Extinction and colonisation of habitat specialists drive plant species replacement along a Mediterranean grassland-forest succession

Questions Land-use change causes shifts in species richness, which can be delayed. However, beta-diversity patterns and especially the relative role of species replacement and nestedness in these situations with time-lagged extinctions and colonisations remain unknown. We aim to (a) quantify beta-diversity change, species replacement and nestedness for vascular plants along a grassland-forest succession with time-lagged biodiversity change for more than 50 years; (b) check its consistency between all species, grassland specialists and forest specialists, and (c) identify the role of forest encroachment relative to other drivers. Study site Prades Mountains, Catalonia (NE Iberian Peninsula). Methods We sampled 18 sites representing a gradient in past and current grassland area and connectivity, and in forest encroachment intensity, to obtain plant composition of all species, grassland specialists and forest specialists. We quantified overall beta-diversity and its components at each species classification group along the forest encroachment gradient and other drivers. Then, we used general linear models to study (a) the change rate of beta diversity along the forest encroachment gradient and (b) the relative importance of the drivers in explaining beta diversity. Results Following the forest encroachment gradient, we found an overall noticeable species replacement, while nestedness was the main component for habitat specialists. Landscape differences contributed to explaining most compositional differences (both nestedness and replacement), while soil characteristics and geographic distance had a more restricted contribution. Conclusions Species replacement due to environmental sorting occurred over the succession, triggered by selective extinctions of grassland specialists and selective colonisations of forest specialists. Nonetheless, historical landscape characteristics, current landscape characteristics and geographic distance modulate plant extinctions and colonisations, suggesting biological inertia, mass effects and habitat isolation, respectively. Partitioning beta-diversity into nestedness and replacement components and exploring the extinction and colonisation patterns of habitat specialist groups might provide relevant insight into the drivers and processes of community shift after land-use change.