hadno significant impact on total and available Pexcept in E. melanophloia where the total andavailable P were exceptionally higher at mediumthan recent and old cleared treatments (Figs. 3Band 4B). This was because of the presence ofbasalt in the parent rock (noted visually). Theuncleared E. melanophloia soil was also rich intotal and available P mainly because of basalt inthe parent rock (the uncleared site paired formedium age of clearing had a basaltic parent rockthat increased the overall average for all unclearedsites of E. melanophloia) (Figs. 3B and 4B).

Total N also declined with increase in soildepth, in line with SOC, without a significantimpact of clearing other than a greater concen-tration at recent age since clearing comparedwith medium clearing for the top 0–5 and bottom30–60 cm depth inA. harpophylla and to unclearedE. populnea at 10–30 cm depth (Fig. 5A–C). Thenitrate content (NO3

j) was highly variable in thetop 0–10 cm depth. It was greater at 0–10 cmdepth at recently cleared sites in E. populnea andA. harpophylla and was less at old cleared sites(Fig. 6A and C), whereas the medium clearedsite had greater NO3

j than the unclearedE. melanophloia for 0–5 cm depth (Fig. 6B).

Micronutrients

There were not many conspicuous differ-ences due to clearing or time since clearing in Cu,Fe, Zn, and Mn concentrations in E. populnea,E. melanophloia, and A. harpophylla soils, in parti-cular within a tree community. The average Cuconcentrations for 0–60 cm depth were 0.21 T0.027 mg kgj1 in uncleared and 0.46 T 0.047mg kgj1 in cleared treatments of E. populnea;0.56 T 0.044 mg kgj1 in uncleared, and 0.64 T0.039 mg kgj1 in cleared treatments of E.melanophloia; and 0.65 T 0.052 mg kgj1 inuncleared and 0.70 T 0.058 mg kgj1 in clearedtreatments of A. harpophylla. Tree clearing had

no impact on Zn concentrations when com-pared within a tree community in relation to agegroup across each soil depth. The average valuesfor 0–60 cm depth were 0.47 T 0.128 mg kgj1

for uncleared and 1.17 T 0.279 mg kgj1 forcleared treatments for E. populnea, 0.91 T 0.186mg kgj1 for uncleared and 0.81 T 0.179 mgkgj1 for cleared treatments for E. melanophloia,and 1.62 T 0.614 mg kgj1 for uncleared and1.37 T 0.145 mg kgj1 for cleared treatments forA. harpophylla. Similarly, there were no differ-ences in Fe concentrations for 0–60 cm depth ofuncleared (average = 51.00 T 7.153 mg kgj1)and cleared (average = 49.02 T 5.060 mg kgj1)treatments and in Mn concentrations ofuncleared sites (average = 9.77 T 2.103 mgkgj1) and cleared (average = 9.80 T 1.011 mgkgj1) treatments. There was no definitive trendfor change in concentrations of micronutrientswith soil depth (data by depth not presented).

Exchangeable Cations

Exchangeable Na concentration was signifi-cantly greater at 30–60 cm at recent andmedium cleared treatments in E. populnea andat old treatment in E. melanophloia comparedwith their respective uncleared treatments(Fig. 7). More than a twofold increase in Naconcentration at these treatments indicated thatclearing may tend to cause sodicity. Such aneffect was also apparent with the percentage ofcation exchangeable sites occupied by Na (ESP)(Fig. 8). Overall ESP tended to increase withdepth in almost all treatments. The results alsosuggested that uncleared E. populnea and A.harpophylla soils had inherently large values ofESP in the subsoil (30–60 cm depth) comparedwith E. melanophloia (Fig. 8A–C).

At 30–60 cm depth, CEC was greater inall the cleared sites in E. populnea and in re-cent and old cleared sites in E. melanophloiaand A. harpophylla, respectively, compared withtheir uncleared sites (Fig. 9A–C). With increas-ing depth, CEC tended to increase in clearedsites, especially at 30 to 60 cm (with a fewexceptions).

Concentrations of exchangeable Ca increasedat 30 to 60 cm soil depth in old and mediumcleared E. populnea, at 20 to 60 cm soil depth inrecent cleared E. melanophloia, and at 10–60 cmdepth in old cleared A. harpophylla comparedwith their respective uncleared sites (Fig. 10).However, Mg concentrations did not changemuch with clearing, except for an increase inrecent cleared E. populnea (data not presented).



Fig. 6. Concentration of NO3-N (mg kgj1) at various cleared and uncleared treatments for (A) E. populnea, (B) E.melanophloia, and (C) A. harpophylla communities. Different letters at any depth within each community denote asignificant difference (P G 0.05) in means between any cleared and uncleared treatments.



Fig. 7. Soil exchangeable cation Na (mEq 100 gj1) at various cleared and uncleared treatments for (A) E.populnea, (B) E. melanophloia, and (C) A. harpophylla communities. Different letters at any depth within eachcommunity denote a significant difference (P G 0.05) in means between any cleared and uncleared treatments.



Fig. 8. Percentage of cation exchangeable sites occupied by Na (ESP) (%) at various cleared and uncleared treatmentsfor (A) E. populnea, (B) E. melanophloia, and (C) A. harpophylla communities. Different letters at any depth withineach community denote a significant difference (P G 0.05) in means between any cleared and uncleared treatments.



Fig. 9. Cation exchange capacity (mEq 100 gj1

) in soil at various cleared and uncleared treatments for (A) E.populnea, (B) E. melanophloia, and (C) A. harpophylla communities. Different letters at any depth within eachcommunity denote a significant difference (P G 0.05) in means between any cleared and uncleared treatments.



Tree clearing increased the concentration ofexchangeable K profile-wide at recent treatmentsfor E. populnea and E. melanophloia and had noother noticeable impact (the average K for 0–60cm depth was 0.17 T 0.042 mEq 100gj1 inuncleared and 0.23 T 0.034 mEq 100gj1 incleared soils of all tree communities).

DISCUSSION

Availability of soil nutrients for pasturegrowth is an important issue for pasture pro-duction in grazing systems of central Queens-land. The release of soil nutrients upon treeclearing is mainly responsible for enhancedpasture production over the initial years ofclearing, which has led landholders to opt forclearing. Increased concentrations of available N(700%), P (225%), and K (70%) upon clearingcompared with uncleared sites in A. harpophyllahave been reported by Lawrence et al. (1994).Overall, the results of the present study showedincrease in some nutrients but not many obviouseffects of clearing. We acknowledge that thenutrient release would have been even greater atthe time of clearing; some nutrients will alreadyhave been taken up by plant growth before oursampling (Sangha, 2003).

The exotic grass species C. ciliaris, com-monly sown to cleared land, is very efficient atcapturing the released soil nutrients because ofits fine roots (Conyers, 1999) and high root/shoot and root length/root biomass ratios(Wilson, 1998, cited in Schmidt and Lamble,2002). This leads to an increase in pasturebiomass, as reported for pastures in this studyarea (Kaur et al., 2005) and for similar pasturesin the region (Scanlan and Burrows, 1990;Burrows, 2002). However, questions as to thesustainability of the cleared systems requireanswers. For example, what are the otherimpacts/imbalances (disequilibrium) in soilsproperties that occur because of clearing? Forhow long will the increased concentrations ofnutrients, which may become available uponclearing in some cases, be sustained to satisfydemands by cattle on pasture growth?

Lower pasture yield at uncleared sites, aprime motive for clearing, is most likely becauseof competition between trees and grasses for soilmoisture and available nutrients (Jackson andAsh, 1998; Schmidt and Lamble, 2002). Most ofthe native grasses (species of Themeda, Aristida,Paspalidium, Eragrostis, and Dicanthium) growingunder tree canopies use C4 pathway for photo-

synthesis, hence tree-plant competition for lightis also likely to occur but it is unlikely to besevere because of the open woodland structurein central Queensland. Soil moisture seems to bea major limiting factor in woodland communi-ties in semiarid climates (Scholes, 1993; Scholesand Archer, 1997). In fact, the competition forboth soil moisture and soil nutrients betweengrass species and trees invariably limit pastureproduction under natural tree stands (Burrowset al., 1988; Scholes and Archer, 1997).

To interpret the impacts of clearing, changesin soil properties at depth must be considered.Most previous studies conducted in the region(Graham et al., 1981, 1985; Lawrence et al.,1993; Robertson et al., 1993) presented data forthe top 0–10 cm depth and showed increasedamounts of nutrients with clearing. Analysis ofsoil samples from greater depth (930 cm) in ourstudy indicated increased amounts of Na in E.populnea and E. melanophloia and total N in A.harpophylla communities. (However, the avail-ability of N is questionable in cleared pastures;this will be discussed later.) Graham (1978) andBrown and Lugo (1990) reported a decline intotal N with depth. The changes in concen-tration of soil nutrients at depth may haveimplications in the long term: cleared pasturescould not effectively recycle nutrients from deeplayers to maintain soil nutrient levels, becauseclearing has caused salinity in some southernparts of Australia (National Land and WaterResources Audit, 2002).

We acknowledge that to some extent theeffects of clearing on nutrient concentrationswere influenced by grazing and postclearingtreatments. Because grazing took place at allthe sites within a narrow range of stocking rate(1 head 3 haj1 yearj1 to 1 head 5 haj1 yearj1)and with almost similar rates at paired sites,grazing could be considered less important thanpostclearing practices that were superimposedupon recent, medium, and old cleared sites. Dif-ferent management practices (e.g., blade plough-ing at medium and old cleared sites; Table 1) areexpected to change the soil properties. Grahamet al. (1985) reported improvement in soil nu-trients for the central Queensland, and Robson(1995) reported increased pasture biomass fornorthwest New South Wales, after blade plough-ing. Hence, blade ploughing at old and mediumclearing could have contributed to an improve-ment in nutrient status at these sites. Because thesoil status in medium and old clearing was notbetter than that of uncleared plots, and indeed,



pH increased at the same sites (Sangha et al.,2005a), as did Na, it is likely that without theblade ploughing, the soils may have shown aneven poorer nutrient status.

Our data on the clearing of trees for pasturedevelopment question the availability of soilnutrients for pasture production over time sinceclearing. Thus, the sustainability of cleared

Fig. 10. Concentration of Ca (mEq 100 gj1) in soil at various cleared and uncleared treatments for (A) E. populnea,(B) E. melanophloia, and (C) A. harpophylla communities. Different letters at any depth within each communitydenote a significant difference (P G 0.05) in means between any cleared and uncleared treatments.



pastures in terms of soil stability and pastureproduction is jointly discussed here. Increasednutrient concentration (in the topsoil layer)upon clearing provides an important option toincrease pasture production (Burrows et al.,1988; Scanlan, 2002; Nelson and Roth, 2004).However, the pasture biomass measured in thesame three communities declined with age ofpasture (Sangha, 2003; Kaur et al., 2005):maximum at medium followed by a decline atthe old age of clearing in E. populnea andA. harpophylla and maximum at recent followedby a decline from medium to old clearing inE. melanophloia. The reason for lesser pastureyield at old cleared sites in all the threecommunities is mostly the reduced availabilityof nutrients compared with that at recent ormedium clearing, especially at depth of morethan 30 cm and/or changes in ecosystemfunctions of cleared pastures (see Sangha et al.,2005a, b, for details).

The availability of nutrients declines 5–7years postclearing in the fertile cleared soils ofA. harpophylla as reported by Lawrence et al.(1994), and this is closely linked to pasture run-down (decline in pasture production over timeof clearing). Most studies (Burrows, 1993;Lawrence et al., 1993, 1994; Graham, 1996)explained pasture run-down as a consequence ofnonavailability of N and P through immobiliza-tion of these nutrients in soil microbial biomassand not because of their reduced amounts in thesoil. Later, Robertson et al. (1997) related thereduced availability of N to immobilization,largely in soil organic matter, clay, plant mate-rial, and, to a lesser extent, soil microbialbiomass. According to these hypotheses, a largeamount of soil organic reserve would be presentat old cleared pastures. Data from our study sitesdo not support this because SOC at all depthsdid not significantly differ between unclearedand old cleared plots in any of the three plantcommunities examined (details on pasture run-down and SOC are presented by Sangha, 2003;Kaur et al., 2006). Similarly, declines in soilmicrobial biomass with time since clearing werenoticed at the same sites (Kaur et al., 2006). Inaddition to these, tree clearing significantlyaffected soil pH (Sangha et al., 2005a), whichin turn affects nutrient availability and subse-quently pasture growth. As a part of the presentstudy, results from soil pH and nutrient levelssuggest that most nutrients (total N, P, Zn, Mn,K, and Ca) become unavailable at more alkalinepH in cleared soils (from uncleared to old

cleared treatments, increased from 5.6 to 7.2 inE. populnea, from 7.1 to 8.8 in E. melanophloia,and from 6.9 to 9.1 in A. harpophylla for 30–60cm depth) (Sangha et al., 2005a). Similar effectsof soil pH on soil nutrient availability werereported by Page et al. (2002) for accumulationof ammonium at high pH in grazing soils ofsouth Queensland, by Robertson (1989) on Nmineralization, and by Conyers (1999) onuptake of various nutrients. Changes in pHoccur due to reactions in soil solution, in whichvarious biotic and abiotic factors play an impor-tant role. Soil microclimate, microbial activity,interaction among various ions, and physicalfactors exert a significant impact on nutrientavailability for plant growth through their effecton soil pH. Thus, there is a complex system ofvarious ecosystem functions (nutrient recycling ormineralization) that operates in an equilibrium.

An important question is whether theequilibrium in soil processes can be maintainedby monoculture pastures of exotic grass speciesin the absence of trees. If it cannot, this suggeststhat trees with an understorey of grass speciesin pastures must prevail to ensure the main-tenance of soil processes (nutrient recycling, soilmineralization, soil-microclimate, etc.). There isevidence that trees provide a stable environmentbecause of their shade, litter, stable hydrologicalcycle, recycling of nutrients, and by providingsubstrate for various soil microbial activities thatresults in improved physical and chemical con-ditions of soil (Vetaas, 1992; Williams et al.,1993; Nelson and Roth, 2004). Further, Williamset al. (1993) suggest that clearing of trees in-fluences and disturbs the equilibrium of soil pro-cesses (nutrient recycling and decomposition).

It is concluded from our study on oneproperty in central Queensland for paireduncleared and cleared sites located in three plantcommunities that, in some cases, soil nutrients(e.g., available N at 0–5 cm in an A. harpophylla)increased after 5 years since clearing butdeclined with age of clearing. Similarly, theresults from this study indicate that implicationsof tree clearing for soil properties at depth suchas increased Na concentrations in old clearedE. melanophloia and increased pH at all the oldcleared sites (Sangha et al., 2005a) could beserious. The changes in soil processes uponclearing, although occurring slowly and notappearing significant at this stage, may havelong-term implications for pasture production.Such changes might take even longer to redressthe system than the time taken to harvest the



benefits of clearing for pasture production. It isimportant for policy decisions to consider thelong-term implications of tree clearing whenprojecting their benefits for pasture production.

ACKNOWLEDGMENTS

The authors sincerely thank the landholders,Mrs. and Mr. Spooner, for permission toconduct this research on their property and fortheir cooperation during the study. They alsogratefully acknowledge the laboratory space andequipment provided by the Department ofNatural Resources and Mines and the Depart-ment of Primary Industries at Emerald, Mr.Robert Lowry for the technical help in samplingand processing of soil samples, and two anony-mous reviewers for their constructive criticismof an earlier draft. Funds were provided byCentral Queensland University, Queensland.

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IMPACT OF TREE CLEARING ON SOIL ATTRIBUTES FOR A PASTORAL PROPERTY IN CENTRAL QUEENSLAND, AUSTRALIA.

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