Eucalyptus globulus agroforestry on deep sands on the southeast coast of Western Australia: The promise and the reality

Abstract

Secondary salinisation is a growing problem in Western Australia, with 1.1 Mha of agricultural and public land now affected to some extent. During the 1980s and early 1990s it was widely believed that agroforestry offered one of the best hopes for reducing the area at risk from salinisation while improving the productivity of conventional agricultural enterprises and providing new sources of income to landholders. This paper details the hydrogeology, tree water use and the productivity of Eucalyptus globulus, crops and pastures in an agroforestry system on the south coast of Western Australia with deep sandy soils and rising groundwater. The tree-belts directly occupied 10% of the catchment and the lateral spread of outer tree roots meant recharge was effectively eliminated over 20% of the catchment. However, groundwater at the site continued to rise for 13 years after the trees were planted. Sapflow and tree growth measurements showed that the trees did not access the brackish groundwater (>500 mg/L) present over most of the site. Consequently, tree growth was relatively poor (5 m 3/ha/yr for inner trees) and the estimated economic returns from pulpwood production were less than from agriculture. While the outer tree rows grew at 8 times the rate of inner rows, this was at the expense of reduced crop and pasture growth in the adjacent 15–20 m wide competition zone (CZ). If the area of the CZ where agricultural production was less than breakeven is considered, then the outer tree rows grew at a similar rate (expressed on an area basis) as the inner rows. This suggests there was no economic advantage in growing pulpwood in belts rather than blocks at this site. Thinning the trees to 125 stems/ha allowed improved crop and pasture production and reduced wind erosion in the CZ, and facilitated greater growth rates of the remaining trees. It remains to be determined whether increased economic returns from the trees will compensate for the cost of thinning and pruning. This study has shown that E. globulus is not a suitable agroforestry tree species for medium rainfall sites with deep sands and brackish groundwater, other tree species may be more suited to these sites and achieve better economic and hydrological outcomes for landholders.