Planting configuration affects productivity, tree form and survival of mallee eucalypt in farm forestry plantings

Mallee eucalypts have been extensively planted in the Western Australia wheatbelt for salinity mitigation and as a short-rotation coppice crop for the production of cineole and bioenergy feedstocks. Mallee has been planted in wide-spaced narrow belts (2–6 rows) within annual crops and pasture, but optimal planting configurations have not been determined. Here, we assess the biomass yield responses of Eucalyptus loxophleba ssp. lissophloia and E. polybractea to; four row treatments (1, 2, 4 and 6 row belts) and five within-row spacing treatments (1, 1.5, 2, 3 and 4 m). Thirteen years after planting, the row effects on plot-level biomass productivity of E. loxophleba ranged from 4.3 to 21.2 Mg ha −1 year −1 . For E. polybractea , both row number and within-row spacing affected yield, which ranged from 2.7 to 18.8 Mg ha −1 year −1 . For both species, the highest growth rates were observed in the one-row belts with shorter (< 3 m) within-row spacing. Within the belts, reductions of growth rate were observed with additional rows, due to increased competition and significant suppression of internal rows; and with wider within-row spacing, due to lower initial planting density. However, when including the area between belts, wider belts generated more biomass. For both species, average tree size decreased with additional rows and shorter within-row spacing. For both species, the number of stems per tree increased with wider within-row spacing, and also for E. polybractea, with fewer rows. The substantial variation in productivity, tree size and form found in these results will affect harvestability and ultimately the economic viability of future mallee plantings.