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Revegetating with non-grazing options


11.3 What are the benefits from non-grazing options?



CSIRO Forestry and Forest Products has evaluated a wide range of provenances, families and clones of selected species for survival, growth and water use on saline sites. As a general rule, growth rates on saline sites decrease significantly with increasing salinity, but growth is also influenced by other soil and site conditions. Mean annual increment [stem volume] (MAI) is the usual measure of growth rate, and not only is there considerable variation between species, there is also variation between provenances of the same species and then from plant to plant.

Following this evaluation work, selection has led to lines with improved salinity tolerance, waterlogging tolerance and combined salinity and waterlogging tolerance. However, even clones from these selections can show variable stem form, crown volume and susceptibility to insect damage.

Selected clones may survive and grow better than seedling trees under saline conditions, but the gains from this improved performance must be assessed against the added costs.

Commercial clonal lines of hybrid eucalypts combine the salt tolerance and timber characteristics of E. camaldulensis with the growth rate, wood quality and form of E. grandis and E. globulus. While hybrid clones have not performed consistently in field trials, Saltgrow Pty Ltd showed that at a moderately saline, clay-dominant Mt Scobie site near Kyabram in northern Victoria, volume growth of many of the E. camaldulensis X grandis and E. camaldulensis X globulus lines are high and exceed that of clones of either parent.

It will take several more years to confidently assess later-age growth rates and wood properties of hybrid eucalypts, particularly as soil salinity increases in the root zone or as soil sodicity develops with receding water tables.

Nico Markar has undertaken extensive testing on E. occidentalis in many regions.

CSIRO’s Water for a Healthy Country program has highlighted the potential of Acacia stenophylla, which is adapted to saline, heavy clay soils in arid and semi-arid areas where supplementary groundwater is available. It tolerates drought and periodic waterlogging and flooding, and some provenances show both higher survival rates and grow taller under saline conditions than non-saline conditions.

A. stenophylla could have potential in the craftwood and specialty timber markets as well as food production from seeds. DPI Victoria has been encouraging its increased use as part of the response to salinity on riverine plains.

A. stenophylla, along with E. camaldulensis, is one of the few species from the Murray-Darling Basin for which any significant amount of data is available. Aside from its ability to cope with waterlogging and high levels of salinity, it has other useful characteristics for revegetation (e.g. high seed production, prolific germination, fast growth, ability to sucker and nitrogen fixing capabilities).

DPI Victoria has measured production from trees irrigated with saline water as part of its ‘Trees for Profit’ species trials. These repeated measurements of height and diameter growth show the performance of species considered to have commercial potential over the course of a rotation, during which the trees must cope with root zone salt accumulation.


Water use

Tree planting on or near to saline discharge areas has the potential to lower saline water tables, which can be important in protecting stream water quality by minimising saline runoff or seepage. These effects, where they occur, are only local unless tree planting is extensive throughout the catchment. A useful rule of thumb is that tree roots will extend from the base of a tree for distances of about 2 tree heights. This is therefore the distance over which the tree roots will take up water; however the drainage benefits may occur over wider distances.

Water tables will be reduced by a combination of reduced recharge, due to interception by tree canopies and extraction of fresh soil moisture, along with direct extraction of groundwater. But research has shown that this latter diminishes rapidly with increasing salinity; little lowering of watertables is expected once the salinity of groundwater exceeds EC 10 dS/m (see Figure 11.6). 


Figure 11.6 Watertable drawdown as influenced by the salinity of the groundwater. After Barrett-Lennard et al. (2005)

The actual rate of water use from groundwater is uncertain, because it is difficult to partition water use between surface soil moisture and groundwater, although it seems reasonably certain that trees will mostly use the least saline water accessible to them in the root zone. In areas with a strong winter/summer seasonal pattern in rainfall, this will generally mean that they will use the shallow soil moisture in winter and the deeper groundwater in summer.

When trees take up water from the groundwater salts accumulate in the root zone. We can think of rows of trees acting a bit like a drain; when watertables are lowered beneath a tree, groundwater flows towards the tree carrying the salt dissolved in that water. This movement of salt continues to occur while the trees are able to lower local watertables. Eventually the salt that has moved towards the root-zones may reach such high concentrations that the growth and transpiration of the trees are inhibited. There are now a few examples where this effect has been monitored in the field; however the implications of this effect for the long-term performance of trees in the field is not known.

Most of the current evidence indicates that at groundwater salinities too high for economically productive tree growth, there are also very limited benefits in terms of lowered watertables. On the other hand, non-commercial species such as Melaleuca halmaturorum and Casuarina glauca and C. obesa could be usefully planted for environmental and amenity purposes.


Amenity and environmental

There is little question that including trees in a revegetation plan can transform salt-affected sites otherwise supporting only sparse sea barleygrass. This improved visual amenity is a strong motivator for many farmers revegetating saltland, and the opportunities for growing trees on saltland are greatly increased if environmental improvements and visual amenity are the primary goals.

The overall impact of tree planting on streams is more difficult to predict. Reducing saline flows into streams can be an important function of trees on saltland, and in most instances, keeping salt out of water courses is considered a very good outcome. On the other hand, reducing the flow of salts into streams can also be associated with an even greater reduction in the flow of water which might not be a desirable result. Landholders contemplating the planting of trees on saltland adjacent to streams might be well advised to seek advice about the possible outcomes for stream flow and salt loads from local hydrologists or catchment authorities.

It is also worth noting that increased urbanisation increases the demand for ‘multi-functionality’ from land and water resources – including improving the quality of water supply, improving the health of riverine habitats, and landscape amenity. Indeed, research in the United States has shown that landscape amenity is the best predictor of rural area population change – a trend that is likely to be replicated in Australia.


How do the $$$s stack up?

Farmer experiences

It is difficult to find any examples of farmer experiences showing commercial gains from planting trees on saltland. This might be partly because there are such long time delays between planting trees and commercial harvesting, but more likely it is because profitable forestry operations on saltland are likely to be rare.

The Australian Government is aiming for an emissions trading scheme to commence in 2010 and it is committed to ensuring that incentives for abatement are maintained in the period leading up to scheme commencement. These carbon credits offer the potential for a completely different financial outcome from forestry on saltland.

Carbon credits represent abatement, and can be used to counterbalance emissions that are covered by the emissions trading scheme. They can also be purchased by firms, events or individuals wishing to voluntarily reduce or offset their carbon emissions, even if they are not liable parties under the trading scheme.

At this stage, the agricultural sector can only create offsets from projects involving reductions of agricultural and land use emissions or removals of emissions through biosequestration, and not from avoided energy emissions as these are covered by the scheme.
It is not possible to advise on the potential for tree planting on saltland within the emissions trading framework until the Australian Government announces details of the program. This is particularly the case if trees are to be harvested and processed into consumables, or if there is the possibility that the trees will die and the credits may need to be bought back.

Research results

Assessment of the benefits of tree planting on saltland depends on what is to be achieved – productivity in terms of stem diameter or improvements to stream water quality.

CRC Salinity researchers undertook economic analyses for two case studies of farm forestry on saline discharge sites in Western Australia. These case studies utilised Eucalyptus occidentalis (flat-topped yate) and E. camaldulensis (river red gum) and used sensitivity analysis to identify the productivity, market conditions and/or management strategies that may be required to make these species a viable commercial option for growers or investors.

Results from the E. occidentalis case study indicate that timber production on this site is not viable, unless growth rates are substantially more than had been measured. Integrating the trees with pasture for sheep production may result in the investment breaking even, but the potential return from saltbush belts integrated with pasture represent a better option.

The E. camaldulensis case study represented a viable alternative, providing the land holder carried a level of debt close to $100/ha for 14 years until the trees are harvested. This analysis ignored any hydrological and environmental benefits which might increase the value of the trees to the land holder or the risk that changes in the salinity at the site might make it unsuitable for forestry.

Most commercial tree species are generally only slightly salt-tolerant, however A. stenophylla could be a valuable exception to this rule and there is evidence that E, occidentalis might be pulpable. 

Saltgrow Pty Ltd has been at the forefront of developing commercial prospects for trees on saltland. Much of their work has been directed at tree options for disposal of saline water from irrigation districts, an activity which has a completely different set of economic drivers. For example, pumping groundwater with an EC of 10 dS/m to a tree plantation for 8 years has protected a 50 ha dairy pasture paddock from salinity. The watertable under the pasture has fallen, although some of this might be attributable to recent dry years. Salt is being applied to the 4 ha plantation at 60t/ha/yr and is accumulating despite some leaching to groundwater. There has been no noticeable decline in the trees during this period, but the real sustainability of the process needs to be monitored over a longer period.