Dominant bryophyte control over high‐latitude soil temperature fluctuations predicted by heat transfer traits, field moisture regime and laws of thermal insulation

Summary1. Bryophytes cover large territories in cold biomes, where they control soil temperatureregime, and therefore permafrost, carbon and nutrient dynamics. The mechanisms of thiscontrol remain unclear.2. We quantified the dependence of soil temperature fluctuations under bryophyte mats on theinterplay of bryophyte heat conductance traits, mat thickness, density and moisture regimes.3. For seventeen predominant bryophytes in six typical subarctic ecosystems, we assessed insitu soil temperature dynamics under bryophyte mats in comparison with bryophyte-removalpatches and per-species mat field moisture. In a complimentary laboratory investigation, westudied how per-species bryophyte thermal conductivity and volumetric heat capacity dependon mat density and moisture content. Subsequently, we tested whether heat transfer throughbryophyte mats could be modelled as a function of mat thickness, thermal conductivity andvolumetric heat capacity, the latter two being determined by mat density and field moisturecontent.4. Laboratory assessment revealed that bryophyte thermal conductivity and volumetric heatcapacity were independent of mat density, and depended linearly on mat moisture content, butthe dependencies were not species-specific. In the field, bryophytes reduced amplitudes of soiltemperature fluctuations and freeze–thaw frequency during the growing season, but not meansoil temperature. These effects differed between species and between ecosystems, being strong-est in Sphagnum fuscum-dominated dry tundra, but were well explained by bryophyte matthickness and field moisture content as affecting thermal conductivity and volumetric heatcapacity.5. We suggest that reduction in soil temperature amplitudes is a generic feature in (sub) arcticecosystems and should be considered as an important mechanism of bryophyte control oncarbon and nutrient turnover. Although heat transfer through bryophyte mats differs greatlyamong species and ecosystems, species differences are fully explained by differences in matthickness and moisture content and generally comply with physical laws, without deviationsdue to biological processes. These results imply that in global vegetation models of carbon andnutrient cycling, the heat transfer through bryophyte mats can be modelled without taking intoconsideration bryophyte species composition, but considering bryophyte mat depth and mois-ture availability only. This will allow us to enhance modelling precision through an improvedrepresentation of the soil temperature regime.Key-words: freeze–thaw regime, moss, plant–soil (below-ground) interactions, soil tempera-ture amplitude, thermal conductivity, thermal diffusivity, thermal insulation, subarctic, tundra,volumetric heat capacity