Eucalyptus bakeri: a potential source species for eucalyptus oil production in the subtropics

ABSTRACT There is a paucity of eucalypt species in tropical Australia that produce a foliar essential oil in high concentration and rich in 1,8-cineole, the sought-after component of medicinal eucalyptus oils. Eucalyptus bakeri, a mallee from the dry subtropical parts of northern New South Wales and Queensland, is an exception, with limited earlier studies reporting that this species produced foliar oil with commercial characteristics. The aim of this study was to determine the variation in foliar oil characteristics throughout the natural range of E. bakeri as a precursor to the more intensive field-testing of the species as a commercial source of eucalyptus oil. Mature leaves and, when available, coppice leaves for comparison were collected over three separate field trips in 2020 from individual trees representing each of 17 provenances throughout the natural range of E. bakeri in Queensland. Foliar oil concentrations on a fresh-weight basis throughout the provenances sampled were relatively high, averaging 2.3% for mature leaves and 2.8% for coppice leaves, with an individual tree high of 4.1% for both leaf categories. 1,8-Cineole was the dominant compound in all foliar oils sampled, averaging 89.3% (range 70.2–94.6%) in oils from mature leaves and 87.6% (range 79–92.5%) from coppice leaves. Sabinene levels, restricted to a maximum of 0.3% in some oil standards, averaged 0.3% (range trace – 1.5%) in oils from mature leaves and 0.4% (range trace – 0.7%) from coppice leaves. The high foliar oil concentrations and consistency in oil qualities mark this species as a potential candidate as a eucalyptus oil source species for the dry subtropics. Provenance/progeny trials are now needed to determine which provenances produce adequate growth rates and the silviculture required to manage the species as an oil-producing crop, as well as to investigate variations in oil characteristics, including sabinene levels, when all sources are grown at a common site.