AbstractAimDifferences in local β‐diversity (i.e., within‐site β‐diversity) along climatic and biogeographical gradients may result from regional differences in the importance of local community assembly processes (e.g. dispersal limitation and habitat filtering) or from regional differences in species pool sizes (i.e. species potentially able to colonize a focal site). Here, we assess the importance of processes influencing regional differences in local plant β‐diversity.LocationEurope.MethodsWe sampled herbaceous assemblages of Alnus‐dominated floodplain forests. At each of 67 locations, we placed a 50‐m long transect with ten subplots. We compared the influence of post‐glacial colonization history, climatic conditions and local environmental conditions on the turnover and nestedness‐resultant components of local β‐diversity. To evaluate the importance of local and regional processes influencing local β‐diversity, we took a variance partitioning approach and applied null‐model fixing of site γ‐diversity. By considering functional community composition, we estimated the degree to which regional differences in local β‐diversity correlate with species functional traits.ResultsWe found that regional factors and local environmental conditions contribute more or less equally to local β‐diversity. The effect of regional factors was not strongly affected by variation in γ‐diversity of sites, indicating the presence of regional differences in local community assembly processes rather than strong effects of species pool sizes. The time of the post‐glacial expansion of dominant Alnus trees was the most important regional variable influencing the components of local β‐diversity. Plant clonality was negatively correlated with the time of the Alnus expansion.Main conclusionsRegional factors influenced local β‐diversity primarily due to changes in local community assembly processes. Specifically, the post‐glacial colonization history influenced local β‐diversity likely by sorting species according to their functional traits with a non‐random nested community structure in refugial areas and a structure close to the null expectations in recently colonized areas.