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SOLUTION 11

Revegetating with non-grazing options

 

11.2 Most likely situations for non-grazing options

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Landscape niche

All plants have landscape niches or zones (combinations of climatic and soil conditions and management) where they are most competitive or perform best. Saltland plants are the same, each tending to have a particular mix of salinity and waterlogging which determine where they will be able to survive, and are likely to thrive.

This concept is used in the other Saltland Solutions to identify the niche where a particular saltland species is most likely to be suited. However, for this Saltland Solution it is not possible to nominate a ‘suitable’ mix of salinity and waterlogging because the species established in a non-grazing situation will be selected specifically to suit the particular salinity and waterlogging situation.

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Indicator species

Sites that might be suitable for trees are typically characterised by salt-tolerant grasses of the medium to high rainfall zone such as barleygrass, Yorkshire fog, prairie grass or annual ryegrass and perhaps sea barleygrass, although the latter tends to colonise land too salty for most trees (See Figure 11.1 -11.5). SALTdeck has been produced to assist with the identification of the 50 most common saltland species. These can be viewed on this website .

Figure 11.1 SALTdeck card for barleygrass

Figure 11.2 SALTdeck card for Yorkshire fog

Figure 11.3 SALTdeck card for prairie grass

Figure 11.4 SALTdeck card for annual ryegrass

Figure 11.5 SALTdeck card for sea barleygrass.

Mature trees generally have much deeper root systems than grasses and can be more susceptible to sudden rises in watertable and soil salinity because their root systems cannot adjust quickly. This makes it difficult to relate visual attributes such as indicator plants or soil surface conditions to the prospects for tree crops.

Where trees are to be used for the disposal of pumped saline groundwater, the site characteristics will be largely dictated by the location of the wastewater. Generally, sandy soils are best suited for disposal of moderately saline water as this allows leaching of salts from beneath the root zone. On the other hand, moderately saline water can improve structural problems associated with sodic soils.

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Soil and climate requirements

Successful agroforestry and farm forestry enterprises are most commonly found where annual average rainfall exceeds 600mm. However, trees grown on saltland often have the advantage of access to a shallow, albeit salty, watertable. The limiting factor then is more likely to be levels of salinity and waterlogging rather than the availability of soil moisture. However, salt-tolerant trees may lower water tables locally by reducing recharge and drawing on groundwater so much over a number of years that lack of soil moisture could eventually become an issue as the trees mature.

Soil salinity is most easily and commonly measured in terms of electrical conductivity of samples taken in the top 10cm. However, site characterisation based on surface soil salinity measurements is clearly absurd for deep-rooted plants that will be affected by soil conditions to depths of several metres. Both salinity and soil texture might vary considerably over this depth, and there might also be considerable spatial variation across saltland sites. If considering a serious investment in this non-grazing option, an EM38 survey might be a useful starting point. Additional useful information might be obtained by conducting some drilling over the upper few metres of the soil profile. When drilling, look especially for the presence of cemented pans which can severely constrain tree rooting depths.

There are many other soil conditions that can limit the ability of trees to establish the type of deep root system that will allow them to survive in the relatively low rainfall zones where salinity most commonly occurs.

The overall conclusion is that there are many risks and challenges associated with commercial tree operations on saltland, and there are no measurements that can be made that will guarantee success over the long time frames associated with commercial forestry.

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Waterlogging and surface water management

Waterlogging can be a significant impediment to tree establishment, growth and long-term survival. Removing surface water through diversion banks, surface drains or planting on beds might be necessary, although clearly this can represent a significant expense.

If subsurface drainage is planned, not only the expense, but also downstream implications must be considered, and in most jurisdictions, development approval will be needed.

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