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AIM: The relative importance of stochastic and deterministic niche processes can affect the assembly of communities in response to land use context and change. In this study, we quantified the relative importance of dispersal‐ vs niche‐based processes in structuring forest plant metacommunities and sought to understand how these processes have changed in these forests over the past 50 years. LOCATION: Wisconsin, USA. METHODS: We used plant understorey community survey data in both the 1950s and 2000s from 142 upland forest stands in southern and northern Wisconsin forests to relate compositional similarity to geographic and environmental distances among sites (reflecting dispersal and niche‐based processes, respectively). We then partitioned the variance to determine their relative importance over time. Finally, we classified species into three dispersal groups based on seed mass, dispersal mode, and plant height to assess how these affect community structure. RESULTS: Both niche and dispersal processes influence the structure of Wisconsin's forest plant understories. Niche‐based processes related to environmental differences among sites dominate the mostly continuous northern upland forests. In contrast, dispersal processes dominate community assembly in the more fragmented southern upland forests. Dispersal processes increased in importance in both regions and especially in more fragmented southern upland forests over the past 50 years. We detected no differences among dispersal groups in how similarity decays with geographic distance. CONCLUSIONS: In regions retaining heavy forest cover, niche‐based assembly still predominates while in areas with smaller scattered patches of forest, stochastic dispersal plays a greater role. Dispersal‐based processes increasingly dominate forest metacommunity dynamics in both regions.

Questions: How can one explicitly quantify, and separately measure, stress and disturbance gradients? How do these gradients affect functional composition in early successional plant communities and to what extent? Can we accurately predict trait composition from knowledge of these gradients? Location: Southern Quebec, Canada. Methods: Using eight environmental variables measured in 48 early successional plant communities, we estimated stress and disturbance gradients through structural equation modelling. We then measured 10 functional traits on the most abundant species of these 48 communities and calculated their community-level mean and variance weighted by the relative abundance of each species. Finally, we related these community-weighted means and variances to the estimated stress and disturbance gradients using general linear models or generalized additive models. Results: We obtained a well-fitting measurement model of the stress and disturbance gradients existing in our sites. Of the 10 studied traits, only average plant reproductive height was strongly correlated with the stress (r2=0.464) and disturbance (r2=0.543) gradients. Leaf traits were not significantly related to either the stress or disturbance gradients. Conclusions: The well-fitting measurement model of the stress and disturbance gradients, combined with the generally weak trait–environment linkages, suggests that community assembly in these early successional plant communities is driven primarily by stochastic processes linked to the history of arrival of propagules and not to trait-based environmental filtering.