Changes in the species and functional trait composition of the seed bank during semi‐natural grassland assembly: seed bank disassembly or ecological palimpsest?

QUESTION: Unlike above‐ground plant community assembly, the processes that govern assembly of the soil seed bank following severe habitat disturbance are poorly understood. Two hypotheses have been put forward in this context: (1) the ‘ecological palimpsest hypothesis’ assumes a gradual accumulation of species in the seed bank; and (2) the ‘community disassembly hypothesis’ assumes a gradual deterministic loss of species from the seed bank. Here, we investigated which hypothesis is applicable to the seed bank assembly of semi‐natural grasslands, following forest clearance. Furthermore, we asked whether seed bank community divergence occurred at the species and at the functional trait level. LOCATION: Restored and ancient (old) calcareous grasslands in south Belgium. METHODS: The species composition of 106 seed bank samples from three restoration age classes were obtained through a germination experiment. Community‐weighted means were calculated for 26 functional traits. We evaluated changes in species and functional composition with increasing grassland age. Differentiation in species and trait composition was compared between age groups. Finally, we tested for the occurrence of nestedness of the seed bank communities according to age. RESULTS: Seed bank species richness decreased with time since restoration, at the trait level reflected by the replacement of traits associated with generalist therophytes by traits typical for chamaephytes and grassland specialists. Whereas species differentiation remained relatively constant, trait differentiation decreased with time since restoration. The seed bank composition of old grasslands was a nested subset of that of young grasslands. CONCLUSIONS: Our results suggest that the ‘community disassembly hypothesis’ is applicable to the community assembly of semi‐natural grasslands. Directly following forest clearance, a diverse seed bank is formed, followed by a gradual net loss of species. Surprisingly, this species loss is not governed by seed persistence traits but by functional changes in the above‐ground community. This disassembly process results in one deterministic end state at the trait level, but not at the species level.