The revelation of processes governing the maintenance of biological diversity and the assessment of their relative importance stand as the primary challenges of ecology. Plant communities are shaped by both environmentally mediated patterns and biotic interactions. The processes governing plant distribution are ordered hierarchically: (1) regional processes (can a plant reach a certain site?); (2) local habitat conditions (can a particular plant germinate / grow / reproduce at this particular site?) and (3) assembly rules (due to interactions among species: competition, facilitation, mutualism and all other biotic interactions). A hierarchical approach to plant community assembly in relatively undisturbed networks of forest streams is proposed. In this context, three main research questions are addressed: (1) how do network connectivity and river section hierarchy influence species distribution, taking into account the dispersal capacity of individual species; (2) can assembly rules be derived from present community structure and to what extent do these deterministic forces explain variation in along-stream species composition; (3) how do regional and local processes interact (e.g. hydrochory compensating for competitive exclusion)