Permafrost and shrubification: Friends or enemies? A look at the summer energy budget of shrub tundra

Polar regions are warming about four times faster than the global average, favoring vegetation changes such as shrub expansion across Arctic tundra. Shrubification modifies the depth of the active layer, the summer-thawed layer whose magnitude affects the release of ancient organic carbon through microbial activity. In winter, snow insulates the ground from the atmosphere, but shrubs modify snowpack thermal properties and facilitate heat transfer through their branches. In summer, the relationship between shrub and ground thermal dynamics remains debated, with many interpretations focusing on shading effects by shrubs, but detailed surface energy budget studies are rare. This limits our ability to improve land surface models and quantify vegetation-permafrost feedbacks.

Here we compare summer surface energy partitioning of shrub and moss tundra in Qarlikturvik Valley on Bylot Island (73°N), Canadian High-Arctic.

From July 2024 to August 2025, we instrumented a low-shrub-dominated site and an adjacent moss-dominated site. We continuously monitored turbulent, radiative, and ground heat fluxes. We also characterized soil thermal properties and vegetation cover, and monitored snowpack thermal properties to help separate winter legacy effects from summer processes. Preliminary data show that ground temperatures are warmer annually under shrubs, with larger differences in winter. In summer, shrubs impact energy partitioning by modifying latent and sensible heat exchanges. Above the surface under the canopy, incoming shortwave radiation is attenuated. This reduced energy input is compensated by the thinner moss layer under shrubs which provides less thermal insulation and facilitates soil warming. By late summer, the active layer beneath shrubs is nearly twice as deep as at the moss site. This is mostly attributed to the winter legacy and to the lower moss insulation at our site.

This local study with a detailed dataset will contribute to improving vegetation-snow-permafrost parameterisations in land surface models and hopefully to more reliable Arctic permafrost projections.