TREE WINDBREAKS AND SHELTER BENEFITS TO PASTURE IN TEMPERATE GRAZING SYSTEMS

The effects of windbreaks on pastures are reviewed, with an emphasis o n temperate grazing systems. Mechanisms of plant response to shelter a re dealt with in brief. Few papers on measured responses of pasture sp ecies to shelter were located in a search of the global literature for the period 1972-97. Except in cold climates, where the benefits of sn ow-trapping on water availability can be demonstrated, there were few reports of increased production of pasture in response to shelter. A s ignificant result was obtained in a summer rainfall environment in Aus tralia, where a 43% increase in wool production was obtained over thre e years in small plots sheltered with iron sheeting on the fences. The gain was attributed to increased pasture growth. In New Zealand, one study over three years with a narrow, permeable shelter-belt in a wind y, dry summer environment showed a 60% increase in pasture growth in t he sheltered zone. However, another study on a high rainfall site with a dense, wide shelter-belt found no substantial shelter effect on pas ture. In dry, hot and windy climates there appears to be scope for pro tecting spray-irrigated pasture with windbreaks. The feasibility of ev aluating shelter effects on pastures or crops from old windbreaks is q uestioned. Variability of soil over the site can not be satisfactorily accounted for and there are problems in defining the true 'unsheltere d' yield. Shelter effects on pastures could best be determined by comp aring production in small completely sheltered plots and open plots. E ffects in and near the competitive zone should be measured for living windbreaks. Modelling could then be used to evaluate windbreak systems . We are not yet in a position to provide unequivocal advice to farmer s on windbreak outcomes for particular purposes or regions.