Phytogeography of Acacia in Australia in Relation to Climate and Species-Richness

Abstract

This paper reports on the kinds of geographic patterns encountered in the distribution of Australian species of Acacia and on some climatic correlates of these patterns. The analyses were based on distribution data of 837 species mapped on a 1° x 1.5° grid. The area of highest density of species was the south-west corner of the continent, especially adjacent to the major boundary separating the Arid Zone from the more humid South West Botanical Province. The second major centre of richness occurred in eastern Australia south of the Tropic of Capricorn along the topographically heterogeneous Great Dividing Range. Secondary centres of species-richness occurred in northern and north-eastern Australia, a number of rocky tablelands of the Arid Zone and in western Victoria. The principal species-poor areas were located in sandy and some riverine areas of the Arid Zone, in temperate forests of Tasmania and in coastal areas of the north of the continent. The geographic patterns of each section of Acacia, when combined with those of species density, highlighted the tropical (section Juliflorae) v. temperate areas (sections Phyllodineae, Pulchellae, Botrycephalae and Alatae). The numerical classification of grids resulted in the recognition of eight major Acacia areas, arranged under four Acacia regions: (1) South-west; (2) Eastern, comprising a southern and a northern area; (3) Northern, comprising an eastern and a western area; (4) Central, comprising a south-east, a central and a north-west area. A discriminant function analysis indicated that precipitation was more important than temperature in distinguishing between areas. Discussion of the potential evolutionary significance of these findings and brief comparison with other biogeographic studies are given.