Abstract
Boreal forests efficiently insulate underlying permafrost. The magnitude of this insulation effect is dependent on forest density and composition. A change therein modifies the energy and water fluxes within and below the canopy. The direct influence of climatic change on forests and the indirect effect through a change in permafrost dynamics lead to extensive ecosystem shifts such as a change in composition or density, which will, in turn, affect permafrost persistence. We derive future scenarios of forest density and plant functional type composition by analyzing future projections provided by the dynamic global vegetation model (LPJ-GUESS) under global warming scenarios. We apply a detailed permafrost-multilayer canopy model to study the spatial impact-variability of simulated future scenarios of forest densities and compositions for study sites throughout eastern Siberia. Our results show that a change in forest density has a clear effect on the ground surface temperatures (GST) and the maximum active layer thickness (ALT) at all sites, but the direction depends on local climate conditions. At two sites, higher forest density leads to a significant decrease in GSTs in the snow-free period, while leading to an increase at the warmest site. Complete forest loss leads to a deepening of the ALT up to 0.33 m and higher GSTs of over 8 ∘C independently of local climatic conditions. Forest loss can induce both, active layer wetting up to four times or drying by 50%, depending on precipitation and soil type. Deciduous-dominated canopies reveal lower GSTs compared to evergreen stands, which will play an important factor in the spreading of evergreen taxa and permafrost persistence under warming conditions. Our study highlights that changing density and composition will significantly modify the thermal and hydrological state of the underlying permafrost. The induced soil changes will likely affect key forest functions such as the carbon pools and related feedback mechanisms such as swamping, droughts, fires, or forest loss.
Highlights
The boreal forest cover exerts a strong control on numerous climate feedback mechanisms (Bonan et al 2018, Zhang et al 2018)
Our results show that a change in forest density has a clear effect on the ground surface temperatures (GST) and the maximum active layer thickness (ALT) at all sites, but the direction depends on local climate conditions
We focus on the direct physical impact of forest change on the detailed thermal and hydrological conditions of permafrost ground underneath, rather than investigating the exact timing of these ecosystem changes because the simulations themselves are decoupled from projected climate forcing data
Summary
The boreal forest cover exerts a strong control on numerous climate feedback mechanisms (Bonan et al 2018, Zhang et al 2018). 80% of boreal forests are underlain by permafrost (Helbig et al 2016). The forest cover is considered to efficiently insulate the underlying, ecosystem-protected permafrost (Chang et al 2015) and play an important role in the development of boreal regions and the stability of permafrost in a warming climate. Boreal regions are projected to warm between 4 ◦C and 11 ◦C by 2100, with a modest precipitation increase (Scheffer et al 2012, Meredith et al 2019). The change in air temperature and precipitation directly influences the vegetation cover development (Esper et al 2010, Kharuk et al 2015, Sato et al 2016, Ito et al 2020) and permafrost thaw (Meredith et al 2019), directly affecting soil water availability and root space limitation (Carpino et al 2018). The changing thermohydrological soil conditions may provoke changes in forest density and forest composition (Takahashi 2006, Kharuk et al 2013, Liu et al 2017, Kropp et al 2021) leading to extensive ecosystem shifts (Pearson et al 2013, Gauthier et al 2015, Boike et al 2016, Kruse et al 2016)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
