Broad-scale environmental relations of floristic gradients in the Mitchell grasslands of Queensland

Abstract

A large floristic data set was collated from vegetation surveys of Mitchell (Astrebla spp.) grasslands in Queensland spanning more than 20 years. The data was ordinated using non-metric multi-dimensional scaling and a four-dimensional solution could be deciphered. The longest floristic gradient was clearly aligned with climatic variables associated with mean annual precipitation. The vector for mean annual temperature was orientated orthogonally to the primary vector and was strongly correlated through the ordination space. A grazing vector tentatively derived from the frequency of species known to be favoured by grazing in Mitchell grasslands was orthogonal to the major climatic trends. However, the suspected grazing influence is complicated by the potential influence of sites occurring on limited areas of limestone habitat representing marginal Mitchell grassland habitat. A vector derived from the binomial categorisation of landform into downs and alluvia was clearly aligned with the fourth dimension of the ordination. Short- and long-term rainfall history variables were generated for each site from local rainfall records. The oblique trajectory of these variables through the ordination space tends to suggest that they may be artefacts of sampling rather than a major independent influence on the character of Mitchell grasslands. Thus the results at the broad scale of this study do not conform to previous studies at the paddock scale that emphasise the influence of climatic fluctuations on the floristic character of Mitchell grasslands. The results suggest Mitchell grasslands will require conservation planning and action throughout their geographic range.