Résumé
Managing ecosystems effectively for the maintenance of biodiversity and ecosystem functions and services requires to understand how these ecosystems are changing and what are the drivers behind these changes. The resurvey of plant communities sampled some decades ago is increasingly used for this purpose. We used a life-history trait-based approach to test a number of hypothesized drivers behind decadal changes (1970-2015) in the herb layer of an ancient broadleaved lowland forest exhibiting contrasted soil and management types, while minimizing potential relocation and observer errors. Process-based hypotheses were tested using paired comparisons (old vs. new records) of community-weighted means and conditional inference classification trees for a number of traits across soil and management types. We then investigated how these processes impacted species composition and diversity within stands, among stands and at the entire forest scale, using metrics of taxonomic, functional and phylogenetic diversity. Though they have been continuously managed as regular high forest, managed stands experienced more pronounced vegetation changes than those left unmanaged. This could be directly or indirectly related to modifications in forest harvesting practices since 1970, via light availability at the forest floor, soil disturbance and local N deposition. While forest specialists increased their abundance over the entire forest, managed stands showed the strongest compositional changes, especially on the soil types that were more susceptible to compaction. Increasingly heavy forestry vehicles that drive more frequently across forest stands likely generate microhabitats suitable for ferns, graminoids and N-demanding forbs and also act as dispersal agents. Species richness of vascular plants increased at both stand (alpha-diversity) and forest (gamma-diversity) scales, due to the non-random, directional colonization by the same suite of species, causing compositional, functional and phylogenetic homogenization among habitats (i.e. decreased beta-diversity). Synthesis. Forest management, via the repeated passing of heavy forestry vehicles, emerged as the key driver of local vegetation changes. By altering canopy structure, it also increased the vulnerability of understories to climate warming and atmospheric deposits. Changes were not only taxonomic, but also functional, suggesting long-term effects on ecosystem functioning and thus deserve attention from forest managers and conservationists.