Long-term ecological consequences of forest fires in the continuous permafrost zone of Siberia

Abstract

Wildfires are an important factor in controlling forest ecosystem dynamics across the circumpolar boreal zone. An improved understanding of their direct and indirect, short-to long-term impacts on vegetation cover and permafrost-vegetation coupling is particularly important to predict changes in carbon, nutrient and water cycles under projected climate warming. Here, we apply dendrochronological techniques on a multi-parameter dataset to reconstruct the effect of wildfires on tree growth and seasonal permafrost thaw depth in Central Siberia. Based on annually-resolved and absolutely dated information from 19 Gmelin larch (Larix gmelinii (Rupr.) Rupr.) trees and active soil layer thickness measurements, we find substantial stand-level die-off, as well as the removal of ground vegetation and the organic layer following a major wildfire in 1896. Reduced stem growth coincides with increased δ 13 C in the cellulose of the surviving trees during the first decade after the wildfire, when stomatal conductance was reduced. The next six to seven decades are characterized by increased permafrost active soil layer thickness. During this period of post-wildfire ecosystem recovery, enhanced tree growth together with positive δ 13 C and negative δ 18 O trends are indicative of higher rates of photosynthesis and improved water supply. Afterwards, a thinner active soil layer leads to reduced growth because tree physiological processes become limited by summer temperature and water availability. Revealing long-term effects of forest fires on active soil layer thickness, ground vegetation composition and tree growth, this study demonstrates the importance of complex vegetation-permafrost interactions that modify the trajectory of post-fire forest recovery across much of the circumpolar boreal zone. To further quantify the influence of boreal wildfires on large-scale carbon cycle dynamics, future work should consider a wide range of tree species from different habitats in the high-northern latitudes. OPEN ACCESS RECEIVED

Tree Vitality and Forest Health: Can Tree-Ring Stable Isotopes Be Used as Indicators?

“…Other studies demonstrated physiological consequences of fire deficits in forests of the western United States [99] and in trees growing on permafrost in boreal ecosystems [100]. All of these studies demonstrated that the responses of the trees, and thus of the stable isotope signature, depend on fire severity.…”

Recent atmospheric drying in Siberia is not unprecedented over the last 1,500 years

“…Along with temperature increases, increasing VPD 14 has been shown to result in low stomatal conductance 3 , thereby responding to water shortage in trees—even in regions affected by permafrost 3 . While water and nutrient supply for plants in the Siberian North predominantly depends on freeze–thaw processes in the active soil layer 16 , 17 , wildfire-induced changes can also significantly affect the active soil layer depth and seasonal dynamics with long-term consequences for carbon, nutrient and water balance of the ecosystem 8 , 18 , 19 .…”

Ecological and conceptual consequences of Arctic pollution

“…These have been shown to curtail surface irradiance and to cause light diffusion (Stine and Huybers, 2014), with the strongest impact over central and eastern Siberia, as well as parts of Alaska and Canada (Briffa et al ., 1998). In addition to the antagonistic role of anthropogenic pollutants, the net primary productivity of the boreal biome is also vulnerable to other abiotic stressors (Beck et al ., 2011; Gauthier et al ., 2015; Girardin et al ., 2016; Charney et al ., 2016), such as the redistribution of nitrogen as a consequence of widespread wildfires that are expected to increase in frequency and intensity under future climate change (Ponomarev et al 2018; Shvetsov et al ., 2019; Knorre et al ., 2019; Kirdyanov et al ., 2020).…”