Abstract
Abstract. Boreal forests in permafrost regions make up around one-third of the global forest cover and are an essential component of regional and global climate patterns. Further, climatic change can trigger extensive ecosystem shifts such as the partial disappearance of near-surface permafrost or changes to the vegetation structure and composition. Therefore, our aim is to understand how the interactions between the vegetation, permafrost and the atmosphere stabilize the forests and the underlying permafrost. Existing model setups are often static or are not able to capture important processes such as the vertical structure or the leaf physiological properties. There is a need for a physically based model with a robust radiative transfer scheme through the canopy. A one-dimensional land surface model (CryoGrid) is adapted for the application in vegetated areas by coupling a multilayer canopy model (CLM-ml v0; Community Land Model) and is used to reproduce the energy transfer and thermal regime at a study site (63.18946∘ N, 118.19596∘ E) in mixed boreal forest in eastern Siberia. An extensive comparison between measured and modeled energy balance variables reveals a satisfactory model performance justifying its application to investigate the thermal regime; surface energy balance; and the vertical exchange of radiation, heat and water in this complex ecosystem. We find that the forests exert a strong control on the thermal state of permafrost through changing the radiation balance and snow cover phenology. The forest cover alters the surface energy balance by inhibiting over 90 % of the solar radiation and suppressing turbulent heat fluxes. Additionally, our simulations reveal a surplus in longwave radiation trapped below the canopy, similar to a greenhouse, which leads to a magnitude in storage heat flux comparable to that simulated at the grassland site. Further, the end of season snow cover is 3 times greater at the forest site, and the onset of the snow-melting processes are delayed.
Highlights
Around 80 % of the world’s boreal forest occurs in the circumpolar permafrost zone (Helbig et al, 2016)
Our simulations reveal a surplus in longwave radiation trapped below the canopy, similar to a greenhouse, which leads to a magnitude in storage heat flux comparable to that simulated at the grassland site
At our primary study site, the model is validated against ground surface temperature (GST) measurements of forested and non-forested study sites
Summary
Around 80 % of the world’s boreal forest occurs in the circumpolar permafrost zone (Helbig et al, 2016). Stuenzi et al.: Variability of the surface energy balance in permafrost-underlain boreal forest atures increased by 0.39 (±0.15) ◦C (Biskaborn et al, 2019; IPCC, 2019). Due to its sheer size, the biome is sensitive to climatic changes and exerts a strong control on numerous climate feedback mechanisms through the altering of land surface reflectivity, the emission of biogenic volatile organic compounds and greenhouse gases, and the transfer of water to the atmosphere (Bonan et al, 2018; Zhang et al, 2011). The canopy exerts shading by reflecting and absorbing most of the downward solar radiation, changes the surface albedo, and decreases the soil moisture by intercepting precipitation and increasing evapotranspiration (Vitt et al, 2000). The forest promotes the accumulation of an organic surface layer which further insulates the soil from the atmosphere (Bonan and Shugart, 1989). Increased soil carbon release from thawing permafrost through the delivery of soil organic matter to the active carbon cycle (Schneider Von Deimling et al, 2012; Romanovsky et al, 2017) is modified by vegetation changes, which can compensate for carbon losses due to an increased CO2 uptake (as observed at ice-rich permafrost sites in northwestern Canada and Alaska; Estop-Aragonés et al, 2018) or even further accelerate total carbon loss
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