Sapling and coppice biomass production by alley-farmed ‘oil mallee’ Eucalyptus species in the Western Australian wheatbelt

Abstract

Summary Nine promising taxa of ‘oil mallees’ were planted in twin row alley culture at twelve sites across the Western Australian wheatbelt. Two harvest regimes were imposed (i) 2.5-year-old saplings cut to ground level in summer and their coppice shoot biomass harvested twelve months later, and (ii) 3-year-old saplings cut in winter and coppice shoots harvested twelve months later. The eight northwestern sites were generally more favourable and mostly comprised acidic subsoils with deep non-saline groundwater, while the four southeastern sites had alkaline and saline subsoils and shallow, saline groundwater. Species native to the northern wheatbelt (Eucalyptus horistes, E. kochii subsp. kochii and E. kochii subsp. pienissima) performed strongly in northern sites but grew and survived poorly in the south. E. vegrandis and E. angustissima grew well initially but survived and regenerated unsatisfactorily after first cutting at most sites. E. polybractea (a species from New South Wales and Victoria), and the central Western Australian wheatbelt species, E. gratiae and E. loxophleba subsp. lissophloia, (smooth barked York gums) performed well as saplings and in coppice format at virtually all sites. Biomass production of saplings at first cut was strongly correlated with water availability as assessed by rainfall, topsoil depth and pan evaporation. Survival of coppicing trees was sensitive to harsh conditions such as presence of shallow, saline groundwater and cold winter conditions during early regeneration. Coppice productivity of all species was strongly correlated with sapling size at first cutting. The study suggests that optimal management regimes in terms of age and size at first cut, and frequency of subsequent harvests, will be strongly determined by species and site conditions, with late spring/early summer harvests being most favourable for survival and coppice vigour.