Trees, Water and Salt · •Salt from the sea carried in rainwater has accumulated in the soil profile over a very long time. Rising watertables dissolve salt and bring it back towards

The JVAP Research Update Series No.1

Trees, Water and Salt:

An Australian guide to using

trees for healthy catchments

and productive farms

With support from:

Natural Heritage TrustMurray-Darling Basin CommissionGrains Research and Development CorporationAustralian Greenhouse Office

2 Trees, Water and Salt: an Australian guide to using trees for healthy catchments and productive farms

Trees, Water and SaltAn Australian Guide to using Trees for healthy catchments andproductive farms

The environmental benefits of growing trees onfarms are universally recognised. To achievethese desired effects, plantings must be wellplanned. Land managers wanting to addressproblems of salinity and waterlogging needanswers to six key questions:

• What area of a catchment needs to be planted?

• Where is the best location in a catchmentto plant?

• Should trees be arranged in blocks or belts?

• What is the time interval between plantingand seeing results?

• How effective are particular farm forestrydesigns in different settings?

• How are appropriate species andmanagement practices chosen?

A valuable new book provides informationneeded to answer these questions. Entitled Trees,Water and Salt: an Australian guide to using treesfor healthy catchments and productive farms, it hasbeen written by researchers from CSIRO Landand Water and CSIRO Forestry and ForestProducts and other agencies (see page 21), andproduced by the Joint Venture AgroforestryProgram. The book is edited by RichardStirzaker, Rob Vertessy and Alastair Sarre.

This research update outlines the book’s keymessages. The detail is important, so thoseconsidering planting trees to address salinity areurged to consult the book.

To answer the six questions above, we need tounderstand the whole picture from thehydrological behaviour of a catchment to theperformance of a single tree in a paddock.

Trees, Water and Salt is designed to make thiscurrent scientific knowledge available to landmanagers. The contents of the book will:

• Provide a design framework for tree plantingto combat salinity

• Outline basic hydrological concepts, givingthe necessary technical background tointerpret the rest of the book

• Give an overview of the ways differentcatchments respond to planting strategies

• Present descriptions of planting designsappropriate to various situations.

• Offer suggestions regarding tree species suitedto specific conditions.

The format is clear and easily accessible, and thebook is fully illustrated.

The problem, and howagroforestry can help

The replacement of native vegetation withcrops and pastures that use less water hasresulted in rising groundwater levels, causingsalinity damage over wide and growing areas.The problem can be alleviated by tree planting,but this requires careful planning based onknowledge of the affected catchment.

Around 2.5 million hectares of farming land inAustralia is now salt-affected, and this area couldincrease sixfold in coming decades despitecurrent efforts to slow the spread. The water insome Western Australian rivers is no longer fitto drink, and several important eastern riversface the same fate.

Clearing native vegetation for crops and pasturescauses this situation. Unfortunately, three featuresof the Australian landscape make it particularlysusceptible to salinisation:

• Native vegetation, adapted to Australia’s highlyvariable climate, is equipped to use water whenit is available, including that stored deep in thesoil. Under this vegetation, leakage of rainfallis low. Leakage is generally much higher underthe shallow-rooted seasonal crops and pasturesthat have replaced it.

• Salt from the sea carried in rainwater hasaccumulated in the soil profile over a verylong time. Rising watertables dissolve salt andbring it back towards the surface. Salty wateralso starts to move laterally, forming salineseeps and entering rivers.

• Horizontal movement of water through thesoil is generally very slow because the landtends to be flat and the soils not verypermeable. Hence, when the vegetation doesnot use all the rainfall, watertables start to rise.

Trees, Water and Salt: an Australian guide to using trees for healthy catchments and productive farms 3

Clearly, reintroducing trees to the landscape canhelp alleviate salinity and waterlogging problems.But to be effective this needs careful planning.The first three stages in planning a catchmenttree-planting strategy to address salinity involveutilising the conceptual knowledge explained inthis book in conjunction with local expertsupport to:

• Determine the scale of the aquifer system andthe discharge capacity — this helps todetermine the area of planting needed to havethe desired effect and the time-scale forrealising the benefits.

• Estimate current groundwater recharge in thecatchment — from predicted long-termleakage rates for each land use and the areaunder each use.

• Identify a target — for example, to reduceleakage to a level that will result in no furtherrise in the watertable.

Once conditions are identified and a target hasbeen decided, the principles explained in thebook can be used to help:

• Assess the best locations and arrangementsfor tree planting.

• Design the most efficient revegetationstrategy to meet the recharge target. Thereare four main agroforestry designs discussedin the book:

1 Alternating woodlots with agriculture(phase farming) – woodlots are used tocontrol recharge by drying out thesoil profile– appropriate for deeper soilprofiles with heavy textured subsoil.

2 Hill-slope tree belts – appropriate forrecharge and discharge control in hilly localflow systems.

3 Mixing tree-belts with agriculture – thesuitability of either tree-belts or blockscan be determined to reach a givenleakage target in recharge areas wherewatertables are still relatively deep butrising.

4 Planting shallow, saline watertables -trees planted in such environments canlower the watertable locally, primarilythrough reduced recharge. This may resultin reduced saline discharge but the designcan only be applied in areas where the wateris not too salty and lateral water movementprevents salt accumulation.


New Guidelines Series-

As a follow up to the best seller DesignPrinciples for Farm Forestry the JVAP isproducing a series of guidelines to help landmanagers decide how to integrate trees onfarms for multiple benefits. Trees, Water andSalt is the first in this series. Other guidelinebooks available in early 2001 are:• Trees for shelter: a guide to using windbreaks

on Australian farms • Farm Forestry Site Selection Manual

Design Principles forAgroforestryProtection of the land resource — fromwind and water erosion as well as fromsalinity — is a major incentive to plant trees.There are others as well, notably:

• Products such as wood, pulp or eucalyptusoils can provide new income streams forfarmers.

• Agroforestry can increase farmproductivity by providing shelter forstock and crops and alleviatingwaterlogging in low-lying paddocks.

• Tree planting can enhance biodiversityand the aesthetic appeal of the landscape.

Further information about how to designagroforestry systems to meet multipleobjectives can be obtained from:-Abel, N., Baxter, J., Campbell, A., Cleugh,H., Fargher, J., Lambeck, R., Prinsley, R.T.,Prosser, M., Revell, G., Schmidt, C.,Stirzaker, R. and Thorburn, P. 1997 DesignPrinciples for Farm Forestry- A guide to assistfarmers to decide where to place trees and farmplantations on farms. Canberra, RuralIndustries Research and DevelopmentCorporation.

JOINT VENTUREAGROFORESTRY PROGRAM(RIRDC/LWRRDC/FWPRDC)Since 1993, JVAP has led Australia in thedevelopment and dissemination of research andpractical information to underpin newsustainable farming systems incorporatingperennial woody vegetation.

The program focuses on commercially driventree production systems for addressing landdegradation issues. It is developing new tree-based industries for integration into low tomedium rainfall farming systems. The programaims to deliver the following outcomes:

• Targeted strategies for implementation offarm forestry

• More sustainable management of naturalresources eg. soil, water and biodiversity

• Optimised productivity of crops and pastures

• Optimised direct returns from tree products

• Cost effective multi-purpose agroforestrysystems to meet commercial andenvironmental objectives.


This simple model helps to illustrate thehydrological processes at work in a catchment.However, it belies the extreme complexities inunderstanding exactly what is going on underneaththe ground surface, simply because we cannot see,and cannot measure the water and solute flowingthrough the soil in any precise way.

The salt content of the soil adds another layer ofcomplexity. Virtually all soil has some soluble saltin it which is derived from a number of sources.Huge quantities of salt have built up over longperiods, especially in Western Australia (WA)where up to 10,000 tonnes is stored under each

The leakage under native vegetation is 1–10 mmper year and under annual crops and pastureabout 10–150 mm per year. Continuing the bathtub analogy, if this rate exceeds the outflowcapacity of the plug hole, then water will rise inthe bath. In catchments, the critical outflow rateis called the discharge capacity, and when theleakage from vegetation exceeds it we get risingwatertables. It is this connection between input(leakage) and output (discharge capacity) whichleads to the enormous problems we have createdfor ourselves right across Australia which showup as saline seeps, waterlogging and discharge ofsalty water into streams.

Surface(over)flow

Surface(over)flow

Evapotranspiration

Groundwater flow

Salt

Precipitation

Replace trees with annual crops/pasture

Groudwater flow

Salt rises

Evapotranspiration

Precipitation

A catchment may be thought of as a bathtub filled with soiland tilted on a gentle incline. Water "leakage" past the rootzone of the plants is like a dripping tap. Under Australia’snatural evergreen and deep rooted vegetation this rate isvery low. As a consequence the watertable is deep beneaththe surface, and what little groundwater there is flowsslowly, as if the bath has a small drainage hole (ordischarge). All rain water carries small amounts of salt whichis stored deep in the soil.

When the deep rooted vegetation is removed, the shallowrooted grasses which replace it do not use as much water, andleakage past the root zone increases. Using the bathtubanalogy, - the "tap" now drips at a faster rate. If this rate isgreater than the discharge capacity of the bathtub(catchment) the watertable rises, dissolving salt as it does andbringing it near the surface. The result is that the bathtub fillsuntil it overflows at its lower edge– meaning shallowerwatertables, waterlogging and salinity, as well as longerperiods of stream flow, now carrying salt, and increasedlikelihood of flooding.

The soil, water, plant connection

hectare of wheat/sheep country. Most of this saltis deep in the soil, but it is highly soluble, andwhen water passes through the salt bearing soilit is easily mobilised. When the catchment“bathtub” fills, the water level rises through thesalt bearing soil, and the salt is dissolved andbrought closer to the surface. When the, nowsalty, watertable reaches stream beds, thestreams run more frequently and with higher saltcontent. Where the soil surface is within about2 m of a saline watertable, surface evaporationdrives capillary action bringing water upwardsfrom the watertable, leading to ever increasingsalt accumulation at the surface.

The rate at which water flows through soildepends on its hydraulic conductivity, which canvary from less 1 mm per day for clays to severalmetres per day for sand and structured soils.

Key influences on catchment hydrology are theunderlying aquifers — saturated geologicalstructures through which water can moverelatively freely. The aquifers under farmland cancover much larger areas than the surface


catchment, and there is a wide variation across acatchment in the rates at which rainwater leaksthrough the soil to recharge aquifers. Areaswhere leakage is greatest may offer theopportunity to achieve a relatively large impacton groundwater recharge with tree planting.

Trees can take water from the watertabledirectly, or indirectly via the capillary effect,and from unsaturated (moist) soil above thewatertable. Planting trees on former pastureor cropping land gives them access to soilmoisture, and possibly an elevatedwatertable, that would not have been there ifthe native vegetation had not been cleared.Once they have removed the excess water,their growth will slow to a rate sustainableby rainfall alone, which may be significantlylower.

How rainfall is distributed through the year hasimportant influences. In areas with aMediterranean climate, such as southern WA,most rain comes in winter and water stress insummer can restrict tree growth. Also,importantly, the soil can quickly become saturatedin winter. As hydraulic conductivity increasesrapidly with water content, this may result inrapid movement of groundwater downslope, withadverse consequences lower down in thelandscape. The soil is less likely to becomesaturated in regions with a similar annual rainfallbut a more even distribution through the year, sowater will usually move more slowly downslopethere.

The complexity of catchment hydrology canbe summarised as follows:1. The spatial scale of the catchments we

want to study and manage varies fromless than 1 km2 to 1000’s of km2.

2. The catchments are filled with internalheterogeneity, or “patchiness” ofvegetation, soils, slopes, and basementrock formation.

3. They carry not only water, but also saltand nutrients.

4. There can be a number of groundwaterinteractions below the surface which arenot visible, and can only be inferred fromindirect measurements.

5. The time scales for hydrologic processescan be very short or very long – fromstorm events to 100s or 1000s of years.

6. There is enormous uncertainty invirtually every quantity associated withthe characteristics of any catchment.


0

200

400

600

800

1000

0 250 500 750 1000 1250 1500

Wat

er y

ield

(m

m)

Forest

Grassland

A

B

C

Predicting catchmentresponse to tree planting

The first vital step in undertaking a tree plantingprogram for salinity control is to obtaininformation on key features of the catchmentthat will influence the response. These includethe discharge capacity, variations ingroundwater recharge rates across thecatchment, the scale of the groundwatersystems and the salinity of the groundwater.Generally, the smaller and more permeable agroundwater system, the faster will be itsresponse to revegetation. Relatively fresh andshallow watertables provide the bestopportunities to use tree belts to interceptgroundwater recharged from upslope.Intermediate and regional scale groundwatersystems usually have a low discharge capacity,and most of the landscape may have to bereplanted to significantly reduce outflows ofsaline water. Reclamation of already salinisedland there may take a very long time.

Responses of catchments to tree planting varyenormously. In some cases, local strategicplantings will reduce waterlogging andsalinisation almost immediately. Elsewhere, mostof a catchment will have to be replanted to have asubstantial impact on salinity and waterlogging.Salinity usually takes decades to appear after landclearing and may take many more decades toabate after revegetation. In tackling the problemin a catchment, the cooperation of many farmswill usually be required.

Differences in soil and aquifer properties,catchment size, landscape gradients and saltstorage contribute to this diversity of response.A key influence is the aquifer’s dischargecapacity — the maximum amount of water thatit can discharge when full. As describedpreviously, recharge in excess of this will resultin rising watertables, which may producedamaging groundwater outflows at the landsurface, such as saline seeps.

Average annual rainfall strongly influences thenature of the outflows. Where it is above about900 mm, salt will have been flushed from the soil;hence the groundwater that emerges is generallyfresh. In drier areas the native vegetation’s wateruse will have prevented such flushing, so rising

watertables following clearing bring salt to thesurface. Therefore, in wetter catchments themain hydrological issue associated with treeplanting is a reduction in fresh streamflow whilein drier catchments it is salinity control.

In the lower rainfall catchments, the leaf areaindex (LAI) — the ratio of the area of leaves tothat of the ground covered — of the nativevegetation was about that needed to utilise allthe rain that fell. The reduction in LAIfollowing clearing closely matches the boost togroundwater recharge.

Designing tree planting for salinity controlrequires an understanding of:• the discharge capacity of the aquifer• how recharge rates vary across the

catchment• the scale — local, intermediate or

regional — of the groundwater systems• the salinity of the groundwater.

The relationship between annual rainfall and catchment water yieldunder grassland and forest. In high rainfall country, where the concern isover streamflow reductions due to afforestation, this is a useful androbust relationship for estimating impacts. For instance, at 700 mm ofrainfall (C), a grassland or previously cleared site may yield about 190mm of runoff (B), while a forested catchment may yield only 60 mm (C);intermediate levels of afforestation would result in a proportionaldecrease between these two yield values.

Knowledge of how leakage is distributedover a catchment is very important. Forexample, if it is evenly distributed, half thecatchment will have to be planted with treesto reduce overall leakage by 50%. But if halfthe recharge occurs on 30% of the landarea, planting only that part of thecatchment will have the same impact.


Examples of catchment types, appropriatereplanting regimes and prospects for salinitycontrol include:

• Local flow system catchments with a highdischarge capacity (1-3 km horizontal scale).Most commonly characterised by hillsideseeps, these are relatively responsive torevegetation. Alley farming, tree belts or evenwidely scattered trees can often providesufficient recharge control.

• Local flow system catchments with a low dischargecapacity (1-3 km horizontal scale). An example is‘break of slope’ catchments where discharge iscontrolled by the topographic gradient.Extensive recharge control is needed to limitwaterlogging and salinisation. Alleys may haveto be closely spaced and/or phase-farmed withperennial, high-water-use crops or pasture, ortrees planted over most of the catchment.

• Intermediate flow system catchments. (5–10 kmhorizontal scale). Most of these systems requiresignificant proportions of the landscape to bereplanted to reduce outflows of saline water.The most dramatic and intractable drylandsalinity in WA is associated with onecatchment type in this category, characterisedby discharge into low-lying areas such asbroad valley floors. Extensive tree planting, atlevels approaching the original cover, isrequired to control watertables. Alley farmingwith closely spaced trees, combined with

perennial pasture in the alleys, may achievesufficient recharge control to contain thespread of salinity. Deep surface drains orgroundwater pumping and saline disposalmay have to be considered for managingdischarge areas.

• Regional flow systems. (greater than 10 kmhorizontal scale). Some of these systemsrecharge in higher rainfall areas, providinglargely fresh groundwater that can be utilisedfor town supplies and agriculture. Whererecharge control is required, levelsof tree cover approaching the original willbe needed in the recharge areas. If deepergroundwaters are relatively fresh, pumpingto control water levels and provide irrigationto the trees may be an option.

Significant work has been undertaken toestablish the distribution of these aquifer typesacross the Australian landscape.


Rotating woodlotswith agriculture

A proposed new ‘phase farming’ system foraddressing salinity involves growing woodlotsin rotation with crops or pasture. The woodlotscan ‘mine’ soil water that has built up duringthe agriculture phase. Deeper profiles withheavy-textured subsoil offer the bestprospects; a typical rotation might be three to five years of cropping/pasture betweenwoodlots, with stored water giving the treestwo to three years of enhanced growth. Herethe focus is the role that woodlots play incontrolling the local hydrological balancethrough recharge control.

Woodlots are blocks of tree plantings. Theproposition that woodlots can be used inrotation with agriculture opens up challengingpossibilities for achieving hydrological controlwhile retaining traditional agriculture.


Growing woodlots has been adopted widely as away to reintroduce trees into the Australianlandscape for a variety of purposes, includinglowering watertables. The build-up of soil waterduring cropping and pasture growth, andnutrients that have been added in fertilisers,often give woodlots planted on farmland aninitial growth boost. However, their productivitymay decline sharply once they have used upthese resources.

The proposed approach, which is yet to betested experimentally, involves rotating woodlotswith agriculture. A woodlot can produce a treecrop while drying out the soil. Then crops andpasture can be grown on the same land until thesoil water store is replenished again to the pointwhere planting trees again becomes necessary tostop excessive groundwater recharge. Suchwoodlots can be moved around a farm to ‘mine’water, thereby increasing the impact onrecharge.

The following questions need to be consideredin determining whether such a system isappropriate for a particular area, and the bestlength of the tree and agriculture phases:

• For what length of time will stored soil waterfrom the preceding agricultural systemenhance a woodlot’s productivity?

• How long will it take for groundwater rechargeto be minimised after a woodlot is planted?

• Once the trees have been harvested, what isthe interval before soil water accumulates tolevels where groundwater recharge againbecomes a problem?

The JVAP is currently conducting researchwhich considers products deriving from suchsystems.

Alternating woodlots with agriculture (phase farming) – woodlots areused to control recharge by drying out the soil profile– appropriate fordeeper soil profiles with heavy textured subsoil.


Little experience is available from actualplantations to answer these questions, soresearchers have used a complex ‘ecohydrological’model to predict hydrological and tree growthresponses on a range of soil types. Two climatescenarios were tested — one with 70% of the 320mm average annual rainfall coming between Aprilto September and the other with a uniformlydistributed 370 mm rainfall. Therefore althoughboth scenarios simulated very low annual rainfall,the distribution of rainfall was quite different.

Major conclusions from the study are that:

• Very shallow or very sandy soil profilesrequire almost continuous protection withtrees; the risk to groundwater associated withany period of cropping is not worth taking.

• Deeper profiles with heavy-textured subsoiloffer the opportunity to alternate woodlotswith crops and pasture. A typical rotation mightbe 3–5 years of cropping/pasture betweenwoodlots, with the woodlots enjoying 2–3 yearsof enhanced productivity due to the storedmoisture. The actual length of the woodlotrotations will depend on economic andsilvicultural considerations. A real risk exists,though, that trees will die in drought after thestored water has been taken up; this should beconsidered in deciding rotation length.

• Woodlots able to tap fresh watertables can bevery productive and sustainable with or withouta crop/pasture phase. Planting woodlots oversaline watertables, however, is risky because saltwill build up in the root zone.

Factors in designing strategies for phasefarming with woodlots• soil texture and depth• groundwater quality• degree of recharge control needed to

protect the land or streams • the woodlot rotation period required for

the intended forest product.

Based on these considerations, a land managercan calculate the area that should be occupied bytrees at any time. Woodlots can then be movedaround the land in a sequence. The soil watermining and groundwater protection potentiallyoffered through this agroforestry strategy couldsignificantly reduce the areas of land requiredunder trees relative to the equivalent protectionafforded by continuous forestry. For instance, insome cases a 50% recharge target can be attainedby planting 25% of the land with such anagroforestry system.

An important effect of a woodlot is to ‘mine’water that has accumulated deep in the soilunder annual crops or pasture. The ‘buffer’created will take several years to refill oncethe trees are removed. So a worthwhilestrategy may be to grow short-rotationwoodlots alternately with crops, and move thetemporary tree cover around the landscape.

Summary of responses to woodlots in two climates and seven soil/groundwater combinations.

Response Scenario*

1 3 6

Recharge under cropping/pasture (mm/yr) 21 7.5 1

Time to recharge control (years) (water-mining phase) 2–3 1–2 1

Time to equilibrium with rainfall (years) 2–3 3 2

Time to full recharge following woodlot harvest (years) – groundwater protection phase 4 5 12

*Scenario 1 = 2 m sand, 3 m sandy loam (Merreden climate); 3 = 1 m sand, 2 m clay (Merreden climate)6 = 0.5 m light over 2.5 m heavy clay (Merreden climate).

Hill-slope tree belts

In hilly local flow systems, trees planted inbelts on hillsides can take up water thatwould otherwise contribute to waterloggingor salinity problems downslope.Requirements include slope and soilconditions that result in lateral flow shallowenough for the trees’ roots to reach, andwater fresh enough for healthy tree growth.Designing tree belts requires information onthe amount of water flowing downhill andhow much the trees can use.

By intercepting water flowing through the soildown hills, belts of trees can prevent or reducesalinity and waterlogging problems furtherdownslope. As the trees will have access to morewater than is available from rainfall alone, theymay also grow faster.

Research combining field observations withcomputer modelling has provided guidance onhow to design plantings to maximise water use —keeping groundwater recharge as low as possible— and obtain maximum tree growth. Key designconsiderations are the position of tree beltson the slope, the width of the belts and theirdistance apart.

This approach - used in isolation- is relevantonly to local groundwater systems - where thearea affected by waterlogging or salinity is in thesame locality as the tree plantings . The waterthe trees draw on comes from rainfall notutilised by crops or pasture upslope.Requirements for successful plantings are:

• sufficient slope, and sufficiently permeablesoil, for lateral flow to occur;

• lateral flow shallow enough for the trees’roots to reach; and

• water fresh enough for healthy tree growth.

Those designing hillside tree belts need to knowhow much water will be available from upslope,how fast it will be delivered, and how muchwater the trees can use. Data required are:

• an estimate of the hydraulic conductivity of thesoil. This ranges from around 1 metre per dayfor sandy topsoil to 1 mm per day for clay,with loam in between at about 0.1 metres perday. Expert knowledge is required for anaccurate assessment.

• an estimate of the length of the wet season. Localknowledge or the Commonwealth Bureau ofMeteorology can provide this.

• an estimate of the thickness of saturated flow thatcan be tolerated. The goal here is to ensure thetrees and the pasture between the tree beltsare not adversely affected by waterlogging.

• an estimate of drainage below the root zone of thepasture or crops. This requires expertknowledge, and will vary with management ofthe upslope areas.

• likely groundwater use by trees at the selectedlocation. This is difficult to ascertain, but anestimate is needed to determine how wide thebelt must be to utilise all the available water.


Rainfall

WaterFlow

TreeUptake

Infiltration

Pasture

Hill-slope tree belts – appropriate for recharge and discharge control inhilly local flow systems.

Tree belts may be established on hillslopes perpendicular to the slopeof the hill to capture water flowing from upslope, supplementingrainfall and enhancing growth while at the same time alleviatingwaterlogging and salinity downslope.


Tree belts are likely to do better — and thereforetake up more water — lower in the landscapethan higher up. This is partly because soilsgenerally become deeper with distancedownslope. The other contributing factor is alarger upslope area, making more water availableto the trees; for the same reason relatively widelyseparated belts will do better than those plantedcloser together. Salinity and waterlogging are,however, more likely to be problems in lowerareas — placing a limit on how far downslope treebelts should be located.

Decisions on tree belt width should take accountof the fact that trees in the middle of the belt haveless access to the extra water, and to light, thanthose at the edge. As a result, they will grow moreslowly and use less water.

The plantation at Warrenbayne in northeastern Victoria, planted at thebreak of slope to intercept water flowing from upslope. Thewaterlogging-prone area can be seen as the brown area downslope ofthe plantation.

Guiding principles for planting hill-slope treebelts are:• the steeper the slope the greater the

discharge capacity, the further apart the belts can be, and the wider the beltsshould be;

• the greater the drainage from pasturethe closer the belts should be;

• the higher the groundwater uptake,the narrower the belts can be; and

• the shorter the wet/winter period,the closer the tree belts should be.

On slopes, belts of trees can ‘capture’ water asit moves downhill, and so have an impact overan area considerably greater than that planted.Similarly, lines or clumps of trees surroundedby cropping or pasture land will take waterfrom beyond the area of their own canopies.

Because the tree roots spread into thesurrounding soil, a tree belt will prevent orrestrict leakage to groundwater over an areagreater than that occupied by the trees. A simpleway to predict the size of the net ‘no-leakage’zone involves comparing the belt’s leaf areaindex (LAI) with an estimate of the LAI of thearea’s original vegetation. A strong relationshipexists between climate and the LAI of thenatural tree and shrub cover, making it possibleto estimate the original LAI from average annualrainfall and evaporation.

The downside of increased recharge reduction isreduced growth of crops or pasture in the areaswhere the trees compete with them for theavailable moisture. Field experiments show cropor pasture yields rise from zero immediatelyadjacent to a tree belt to the open paddock levelsome distance away. Yield data from transectsaway from a tree belt can be used to estimate thesize of the net ‘no-yield’ zone; as use of yieldmapping grows such information will becomeincreasingly available. The impact of trees onyield of crop/pasture varies depending on speciesand environmental conditions.

The method proposed for assessing whether treebelts or woodlots offer the best prospects on a siteinvolves comparing the sizes of the no-leakageand no-yield zones. For example, calculations forwheat country near Esperance, WA, gave a no-leakage zone width of 32 m, compared with 18.6 m for the no-yield zone. As the ratio of thesefigures is greater than one, tree belts are thebetter option. A second example comes from atagasaste alley crop experiment near Moora, WA.In this situation woodlots would be advisable, asthe zone widths were 3 m for the no-leakage zoneand 5 m for the no-yield zone (a ratio of less thanone).


Mixing tree belts withagriculture

Rain-fed tree belts in recharge areas ofcatchments can make a useful contributionto reducing inputs to groundwater. Becausethe roots spread into the surrounding soiltheir impact on recharge extends beyond thebelts, but so does their negative influence oncrop and pasture growth. Researchers havedevised an easy way to determine whethertree belts or woodlots are likely to give thebest overall result in a particular situation.This section targets flatter terrain inrecharge areas where lateral water flows arenot appreciable.

Trees spread across a catchment in belts arelikely to grow faster and have a bigger impactin reducing groundwater recharge than the samenumber of trees in a woodlot. The drawback,though, is that crop and pasture growth maybe reduced over substantial areas because ofcompetition from the trees for water, nutrientsand light.

Land managers considering planting tree beltswould like reliable predictions of both thebenefits the trees can provide and the likely lossof crop or pasture production, but these areimpossible to make accurately for the wide rangeof soils, climates and tree species found acrossAustralia. Researchers have devised a usefulmeans of determining whether planting trees inbelts or in woodlots should be the best option ina particular situation. The method is aimed atrecharge areas of a catchment, where thewatertable is deep and trees are rain-fed only.

Mixing tree-belts with agriculture – the suitability of either tree-belts orblocks can be determined to reach a given leakage target in rechargeareas where watertables are still relatively deep but rising.


Year-to-year rainfall variability presents thegreatest challenge to the integration of trees andcrops. The area of trees needed to preventleakage to groundwater in an average year canbe calculated, but this will allow some leakage inwetter years. In dry years, competition by thetrees for the available moisture may be so greatthat no crop can be grown.

This means agroforestry in which trees arearranged in belts is more likely to succeed inhigher rainfall regions than in lower-rainfallregions. Mixing trees and crops becomes moredifficult in very low rainfall areas and as season-to-season variability increases.

No-yield zone

No-leakage zone

Leak

age

Yiel

d

The no-yield and no-leakage zones linked to a tree belt. The slowincrease in yield and leakage away from the belt is reduced to a stepfunction to facilitate easy comparison of the above- and below-groundeffects.

Planting over shallow,saline watertables

Planting trees over shallow watertables is apractical way to reduce saline discharges inlocations where the water is not too salty andlateral water movement prevents salt buildingup in the soil. However, in the much morecommon situations where aquifer systems arelarge and saline, salt accumulation will makesuch plantings unsustainable. In most cases,tree planting in discharge areas cannot be seenas a substitute for planting in recharge areas.

Planting trees over shallow watertables — onland likely to be subject to periodic waterloggingand rising salt levels — has obvious appeal. Byusing the groundwater, plantations could beexpected to help prevent damaging discharge.

The main difficulty with this strategy is that saltwill probably build up in the root zone; most ofthe salt in water taken up by trees is left behindin the soil. Growth rates will slow as the saltaccumulates, and eventually even salt-toleranttrees may die.

Nevertheless, growing trees over shallow, salinewatertables can be a practical proposition insome situations. Research has identified theconditions that should lead to a good outcome.

Trees take up groundwater mainly from thecapillary fringe, the layer of wet soil immediatelyabove the watertable. The initial depth of thewatertable, soil texture and salinity are critical indetermining the survival prospects of plantations.Very shallow watertables — within a depth ofabout 1.5 metres — present a particular problembecause water evaporates from the soil surface,

leaving behind a salt crust. Trees can help byintercepting water as it moves towards the surface,but then the salt is left behind in the root zone.

The following scenarios provide an indication ofhow plantations might perform in catchmentswith a range of hydrological characteristics:

• The watertable depth stays constant becauseadditions from surrounding recharge areas replacegroundwater taken up by the trees. In thisscenario, all the salt left behind stays in thesoil from which water is extracted. How longtrees can survive will depend on the depth ofthis soil zone, what salt concentration thetrees can withstand, the salinity of thegroundwater and the trees’ transpiration rate.Different assumptions result in salt levelsreaching a threshold figure, above which treegrowth stops, within just 3.5 to 14 years.

• The watertable level falls in response to water useby trees. A plantation monitored in Victoriafor 20 years lowered the watertable from 1 to5 metres depth. However, groundwater useappeared to decline dramatically after the firstten years and leaf area and water usedecreased significantly indicating theplantation is unsustainable. The salinity of thewatertable is in the range 3000–5000 EC(aquatic ecologists consider 5000 EC thedivide between fresh and saline water).Modelling studies indicate that, whereaccumulated salt cannot be ‘exported’ from asite, tree plantations grown over groundwaterwith a relatively low salinity of 3000 EC orabove must generally be consideredunsustainable. Also, if the trees are replacedby pasture, growth may be harmed as thewatertable rises again, bringing salt with it.

• The watertable level falls and salt is carried out ofthe root zone by lateral water movement. At anexperimental site in Western Australian with athin, transmissive aquifer and relatively lowgroundwater salinity, the land area affected bysalinity decreased substantially and rapidly eventhough only 2% of the catchment was plantedwith trees. Unfortunately, such ‘best case’conditions are not often found in Australia.

Plantings over shallow aquifers should bedesigned using estimates of the saturatedconductivity of the soil and expected groundwaterextraction rates. It is essential to plan for, andmonitor, salt build-up in the root zone and itslikely impacts on tree water use and survival.


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Daily water use by E. camaldulensis from a watertable 93 cm below thesoil surface. Prior to day 0 the watertable was fresh. After that, thewatertable was slightly saline (conductivity = 2 000 EC).


The research suggests tree planting can have amajor impact in reducing the area of salinedischarge where aquifers are relatively shallow,the water is not too salty, and lateral watermovement prevents salt accumulation. However,prospects are not nearly as good in the muchmore common situations where aquifer systemsare large and saline. In most cases, tree plantingin discharge areas cannot be regarded as asubstitute for planting in recharge areas. At best,the strategies are complementary.

For example, in an area with 850 mm averageannual rainfall, a mean temperature of 15ºC, nosalinity or waterlogging, neutral pH and low frostrisk, the choice of species will be wide and themarket for tree products may strongly influencethe decision. However, a site with 500 mm rainfall,an average temperature of 20ºC and few frosts, buthigh salinity and seasonal waterlogging, presentsfar fewer options; few species have high toleranceof salt and waterlogging. The most suitable speciesappear to be river cooba (Acacia stenophylla), oldman saltbush (Atriplex nunmularia) and swampshe-oak (Casuarina obesa).

Most tree species currently of commercialvalue have only slight to moderate salttolerance. Species with moderate high salttolerance, such as river red gum (Eucalyptuscamaldulensis), show more growth decline asroot-zone salinity increases than species withhigh salt tolerance, such as swamp yate (E.occidentalis). Other species with a strongcapacity to survive and continue to grow anduse water in the face of increasing root-zonesalinity include river cooba and salt paperbark(Melaleuca halmaturorum).

As well as choosing appropriate species, growersshould ensure the trees they plant are from thebest provenance (seed source) within a species.Growth rates and traits such as stem form canvary markedly between provenances. Trials onsaline sites have shown, for example, that thebest-performing river red gums come fromnorth-western Victoria.

In general, commercial opportunities for farmforestry are more restricted in low rainfall zonesthan in wetter areas, so appropriate speciesselection is vital. Options that may be availableinclude production of eucalyptus oil from mallees,sawn timber from some eucalypt species, andbiomass feedstock for production of bioenergy.

Tree management factors most likely toinfluence stand growth and water uptake are sitepreparation, fertiliser use, weed control andplanting density. Thinning and pruning may alsobe necessary if an aim is to produce high qualitytimber or to reduce the risk of drought-inducedtree death. Relatively low tree stocking densitiestend to be most cost-effective when the primaryaim is to maximise water use. On lower rainfallsites, densities around 200–500 stems per ha arelikely to be suitable.


Species selection and farmforest management

Information compiled on the suitability of 32species for different rainfall and temperaturezones, along with their tolerance of frost,salinity, acidity, alkalinity and waterlogging,will assist the choice of trees to plant. As wellas choice of appropriate species, and the bestseed source within species, goodmanagement is vital to the success of treeplanting for salinity control.

Choosing species suited to the particular climateand site conditions and appropriate managementpractices are essential to realising the potentialbenefits of tree planting for salinity control.

Important considerations for species selectioninclude:

• Climate. Annual rainfall and its variability,temperature — especially the extremesexperienced — and the frequency and severityof frosts are the key variables.

• Soil conditions. Soil texture and structure,water infiltration rates and water availability,and soil chemical conditions — nutrientstatus and acidity, salinity, alkalinity andsodicity — influence root and foliage growth.

• Watertable depth and salinity. Tree species showa range of responses to these key variables.

• Commercial opportunities. Wood products,non-wood products such as eucalyptus oil,and carbon credits provide various incomeopportunities.

• Potential competition with crops and pastures.This can be an issue when trees are grown indesigns other than plantations and woodlots.

The survival and growth of trees depend on amyriad of factors, and species performance forparticular sites — especially on saline land — oftencannot be predicted with precision. Nevertheless,much information is now available, and thesuitability of 32 species for different rainfall andtemperature zones has been summarised in thebook, along with their tolerance of frost, salinity,acidity, alkalinity and waterlogging.


Site preparation may involve removing existingliving and dead vegetation, ripping, mounding,and construction of vermin-proof fences.Addition of fertiliser will often be worthwhile,but sites where improved pasture has beengrown may not benefit as nutrients are likely tobe in good supply there already. Weed control,initially through a combination of knock-downand residual herbicides, is essential formaximising tree growth. Plastic tree guards canhelp suppress weed growth around trees and arelikely to be particularly beneficial in tree belts.

Suitability for climatic and soil conditions of selected tree and shrub species currently planted or potentially suited tofarm forestry and dryland salinity management in southern Australia1.

Mean annual rainfall (mm) Mean annual temperature (ºC) Frost Salinity Acidity Alkal Water-inity -logging

<400 400–600 600–800 >800 >23 17–22 12–16 <12Acacia dealbata ** ** *** *** ** *** * ** * *A. mearnsii *** ** ** *** * ** * ** * *A. melanoxylon ** *** * *** *** ** * ** * *A. saligna ** *** * *** *** * ** * ** *A. stenophylla * *** ** *** *** *** * **** * ** ***Atriplex nummularia *** *** *** ** * **** * ** *Casuarina cunninghamiana *** *** *** ** *** *** * ** ** * **C. glauca *** *** * *** *** * *** ** * ***C. obesa * *** *** * *** *** * **** * *** **Chamaecytisus palmensis *** ** ** *** * * * * *Corymbia maculata * *** *** *** ** * * ** * *Cupressus macrocarpa ** *** * *** *** ** * ** * *Eucalyptus camaldulensis (northern) *** *** *** *** *** * ** ** ** ***E. camaldulensis (southern) *** *** *** *** *** ** ** ** ** ***E. cladocalyx * *** ** *** *** * * * ** *E. globulus ** *** *** *** ** ** * ** * *E. grandis *** ** *** ** ** * ** * *E. largiflorens ** *** * *** *** * ** ** ** **E. leucoxylon * *** *** * ** *** ** ** ** ** *E. nitens ** *** * *** *** *** * ** * *E. occidentalis * *** *** *** *** * *** * ** ***E. polybractea * *** ** *** *** * ** * * *E. robusta * *** *** ** * ** ** * **E. sideroxylon/tricarpa * *** *** * *** * * ** * ** * *E. spathulata ** *** ** *** *** * **** * * **E. viminalis ** *** * *** ** *** * ** * *G.revillea robusta ** *** *** ** ** * * ** * *Melaleuca halmaturorum ** *** *** ** *** ** **** ** ** ***M. uncinata ** *** * *** * ** * ** *Pinus pinaster * *** *** ** ** *** * ** ** ** * *P. radiata ** *** * *** ** *** ** ** * *Populus deltoides * *** *** *** *** * * * * **

Average annual rainfall: * = reasonable suitable; ** = suitable; *** = very suitable. The ratings do not imply a particular growth rate; they merelyprovide a comparison between species of relative performance within zones. In general, there is a positive correlation between growth and rainfall.Species rated as very suitable in low rainfall zones will have slower growth rates when grown at low rainfall sites than species rated as very suitable forhigh rainfall zones grown at high rainfall sites. For example, P. pinaster grown at a site with 500 mm annual rainfall will not grow as fast as P. radiatagrown at a site with >800 mm rainfall.Annual annual temperature: * = reasonable suitable; ** = suitable; *** = very suitableFrost: * = can be planted in low frost-risk areas (<5 frost days per year); ** = can be planted in moderate frost risk areas (5–20 frost days); *** = canbe planted in high frost risk areas (>20 frost days with minimum temperatures ranging between –5 and -10 °CSalinity 2: defined here in terms of the electrical conductivity of a saturated soil paste (ECe) in units of decisiemens per metre (dS/m) as an average overthe root zone: * = slight (2-4); ** = moderate (4-8); *** = severe (8-16); **** extreme ( >16) Acidity 2:* = pH 6-7; ** = pH 5-6; *** = pH <5Alkalinity 2: * = pH 7–8; ** = pH 8–9; *** = pH >9Waterlogging 2:* = days; ** = periodically (days to weeks); *** = seasonally (several weeks). 1 Data compiled from various published, unpublished and personal communication sources. For a number of species, there is likely to be considerablevariation between seed sources in response to climate and soil factors. Readers should consult local information sources. 2 Ratings given for each species indicate that this average root-zone condition should not reduce growth markedly from optimal conditions, howevercombined stresses may have greater impact.



Glossary

Aquifer: A saturated permeable geologicalstructure that can transmit significant quantitiesof water under gravity

Capillary fringe: the region of soil just abovethe watertable where the soil pores are saturatedbut the fluid pressure is less than atmospheric(ie: the pore water is held under some degree oftension by neighbouring soil particles).

Discharge: Water flowing out of an aquifer(aquifer discharge), catchment (catchmentdischarge), or from groundwater through thesoil surface (groundwater discharge).

Hydraulic conductivity: The ratio of the rateat which water can flow though soil to thepressure gradient driving the flow.

Leakage: Water that drains past the plant rootzone; also called deep drainage.

Leaf area index: The ratio of the leaf area ofvegetation to the land area covered.

Phase-farming: Alternating farming land uses— such as trees and crops.

Recharge: Accession of water to an aquifersystem.

Saturated hydraulic conductivity: Hydraulicconductivity of saturated soil.

Unsaturated soil: The soil profile above thewatertable and capillary fringe.

Watertable: The boundary betweenunsaturated and saturated soil, at which the porewater pressure is exactly atmospheric.

Authors

Authors: The chapters of Trees, Water and Salt:an Australian guide to using trees in achieving healthycatchments and productive farms were written byresearch scientists with expertise in the subjectmatter. Those who contributed to the book are:

Richard G. BenyonCSIRO Forestry and Forest Products

Warrick DawesCSIRO Land and Water

Tim EllisCSIRO Land and Water

Richard HarperWA Department of Conservation and LandManagement

Thomas J. HattonCSIRO Land and Water

Geoff HodgsonCSIRO Land and Water

Ted LefroyCentre for Legumes in MediterraneanAgriculture, University of WA

David McJannetCSIRO Land and Water

Nico E. MarcarCSIRO Forestry and Forest Products

Brian J. MyersCSIRO Forestry and Forest Products

Paolo ReggianiCSIRO Land and Water

Richard SilbersteinCSIRO Land and Water

Richard StirzakerCSIRO Land and Water

Rob VertessyCSIRO Land and Water

Photo credits

Images supplied by: David Bush, Tim Ellis, PhilEvans, Paul Feikema, David McJannet and BrianMyers


© 2000 Rural Industries Research and DevelopmentCorporation.

All rights reserved.

ISBN 0 642 58201 7

ISSN 1440-6845

Trees, Water and Salt: An Australian guide to using trees forhealthy catchments and productive farms- Research Update

Publication No. 00/170

Project No. CSM-4A

The views expressed and the conclusions reached in thispublication are those of the author and not necessarily those ofpersons consulted. RIRDC shall not be responsible in any waywhatsoever to any person who relies in whole or in part on thecontents of this report.

This publication is copyright. However, RIRDC encourages widedissemination of its research, providing the Corporation is clearlyacknowledged. For any other enquiries concerning reproduction,contact the Publications Manager on phone 02 6272 3186.

Researcher Contact DetailsDr Richard StirzakerCSIRO Land and WaterGPO Box 1666, Canberra, ACT 2601Phone: (02) 6246 5570Fax: (02) 6246 5560Email: [email protected]

Dr Rob VertessyCSIRO Land and WaterGPO Box 1666, Canberra, ACT 2602(02) 6246 5790(02) 6246 [email protected]

RIRDC Contact DetailsRural Industries Research and Development CorporationLevel 1, AMA House42 Macquarie Street BARTON ACT 2600PO Box 4776KINGSTON ACT 2604

Phone: 02 6272 4539Fax: 02 6272 5877Email: [email protected]: http://www.rirdc.gov.au

Published in October 2000

For further information about the JVAP Program contact the Program Research Managers:

Dr Roslyn Prinsley ph: 02 6272 4033 email: [email protected] or Sharon Davis ph: 02 6271 6671 email: [email protected]

Rural Industries Research and Development Corporation PO Box 4776, Kingston 2604

Documents

Trees, Water and Salt · •Salt from the sea carried in rainwater has accumulated in the soil profile over a very long time. Rising watertables dissolve salt and bring it back towards