1. Ecological processes in mixed-species assemblages are not always an additive function of those in monocultures. In areas with high ground cover of bryophytes, renowned for their considerable water retention capacity, non-additive interactions in mixed-species cushions could play a key role in the ecosystem water economy. 2. We investigated mixture effects on external water loss in natural cushions with different species pairs of mosses and liverworts and the underlying mechanism of any non-additivity through shoot characteristics and canopy traits. 3. Species mixtures in bryophyte cushions had both additive and non-additive effects on the water economy, and these interactions were dependent on the composition of species assemblages and on plant tissue mass. Non-additivity of species mixtures was positive, resulting in the improvement of water retention. 4. Our results suggest advantages for bryophyte species to grow smaller and denser when in mixtures. They appear to alter the surface exchange area to converge in size with their neighbours, thus controlling boundary-layer properties and evaporation to reduce water loss. 5. Synthesis. A shift in bryophyte assemblages thus may influence ecohydrological processes of various ecosystems that cannot be simply predicted from the water economy of the component species when in monospecific cushions. In contrast to vascular plants, bryophytes do not compete for water but share it, and trait plasticity amongst bryophyte mixtures acts as a critical physical strategy in the community water economy. © 2011 The Authors. Journal of Ecology © 2011 British Ecological Society.