Tree ring-based reconstruction of the long-term influence of wildfires on permafrost active layer dynamics in Central Siberia

Abstract

Although it has been recognized that rising temperatures and shifts in the hydrological cycle affect the depth of the seasonally thawing upper permafrost stratum, it remains unclear to what extent the frequency and intensity of wildfires, and subsequent changes in vegetation cover, influence the soil active layer on different spatiotemporal scales. Here, we use ring width measurements of the subterranean stem part of 15 larch trees from a Sphagnum bog site in Central Siberia to reconstruct long-term changes in the thickness of the active layer since the last wildfire occurred in 1899. Our approach reveals a three-step feedback loop between aboveand belowground ecosystem components. After all vegetation is burned, direct atmospheric heat penetration over the first ~20 years caused thawing of the upper permafrost stratum. The slow recovery of the insulating ground vegetation reverses the process and initiates a gradual decrease of the active layer depth. Due to the continuous spreading and thickening of the peat layer during the last decades, the upper permafrost horizon has increased by 0.52 cm/year. This study demonstrates the strength of annually resolved and absolutely dated tree-ring series to reconstruct the effects of historical wildfires on the functioning and productivity of boreal forest ecosystems at multi-decadal to centennial timescale. In so doing, we show how complex interactions of above-and belowground components translate into successive changes in the active permafrost stratum. Our results are particularly relevant for improving long-term estimates of the global carbon cycle that strongly depends on the source and sink behavior of the boreal forest zone.

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

“…An abrupt decline of δ 13 C in the early 1970s is probably associated with a tipping point in the adaptation of larch trees to the soil conditions that become similar to those in the prior-wildfire period. This fast change could, for instance, be related to the die-off of parts of the roots due to shallower ASL (Knorre et al 2019). Increased iWUE and reduced wood accumulation (BAI) (figure 4) demonstrate a decline in photosynthetic rate as a result of adaptation to cooler soil conditions.…”

“…Dendroecology, however, can provide insights of unique temporal resolution, because tree rings may allow fire histories to be reconstructed (McBride 1983, Stivrins et al 2019. A recent example of the successful utilization of tree rings in wildfire dendroecology is the precise dating of moss buried stems to quantify postwildfire dynamics of the ASL thickness and ground vegetation recovery in northern Siberia (Knorre et al 2019). Moreover, innovate dendrochronological approaches have combined annually-resolved and absolutely-dated ring width measurements with stable isotopic ratios to provide eco-physiological insights into tree-fire interactions (Beghin et al 2012, Battipaglia et al 2014.…”

Dynamics of Post-Fire Effects in Larch Forests of Central Siberia Based on Satellite Data

“…Such changes are caused by a decreasing of albedo and heat-insulating properties of the litter and ground cover at the post-fire forest plots [9; 10; 11]. Anomalies of the thermal balance of the underlying surface can be critical for ecosystems under the conditions of the permafrost of Central Siberia [12], as an ecosystem under long-term disorders are characterized by low stability. The ecological importance of Northern forests prevails over their raw material potential [9], and the impact of wildfires can lead to significant changes in environmental conditions.…”

Intraseasonal Dynamics of River Discharge and Burned Forest Areas in Siberia

“…In some papers, inter-annual fluctuations and trends in river discharges in Siberia are mainly associated with climatic processes [2,[4][5][6]. At the same time, wildfire impact is the main factor, which strongly affected the state of Siberian boreal ecosystems as a whole [7][8][9]. The increased water yields or total runoff primarily resulting from the reduced evapotranspiration have been a reported effect on post-wildfire hydrology [10].…”

“…The seasonally thawing soil layer deepens quickly over the first postfire years, and slowly recovers following re-vegetation. Permafrost active layer dynamics as well soil condition changes after wildfires were observed in the region [9,13,14]. Long-term temperature anomalies were evaluated postfire using remote sensing data [12].…”