Bryophyte community responses to long-term warming manipulations vary greatly between contrasting tundra habitats

Ingibjörg Svala Jónsdóttir, Ingeborg Klarenberg, Ana Judith Colmanares Russi

Bryophytes are a diverse group of plants, both taxonomically and functionally. Although they are a major vegetation component in many tundra habitats and affect ecosystem function in various ways, they have not received full attention in studies on responses of tundra plant communities to climate warming. Here we compared bryophyte community structure, taxonomic composition, diversity and bryophyte functional group (BFG) composition among five contrasting tundra habitats across Arctic bioclimatic sub-zones and how those communities responded to long-term (decadal) experimental warming. We further explored how selected functional traits of the dominating bryophyte species responded to warming. Bryophyte cover and layer depth declined in response warming in only one of the habitats, the oro-Arctic Betula nana heath (BH) in Iceland, while increasing or unaffected in the other four habitats that all lacked erect-growing and warming-responsive deciduous shrubs. Warming also affected both taxonomic and BFG composition in BH and increased bryophyte diversity, while communities in the other habitats were not affected except in a high-Arctic moist snowbed in Svalbard. Shoot annual growth increased in the warmed plots in BH and tended to increase in the high-Arctic Cassiope heath in Svalbard, apparently due to shoot etiolation in response to increased shrub shading. Nitrogen fixation by bacteria associated with the most abundant bryophyte species was at lower rates in the warmed plots than in controls in all habitats and water holding capacity (only measured in the two Icelandic habitats) was not affected by warming despite reduced moss colony density. We conclude that an extensive and deep bryophyte layer generally enhances tundra plant community resistance to warming in tundra habitats. However, in habitats with warming-responsive erect deciduous shrubs, warming intensifies bryophyte-shrub interactions causing a decline in bryophyte cover and layer depth, which may further accelerate community change with implications for the soil environment and ecosystem processes.