The extent and cost of mallee-crop competition in unharvested carbon sequestration and harvested mallee biomass agroforestry systems

Mallee-based agroforestry has potential to provide farmers with new income sources derived from biofuels, biofeedstocks, and carbon sequestration. Although mallees are planted on >12 700 ha across the south-west of Western Australia, very little commercial harvesting of mallee has occurred to date. The development of biomass processing industries is constrained by lack of robust information regarding the productivity of integrated mallee and agricultural systems. This study addresses this constraint by quantifying the productivity and economics of agricultural crops and pastures growing in the competition zone adjacent to mallee belts at 15 sites across the Western Australian wheatbelt. The sites covered a range of climate and edaphic conditions, three mallee species (Eucalyptus polybractea R Baker, E. loxophleba ssp. lissophloia LAS Johnson and KD Hill, or E. kochii ssp. plenissima (CA Gardner) Brooker), various crop and pasture rotations, and various mallee harvest-management treatments. Mallee-crop competition was negatively correlated with rainfall and positively correlated with mallee age and size, and greater for crops than pasture. Consequently, extent and magnitude of competition were highly variable across sites and years. On average, mallee-crop competition extended 11.3m from unharvested belts and reduced crop and pasture yields by 36% within 2-20m of the mallee belts relative to open paddock yields. This is similar to what has been reported for taller tree species. Harvesting mallees reduced competition such that crop and pasture yield was reduced by 22 or 27% relative to open paddock yields for mallees harvested at 3- or 6+-year intervals, respectively. The economic cost of mallee-crop competition on agricultural enterprises was also highly variable between sites, and between years within individual sites. Averaged across all site-years, the opportunity cost of competition was equivalent to forgoing agricultural production for 14.4m on each side of unharvested mallee belts, or 9-10m on each side of harvested belts. Farmers with mallee agroforestry systems will need to manage the economic impacts of competition by reducing agricultural input costs in the competition zone, timing crop-grazing rotations with mallee harvests, ensuring that the width of alleys is at least 25 times the height of the mature trees, and possibly root-pruning mallees in unharvested or long harvest interval systems. This research has shown that mallee-crop competition presents a significant cost to farmers and must be considered when designing mallee agroforestry systems. The findings have relevance for the development of appropriate biomass and carbon sequestration pricing benchmarks for mallee plantings.