Multiple global change factors cause declines of a temperate bryophyte

BackgroundClimate change, nutrient enrichment and land use have been predicted to alter bryophyte abundance and performance; we expect these factors to interact, yet experiments addressing their joint effects are missing.AimsWe tested the responses of Brachythecium rutabulum, a common temperate moss species, to single and combined effects of future climatic conditions, grazing, light limitation and nutrient enrichment. We predicted that future climatic conditions, intense grazing, light limitation and nutrient enrichment have all negative effects on the survival and photosynthetic condition of B. rutabulum, and their joint effects become strong even if individual factors have only weak effects (multiple stress hypothesis).MethodsWe measured after two growing seasons biomass and chlorophyll fluorescence of transplanted moss colonies in full-factorial treatments of fertilisation, exclusion of sheep grazing and light amendment by LED lamps, replicated in ambient and future climatic conditions.ResultsFuture climate and fertilisation had negligible effects on colony biomass and chlorophyll fluorescence of bryophyte colonies, whereas light amendment had positive effect on chlorophyll fluorescence and grazing exclusion had positive effect on colony biomass. Colony biomass and chlorophyll fluorescence decreased with increasing number of global change factors.ConclusionSupporting the multiple stress hypothesis, individually weak global change factors can combine to strong joint effects. Multiple global change factors have relatively strong negative effects on a bryophyte even if the effects of single factors are weak.We did not find evidence of multiplicative interactions (e.g. synergies) between multiple global change factors. Nevertheless, the additive effects of combined treatments showed increasing linear response with greater number of factors acting in combination.Amended light availability promotes bryophyte photosynthetic condition, whereas intense grazing may be a disturbance factor having negative effects on bryophytes.Considering multiple global change effects operating simultaneously is required for improved predictions of plant responses to global environmental change.