Seed limitation can narrow down the number of coexisting plant species and limit plant community productivity. It is also likely to constrain community responses to changing environmental and biotic conditions. In a 10-year full-factorial experiment of seed addition, fertilisation, warming and herbivore exclusion, we tested how seed addition alters community richness and biomass, and how its effects depend on seed origin and environmental and biotic context. We found that seed addition increased richness in all treatments, and increased community biomass depending on nutrient addition and warming. Novel seeded species, originally absent from the communities, increased biomass the most, especially in fertilised plots and in the absence of herbivores, while adding seeds of local species did not affect biomass. Our results show that dispersal limitation can constrain the invasion of novel species and their effects on community biomass, and demonstrate that these relationships are contingent on trophic interactions and environmental conditions.

Anthropogenic disturbances greatly alter community composition and diversity. However, it remains largely unknown which underlying processes - colonizations, local extinctions or abundance changes - drive compositional changes in response to disturbance, and whether these processes are constrained by environmental gradients. Here, we investigated the processes underlying temporal turnover of vascular plant communities in boreal forests in response to silvicultural practices along a soil fertility gradient. Our analyses were based on long-term data from 1985 to 2006 covering up to 1700 sites across Finland. While average richness remained static, we found that silvicultural practices induced greatest turnover in the most fertile habitats. In recently disturbed sites, colonizations and species losses altered dominance structure of the communities, while the undisturbed old forests were characterized by stable dominant species even when the majority of species shifted their identity. We conclude that disturbance history and fertility constrain temporal turnover in boreal forest communities.