Exploration of Thermal Bridging Through Shrub Branches in Alpine Snow

In the high Arctic, thermal bridging through frozen shrub branches has been demonstrated to cool the ground by up to 4 degrees C during cold spells, affecting snow metamorphism and soil carbon and nutrients. In alpine conditions, the thermal conductivity contrast between shrub branches and snow is much less than in the Arctic, so that the importance of thermal bridging is uncertain. We explore this effect by monitoring ground temperature and liquid water content under green alders and under nearby alpine tundra in the Alps. During a January 2022 cold spell, the ground temperature at 5 cm depth under alders is 1.3 degrees C colder than under alpine tundra. Ground water freezing under alders is complete, while water remains liquid under tundra. Finite element simulations reproduce the observed temperature difference between both sites, showing that thermal bridging does affect ground temperature also under Alpine conditions. With climate warming, shrubs are expanding in seasonally snow-covered Arctic and alpine areas. In the Arctic, it has been shown that shrubs buried in snow act as thermal bridges between the ground and the atmosphere, so that the ground winter temperature is colder than if thermal effects due to snow only are considered. Thermal bridging is efficient in the Arctic because the thermal conductivity of shrub branches is 30-70 times as large as that of snow. In alpine areas, the difference in thermal conductivity between the shrub branches and the snow is much lower, so that the efficiency of thermal bridging by shrubs is uncertain, even though shrub branches are much thicker. We monitored the soil temperature at an Alpine site at a spot with green alders and a nearby spot with herbaceous vegetation. The ground was 1.3 degrees C colder under alders during a cold spell. Using finite element simulations, we show that this is explained by thermal bridging through alder branches. Snow and land surface models must therefore include this process for adequate simulations of the soil temperature, soil freezing and carbon recycling, snow metamorphism and snow physical properties. The winter ground temperature in a snow-covered alpine area during a cold spell is 1.3 degrees C colder under alders than under nearby grassUsing finite element modeling, we demonstrate that this is due to thermal bridging through alder branchesPlane-parallel snow layer geometry cannot be used to model ground temperature, soil freezing, and snow metamorphism if shrubs are present