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A Revolution in Land Use
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| COPYRIGHT |
| © 2000 CSIRO Land and Water. To the extent permitted by law, all rights are reserved and no part of these publications covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water. |
| IMPORTANT DISCLAIMER |
| To the extent permitted by law, CSIRO Land and Water (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using these publications (in part or in whole) and any information or material contained in them. |
Options and future prospects – part 2
Forestry for sawlogs and pulpwood is a potentially valuable land use where annual rainfall exceeds 800 mm. Where rainfall is below this limit, forestry is more difficult, although it is being attempted. There are two major constraints to high rainfall tree crops as a tool for managing salinity in the Basin. First, only 6% of the Basin receives more than 800 mm of rainfall per year, and some of this is already under forest cover. Second, timber production has to be balanced with another major product of this zone — fresh water. In North Eastern Victoria, for example, the catchments of the Kiewa, King and Ovens rivers make up only 2% of the Basin yet contribute 38% of the total river flows each year.
While planting trees would probably improve water quality, the volume of runoff would certainly decline due to increased evapotranspiration by the trees. In catchments that currently yield high-quality water, the value of that water may well be greater than the value of the timber. One estimate puts the decline in mean annual runoff after planting at up to 220 mm for eucalypts and 290 mm for pines, where annual rainfall is 1000 mm (Figure 13). The magnitude of surface overland flow during storm events in areas under re-afforestation is open to manipulation; it should not be assumed that water yield is determined by evapotranspiration alone. This area of research is critical to managing re-afforestation in the Basin. To manage this trade-off, forestry needs to be researched and implemented with great care in catchments of the Basin with their heavily allocated flows.
Afforestation in medium–high rainfall areas will also affect the magnitude and distribution of flows over time and reduce the incidence and severity of flooding.
Field measurements show that flood peaks associated with low and intermediate storm events would be roughly halved by pine afforestation, with lesser reductions likely for large storm events. However some perennial streams may become intermittent.
Figure 13. The reduction in runoff when moving from grass to trees.
Source: Vertessy R (2000) in Afforestation impacts on catchment
runoff. Presented to Plantations, Farm Forests and Water:
a national workshop organised by CSIRO, Agriculture, Fisheries
and Forestry – Australia, and the RIRDC/LWRRDC/FWPRDC Joint
Venture Agroforestry Program.
Figure 14. Relationship between wood volume and annual rainfall
Source: Wong J, Baker T, Duncan M, McGuire D and Bulman P
(2000). Forecasting Growth of Key Agroforestry Species in south-eastern
Australia. A report for the RIRDC/LWRRDC/FWPRDC Joint Venture
Agroforestry Program, RIRDC Publication No 00/68
As the growth rates of trees decline with rainfall, their commercial viability depends increasingly on high-value/low-volume products. Historically, this has included industries in the Basin based on specialty timbers, essential oils and tannin from native wattle and pine. More recently, there has been market interest in native shrubs and trees for foods, flowers, oils and pharmaceuticals, as well as for a suite of exotic species including carob, jojoba, olives and nut trees.
One emerging industry utilises Australian native species to produce food. Most of the native foods that are harvested come from the wild, but there are increasing efforts to cultivate these plants, which include both trees and perennial shrubs. Thus far, the main aims have been to reduce the demand on wild stands, and to increase the reliability and quality of supply. Species for which product demand exceeds supply include quandong, Acacia (for wattle seed), native citrus, mountain pepper, riberry (clove lillypilly) and lemon aspen. The list includes some arid zone species and others from higher rainfall areas in eastern Australia. These species are relatively widespread in the native flora and some, such as Acacia pycnantha, are found in the understorey. Knowledge on best practice for cultivating these plants is in its infancy, but growing.
Producing food from native trees offers medium-to-high returns. Data on gross margins and economic analyses are beginning to emerge. The capacity of trees such as Acacia (grown for seed) to contribute to solving salinity problems has not yet been explored, but they appear to be good candidates.
The problem with specialty products such as native foods is that small markets can be readily satisfied with a small area of trees. Australian imports of tannin could be replaced with some 25 000–50 000 ha of acacias. Similarly, imports of carob bean gum worth A$10 million each year could be replaced with 5000 hectares of carob trees. If salinity targets are going to be met, requiring millions of hectares of new perennial vegetation in the medium-to-low rainfall zone, this approach would need an enormous number of different commodities suited to low rainfall areas.
A new approach that is being developed involves multi-purpose harvesting of native trees for industrial products. Mallee eucalypts, similar to those that once occupied large areas in the south west of the Basin, are harvested at ground level on a two-to-four-year rotation and streamed into three products. Pelletised charcoal for use in water filtration and gold mining is made from the wood, the essential oil cineole is extracted from the leaves for use as an industrial solvent, and the waste is used to drive the oil distillation and to generate electricity for sale into the grid.
Figure 15. Harvesting tea tree.
Source: Australian Plantations Pty. Ltd.
It is not profitable to produce any of these products alone. Put together, the value of mallees may come close to returns from cropping on lighter land, precisely the areas that leak the most under current agriculture. With a world market for activated carbon of 700 000 tonnes a year and competitive price for electricity, such generalist products are likely to be applicable to large areas. Yields of between 5 and 7 tonnes per hectare have been achieved from oil mallees in the Murray Darling Basin on an annual harvesting cycle.
The likely structure for such an industry is cells based on major regional centres with plants capable of processing several hundred thousand tonnes of trees a year, requiring some 10 000 ha of trees within a 100 km radius.
The production of ethanol and methanol as replacements for fossil fuels is potentially a large industrial use for tree crops. Modelling has suggested that 30 million hectares of trees and shrubs would be required if Australia were to make the transition to an ethanol fuel-based transport system. This would provide 50 jobs in each of 1000 regionally based production plants and 200 000 jobs in growing and delivering the feedstock.
It would also reduce carbon dioxide emissions by 500 million tonnes a year.
The costs of producing ethanol from woody biomass 'crops' is likely to be significantly more expensive than current transport fuels (maybe by a factor of two to three). However, government excise policies (or other incentives) may make such land uses economically feasible in the medium term. A pilot plan is being established in NSW to explore ethanol production from woody biomass. In addition, the Federal Government has a taskforce currently exploring options for increasing the renewable energy component of Australia's transport fuels.
Finally, any attempts at costing the production of biomass fuels should factor in the benefits mentioned above: improving land and water quality, converting to a carbon-neutral fuel cycle, increasing regional employment and replacing imports of fossil fuels.
Trees are profitable in high rainfall areas, but this is only a small fraction of the Basin. However, trees can be grown profitably in drier regions in landscape positions where they can capture more water than annual rainfall. A second way to introduce trees is to identify parts of cropped areas where crops routinely perform poorly. Identifying niches in the landscape particularly suited to trees or unsuited to cropping is a way of shifting the balance to woody perennials at least cost to the landholder.
Niche locations of trees
Where slopes exceed 3–5%, water can move horizontally across the landscape. Hillslopes that are convergent or concave tend to contain wet areas. Such areas may be problematic in that they are prone to waterlogging, salinisation and erosion. They may also be ideal locations for tree planting.
Tree belts are particularly suited to the high rainfall pasture zone, where it is clear that pastures do not use all the rainfall. For local groundwater systems, the excess water may move laterally over impermeable soil layers or shallow bedrock, and will often be fresh (Figures 16,17).
Figure 16. Trees can intercept water moving laterally across the landscape.
Source: From Silberstein R et al (in prep) in Trees, Water and Salt, an Australian guide to using trees in achieving healthy catchments and productive farms.
In southern Australia, trees have the potential to use around 1000–1400 mm of water per year, so the excess water would certainly enhance tree productivity and possibly reduce waterlogging down slope. However, suitable locations represent a small area of the Basin.
A second way to deploy trees tactically is to site them over areas with particularly large salt stores so that this salt is not mobilised. New techniques are being developed to identify such areas.
Figure 17. The combination of slope
and soil conductivity required for significant
lateral movement of water to tree belts
(purple – good lateral movement),
green – some lateral movement, grey
– insufficient lateral movement.
Source: From Silberstein R et al (in
prep). in Trees, Water and Salt, an
Australian guide to using trees in achieving
healthy catchments and productive farms.
Selective removal of crops
Yield mapping — the real-time measurement of grain yield during harvesting — has revealed enormous variability across single paddocks. When yield maps are turned into gross margin maps, we see that most of the profit comes from a small proportion of the paddock and some areas even lose money. The bad spots can be season specific, such as low-lying areas in wet years that may outperform other areas in dry seasons. The bad spots can also be due to shallow soil, sodicity or other toxicities that consistently reduce crop yield.
Such areas may never be profitable under cropping. Revegetation with perennial vegetation that is suited to the particular impediment would reduce both recharge and the variable costs associated with cropping. It also provides opportunity for creation of habitat for the maintenance of native biodiversity.
Figure 18. The yield of annual crops varies enormously
across paddocks. Areas in red have a negative gross margin
and if removed from cultivation, farmers' profits would
increase.
Source: From Simon Cook, CSIRO Land and Water (2000).
Over most of the Basin, water does not move laterally. A plantation will only stop leakage over the area of land it occupies. The situation is different for spaced trees or tree belts in cropping or pasture land. Spaced trees can scavenge water from an area far beyond their canopies, and use water left behind by crops and pastures. Thus fewer trees can achieve a greater impact on leakage. A spaced tree is likely to grow faster than its counterpart in a plantation because of the extra resources available.
Mixing trees and crops introduces the problem of competition for light, water and nutrients. Tree/crop combinations (agroforestry) will only be profitable if the value of tree products and any benefits from shelter exceed the value of displaced crops and decline in crop yield through competition (Figure 22).
Figure 19. The net benefit of tree belts is a combination of the value of the tree product plus yield enhancement due to shelter less the area of land displaced and the crop lost to competition
Source: Redrawn from Lefroy EC and Scott PR (1994). Alley farming: new vision for western Australian farmland. Western Australian Journal of Agriculture 35: 119-126
Most studies have shown that the increase in crop yield resulting from nearby shelter hardly compensates for the loss of crops closer to a row of trees. Since the value of trees is less than that of the crop, this usually makes agroforestry uneconomic. Edge rows of tree belts have shown enhanced growth, but edge rows also require more silvicultural management.
Win-win situations, where drainage is reduced and profitability is increased, are therefore rare. However, mixtures of trees and crops can be a better way of reaching a target reduction in leakage than having part of a catchment in plantation and the rest in crop. This represents a trade-off between environmental service and profitability. Belts of trees may be more cost effective than plantations but are less profitable than pure cropping.
The aim of agroforestry is for the trees to use water that would have contributed to leakage, not water that would have been used by the crop. This balancing act is difficult to achieve in a variable climate. In wetter-than-average years there will be more water than the tree/crop mixture can cope with and leakage will continue. In dry years there will not be any excess resources and the tree, by virtue of its perennial root infrastructure, is likely to be a stronger competitor for water than the crop, resulting in crop failure.
Agroforestry may be more suitable in wetter pasture regions. Competition in pastures may be lower than for crops, partly because there is more rainfall and partly because pastures that comprise several different species and exhibit different growth patterns spread the period over which competition can be tolerated. Fodder shrubs may also provide feed when the supply from annuals is at its lowest.
The best prospects for successful agroforestry or pasture mixtures occur when the tree component has a direct economic value. As the value of tree products approaches that of annual products, the problem of competition between trees and crops dissipates.
Back to top
While profitable tree cropping for the less than 800 mm rainfall zone remains an unproven option, herbaceous perennial plants — lucerne and perennial grasses — are already part of current farming systems. To what extent can a herbaceous perennial match the water use of a woody perennial?
Perennial grass pastures are capable of using higher rates of water than trees in the short term (days), but cannot match the performance of trees over the medium or longer term. The reason is that perennial grasses seldom have roots below 2 m, whereas tree roots frequently reach below 6 m.
Although they are a great improvement on annual pastures, perennial grass-based pastures cannot meet recharge targets where rainfall exceeds 600 mm and is winter dominant. Evapotranspiration can be 20–100 mm less than annual rainfall, making this zone responsible for large salt discharges to rivers. However, in some areas, particularly over duplex soils, much of the excess water leaves as runoff and contributes fresh water for rivers.
Perennial pastures suffer two further impediments. Acid sub-soils limit the distribution and performance of the most favourable species, and heavy grazing can severely compromise their water use.
There have been few comparisons of introduced and native perennial grasses. The major difference, at least with Themeda grasslands, is that the native grasses use less water in winter and spring and more in summer — thus the annual evapotranspiration is similar. There is some evidence that native grasses have greater surface runoff than introduced grasses and thereby reduce the leakage beneath the root zone.
Lucerne has proved to be a herbaceous perennial in a class of its own, frequently drying the soil profile to 3 m and more. The main interest in lucerne in recent times is due to its role in phase farming, as discussed on page 11. However, the perennial phase suffers two impediments at farm level. First, the marginal return decreases as the proportion of the farm under lucerne increases, as there is a limit to the area of well-suited soils. Second, the value of the perennial must either be similar to the annual, or the area planted to the perennial must protect a large area of land currently under annuals (Figure 20). Generally, neither is the case.
Figure 20. The required value of the perennial species depends
on the area of land it protects beyond the area over which it is planted.
For details see Bathgate
and Pannell (2000)
Source: Bathgate A and Pannell DJ (2000). Economics of deep-rooted
perennials in Southern Australia.
Current predictions state that the Basin will contain 6–9 million ha of salt-affected land. Salt affected land is defined not as a white landscape devoid of plants, but as land where the yield of traditional crops and pastures is no longer economic.
Salt makes water less available to plants so that they experience the stress of drought in a wet soil and their growth rate is reduced. When salt gets into a plant it causes progressive leaf fall and ultimately, death. However, some plants and trees can exclude most of the salt at the root surface and tolerate a high concentration of salt in their leaves.
On the rare occasions when groundwater is fresh, plants can obtain up to half of their daily requirement from the watertable. As the salinity increases to above 5000-10 000 EC, the amount of groundwater use falls dramatically, even by salt tolerant vegetation. Unfortunately, groundwater salinities above this value are all too common in Australia.
Figure 21. The use of saline groundwater by trees falls rapidly
as the salinity of the groundwater increases.
Source: Thorburn PJ (1996). Can shallow watertables be controlled
by the revegetation of saline lands?
On the other hand, plants using only small amounts of groundwater can have a large effect on the depth of the watertable. If plants can maintain a net groundwater use of just 0.1 mm/d over a year, a watertable would drop by 30–60 cm. This rather optimistic picture is tempered by the fact that salt becomes concentrated in the soil above saline groundwater. Since plants use essentially fresh water, the excluded salt is left behind in the soil. The freshwater transpired is continuously replaced by salty groundwater. Even if the groundwater starts off slightly saline, the concentration of salt in the root zone will approach that of seawater, thus precluding further uptake.
There are success stories where farmers have made good use of or even reclaimed saline land. These are likely to be hydrogeological settings where the accumulated salt can be flushed out of the soil and transported to rivers. Thus rarely does reclaiming saline areas assist in controlling salinity in rivers. Success in one location is not necessarily transferable to another.
Planting discharge areas with salt tolerant vegetation remains an important strategy for reducing the risk of spreading localised salinity, reducing the visual impact of salted land, reducing soil erosion and maybe salt transport to creeks, and for obtaining some productivity from salt tolerant grasses and shrubs. Small-scale groundwater systems may be ameliorated, but salt will continue to accumulate above larger-scale systems.
Once the salt concentration exceeds the threshold of the tree to take up water, the very process that brought the watertable within range of the tree roots will continue to operate and drown the trees in low-lying discharge areas, unless the salt can be removed. From a hydrological perspective, trees in discharge areas are not a substitute for planting recharge areas in most cases. At best, the strategies are complementary. They do little to halt the transport of salt to rivers and creeks.
Our existing maps and databases of land resources in the Murray-Darling Basin do not provide enough information for targeting where land use should be changed for maximum benefit. We need good maps of land suitability for the emerging systems of land use. This requires a much better understanding of variations in regolith hydrology and salt storage throughout the landscape. It also requires good understanding of the relative performance of these land uses across a range of soils and climates. Current surveys do not have the necessary resolution to support the type of landscape planning implied in this report. They also fail to integrate our understanding of the hydrology of farming systems into the landscape processes and functioning as a whole. New methods of land resource assessment must be developed if we are to provide a scientific basis for the revolution in land use described here. Methods for mapping land resources have to be integrated with procedures for simulation modelling, and these in turn have to be supported with strategic programs of natural resource monitoring.
| COPYRIGHT |
| © 2000 CSIRO Land and Water. To the extent permitted by law, all rights are reserved and no part of these publications covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water. |
| IMPORTANT DISCLAIMER |
| To the extent permitted by law, CSIRO Land and Water (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using these publications (in part or in whole) and any information or material contained in them. |
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Last updated: 30 August, 2012