Intertwined degradation of plant diversity and permafrost in sub-Arctic palsa mires

Plant communities of sub-Arctic and Arctic regions are undergoing changes due to rapid climate change. Permafrost degradation is one of the key drivers of habitat change in these environments and it is considered to impact the biodiversity of northern peatlands. However, the empirical evidence linking permafrost thaw to biodiversity change remains limited. Here, we conducted a vegetation survey of the plant communities (including vascular plants, bryophytes, and lichens) of 14 palsa mires to quantify biodiversity change along different stages of permafrost degradation. We quantified changes in community composition, indicator species, alpha- and beta diversity as well as in two biodiversity indicators. Our results showed that permafrost degradation significantly affected both the plant community composition and the biodiversity of palsa mires. The most pronounced changes in biodiversity were associated with mires from which the permafrost had thawed completely. However, those mires that represented high stage of degradation but still retained remnant permafrost features, were still maintaining plant communities comparable to palsas with low stages of degradation. Thus, our results indicate that collapsing palsas are a clear tipping point for local biodiversity. Almost a complete loss of lichens was observed in completely thawed mires. Furthermore, the shifts in the vascular plant community were recognized to have potential to affect the biodiversity of palsa mires beyond vegetation. This study underscores the role of permafrost sustaining the compositional and taxonomic diversity of plants in palsa mires while also highlighting the varying responses of vascular plants, bryophytes, and lichens to permafrost degradation. Overall, our findings illustrate how the climate-induced changes in habitats and loss of environmental heterogeneity impact the biodiversity and the ecological resilience of the permafrost peatlands.