11.6.1 Growing trees with other crops
The World Agroforestry Centre (formerly called ICRAF, the International Council for Research in Agroforestry) in Nairobi, Kenya is the main international agroforestry body. They define agroforestry as a collective term for all land-use practices in which woody perennials (trees and shrubs) are grown on the same land as pastured animals (sylvo-pastoralism) or crops (agri-silviculture). This is most commonly practised in the tropics but can also be found in temperate areas north and south of the equator, including such things as the production of maple syrup from trees between fields in New England. Agroforestry systems produce major improvements in both production and soil protection and there is no doubt that more and more areas will be involved worldwide. In many ways, agroforestry provides the best of both worlds. It produces benefits from the trees that can include fuel, fodder, food, fertilizer, fibre and fruits (the six Fs of conservation forestry) and possibly medicines and insecticides (such as from the neem tree Azadirachta indica, native to South Asia and widely planted in drier parts of the subtropics and tropics). The trees can also help to rehabilitate degraded land by building soil organic matter, act as biological pumps to remove excess irrigated water and so prevent salt buildup on the surface, and protect against erosion (and consequent loss of soil organic matter and nutrients) by the litter and roots.
Rather than a uniform layer of trees over a crop or grass, agroforestry usually works best with some segregation. The usual system is alleys of crops between rows or strips of trees. The rows are typically 4-8 m apart and run from east to west to minimize shading of the alleys between them. This intercropping can result in higher biomass production than either the trees or crop alone, partly because of differing moisture needs, taken from different horizons in the soil or at different times, the extended growing season of crops from the ameliorated microclimate and the use of nitrogen-fixing trees. Care is obviously needed in choosing compatible crops and trees. A whole field of cereal grains or pulses may be more valuable than the mixed agroforestry produce in the short-term; the advantage of the latter is its long-term sustainability and long-term profitability.
Although agroforestry is most commonly practised in the tropics, forms of it are appearing elsewhere in the world. For example, alternating plantations of pines with sheep pasture or agricultural crops may be a means of lessening problems with tree pests and diseases as well as conserving the soil from the worst effects of long-term agriculture. Although financial returns from the trees and crops are separated in time this is a potentially valuable form of agroforestry. Farm forestry has rapidly expanded in the UK since the production of livestock has become less profitable. Areas used for agriculture usually have much higher phosphate levels than woodland soils and indeed many woodlands planted a century ago have approximately twice as much phosphate as ancient woodlands. Not surprisingly these woodlands have shown good growth. However, as modern agricultural soils have still higher fertilizer contents, weeds, particularly stinging nettle Urtica dioica, are abundant in farm woodlands whose soils verge on the edge of eutrophication, limiting the development of a more natural understorey vegetation.
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