Sensitivity of ecosystem-protected permafrost under changing boreal forest structures

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.

Novel coupled permafrost-forest model revealing the interplay between permafrost, vegetation, and climate across eastern Siberia

“…As fires become more intense and frequent under global warming, spruce or other species may become dominant rather than shade-intolerant larch species. In the currently naturally deciduous, larch covered areas, evergreen taxa may invade and change the heat fluxes and energy balance with the threat of entering a positive feedback loop such as a deepening of the ALT (Bonan, 2008, Stocker et al, 2013, Stuenzi et al, 2021b.…”

“…Here, we aim at creating a model system that can accurately assess detailed thermal and hydrological fluxes between permafrost and forest cover as recently developed by Stuenzi et al (2021aStuenzi et al ( , 2021b. It will include a dynamic vegetation model, which has a full life-cycle to allow intraspecies and interspecies interactions at all stages (seed-seedling-mature tree) leading to nonlinear behaviour of population dynamics as well as resolving a 3-dimensional landscape that is available for Siberian treeline areas developed by Kruse et al (2016Kruse et al ( , 2019aKruse et al ( , 2019b.…”

“…CryoGrid model has recently been extended by a multilayer canopy module developed byBonan et al (2014) for use in boreal permafrost regions (seeStuenzi et al 2021a and2021b for model details). The multilayer canopy model provides a comprehensive parameterization of fluxes from the ground, through the canopy layer up to a roughness sublayer.…”

“…In combination with CryoGrid the canopy model replaces the standard surface energy balance scheme while soil state variables are passed back to the forest module. FollowingStuenzi et al (2021b), a realistic canopy structure is simulated by allowing fractional compositions of deciduous and evergreen taxa within a simulated forest stand.…”

“…This entire model setup has previously been extensively validated for different study sites throughout our study region, including Nyurba (63.08°N, 117.99°E), Spasskaya (62.14°N, 129.37°E), and Ilirney (67.40°N, 168.37°E)(Stuenzi et al 2021a and2021b). Validation exercises were carried out based on measured and modelled ground surface temperature (GST), activelayer thickness (ALT), soil moisture, bowen ratio, and short-and longwave radiation below and above the canopy.…”