Bryophytes cover large territories in cold biomes, where they control soil temperature regime, and therefore permafrost, carbon and nutrient dynamics. The mechanisms of this control remain unclear. We quantified the dependence of soil temperature fluctuations under bryophyte mats on the interplay of bryophyte heat conductance traits, mat thickness, density and moisture regimes. For seventeen predominant bryophytes in six typical subarctic ecosystems, we assessed in situ soil temperature dynamics under bryophyte mats in comparison with bryophyte-removal patches and per-species mat field moisture. In a complimentary laboratory investigation, we studied how per-species bryophyte thermal conductivity and volumetric heat capacity depend on mat density and moisture content. Subsequently, we tested whether heat transfer through bryophyte mats could be modelled as a function of mat thickness, thermal conductivity and volumetric heat capacity, the latter two being determined by mat density and field moisture content. Laboratory assessment revealed that bryophyte thermal conductivity and volumetric heat capacity were independent of mat density, and depended linearly on mat moisture content, but the dependencies were not species-specific. In the field, bryophytes reduced amplitudes of soil temperature fluctuations and freeze-thaw frequency during the growing season, but not mean soil temperature. These effects differed between species and between ecosystems, being strongest in Sphagnum fuscum-dominated dry tundra, but were well explained by bryophyte mat thickness and field moisture content as affecting thermal conductivity and volumetric heat capacity. We suggest that reduction in soil temperature amplitudes is a generic feature in (sub) arctic ecosystems and should be considered as an important mechanism of bryophyte control on carbon and nutrient turnover. Although heat transfer through bryophyte mats differs greatly among species and ecosystems, species differences are fully explained by differences in mat thickness and moisture content and generally comply with physical laws, without deviations due to biological processes. These results imply that in global vegetation models of carbon and nutrient cycling, the heat transfer through bryophyte mats can be modelled without taking into consideration bryophyte species composition, but considering bryophyte mat depth and moisture availability only. This will allow us to enhance modelling precision through an improved representation of the soil temperature regime. © 2013 British Ecological Society.