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Silvoarable

Silvoarable

In Silvoarable systems agricultural or horticultural crops are grown simultaneously with a long-term tree crop to provide annual income while the tree crop matures. Trees are grown in rows with wide alleys in-between for cultivating crops.

Intercropping & Alley cropping

Alley component: Any arable or horticultural crop is possible. Overwintering crops (ie autumn-sown) are very efficient users of the almost full light available over the dormant season of deciduous trees, and may be the best choices for narrow alleys where trees are quite large. It is important that the alleys are physically cultivated (or at least ripped with deep tines) – horticultural no-dig raised beds are likely to fill with fine tree roots.

Tree component: may be timber or fuelwood trees, or a fruit or nut crop. Pollards and coppiced trees are both possible, the former interfering least with arable operations.

For more information on tree species choice in Britain, see Selecting timber tree species.

Fruit crops can be used as the tree component. Apples, cider apples and plums are all possibilities.

Nut crops can include walnuts, chestnuts and hazelnuts.

silvoarable1Silvoarable agroforestry experiment with poplar and barley in Bedfordshire in 2002

Design & establishment

Tree rows are spaced at a minimum of 10-14 m apart to allow enough room for cultivation operations. Usually a whole number of cultivation equipment widths is chosen for efficient operations. Rows are best aligned North-South.

Both single and double rows of timber crop trees can be used; a further alternative is a triple row, with high-value timber crop trees sandwiched between rows of nurse trees (usually coniferous) which help train straight crop trees and are themselves thinned at a later stage. Shrubs and other plants can also be planted to the side of main trees for better wind protection and other uses. Trees can be planted in the rows at final spacing or at a closer spacing to allow for thinning at a later date. The latter allows for more selection of good quality timber trees.

Weed control is essential. Black plastic mulches give best tree establishment and growth, and will soon be covered with leaf mold. Cultivations up to a few inches of the plastic edge are possible.

Yields (per unit area) of alley crops are not reduced by shading until the tree height reaches the alley width (at which stage the system can be converted to silvopasture.) Competition between trees and alley crop for water does not appear to be a big problem in Britain; it is possible some problems may occur in drier areas (ie the East) during droughty summers.

Benefits

Wood or tree products are produced in addition to agronomic crops, with no reduction in crop yields per unit area for many years.
Crop quality and yields can be increased by enhancing microclimatic conditions, offsetting any reduction by the removal of tree strips from cultivation.
Utilisation and recycling of soil nutrients is improved.
Wildlife habitat and corridors are created.

Aesthetic diversity – improvement to open monocropped areas.

Drawbacks

Many high-value deciduous timber trees grow with poor (crooked) form without the sideways light pressure of a forest. Correction pruning and/or the use of nurse trees can overcome some of this problem.

Arable farmers in particular often have an aversion to trees in arable fields, citing single trees which “get in the way”. Alleys, however, if properly designed, should present no problems with machinery and cultivations.