Riparian degradation, stream position in watershed, and proximity to towns facilitate invasion by Hedychium coronari

Hedychium coronarium is an invasive plant widespread in the Brazilian Atlantic forest, especially in riparian areas. However, its distribution along streams is not continuous and the factors that are related to its local occurrence are unknown. We investigated which natural and anthropogenic drivers, particularly concerning dispersal and disturbance, facilitate establishment of H. coronarium. We sampled 148 randomly chosen riparian sites (each containing two plots) in a subtropical basin in southern Brazil and recorded presence/absence of the plant and some environmental variables in situ; other variables were extracted via GIS software. We performed a GLMM with presence/absence as the response variable, sampling site as a random factor and five predictors: intensity of ecosystem degradation, dominant type of terrestrial vegetation, river substrate size class, Strahler stream order and downstream distance to the nearest urban centre. Our results point out that invader presence is favoured by local human disturbance (high riparian degradation and presence of non-native forest), and possibly dispersal, as there is a higher H. coronarium presence probability in proximity to urban centres. Furthermore, a higher presence probability in downstream sections (higher Strahler order) might be explained by hydrologic dispersal of rhizome fragments. Our study illustrates that in the case of riparian invasions it is important to consider terrestrial and aquatic drivers, both natural and anthropogenic.