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SOLUTION 11

Revegetating with non-grazing options

 

11.5 Establishment & management

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Choosing the right species and varieties

Table 11.1 shows selected, predominantly native, tree and shrub species suitable for planting on soils of different salinity. Bold type indicates species that are at least moderately tolerant of waterlogging.

Table 11.1
from Trees for Saline Landscapes (by Marcar & Crawford)

Size 

Rootzone salinity (ECe dS/m)

 

Slight (2 - 4) 

Moderate (4 - 8) 

High (8 - 16) 

Severe (>16) 

Tree 

Acacia. Mearnsii

A. pendula

A. ampliceps

A. stenophylla  

 

A. melanoxylon

All. luehmannii 

A. maconochieana 

C. obesa 

 

Cor. citriodora subsp.
variegata E
 

All. verticillata 

A. salicina 

 

 

Cor. maculate

C. cristata 

C. glauca 

 

 

E. aggregata 

C. cunninghamiana
subsp. cunninghamiana
 

E. kondininensis 

 

 

E. botryoides 

E. astringens subsp.
astringens

E. occidentalis

 

 

E. brockwayi

E. camaldulensis

E. platypus subsp.
platypus

 

 

E. camphora subsp.
humeana

E. campaspe

E. sargentii 

 

 

E. cinerea subsp.
cinerea
  

E. gomphocephala

E. spathulata 

 

 

E. cladocalyx  

E. largiflorens 

M. leucadendra 

 

 

E. coolabah 

E. leucoxylon subsp.
leucoxylon
 

 

 

 

E. cornuta 

E. melliodora 

 

 

 

E. crenulata

E. moluccana

 

 

 

E. globulus subsp.
bicostata

E. polybractea

 

 

 

E. globulus subsp.
globulus

E. raveretiana

 

 

 

E. grandis

E. robusta

 

 

 

E. loxophleba subsp.
lissophloia

E. rudis subsp. rudis

 

 

 

E. microcarpa

E. salicola

 

 

 

E. ovata var. ovata

E. tereticornis subsp.
tereticornis

 

 

 

E. saligna

E. wandoo subsp. wandoo

 

 

 

E. sideroxylon

M. quinquenervia

 

 

 

E. tricarpa

M. styphelioides

 

 

 

E. viminalis subsp.
viminalis

P. pinaster

 

 

 

P. brutia

P. radiata

 

 

 

 

 

 

 

Shrub

A. implexa

A. acuminata

A. cyclops

M. halmaturorum

 

A. iteaphylla

A. redolens

A. retinodes

M. thyoides

 

A. longifolia

A. saligna

E. halophila

 

 

angustissima subsp.
angustissima

A. victoriae

M. cuticularis

 

 

M. armillaris subsp.
armillaris

E. famelica

M. lanceolata

 

 

 

M. acuminata

 

 

 

 

M. squarrosa

 

 

 

 

M. bracteata

 

 

 

 

M. decussata

 

 

 

 

M. ericifoli

 

 

 

 

M. lateriflora

 

 

 

 

M. linariifolia

 

 

 

 

M. uncinata

 

 

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Site preparation

Saltland sites are often quite variable and should be thoroughly assessed for extent of soil salinity, the depth to the watertable and the salinity of the groundwater. After just a few months, tree growth is going to be driven substantially by the salinity of the deeper root-zone, so the salinity of sites should be assessed to depths of at least 50 cm, preferably deeper. The EM38 may also be a useful tool. Watertable depths can be determined by installing observation bores in representative locations, and the salinity of the groundwater can be determined by taking samples of that water with a sludge pump and measuring its electrical conductivity. At the simplest level, salinity indicator plants may give some indication of salinity levels, although generally only at the top few centimetres.

Fencing from livestock and controlling rabbits and hares are standard procedures for protecting trees during the establishment phase.

As with pasture establishment, reducing the incidence of waterlogging by removing surface and subsurface water through drains can mean the difference between establishment success and failure. Drainage might only be by shallow drains to remove surface water, but could involve deeper groundwater drains or even pumping. All of these tactics impose significant costs to the project - slotted pipe (tile or plastic drains) and mole drains, or both would usually be justified only in the most potentially profitable situations and where disposal of the effluent is manageable.

Deep ripping (30–50 cm) can assist root penetration vertically and laterally, although this is rarely necessary on waterlogged sites. On dry sites, ripping on the contour increases moisture capture and availability to the plants. Ripping should be carried out several months ahead of planting to allow soil consolidation to fill air cavities, and should never be when soil is too wet or very dry.

Mounding is very important, particularly on waterlogged sites. Provided that there are orientated correctly mounds can also assist with surface water drainage; they will usually increase the amount of salt that can be flushed from the root zone of the establishing plants. On heavy, more salt-affected and waterlogged soils, mounds need to be higher and wider with a shallow trough at the top of the mound to capture rain.

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Weed and pest control

Weed control is an essential component of tree establishment and on saline sites is generally achieved with a combination of knockdown herbicide (e.g. glyphosate and amitrol) and residual (e.g. simazine and propazine). Herbicides can be used over mounds, usually at lower rates than on non-mounded or undisturbed soil, and applied several months before planting and again at or just prior to planting, using strips 1m wide centred on the planting line.

Weed control might be necessary a year after planting, which might be easier if plastic tree guards are used to initially protect the seedlings from rabbits, hares and kangaroos.

Trees under stress from salinity or waterlogging are generally more susceptible to damage from native insects, however this vulnerability is no more apparent in newly planted seedlings than in trees that are well established.

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Sowing and planting

Seedlings are generally preferred, rather than direct seeding, for establishing trees on saline sites, particularly at high salinities and in the presence of waterlogging. However, there are many examples where direct seeding has been successful so that local experience should be checked before deciding on how to proceed.

Seedlings should be hardened off (by reducing shading, watering and nutrient application) for at least a month before planting, but should be well watered immediately before leaving the nursery and at the time of planting.

Mulching is helpful for the usual reasons of suppressing weeds and conserving moisture, but in this instance it also helps reduce the build up of salts associated with the evaporation of soil moisture from the soil surface.

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General principles

Fertiliser is seldom required for seedling establishment, but can be beneficial for early growth to overcome later weed competition and to sustain acceptable growth rates. The application should be targeted to ‘feed’ the trees and not encourage increased weed growth and competition.

Planting is best when the soil is moist and warm but not waterlogged, preferably after good soaking rains have had a chance to leach salts from the surface. These conditions can be difficult to arrange in the best of circumstances, but even more on saline sites which can very quickly become waterlogged.

On cold sites (e.g. tableland areas of the eastern mainland states and in Tasmania), avoid planting in autumn, but be prepared to use plastic tree guards to protect seedlings from frost if planted in early spring.

High soil salinity and waterlogging have a similar effect to drought in restricting moisture availability to seedlings, so time the planting early enough to avoid the risk of hot spells for a few months. Waterlogging can often be minimised with drainage before planting and along with mounding will allow earlier planting, with more time for strong growth as moisture recedes in the dry season.

Gypsum (calcium sulphate) can be added to the soil to amend soil sodicity. Application rates vary depending on the degree of sodicity but range from 2.5 to 10 t/ha, and to be effective the soluble calcium must be incorporated or able to leach into the root zone.

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Silviculture

Tree planting at sites with less than about 600 mm annual average or at sites with low salinity can begin at high densities up to 2500 stems/ha, a density which is generally suitable for short-rotation pulpwood production. This encourages good form and enables the selection of good quality stems for the final crop trees, but they need to be thinned early (time depends on growth rate) to concentrate growth on a smaller number of stems. Spacing between rows for sawlog production is about 4m to allow for slashing, thinning and harvesting.

If form is unimportant, it may be more cost-effective on highly saline discharge areas with shallow water tables to plant trees at a relatively low density (e.g. 500-800 stems/ha) principally to increase site water use with limited prospect of a commercial return. This reduces planting and subsequent thinning costs, with the timber suitable for firewood.

Spacing is not important where sites are devoted entirely to biodiversity enhancement using local species, however there will be casualties if competition for good water, nutrients and light is too great.

Trees grown for high-value wood products such as sawn timber or veneer may need to be thinned several times to progressively reduce the density of the stand until only the largest, best-quality trees remain. Plantings can be thinned from around 1000 stems/ha to 600-800 stems/ha, and then to 200-300 stems/ha at a second thinning depending on climate and soil moisture. Thinnings may yield firewood, posts, poles, pulpwood or sawlogs. Thinning will probably reduce the overall water use of the stand, depending on species, stand age, density, intensity and type of thinning. Realistically, the demanding requirements associated with high value wood products are rarely likely to be satisfied at saline sites.

Pruning will generally increase the value of trees harvested for sawn timber or veneer by reducing the size of knots. On the other hand, plantations for pulpwood will not need thinning or pruning, usually being coppiced on short rotations (8–15 years).

Unused portions of trees such as under-sized stems, branches, bark and foliage should be left on site, where they will decay over time and return nutrients to the soil.

Irrigation with saline water can lead to accumulation of salts in the plant’s root zone, and have other detrimental affects on soil chemistry and structure. Therefore inputs of salt via irrigation water will need to be balanced by export of salt from the root-zone. Such export may be possible in areas with deep watertables but is unlikely to occur if watertables are shallow (within 2 m of the soil surface).

Small volumes of water (0.4–1.5 ML/ha) should be applied regularly during the growing season to compensate for losses through evaporation and deep drainage. The quality of water will have different effects depending on soil texture and degree of salinity and sodicity. Irrigation with low-salinity water is best on saline or non-saline soil, but not on sodic soils where the clay minerals will disperse. Irrigation with moderately saline water (EC < 0.8 dS/m) improves structural problems in sodic soils, while highly saline water (EC > 2 dS/m) will improve the structure of clay soils but will increase soil salinity unless there are opportunities for leaching.

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Managing insect pests

A favoured approach to reducing the impact of insect damage is to choose species or seed sources of trees that are relatively resistant to insect attack. For example, E. grandis and juvenile E. globulus are known to be very susceptible to insect attack while E. occidentalis is resistant in eastern Australia. ‘Silverton’ or ‘inland’ forms of E. camaldulensis are relatively resistant to lerps but many of the northern Victorian provenances are quite susceptible. Furthermore, if local seed is being collected, avoid collecting from trees that show the signs of heavy insect attack.

Integrated pest management (IPM) uses complementary control procedures in a coordinated way to reduce damage by insect pests. In its most desirable form, IPM relies on the natural enemies (predators, parasitoids, pathogens) of insect pests to reduce pest numbers to acceptable levels. Chemical insecticides are only used when significant economic loss is likely to occur. Biological control can be augmented with the use of biological insecticides which are often very specific. For example, biological pesticides containing Bacillus thuringiensis can be used against the early stages of moth and beetle larvae.

Identification and appropriate control methods for the main insect pests of trees and timber in Tasmania, many of which are used as commercial species elsewhere in southern Australia, are well described and illustrated in Insect Pests of Trees and Timber available from the Tasmanian Forestry Commission.

For more information, see Insect pests - control

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