Eucalyptus species for biomass energy in New Zealand—Part II: Coppice performance

Coppice re-growth and related yields of twelve species of the subgenus Symphyomyrtus and seven species of the subgenus Monocalyptus were monitored over five 3-year rotations. Planted in small plots at an original planting density equivalent to 2200 stems/ha, the resulting population densities (trees/ha and shoots/ha) varied with species and with each rotation as tree mortality increased to varying degrees following every successive harvest. Only eight of the 19 species planted had survival rates exceeding 50% of the initial population density after the fifth and final harvest. E. brookerana and E. ovata were the most vigorous species with survival rates exceeding 80% of the original planting. Eight of the species had died out completely before the final harvest. Overall, species from the sub-genus Symphyomyrtus had higher survival rates than those from the sub-genus Monocalyptus. Tree height, shoot stump diameter and above ground biomass dry weights varied between species, between sub-genera, and also between harvests. Biomass yields at comparative population densities tended to increase with subsequent coppice harvests, even though no irrigation, fertiliser, pest management systems or weed control methods were applied. Six species—E. brookerana, E. botryoides, E. botryoides×saligna and E. ovata of the sub-genus Symphyomyrtus, and E. elata and E. obliqua of the sub-genus Monocalyptus—gave satisfactory yields which exceeded 10 ODt/ha/y in any one of the five harvests. This provided mean annual incremental yields over the 15 year period ranging between 12-34 ODt/ha/y for these species when grown in the small plots. Commercial scale crop yields are likely to be considerably lower. However the six top yielding Eucalyptus species identified can be recommended for consideration in commercial plantings of short rotation coppice forestry schemes when grown on fertile soils in a temperate climate.