Multiple global change factors cause declines of a temperate bryophyte

Climate changes, nutrient enrichment, or land use have been predicted to affect bryophytes, yet factorial experiments on their effects are missing. We hypothesised that future climatic conditions, intense grazing, light limitation, nutrient enrichment have negative effects on the survival and photosynthetic condition of a common temperate bryophyte, and their joint effects become strong even if individual factors have only weak effects. We tested the effects of future climatic conditions, grazing, light limitation, and nutrient on a common temperate bryophyte (Brachythecium rutabulum) as well as the multiple stress hypothesis. We measured biomass and chlorophyll fluorescence of transplanted moss colonies after two growing seasons’ full-factorial treatments of fertilization, exclusion of sheep grazing, and light addition by LED lamps, replicated in ambient and future climate conditions arranged in a global change experimental facility. Future climate and fertilization had negligible effects on colony biomass and chlorophyll fluorescence of bryophyte colonies, whereas light addition and grazing exclusion had positive effects. Colony biomass and chlorophyll fluorescence decreased with the increasing number of global change factors. Supporting the multi-stress hypothesis, the effects of individually weak global change factors on bryophytes can become strong under multiple global change factors operating in concert.