Executive Summary
What the report is about
This report presents the results of a field trial using phase farming with trees, a system designed to grow tree crops in short rotations of 3-5 years to intentionally deplete soil water as a buffer for agriculture in areas prone to dryland salinity. The trial showed that on suitable soils, a short tree phase will deplete soil water sufficiently to allow a subsequent return to agriculture for a 11-20 year rotation.

Biomass production from the tree phase was also measured, with a view to emerging biomass energy and biofuel production systems. Some of the technical and policy issues that need resolution before this farming system can be introduced in southern Australia are discussed.

Who is the report targeted at?

The report is targeted at industry, policy and researchers who are concerned with developing integrated systems to achieve sustainable farming, environmental services, and second generation feedstock supply for biomass energy or biofuel production, particularly in Western Australia.

Background
The salinisation of land and water resources is a major environmental problem in Australia, with up to 17 million hectares of farmland likely to be affected by 2050. Salinity threatens long term agricultural production and will also have profound effects on biodiversity conservation, water supplies, and rural infrastructure. This hydrologic imbalance has been caused by the replacement of deep-rooted natural vegetation with shallow-rooted agricultural plants, and reforestation is often advocated as a treatment.

Current approaches with reforestation are limited in lower rainfall environments (e.g. <500 mm mean annual rainfall). Several studies suggest that it will be necessary to replant up to 80% of the landscape, the hydrologic impacts of trees in dryland-farming landscapes are often localized, and belts of trees can compete with crops for water and nutrients. Dispersal of trees across paddocks interferes with crop production.

Another approach is to insert short rotations (3-5 years) of trees into existing agricultural systems on a 20-25 year cycle. This is analogous to phase farming systems using perennial legumes such as lucerne (Medicago sativa), differing in terms of likely rates of soil water depletion. The premise of phase farming with trees is that the trees will rapidly de-water soil profiles to several metres depth and thus create a buffer of dry soil, with this being refilled during the subsequent agricultural phase. Moreover, it will be possible to distribute trees across the whole landscape, without displacing or competing with agricultural production.

This project field tests the recommendations of an earlier JVAP-funded scoping study. (RIRDC Publication 00/48)

Aims/Objectives
In this study we set out to determine
(a) Whether the fundamental premise of the system, that rapid de-watering of soil profiles to several metres depth, was valid.
(b) Whether the amount of soil water depletion, and thus the tree rotation length, could be manipulated by species selection or silvicultural management.
(c) The rates of biomass production from the system.

The field experimental site also formed the basis of an extension program and allowed practical difficulties such as stump removal to be resolved.

Methods used
The project tested phase farming with trees in a field experiment on land normally used for cereal production near Corrigin, WA (300 mm/year annual rainfall). Here we manipulated leaf area through variations in tree species (Eucalyptus globulus, E. occidentalis, Acacia celastrifolia, Pinus radiata and Allocasuarina huegeliana), planting density (500, 1000, 2000 and 4000 trees/ha) and fertility to determine (a) if the premise of soil water depletion to depths of several metres in 3-4 years was justified, and (b) if it is feasible to accelerate the rate of water depletion by intensifying stocking rates and hence decrease the duration of the forestry phase. Tree survival, diameter and other growth attributes were measured to allometric relationships which indicate biomass and product options. Rates of biomass production from 3 year old trees were estimated. Destructive samples of above- and belowground tree components were taken to determine harvest options. Soil water content was measured using neutron moisture meter tubes, and soil water deficits calculated.

Results/Key findings
The results from the phase farming with trees experiment are very promising with significant soil water depletion of 440 to 780 mm occurring beneath high density (4000 stems/ha) plantings of E. occidentalis occurring within 3 years of planting. Assuming a recharge rate of 40 mm/year under annual cropping, this could result in a rotation of 3 years trees, followed by 11 to 20 years of agriculture. Biomass production was dependent on high stocking densities and optimal slope position, with biomass yields of 15-22 t/ha/3 yr possible. When averaged across the landscape, yields were more modest and ranged from 12-14 t/ha/3 yr. Similarly, other species may have faster growth rates.

Implications for relevant stakeholders
The results are promising and show that the basic premise of the system is correct, with significant soil water depletion beneath 3 year rotations of trees planted at high densities. To maximize biomass production and water use, a balance is required between planting density and water availability and matching different species to different sites. Further improvements in water use and thus a decrease in the duration of the forestry component of the rotation are likely to be achieved by manipulating these elements. The report also highlights the importance of systematically measuring both moisture and biomass responses, when attempting to design new hydrologically balanced farming systems based on the introduction of new species.

The implementation of phase farming systems using trees relies on several unresolved issues such as cheap establishment and harvesting techniques, and a market for the products. With regard to cheap establishment, an alternative approach is to use species that are amenable to direct seeding, such as nitrogen-fixing Acacia species. This may have merit and offer a pathway to more rapid adoption.

Unlike phase farming with trees this would involve using existing farm equipment, forgoing harvesting and providing value in terms of recharge control, grazing benefits and soil fertility improvement. Acacia celastrifolia produced a water deficit of 368 to 479 mm, with high density plantings, after 3 years, and could result in a rotation of 3 years Acacia and 9 to 12 years of agriculture. In common with all salinity treatments, there is currently a lack of a reward or environmental payment for restoring landscape hydrology.

The markets for biomass-based products, such as bioenergy, are currently speculative and require significant changes in renewable energy policy and investment. The biomass produced in this system could also provide a feedstock for liquid biofuel production. However, there are currently some technological barriers for industrial scale wood to liquid fuel conversion. Large future markets may develop when enabling technologies, such as the conversion of woody biomass to liquid fuels, are developed. When that occurs phase farming with trees offers considerable promise as a method of producing both food and fuel from the same land, while increasing agricultural sustainability.

Recommendations

• It is essential that the likely environmental benefits of phase farming with trees are valued in some form if the system is to be adopted as a routine component of sustainable farming systems.
• Further improvement is required to increase the reliability of direct seeding establishment methods before large scale application of the system.
• Further research would be appropriate to determine whether biomass yields can be increased using even higher planting densities of species such as Eucalyptus occidentalis.

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