Effect of two tree windbreaks on adjacent pastures in south-western Victoria, Australia

The effects of 2 tree windbreaks on pasture production in adjoining paddocks were assessed over 4 years in a cool-temperate climate, perennial pasture area in south-western Victoria, Australia. The Willandra windbreak was 2 rows of direct-sown black wattle (Acacia mearnsii) aligned south-east-north-west. The Helm View windbreak was 4 rows of mixed Eucalyptus, Casuarina and Acacia spp., aligned east-west. Pastures at both sites had been sown to perennial ryegrass and subterranean clover. Wind direction data indicated that the north-eastern and south-western paddocks at Willandra were sheltered 39 and 28% of the time, respectively. The northern and southern paddocks at Helm View were sheltered 34 and 42% of the time, respectively. The relationship of rainfall with wind speed and direction was examined to indicate the potential of windbreaks of various orientations to protect livestock. The windbreaks had clear effects on pasture production in the competition zone along the margins of the windbreak. Average pasture production at Willandra in the zone 0.5-0.75 H (where H is the windbreak height) was 69% of open yield; production in the 0.9-1.5 H zone was 96%. At Helm View, production in the zone 0.7-1.0 H was 74% of open yield, with 81% in the zone 1.1-1.5 H. Averaged over all years, both windbreaks had no effect on pasture growth in the 2-10 H sheltered zone, although in some years there were small significant growth increases at parts of that zone in certain paddocks. Periodic soil moisture measurements at Willandra showed that water use was greater in the competition zone near the windbreak, with a difference in summer water content of the 1.2 m profile of at least 20 mm between soil 4.6 m from the windbreak and further away. The difference was due to greater water use from the deepest part of the profile. No other effects of the windbreak on soil water content were observed.