Design Field Shelterbelts to Prevent Wind Erosion
Field shelterbelts consist of rows of trees planted on agricultural land to protect crops and soil, to catch and distribute snow, and to improve the micro-climate for crops growing in their shelter. A shelterbelt must be designed to perform its main function with optimum effectiveness.
On the Canadian prairies reduction of soil erosion by wind is the major reason for planting shelterbelts. Wind erosion has been a continuous problem in western Canada since cultivation began. Major erosion occurred in 1917-20 and in the 1930s. Shelterbelts have been planted where soil is easily blown or where special crops leave little vegetative cover.
Shelterbelt height and porosity must be considered when designing a shelterbelt which will be effective against wind erosion. The tree species determines the height of the shelterbelt. The growth habit and leafing characteristics of a tree, along with the tree spacing in the row, influence shelterbelt porosity. Trees are much less effective in reducing erosion in their leafless state than they are in the summer. They should be fully foliated during the times of the year when the potential for erosion is greatest. Excessive distance between trees in the row can also greatly reduce shelterbelt effectiveness.
Ideal field shelterbelts for the prairies would consist of tall, long-lived trees which are not competitive with nearby crops and do not occupy too much land. The trees should be drought hardy, winter hardy, disease, insect herbicide tolerant and have a porosity of 30-50% during the erosion periods of the year (spring and fall). The PFRA Shelterbelt Centre at Indian Head presently recommends mainly green ash and caragana for field shelterbelt use. These species have many desired characteristics.
Caragana is very long lived and non-competitive. Land occupation is normally less than 15 feet. While caragana is very hardy and pest resistant its major drawbacks are its low height (max. 20 ft.) which reduces the overall protection it can give and its low porosity which produces high, narrow snowdrifts in winter and reduces the zone of protection in summer.
Green ash is tall, long lived and hardy. It is also relatively non-competitive and compact in form. The only drawback to this species is that it leafs out late in the spring, usually after the period when the most severe wind erosion occurs. Its effectiveness in combatting wind erosion, therefore, depends on its leafless state which may be too porous (80% porosity) when trees are planted too far apart. Although a spacing of two metres between trees is presently recommended by the Shelterbelt Centre, this represents a compromise since more desirable snowdrift patterns are achieved while some effectiveness against wind erosion is sacrificed. For these reasons, mixed plantings of green ash and caragana may be the most effective in reducing wind erosion.
Alternative trees being evaluated by Shelterbelt Centre staff include Siberian larch, Scots pine, Ussurian pear and Bur oak. Villosa lilac and chokecherry are also being used as shrubs either by themselves or in combination with green ash. It is hoped that a greater choice of tree species may result in field shelterbelts which more closely approach the ideal.