Hydrology is a major environmental factor determining plant fitness, and hydrological niche segregation (HNS) has been widely used to explain species coexistence. Nevertheless, the distribution of plant species along hydrological gradients does not only depend on their hydrological niches but also on their seed dispersal, with dispersal either weakening or reinforcing the effects of HNS on coexistence. However, it is poorly understood how seed dispersal responds to hydrological conditions. To close this gap, we conducted a common-garden experiment exposing five wind-dispersed plant species (Bellis perennis, Chenopodium album, Crepis sancta, Hypochaeris glabra, and H. radicata) to different hydrological conditions. We quantified the effects of hydrological conditions on seed production and dispersal traits, and simulated seed dispersal distances with a mechanistic dispersal model. We found species-specific responses of seed production, seed dispersal traits, and predicted dispersal distances to hydrological conditions. Despite these species-specific responses, there was a general positive relationship between seed production and dispersal distance: plants growing in favourable hydrological conditions not only produce more seeds but also disperse them over longer distances. This arises mostly because plants growing in favourable environments grow taller and thus disperse their seeds over longer distances. We postulate that the positive relationship between seed production and dispersal may reduce the concentration of each species to the environments favourable for it, thus counteracting species coexistence. Moreover, the resulting asymmetrical gene flow from favourable to stressful habitats may slow down the microevolution of hydrological niches, causing evolutionary niche conservatism. Accounting for context-dependent seed dispersal should thus improve ecological and evolutionary models for the spatial dynamics of plant populations and communities.

Deed production and seed dispersal traits are from a common-garden experiment conducted at Heidfeldhof, University of Hohenheim, Germany.

Physical dispersal environments (wind speeds) were from the LAFO project (https://lafo.uni-hohenheim.de/en/1670).

Seed dispersal data was from simulation using the mechanistic dispersal model WALD (Katul et al. 2005, American Naturalist).