Invasive tree species affect terricolous bryophytes biomass and biodiversity in nutrient-poor but not nutrient-rich temperate forests.

Although invasive tree species pose a significant threat to biodiversity, their impact on terricolous bryophytes has been insufficiently recognized. In addition, almost all previous studies focused on differences between invaded and uninvaded stands, not accounting for the effect of invader abundance. Here we aimed to assess the impact of Prunus serotina and Robinia pseudoacacia on terricolous bryophytes on nutrient-poor (Pinus sylvestris stands) and nutrient-rich (Quercus spp. stands) sites, along a gradient of invader biomass. We assessed the cover and species richness of bryophytes by vegetation surveys in 160 study plots (100 m) and biomass in eight 0.264 m circular subplots. Both tree species affected the cover and biomass of bryophytes, while R. pseudoacacia also reduced species richness. Although we did not confirm their impact on species composition except R. pseudoacacia in nutrient-poor sites, we revealed the negative individual responses of three species to invader biomass: Pleurozium schreberi, Dicranum polysetum, and D. scoparium. However, these impacts occurred only on nutrient-poor sites, but not on nutrient-rich sites. We provided the first quantitative assessment of P. serotina and R. pseudoacacia impacts on terricolous bryophytes along its biomass gradient. We showed low vulnerability of terricolous bryophytes in nutrient-rich broadlevaed sites while high in nutrient-poor coniferous sites, showing that there is no safe level of invader biomass.