Biogeography of Fire-Killed and Resprouting Banksia Species in South-Western Australia

Abstract

Banksia includes 38 fire-killed (seeders) and 20 resprouting species, and two species with contrasting ecotypes, in south-western Australia. There may be up to 12 seeders per 50 × 50 km grid cell in the southern sandplains and 12 resprouters in the northern sandplains. The patterns of distribution of species across soil type and eight climatic attributes is similar for both life forms, except that greater numbers of resprouting species occur at higher rainfalls and where there is greater seasonal spread of rainfall. Most seeders occur on white sands and rocky substrates, and resprouters occur on yellow sands and the more fertile lateritic soils. Nutrient requirements for both life forms appear to be similar. Resprouters are more widespread than seeders which suggests that resprouters show greater environmental tolerances. The distribution of grid cells containing each life form across soil types and eight climatic attributes is similar and any differences in climatic profile for all species in each category are considered biologically insignificant. Both life forms in section Abietinae are well represented in the climatically distinct southern and northern sandplains indicating no climatic preferences within the lineage. There are no consistent trends in environmental attributes from fire-killed to resprouting ecotypes of B. ashbyi E.G.Baker and B. violacea C.A. Gardner. Multiple-partitioning classification of the floristic data produced 10 groups varying greatly in geography, species richness, and proportion and endemism of each life form. The Lesueur (northern) district has four endemic seeders, six endemic resprouters and a mean of 10 resprouters per cell. The East Eyre (southern) district has five endemic seeders, no endemic resprouters and one resprouter per cell. Both groups have a mean growing season of 5 months. The relative aridities and fluctuations of present and past (Quaternary and late Tertiary) climates are invoked to explain the much higher proportion of resprouters in the northern than southern sandplains and the absence of seeders in the most marginal cells. The absence of endemic species yet high proportion of resprouters (73%) in the extreme south-western corner of the region might be explained by elimination of seeders through frequent burning by Aborigines in the late Quaternary. The increase in the proportion of fire-killed species along the south coast from 23% to 100% at the edge of the Nullarbor Plain also requires an explanation.