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UNIT 4

Plant & animal performance

 

4.2  What grows where? Why?

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Salt & water as organising influences in the landscape

Plants colonise distinct zones across a saltland area (zonation) primarily because of how the plants respond to salt and water in the environment. This can be summarised conceptually in terms of: (a) the constraints affecting plant growth and survival (Figure 4.3a), and (b) the mechanisms that should adapted plants have to survive (Figure 4.3b).

Figure 4.3

 

Figure 4.3. Zonation on saltland – organising principles: (4.3a) principal constraints affecting plant growth and survival, (4.3b) selection criteria for adapted plants.

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Salinity impacts on plants

Soil salinity inhibits the growth of most plants and spells death for many. This in turn provides a niche for those plants that do possess salt tolerance, and indeed the presence of these plants is sometimes the first indication of saline land.

Salinity has two different impacts on plants. There is an osmotic or “suction effect” where the salt in the soil attracts water, making it more difficult for plants to access the soil water and draw it in through their roots. As the salinity increases the plants need to work harder and harder to extract moisture even when the soil is saturated.

There can also be a specific ion effect where the extra salt in the plant cells slows the activity of enzymes which retards the overall metabolic process and inhibits growth and survival.

Plants have three general mechanisms for managing salinity: by preventing uptake by the roots; by accumulating the salt in the pant; or by excreting salt from the leaves. Highly salt-tolerant plants tend to use all three strategies.

Halophytes are by definition ‘salt lovers’. Most non-halophytes have to expend considerable energy to exclude salt, which reduces productivity even at low salinity levels and often leads to plant death as salinity further increases. However, the productivity of halophytes actually increases as salinity increases, up to an optimal salt level beyond which productivity then declines, but at a much slower rate than for non-halophytes.

Halophytes manage their salt load by exclusion, accumulation and/or excretion and are usually found naturally on saltland where they are able to complete their life cycle.

  • Exclusion. The degree to which plants exclude salt can be determined by comparing the salt concentration in the soil solution to the salt concentration in the sap moving up to the leaves. This comparison shows that to some degree all plants are salt excluders but some are better at this than others. Good excluder plants include puccinellia, sea barleygrass and melilotus which all exhibit strong control of the uptake of salt at the root surface.
     
  • Accumulation. Accumulator plants such as saltbush sequester salts in the vacuoles – the non-metabolising part of the cells. This strategy uses the salt to help the cells osmotically adjust so the plants can survive high levels of soil salinity, but the high salt concentrations in the tissues can make the plants unpalatable or even toxic to stock. Stock grazing plants that adopt this strategy will require ample fresh water.
     
  • Excretion. The excreter plants such as distichlis and mangroves excrete salt through salt glands.

Many plants (glycophytes and salt-tolerant non-halophytes) achieve modest salt tolerance by regulating the uptake of salts (particularly sodium and chloride ions) by their roots. This is a very energy-demanding activity and osmotic adjustment needs to occur through the use of potassium, sugars and other organic osmotica, so growth becomes further retarded as salinity levels increase.

Why are halophytes generally not found at low salinities? Halophytes actually grow sub-optimally at low salinities – their most rapid growth does not occur until soil salinity increases to 5–10 dS/m. For this reason, at very low salinities non-halophytes compete better than halophytes for the available resources of water, light and nutrients (Figure 4.3a). However, as soil salinities increase, halophytes with their greater salt tolerance are better able to compete than other plants, and they increasingly dominate until salt concentrations become so high that even the persistence of halophytes is threatened.

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Waterlogging impacts on plants

Waterlogging is a common feature of salt-affected land because of its low position in the landscape, the presence of shallow watertables and the pooling of run-off water. The impact of waterlogging on plants is due to an oxygen deficiency in the roots and/or to the build up of toxic compounds.

In roots, oxygen deficiency leads to energy deficiency – within a few minutes of roots becoming oxygen deficient, the rate of energy production by the root declines by 95%. Under the best of conditions, this dramatically decreases root growth and can impact on root survival. However soil salinity is hardly one of the “best of conditions”. With nearly all plants growing on saltland, oxygen deficiency associated with waterlogging means plant roots lack sufficient energy to exclude salt. In these situations, there can be a dramatic increase in salt intake which further decreases plant growth and survival. Indeed the combined impact of waterlogging with salinity on plants is often far greater than the impacts of these factors alone.

Many landholders have found that managing excess surface water is the precursor to reliable establishment and maintenance of saltland pastures.

……the groundwater system here was a bit like a bathtub that was filling up. So I thought, ‘why are we trying to mop this up with towels – shouldn’t we be looking for a way to pull the plug?’

All up we have excavated almost 10 km of drain, essentially removing the years of accumulated silt and digging down into the groundwater. We have also dug a series of shallow spur drains that feed into the main channels.

The experience was amazing – the water ran in the drain continuously for nine months and we had a sense that the whole landscape was being reborn
.”

David Liddicoat, Farmer, Ungarra, SA
 

Pulling the Plug on Salt

The accessible moisture on saltland can be a good thing for saltland plants. Perennial plants growing in Mediterranean environments may be subject to drought effects particularly in summer when water is less available from rainfall (Figure 4.3a). Under these conditions, shallow groundwater can act as a form of ‘sub-irrigation’; therefore, there will be a relationship between the depth to which plant roots can grow and the preferred depth to the watertable of those plants (Figure 4.3b).

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Predicting plant zonation using a matrix of subsoil salinity and depth to watertable

The ability of a saltland site to support the growth of different plant species can be reasonably predicted knowing the depth to the watertable and the salinity of the subsoil. Saltland Genie uses this information to assist farmers select the best solution for their particular saltland.

We have collated below our best information about the critical depths to watertable in summer and winter, and the average subsoil salinity associated with the success or failure of all the Saltland Solutions available on the Saltland Genie website – except for Saltland Solution 2 (Fence and volunteer pasture) and Saltland Solution 11 (non-grazing options) because these options do not have specific plant species associated with them.

Key to symbols:

Big Dot

Option should grow well

Small Dot

Option should survive well

Dash Ring

Option may only partially survive

 

Saltland Solution 1. Fence and exclude from grazing (samphire)

Subsoil salinity/ depth to watertable matrix

 

Winter

 

 Summer 

Winter Summer 

Drivers of plant zonation 

  • Shallow rooted halophyte
  • Tolerates high waterlogging, inundation in winter
  • Growth depends on groundwater accessible to roots
  • Rainfall > 300 mm

Saltland Solution 3. Dense saltbush plantings

Subsoil salinity/ depth to watertable matrix

Winter

Summer  

Winter Summer

Drivers of plant zonation 

  • Shallow rooted halophyte
  • Tolerates low waterlogging in winter
  • Growth in summer assisted by groundwater accessible to roots
  • Rainfall 300–400 mm

Saltland Solution 4. Saltbush with under-storey

Subsoil salinity/ depth to watertable matrix

Winter

Summer 

 

Drivers of plant zonation 

  • Halophyte (saltbush) and non-halophytes (under-storey) with differing tolerance to waterlogging.
  • Recommendation reflects best compromise for all pasture elements.
  • Rainfall 350–450 mm

 

Saltland Solution 5. Tall wheatgrass based pastures

Subsoil salinity/ depth to watertable matrix

Winter

Summer 

Drivers of plant zonation 

  • Deeper rooted halophyte
  • Damaged by salt/waterlogging interaction in winter
  • Growth in summer assisted by groundwater accessible to roots
  • Rainfall 400–500 mm

 

Saltland Solution 6. Puccinellia based pastures

Subsoil salinity/ depth to watertable matrix

Winter

Summer 

Drivers of plant zonation 

  • Shallow rooted halophyte
  • Tolerates high salt/waterlogging interaction in winter
  • Resurrection grass – resists dry conditions in summer
  • Rainfall 350–500 mm

 

Saltland Solution 7. Vegetatively established pastures

Subsoil salinity/ depth to watertable matrix

Winter

Summer 

 

Drivers of plant zonation 

  • Spread vegetatively so can fill gaps
  • Can tolerates high levels of waterlogging
  • Medium tolerance to salinity
  • Warm season growers
  • Rainfall >500 mm

 

Saltland Solution 8. Temperature perennial grasses with limited salinity tolerance

Subsoil salinity/ depth to watertable matrix

Winter

Summer 

Drivers of plant zonation 

  • Deep rooted non-halophytes
  • Tolerate limited waterlogging in winter
  • Very limited salinity tolerance
  • >400mm during the growing season

 

Saltland Solution 9. Subtropical grasses with limited salinity tolerance

Subsoil salinity/ depth to watertable matrix

Winter

Summer 

 

Drivers of plant zonation 

  • Non-halophytes so limited salinity tolerance
  • Tolerate low-moderate waterlogging in winter
  • Growth in summer if access to groundwater
  • Rainfall >400 mm

 

Saltland Solution 10. Legumes for saltland

Subsoil salinity/ depth to watertable matrix

Winter

Summer 

 

Drivers of plant zonation 

  • Non-halophytes and very limited salinity tolerance
  • Great variation in tolerance to waterlogging
  • Reestablishment each year for annual legumes can be a limitation
  • Rainfall >300mm
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Indicator species for assessing saltland capability

Given some protection, saltland will naturally grow a range of annual and perennial plant species. To some degree, the occurrence of these plants can be used to ‘indicate’ the suitability of saltland sites for saltland pastures.

Most of the individual species recorded have either a wide tolerance to soil salinity or to watertable depth, but the presence of more than one species enables more accurate identification of the potential soil salinity and watertable depth. Indicator species used in combination with other site characterisation methods provide a powerful tool to predict saltland capability and thus potential production of salt-affected land.

For example, SGSL research on saltland sites in WA showed that the average subsoil salinity and depth to watertable in summer associated with substantial growth (40% cover or more) of indicator species were as tabulated below.

Species 

Watertable depth (m) 

ECe (dS/m) 

Samphire

0.71

71 

Puccinellia 

0.75

33 

Rat’s tail fescue

1.04

1.3 

Curly ryegrass  

1.07

16 

Cotula

1.27

11 

Capeweed

1.66  

Slender iceplant  

1.95 

18

Annual ryegrass

2.13 

10


Comparing these values with those associated with the different saltland pasture options in Section 4.2d we can conclude that:

  1. Sites suited to the growth of samphire and puccinellia could be indicated by the presence of other samphire or puccinellia, curly ryegrass and by the absence of all the other species listed (rat’s tail fescu, cotula, capeweed, slender iceplant and annual ryegrass).
     
  2. Sites suited to the growth of dense stands of saltbush could be indicated by the presence of slender iceplant, annual ryegrass and cotula (poorly) and the absence of the other species listed (samphire, puccinellia, rat’s tail fescu, curly ryegrass and capeweed)
     
  3. Sites suited to the growth of tall wheatgrass could be indicated by the presence of cotula and (to some degree) puccinellia and curly ryegrass, and by the absence of the other species listed (samphire, rat’s tail fescue, capeweed, slender iceplant and annual ryegrass).

Research into defining the “niche” for different indicator species is continuing. We expect to develop increased confidence in the use of plant indicators over the next few years.

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Identifying Saltland Plants

Plants growing naturally on saltland provide a good guide to the severity of the salinity on the site. However, many farmers and advisors lack confidence in identifying those plant species which could be a critical guide to initially diagnosing saltland capability, and then managing the saltland for production or environmental gains.

The Sustainable Grazing on Saline Land (SGSL) initiative developed SALTdeck to take the guesswork out of saltland plant species identification and selection. SALTdeck is modelled on the highly effective WEEDeck process and provides a fast and convenient way of identifying the 50 most common plant species growing on salt-affected land. Download the cards.

Each SALTdeck card has a series of high quality colour images on one side, with additional information to assist with identification on the other. The card for wavy-leaf saltbush (Atriplex undulata) (Figure 4.4) demonstrates these features, along with the index card for all 50 species in SALTdeck (Figure 4.5)

Figure 4.4. SALTdeck card for wavy-leaf saltbush showing the SALTdeck card system with one side containing colour photos and the other side with helpful information about the species.



Fig 4.5b Fig 4.5a

Figure 4.5 – Both sides of the SALTdeck index card giving the botanical and common names for 50 common saltland species

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Regional assessments of saltland capability

Saltland capability is an assessment of the combined factors that may limit plant growth on a saline site. High capability saltland is able to support productive plants such as balansa clover, lucerne, tall fescue, kikuyu and possibly even barley. At the other extreme, low capability saltland will support little other than samphire. Between these extremes (ie moderate capability saltland) there is potential for the likes of saltbush, puccinellia, tall wheatgrass and other plants with significant grazing potential.

The CRC Salinity publication Prospects for Saline Land contains a comprehensive review of the recommended plant-based systems for managing saltland across 16 regions in Australia:
 

Western Australia

  1. Eastern and northern wheatbelt
  2. Central wheatbelt
  3. Woolbelt and West Midlands
  4. Southern Coast

    Tasmania

  5. National Action Plan Region 

Victoria
6. South-west districts
7. Gippsland
8. Central districts
9. North-east districts 

New South Wales
10. Southern slopes
11. Central slopes
12. Northern slopes 
 

South Australia
13. Coorong and southern Eyre Peninsula
14. Northern Yorke Peninsula,
mid-north and northern Eyre Peninsula
15. Upper south-east, southern Yorke Peninsula and Kangaroo Island
16. Adelaide Hills 
 


As an example, the information from one region is included below – the eastern and northern wheatbelt in WA – but detailed information from all the other regions is available. For more information, see Saltland Prospects Part B or Saltland Prospects Part C.

The Eastern and Northern Wheatbelt at a glance

This region encompasses 11.4 million hectares of the low rainfall areas of the wheatbelt (approximately less than 350 mm average annual rainfall), embracing the northern and eastern agricultural regions. While grain is the major commodity, there is limited wool and, more recently, prime lamb, present in mixed farming systems. Beef production is beginning to emerge on some properties, particularly in the north.

Approximately 380,000 hectares of land is severely salt-affected (Land Monitor) and it is forecast that between 15-30 % of the area may develop a shallow watertable and have a high hazard to salinity (although current rainfall patterns have stalled groundwater rise in some northern areas).

The prospects for managing saltland

SALTLAND
CAPABILITY 

INDICATOR 

RECOMMENDED PLANT SYSTEM 

PRODUCTIVE POTENTIAL
(Grazable DM yield; value for grazing)
 

Low 

Scalded, inundated, high salinity, clayey, samphire, curly ryegrass

Samphire 

Low (less than 0.5 t/ha; not suited to grazing) 

 (low capability)

Arrow

High

 

Patchy scalding, sea barley grass, prone to waterlogging, mod-high salinity, samphire on more affected boundary 

Dense saltbush

Low-moderate (0.5 to 1.0 t/ha; will maintain sheep if supplemented with good hay and under-story) 

Morrel soils, moderately saline, low-moderate waterlogging, sea barley grass, bluebush 

Bluebush 

Low-moderate (<1.0 t/ha; will maintain sheep if supplemented with good hay) 

 (high capability)

Duplex soils, low-moderate salinity and waterlogging 

Alleyed saltbush with under-storey 

Moderate (~1.0 t/ha; sheep will gain weight if annual legumes in under-storey) 

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