Associations of forest-type with morphology of Ecklonia radiata and abundance of understorey algae

Abstract

This study tested for ecological differences between monospecific and mixed-species stands of subtidal canopy-forming algae. At three sites on the northern shore of St Francis Island, South Australia, we determined the proportional cover of four configurations of canopy-forming algae: (i) monospecific stands of Ecklonia radiata; (ii) E. radiata clumps (≥4 individuals) surrounded by species of Fucales (e.g. Cystophora spp. and Sargassum spp.); (iii) individual E. radiata plants (or clumps of <3 plants) interspersed among fucoid species; and (iv) fucoid-dominated stands. Mixed E. radiata-fucoid and fucoid dominated stands formed the most extensive types of canopy, comprising >94% of forests sampled at all three sites. Using this information, we then tested whether the morphology of E. radiata plants and structure of understorey floral assemblages differed between monospecific E. radiata stands and stands of individual E. radiata plants interspersed among fucoids. The morphology of E. radiata showed no substantial differences between monospecific and mixed stands. The structure of understorey assemblages differed substantially among monospecific, mixed and open habitats (reef without canopy-forming algae). These results suggest that while the morphology of mature E. radiata is not strongly influenced by forest composition, it is likely that understorey algae are strongly affected by the composition and relative covers of species in the overlying canopy.