An analysis of the distribution of plants on granite outcrops in southern Western Australia using Mantel tests

A total of 141 vascular plant species were recorded from 20 quadrats on five isolated granite outcrops in southern Western Australia. The distributions of common plants, rare plants, trophic guids, and the complete data were analysed with respect to substrate (on sheet granite or scree), location (on granite outcrops), and a broader classification of locations into two western and three eastern outcrops. Most of the observed correlations were in accord with an hypothesis of environmental control that predicts distributions reflecting spatially autocorrelated environmental parameters. However, plant guilds were very strongly correlated with substrate and relatively weakly correlated with location, a result that can be explained by coexistence through geographic replacement and contingent exclusion. Rare plants were more likely to grow on either substrate than were common plants, which contrasts with the supposition that rare plants have limited environmental tolerances, but may have been caused by spatial or temporal mass effects. The Mantel test and its recent extensions allow non-exclusive, competing hypotheses to be distinguished.