Understory vegetation development along 60-year chronosequences of oak, beech, and Norway spruce

Afforestation in European countries often targets agricultural soil. New forests can provide habitats for specialist species, however, land-use legacies and fragmentation hinder biodiversity in post-agricultural habitats. The goal of this paper is to explore the effects of forest age, tree species, and the environment on the understory plant community in post-agricultural forest plantations. We used vegetation survey data from 60-year chronosequences of oak, beech, and Norway spruce sampled in 2001, 2013, and 2021, supplemented with data on soil chemistry, forest structure, management, and distance to the closest old forest. The development of understory communities was analysed with generalized mixed effect models and multivariate analyses. We assessed the abiotic and spatial factors shaping the community and the subsets of forest specialists, generalists, and open habitat species. Plant communities maintained low similarity to the old-forest reference, albeit increasing with time. Forest specialists prevalence increased over time while generalists remained abundant in all surveys. Environmental variables did not influence this trend. Open habitat species disappeared with canopy closure. Tree species influenced both the soil and light availability but correlated significantly only with the occurrence of open habitat species. Both abiotic and spatial factors partly explained the beta diversity of the whole community and of the specialists, generalists, and open habitat sub-communities. While the understory vegetation in post-agricultural forest plantations develops towards old-forest communities, major differences remain after nearly 60 years. Factors such as dispersal limitation and land use legacies may delay the establishment of specialized forest plant communities. • Understory communities slowly approach old-forest composition over 60 years. • Oak and spruce stands retained higher richness than beech due to more open canopies. • Forest specialists increase over time; generalists remain abundant throughout. • Spatial constraints explained more variation than abiotic factors. • Dispersal limitation and land-use legacies slowed recovery of old-forest communities.