In forest communities, light competition is a key process for community assembly. Species' differences in seedling and sapling tolerance to shade cast by overstory trees is thought to determine species composition at late-successional stages. Most forests are distant from these late-successional equilibria, impeding a formal evaluation of their potential species composition. To extrapolate competitive equilibria from short-term data, we therefore introduce the JAB model, a parsimonious dynamic model with interacting size-structured populations, which focuses on sapling demography including the tolerance to overstory competition. We apply the JAB model to a two-"species" system from temperate European forests, i.e. the shade-tolerant species Fagus sylvatica L. and the group of all other competing species. Using Bayesian calibration with prior information from external Slovakian national forest inventory (NFI) data, we fit the JAB model to short timeseries from the German NFI. We use the posterior estimates of demographic rates to extrapolate that F. sylvatica will be the predominant species in 94% of the competitive equilibria, despite only predominating in 24% of the initial states. We further simulate counterfactual equilibria with parameters switched between species to assess the role of different demographic processes for competitive equilibria. These simulations confirm the hypothesis that the higher shade-tolerance of F. sylvatica saplings is key for its long-term predominance. Our results highlight the importance of demographic differences in early life stages for tree species assembly in forest communities.

R programming language and environment is needed to access the files.Funding provided by: Bavarian Ministry of Science and the Arts with the Bavarian Climate Research Network (bayklif )*Crossref Funder Registry ID: Award Number: Funding provided by: Agence Nationale de la RechercheCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001665Award Number: ANR-20-CE32-0005-01Funding provided by: BiodivERsA, ERA-NET*Crossref Funder Registry ID: Award Number: ANR-20-EBI5-0005-03Funding provided by: Slovak Research and Development AgencyCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100005357Award Number: APVV-20-0168

The data set is a curation of the German and the Slovakian NFIs.