RIRDC Short Report:
Alley Farming in Australia

Wet and too wet

For a start, removal of natural vegetation exposes the land to the powerful forces of wind and water; the net effect can be serious erosion and a general degradation of the soil. In addition, since crops rarely use as much water as the vegetation they displace, land clearing can lead to a rise in the water table. The ensuing waterlogging and soil salinisation can spell disaster for farming.

In the last few decades, farmers have sought solutions to such problems. Naturally, perhaps, the concept of returning trees to the landscape is at the forefront of many attempts to rehabilitate degraded land.

The potential benefits are many: tree plantations can stabilise soil while offering shelter to crops and livestock. They can help lower water tables by reducing groundwater recharge and increasing discharge. They can provide habitat for native species of flora and fauna. They can produce timber for sale or use around the farm; in some cases they can even provide animals with an additional source of feed in times of drought.

The alley solution

The answers to these questions depend in large part on the farmer's purpose for tree-planting; many different designs have been proposed and implemented.

In the early 1980s, the notion of alley cropping became fashionable. Lines of trees could be planted in rows which could be oriented to provide the best local protection from the elements, and annual crops could then be sown between them. If the right sorts of tree were planted, the farmer could also become a part-time forester and gain additional income from harvesting them.

This idea was adopted enthusiastically in various parts of the world. In particular, alley farming was promoted as an alternative to an agricultural practice known as shifting cultivation. This is the practice of growing annual crops for a couple of years followed by a fallow period of 10-20 years, during which trees helped restore soil fertility. In many developing countries, the fallow period has shortened as population pressures restrict the area of land available to rural populations. Agroforesters thought that trees and crops grown simultaneously instead of in a cycle would help maintain soil fertility without the need for long fallow periods.

Curiously, alley farming arose in Australia independently of the rest of the world. Here it was driven less by the issue of soil fertility and more by the need to manage wind erosion and soil salinity and the possibility of providing supplementary feed during drought.

Today, alley farming is practised in the south-west of Western Australia (about 10,000 hectares), south-eastern South Australia (several hundred hectares), the mallee and Wimmera of Victoria (less than 100 hectares) and in south-eastern Queensland (less than 100 hectares). Some 5,000 hectares of this has been planted specifically to control rising water tables in the wheatbelt of Western Australia, while the rest are multipurpose plantings for fodder, timber, wind protection and soil improvement.

Does it work?

Two scientists, Richard Stirzaker and Ted Lefroy, set out to answer this question in a report published recently by the RIRDC/LWRRDC/FWPRDC Joint Venture Agroforestry Program. They visited many places in Australia and overseas where alley farming has been attempted, reviewed the research literature on the subject (summarised in Table 1), and interviewed a large number of farmers who have carried out practical experiments.

They found that alley farming has its place in Australia - under the right conditions - along with other agroforestry strategies. It is one solution among many: the trick is to work out when it should be applied.

The issues

Perhaps the biggest issue of all is water. Where will the trees get the stuff? Will their roots be able to tap into a water table? How does the depth and salinity of the water table affect water uptake from it? How far laterally do the tree roots grow, and will they compete with the annual crops?

The concept of complementarity is critical here. Trees and crops may be said to be complementary if there is little competition for such resources as water, nutrients and light. This can occur when these items are not limiting, or when they can be obtained from different sources or at different times of the year. In the case of water, for example, trees might get much of their water from the subterranean water table, while the roots of annual crops tend to occupy the surface layers of the soil. At the same time, the trees might bestow above-ground benefits to the crop such as shelter and soil stabilisation.

The question of the water table

Clearly, based on this result we cannot assume that alley farming would also be successful in a low-rainfall area, where the water table is too deep to be exploited by trees or too salty to use. In such regions, the competition for scarce water between the trees and the crops could make alley farming uneconomic.

This leads to a simple decision-making approach: if the environment is such that complementarity between trees and crops is greater than the competition, then alley farming may be a suitable land use.

If competition is greater than complementarity, then the tree/crop interface (see Figure 2 and Table 2) should be kept low. There is an important qualifier, though: the ability of alley farming to lower water tables may provide benefits that will eventually outweigh losses due to tree/crop competition.

Models can be used to help in the decision-making process. The authors report a modelling exercise conducted by researchers using MIDAS (Model of an Integrated Dryland Agricultural System). They compared the profitability of the conventional farming system employed in the West Midlands sandplain region of Western Australia with three recent innovations: new cultivars of the annual pasture species Serradella; the forage shrub tagasaste in plantations; and tagasaste alley cropping. In this instance, alley cropping was predicted to be the most profitable option (Table 3).

Figure 2. Possible outcomes at the tree/crop interface

Table 2. The Length of the tree crop interface for six arrangements of planting 25% of land to trees.
 

Root pruning: a short cut to higher yields?

But even if competition is high, judicious management may play a role; it might be possible to minimise root competition by root pruning along the tree/crop interface. The effectiveness of this will depend on tree root architecture, the cost and frequency of the procedure and its effect on the health and growth of the trees.

Is it all worthwhile?

But overseas alley cropping has been assessed only in terms of short-term productivity. In Australia, we need to balance short term crop productivity - which may decline as we introduce trees to the landscape - against the environmental benefits that will help ensure the long term viability of the agricultural sector.

What needs to be known

Research, say the authors, needs to be conducted at the following scales:

Ecosystem. How do native and agricultural ecosystems compare in terms of their use of water, the cycling of nutrients and their use of energy?

Habitat. What are the right native species to select on the basis of their root architecture, phenology and the commercial value of the trees?

Catchment. What effect will the introduction of alley farming systems have on the hydrology at the catchment scale?

Farm. If farmers are to surrender land to tree planting, what are the social and economic prospects for them in the short and long term?

Plot/paddock. When trees and crops exist together, how much will they compete with each other for nutrients and water, and what will be the effects on crop yield?

What is it all for?

• to help control water tables;
• to grow a commercial tree crop for fodder and/or timber;
• to provide shelter and reduce wind erosion;
• to improve the nutritional status of the soil through mulching; and
• for conservation and to improve the appearance and value of the land.