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The southern boundary of the Carnarvon phytogeographic district separates the arid Acacia shrublands from the species rich heaths and woodlands of the Irwin region. This boundary has previously been mapped at a scale of 1 : 250000 using the structure, density and composition of the dominant perennial plant species. One hundred quadrats were established across this boundary to determine if floristic composition correlated with the mapped position of this boundary. Our results suggest that the boundary of the Southwestern Botanical Province should move west and south to exclude the Acacia - Casuarina thicket on red sandplain and the tree heaths south of Shark Bay. In terms of species and family composition the vegetation of these areas is more similar to typical Carnarvon vegetation than to the scrub heaths of the yellow sandplains of the Irwin. Major edaphic patterns are strongly correlated with the position of the revised boundary. The boundary between the Southwestern and Eremaean Botanical Provinces runs roughly south­ east from Shark Bay (Beard, 1976a) and bisects the lower third of the Carnarvon Basin study area of Keighery et al. (2000; Figure 1). This boundary represents the transition from the complex and species rich heathlands and woodlands of south­ western Australia to the less diverse Acacia shrublands of the Carnarvon Basin and is clearly related to the increasing quantity and reliability of rainfall to the south-west (Beard, 1976a). This major floristic boundary was first mapped by Diels (1906) and later work by Gardner (1944) and Gardner and Bennetts (1956) further refined its position but on maps at very large scale. Detailed mapping of this boundary did not take place until Beard's pioneering Vegetation of Western Australia project was undertaken, culminating in the publication of vegetation maps of the south-west at 1 : 250000 scale and the rest of the State at 1 : 1 000 000 scale (e.g. Beard, 1975, 1976a, 1976b, 1976c) and allowing detailed boundaries for 21 districts and 4 subdistricts to be defined across the State (Beard, 1980). The terminology of these districts has subsequently undergone some minor modification (Beard, 1990). Beard's vegetation mapping used vegetation structure, density and composition and generally showed remarkable concordance with the earlier published phytogeographic boundaries (Beard, 1976b). The position of this boundary in the Shark Bay area is, however, equivocal due to the intermediate nature of the structure and floristic variation in the vegetation. Diels (1906) originally included Edel Land and Peron Peninsula in the Southwestern Province, Gardner (1944) and Gardner and Bennetts (1956) showed the boundary bisecting the Peron Peninsula, while Burbidge (1960) followed Diels' line. Beard (1976b) considered that Peron Peninsula with its Acacia ramulosa scrub and rriodia plurinervata hummock grasslands clearly represented southern Eremaean vegetation, as did the vegetation of Bernier and Dorre Islands. The tree heaths of the Tamala area (immediately south of Shark Bay) are a unique vegetation type. Beard (1976a, 1976b) considered these tree heaths had clear floristic affinities to the Acacia - Casuarina thicket of the Eurardy system (within the Irwin botanical district). Edel Land and Dirk Hartog Island, however, have a complex of intermediate vegetation types with affinities both to the Eremaean and the Southwestern Provinces (Beard, 1976b). Beard placed Edel Land and Dirk Hartog Island in the Eremaean Province based on general impression of desolation and aridity while recognising their intermediate character. In subsequent work on Dirk Hartog Island Burbidge and George (1978) disagreed with the placement of this island in the Eremaean Province and considered it better treated as a 'transitional zone' or placed in the Southwestern Province based on its flora and vegetation formations with the exception of the hummock grassland. In the most recent review of the biogeographic regions in Western Australia, Thackway and Cresswell (1995) placed Edel Land in the Southwestern Province (in their Geraldton

The temperate to arid transition of the Irwin – Carnarvon phytogeographic boundary, Western Australia