Patterns of association between canopy-morphology and understorey assemblages across temperate Australia

Patterns of association between canopy and understorey vegetation have been described over 1000s of km according to the presence and absence of algal canopies and the different types of canopies. However, the degree to which morphological variation of the canopy is correlated with patterns in the understorey algal assemblage is unknown. We tested the hypothesis that variation in the morphology of Ecklonia radiata, growing in monospecific canopies, is associated with variation in the structure of understorey assemblages at regional scales across temperate Australia. We found that the morphology of kelp did correlate with the structure of understorey assemblages, over broad spatial scales, particularly that of surface area/volume ratio and measures of stipe width. These canopy-understorey associations revealed two 'types' of kelp forest; one characteristic of Western and South Australia and the other of Eastern Australia. We suggest that future research on causal relationships between morphology and understorey assemblages of algae consider the potential importance that morphology may have on mechanisms such as light penetration and physical abrasion by fronds. Whilst correlations between the understorey and morphology do not demonstrate causality, the realisation that these associations occur over broad spatial scales and that southern and eastern Australia differ in their 'type' of kelp forest, at the very least, contributes to a more broadly based understanding of a major ecological pattern across the world's most extensive west-east coastline.