Boreal forests' capacity to protect permafrost in a changing climate

The boreal forest biome contains one-third of the global forest area and exerts a strong control on numerous climate feedback mechanisms (Gauthier et al. 2015; Bonan et al., 2018). Boreal forests cover more than a third of the global continuous and discontinuous permafrost zones (own estimate based on Obu et al. 2019 and ESA 2017, see Fig. 1). Permafrost conditions are highly dependent on the insulating properties of the vegetation layer and vice-versa the thermal and hydrological conditions of the ground determine the biome and thereby the vegetation cover (Chang et al. 2015; Helbig et al. 2016; Stuenzi et al. 2021). The expansion of global forest cover has been classified as a potential negative feedback to climate change, as it would increase carbon sequestration and carbon storage in biomass (Bastin et al. 2019). Concurrently, numerous studies have stated that boreal forests with lower densities, after forest loss, or after changes in species composition would also lead to a negative climate feedback due to the related albedo increase in the long snow-covered periods (Snyder et al. 2004; Bala et al. 2007; Bonan et al. 2008; Rogers et al. 2015). Furthermore, the thermo-hydrological interaction between boreal forest and permafrost introduces additional, highly non-linear feedback mechanisms that have so far received little attention. Boreal forests act as a dynamic and reactive insulating buffer between the atmosphere and the permafrost. Canopy removal leads to an increase in the ground surface temperatures and active layer thicknesses (Fedorov et al. 2019; Stuenzi et al. 2021b), and a loss of the forest cover could trigger the release of carbon currently frozen and stored in the ground. The capacity and direction of this feedback are still unclear, especially regarding canopy structures, regional differences, and climatic changes. I aim to constrain this feedback by studying the thermal and hydrological processes that determine (i) the evolution of boreal permafrost and (ii) the feedback between forests and permafrost under continued climate warming. In the proposed research, I will quantify the capacity of boreal forests to protect permafrost in light of changing temperatures and precipitation by combining the physically-based permafrost-forest model CryoGrid-Vegetation, available observational data from Canada and Siberia, and remotely sensed time-series of boreal forests. I hypothesize that (1) continuing climatic changes and longer growing seasons will not only lead to denser forests with potentially higher carbon sequestration but also higher permafrost insulation. (2) The insulating canopy properties will differ regionally due to different dominant plant functional types and succession patterns, and (3) the insulation capacity of the canopy will become insufficient at a certain degree of warming, which will lead to active layer deepening and the initiation of thermokarst processes accompanied by local boreal forest loss.