Complex environmental gradients predict distributions of river-dependent plants in eastern Australia

We used principal components analysis and multiple logistic regression to investigate the relationships between environmental variables and the distributions of 71 species of river-dependent vascular plants in north-eastern New South Wales, Australia. Our analysis defined seven main environmental factors, summarised (in order of decreasing frequency of statistically significant association with species distributions) as exposure, salinity, stream size, stone scarcity, nutrient enrichment, grazing pressure and rockiness. The main environmental correlates of the presence or absence of macrophyte species in our study were broadly similar to those reported elsewhere, but the relatively low apparent importance of nutrients and grazing was unexpected. We were not able to fully separate the effects of climate-related and non-climatic environmental variables because variables of both types loaded strongly on some principal components, but we suggest that both types of variables should be included in models that aim to forecast potential shifts in plant distributions under projected climatic change. Vascular plants have been neglected in monitoring programs for Australian rivers and their conservation requires a better understanding of patterns and trends in distribution and abundance.