1. We investigated the conservation status of the eucalypt taxa and communities of northern Australia including identification of regions of high species richness and biogeographic note. We did so using records from Australia’s Virtual Herbarium (almost 52,000 records) covering the entire range of taxa recorded in our study area, shapefiles of Map Units from The Vegetation of the Australian Tropical Savannas and of the National Vegetation Information System, shapefiles of land clearing and of crown and private conservation reserves, the literature and a miscellany of other sources. We defined northern Australia as the tropical savanna region plus the embedded Wet Tropics and Central Queensland Coast bioregions.
2. The eucalypt flora of northern Australia comprises 188 species and 38 subspecies (includes one variety) (Chapter 3). All three eucalypt genera are represented, with Eucalyptus being most speciose and Angophora of marginal occurrence in the study area. Eucalyptus is predominant in eastern Queensland and Corymbia in the northern Top End of the Northern Territory. Of these, 105 species and 22 subspecies are strictly endemic to the study area and a further 24 species and 3 subspecies nearly so. Seven species are shared with New Guinea or the Islands of Wallacea to Australia’s north. Species richness is greatest in the central and northern Kimberley, the northern Top End and eastern Queensland, with a peak richness of 46 species in the one degree cell covering the Atherton Tableland and adjacent western slopes of north Queensland. Species richness is markedly lower in inland areas but interpretation of the magnitude of this effect is somewhat confounded by reduced collection effort.
3. Biogeographic analysis showed strong regional patterning with a strong shift in species composition between Queensland east of the Gulf of Carpentaria and areas to the west that is consistent with biogeographic patterns identified among plants and animals in general (Chapter 4). We identified 12 regional groups of taxa with many groups exhibiting high levels of regional endemicity, an analysis that adds substantially to previous biogeographic interpretations of northern Australia.
4. We developed hierarchically-ranked metrics of restrictedness based on a combination of Extent of Occurrence, the number of degree cells, and the number of records of each taxon, with thresholds derived from IUCN criteria for Extent of Occurrence (Chapter 5). Sixteen species and seven subspecies were rated as extremely restricted (rank 1), while 125 species and 21 subspecies were rated as not at all restricted (rank 5). The greatest concentrations of extremely restricted taxa are in the central and north Kimberley and in the White Mountains area south-west of Charters Towers in Queensland. Restricted taxa (i.e. all except rank 5) are widespread in the central and north Kimberley, the Top End centred on the Arnhem Plateau, and in and around the Einasleigh Uplands of north Queensland.
5. Applying IUCN criteria, we assessed 19 north Australian eucalypt taxa as Threatened (three as Endangered, 16 as Vulnerable), and an additional nine as Near Threatened and two as Data Deficient (Chapter 5). Seventeen of these assessments were based solely on decline due to clearing (criterion A2b), four were rated on the basis of a combination of rarity and decline due to clearing (criteria B1a,b(ii,v) and B2a,b(ii,v)), and nine taxa were rated on the basis of extreme rarity alone (criteria D1 and/or D2). Taxa we rated as Threatened are strongly concentrated in eastern Queensland. Our ratings differ markedly from official listings of Threatened taxa, with the latter seriously under-representing the level of threat but also rating a number of taxa as Threatened which clearly are not.
6. Seventy-two of 125 Map Units (communities) in northern Australia are characterised as primarily dominated by eucalypts and a further 12 feature eucalypts as secondary dominants (Chapter 6). Combined, these units cover 69% of the tropical savanna portion of the study area (i.e. Wet Tropics and Central Queensland Coast bioregions excluded).
7. In the study area, reserves are strongly concentrated in the higher rainfall regions of the north-west and along sections of the Queensland coast especially on Cape York Peninsula and in the Wet Tropics (Chapter 5). There is considerable complementarity between crown and private reserves in their coverage of taxa and communities (Chapters 5 & 6). Eleven species and three subspecies endemic to the study area do not occur in either a crown or private nature reserve, and a further 52 endemic species and ten endemic subspecies have reservation indices of less than 30%. Twelve of 84 eucalypt Map Units are not represented in any crown or private conservation reserve, while a further 40 of these Map Units are poorly represented with less than 10% of their area (and often less than 1%) in conservation reserves.
8. Land-clearing is strongly concentrated in the south-east of the study area and also along the Queensland coast north to the Wet Tropics (Chapter 5). Targeted assessment of taxa demonstrated indices of clearing of >30% – sufficient to qualify as threatened under IUCN criteria independent of rarity – for eight taxa. A further nine taxa have indices of between 20 and 30%, sufficient to qualify as Near Threatened under IUCN criteria. Five eucalypt Map Units (communities) have been more than 50% cleared and a further three have been 30–50% cleared (Chapter 6). Map Units subjected to extensive clearing have not been adequately reserved by way of compensation.
9. Land clearing releases large volumes of greenhouse gases into the atmosphere, particularly carbon dioxide sequestered in trees (Chapter 6). The relationship between fire and greenhouse gases is complex; however reductions in the areal extent of fires and a shift to those with lower combustion efficiency (because the grass is still green) offers great potential to reduce emissions of the trace but very potent greenhouse gases methane and nitrous oxide.
10. The major threat to the persistence of eucalypts in northern Australia is land clearing (Chapter 7). Climate change may pose a substantial threat to some populations in the future. Local reduction in populations may occur because of rainforest expansion, over-harvest for didgeridu production, and frequent intense fires driven by invasive Gamba Grass.