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SOLUTION 11

Revegetating with non-grazing options

 

11.4 Level of confidence in revegetating with non-grazing species

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How reliable is the information?

On the basis of research and consistent observations we can confidently state that:

  • The essential plant physiological processes underlying salt-sensitivity in non-halophytes and susceptibility of most plants to waterlogging are well understood, although research continues on why some plants cope better than others with these stresses.
     
  • Research has assessed the suitability of commercial tree and shrub species for saline and/or waterlogged land in southern Australia. Much of this research also takes account of other relevant soil (e.g. acidity, alkalinity) and climatic (e.g. rainfall, temperature, frost) conditions.
     
  • Salt-tolerant trees will lower watertables locally by reducing recharge and drawing on groundwater. The extent to which these trees take up saline groundwater declines rapidly with increasing salinity beyond a threshold, depending on the species and other environmental stresses.
     
  • Evapo-transpiration by trees will lead to salt accumulation in the root zone unless there is a flushing mechanism.
     
  • The sustainability of trees planted over saline watertables depends on the extent to which salt can be flushed either vertically or laterally from the soil profile. This in turn will depend on rainfall, soil properties, depth to watertable and slope.
     
  • The hydrological impact of trees planted over saline watertables is greatest and quickest for local groundwater flow systems.
     
  • Amongst the eucalypts, the trees with greatest tolerance to salinity and waterlogging are E. occidentalis and E. camaldulensis. There are also high levels of tolerance amongst Casuarina and Meleleuca species. Hybrids of E. camaldulensis (river red gum) with E. globulus (Tasmanian blue gum) and E. grandis (flooded gum) have improved salt tolerance and biomass productivity.

Ares of uncertainty:

  • It is difficult to predict the performance (growth, water-use) of trees in the field as this depends on many factors aside from salinity and because tree roots grow to considerable depths, the causes of these effects may not be obvious.
     
  • There appears to be considerable variation in salt tolerance between provenances of single species, however few controlled trials span more than 10 years.
     
  • Saline sites can change significantly over the sorts of timeframes needed for forestry options – this greatly increases the risk of ‘failure’ if success depends on a commercial outcome.
     
  • There is considerable uncertainty about how any emissions trading scheme will operate, although it appears that this could be a significant driver for tree planting on saltland, and one that is not necessarily dependent on the trees surviving to maturity.
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Farmer experiences

Saltgrow Pty Ltd, a company that undertook significant research to develop and trial salt-tolerant commercial species, has several farmer testimonials on its website.

“A variety of trees were planted to gauge their effectiveness at lowering the water table.

The trees were healthy and had achieved consistently high growth rates. River red gum had an 80% survival rate, compare to coolibah 69%, blackbutt 58%, Queensland white gum 60%, tipuana 38% and yapunya 16%. After 7.5 years, the blackbutt reached the greatest average height at 9.1m whereas the tipuana averaged out at 3.2m.

There was no evidence of salt damage. Initially some people believed the trees would eventually kill themselves by concentrating salts in the root zone, but this hasn’t happened.

Most tree deaths occurred at the seedling stage soon after planting. There have been very few deaths in the last seven years. Monitoring of both trees and soil is continuing.

Chloride levels of the surface soil have dropped by an average of 95% in both the mounded and inter-row areas. The watertable has dropped allowing rainfall to leach the salt lower into the soil profile enabling the return of native grasses such as bluegrass, wiregrass and windmill grass.

The seepage area has stopped spreading but high chloride levels below 50cm suggest that if the trees were removed the salinity outbreak would re-occur.”

For more information, see page 18 of SALT Magazine Issue 4: Targeted Trees Keep Salinity at Bay

Many farmer experiences are more focussed on revegetation for environmental and amenity benefits than on commercial tree production. Several of these farmer stories have been reported in SALT magazine:

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Risks and challenges

Most of the risks associated with revegetation of saltland with trees are more applicable to commercial options than to conservation plantings. This is because planting a mixture of tree and shrub species is more likely to match the variety of niches across a saline site, and species that have higher salt tolerance (but no commercial value) can be included – these include Melaleuca halmaturorum, Casuarina glauca and C. obesa.

The main risks associated with commercial forestry on saltland are:

  • Tree growth rates will be slowed by salinity and waterlogging to levels that cannot provide a commercial return;
     
  • Saltland sites are often smaller than required for commercial forestry operations, and are often located at challenging distances from markets;
     
  • Sites that are currently suited to forestry may become more saline and waterlogged (and therefore unsuitable) over the long time span needed for forestry options to mature; and
     
  • The water use by the trees results in significant salt accumulation in the root zone, perhaps rendering this option unsustainable in the long-term.

Some of these challenges also apply if carbon credits are sold from trees planted on saltland, but with the additional risk that if the trees die or fail to sequester sufficient carbon, then the credits may need to be re-purchased, perhaps at a significantly higher price. The rules associated with planting trees for carbon credits need to be very clear before this option could be recommended.

Farm forestry that reduces seepage or run-off from saline sites can represent a benefit for stream water quality. On the other hand, seepage and run-off from sites of low salinity might be beneficial to watercourse ecosystems that have adapted to brackish water. Similarly, brackish stream flow might represent a valuable resource to grazing enterprises downstream. Reducing stream flow in these circumstances, particularly in what appears to be a prolonged dry period for medium rainfall zones, can be a significant disadvantage.

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Prospects

Current research

Research into forestry options for saltland is ongoing in CSIRO and Saltgrow Pty Ltd.

The book Trees for Saline Landscapes presents current knowledge on a large suite of tree species tolerant to different levels of salinity as well as their uses. It contains detailed descriptions of 85 salt-tolerant species as well as a detailed overview of salinity problems in Australia. This is an important publication for anyone involved in Australian agriculture looking to make decisions about which trees might be appropriate for different types of salt-affected environments.

The book is a collaboration between CSIRO and the Rural Industries Research and Development Corporation's Joint Venture Agroforestry Program, and is based on research conducted through the Commercial Environmental Forestry program and through the CRC Salinity.

Future Prospects

There are strong prospects for the continued use of this non-grazing option for saltland – in particular the use of trees and shrubs for conservation and visual amenity. Many saline sites especially in the eastern states are too small for commercial forestry or even to make a significant difference to the farm if used for saltland pastures. Mixed species plantings, with only occasional grazing, or use as emergency shelter for sheep off shears or with lambs, offer a good long-term prospect for revegetating saline sites and including them in the farm conservation/revegetation plan. As there are few commercial options, it makes sense to focus efforts on these salines sites around non-commercial activities.

In the future, breeding/selection may produce a greater array of salt- and waterlogging- tolerant trees for commercial use, and this would increase the prospects for this Saltland Solution. However, long-term investments such as forestry are challenging enough, without the added risks associated with saline sites. Commercial forestry options seem unlikely for saltland.

A significant change to the status of this Saltland Solution could be carbon trading schemes associated with Australian Government targets to reduce emissions. When the details of Australian and international carbon trading schemes are known, the prospects for using trees on saltland to generate carbon credits can be reassessed.

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