Plant community assembly along dendritic networks of small forest streams

Although insight into local and regional factors that structure communities is growing, the specific contribution of each of these factors generally r emains unknown. Moreover, the extent to which community assembly is intrins ically indeterministic (i.e., the degree to which community composition dep ends on chance events) remains an open question. The complexity of this iss ue argues for field studies in a semicontrolled environment. We studied spe cies richness and community composition of a guild of phreatophytes along d endritic networks of small forest streams in western Belgium. The streams, situated at the forest-covered headwaters of six different watersheds and c haracterized by homogeneous environmental conditions, impose spatial disper sal constraints to the plant species. This allowed us to assess the relativ e importance of deterministic processes guiding community assembly. More sp ecifically, we focused on the role of species migration and colonization pr ocesses relative to local processes (competitive exclusion and passive habi tat selection) on species richness and on community composition. Our analys es at three consecutive levels (species number, community composition, and species) revealed that plant community assembly along the stream systems is based on (1) historical and stochastic colonization events of species and (2) subsequent deterministic downstream dispersal processes. Apparently, du e to the influence of historical chance events, deterministic processes do not necessarily result in community convergence. In addition we show that c ontinuous downstream dispersal and colonization can compensate for local ex tinction processes. This is another piece of evidence that, when studying c ommunity composition, it is absolutely necessary to place communities in th eir spatial contexts.