Based on the assumption that with unrestricted water availability, temperature will determine the response of sheltered crops, the effects of a Casuarina glauca Sieb. windbreak on the microclimate, water use and biological production of a lucerne crop were investigated. Degree-days (dd) were used to compute an index for the efficiency of the thermal effects of the shelter on a well- watered lucerne, under favourable (10 to 30 degrees C) and unfavourable (above 30 degrees C) temperature conditions. Water use efficiency was considered for two contrasting water regimes, with no water stress or with a large water deficit. In June, under favourable temperature and water availability conditions, temperature efficiency of the windbreak was 39 g m(-2) dd(-1) at a distance of three times the height of the trees compared to an exposed situation, and water use efficiency increased by 7.1 g mm(-1). However, the sheltered conditions induced a decrease in above ground biomass production and water use efficiency when a highly restrictive water regime was applied. In July, daytime temperatures were disadvantageous to the lucerne, and even with a high degree of water availability, there were no significant differences between sheltered and unsheltered conditions. This research may help decision makers to manage windbreak systems by designing an appropriate irrigated area according to the shelter and choosing crops that will best benefit from sheltered conditions.
