Shelterbelts and their effect on crop yield

Shelterbelts planted to control wind erosion indirectly affect the adjacent crop. Yield data were collected during 3 yr from 15 fields cropped mainly to spring wheat seeded on fallow, to quantify the competition and sheltering effects of the shelterbelts (about 6 m high and 200 m apart) and to relate these to microclimate and plant-available soil N, P and water. In 1989 and 1990, when moisture conditions during the growing season were dry and close to average, respectively, competition by the trees and their approximately 5-m-wide grass edge reduced crop yield about 10 m into the field. Yields were slightly above the field mean about 10-20 m from the field edge. The competition effect was smaller and the zone of improved crop growth was absent in 1991 when water was abundant. Plant-available N and P were close to the field means at the edge of the cultivated fields and were highest about 10 m into the fields. Competition for water is believed to be the main reason for the reduced yields at the edge of the fields as most farmers seed-placed fertilizer P, and response to added fertilizer N appeared independent of distance from the shelterbelt. Competition for water occurs during the growing period of the crop, but also includes water taken up by the shelterbelt when the field is in fallow. The improved crop growth 10-20 m from the field edge appears related to the reduction in potential evaporation as distance to the shelterbelt decreases.