Impact of Tasmanian blue gum belts and kikuyu-based pasture on sheep production and groundwater recharge in south-western Western Australia

Abstract

The effect of Tasmanian blue gum (Eucalyptus globulus) belts and kikuyu (Pennisetum clandestinum) grass on livestock production and groundwater recharge was studied in the high rainfall zone (>600 mm/year) of south-western Western Australia from 1998 to 2001. The objective was to identify optimum combinations of tree belts and pasture for sustainable livestock production and the prevention of secondary salinisation. Treatments were annual pasture, in competition with trees at different orientations (east, west and south), kikuyu pasture in competition with trees at one orientation (west), compared with pasture in the absence of tree competition. Plots had 0, 20 or 36% of their area within 10 m of the tree belt where tree–pasture competition would be expected. Plots (0.48 ha) were stocked with Merino wether hoggets at 12 DSE/ha on annual pastures and 14 DSE/ha on kikuyu pastures. Additional sheep were placed on plots in spring and the annual pasture was destocked in autumn.

Within the growing season, herbage mass was similar across both control treatments as a result of varying stock numbers. However, in summer and autumn the kikuyu control contained between 350 and 4900 kg DM/ha more herbage than the corresponding annual pasture. While both pastures accumulated similar amounts of herbage in 1998 and 2000, kikuyu accumulated more in 1999 (11 900 v. 9800 kg DM/ha) as a result of summer rain. Competition from tree belts significantly reduced adjacent annual pasture herbage accumulation (16% average reduction), although there was no difference among the levels of competition. Trees did not significantly affect adjacent kikuyu pasture herbage accumulation.

Both carrying capacity and clean wool production per hectare were significantly higher on kikuyu pasture in 1999 and 2000. Tree competition also significantly reduced the carrying capacity of neighbouring annual and kikuyu pasture by an average of 10%. Clean wool production per hectare was significantly lower on annual pasture in combination with trees (11% reduction on average), but there was less effect of competition on kikuyu pasture.

The kikuyu pasture used 115, 57 and 132 mm more water than the annual pasture in 1999, 2000 and 2001, respectively. The soil water deficit beneath the trees exceeded that below both control pastures by between 297 and 442 mm.

Although the addition of tree belts to annual pasture provided substantial reductions in groundwater recharge, producers would also have to accept losses in livestock production. While kikuyu alone provided significant increases in livestock production and substantial reductions in groundwater recharge, the best compromise was kikuyu in combination with tree belts.