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.