Abstract
The Sala fire in the Västmanland County of central Sweden that burned about 14,000 ha in 2014 has been the largest fire recorded in the modern history of Sweden. To understand the long-term fire history of this area, we dendrochronologically dated fire scars on Scots pine (Pinus sylvestris L.) trees (live and deadwood) to reconstruct the fire cycle and fire occurrence in the area affected by the 2014 fire. We identified 64 fire years, using a total of 378 pine samples. The earliest reconstructed fire dated back to 1113 AD. The spatial reconstruction extended over the period of 1480–2018 AD. Lower levels of fire activity (fire cycle, FC = 43 years, with the central 90% of the distribution limited by 35 to 57 years) dominated in the earlier period (1480–1690 AD) that was followed by a strong decrease in fire activity since 1700 (FC = 403 years, with 90% of the distribution being within 149 to 7308 years), with a fire-free period between 1756 and 2014. Sala area, therefore, features the earliest known onset of fire suppression in Scandinavia. The high demand for timber during the peak in mining activities in the study area around the 1700–1800s, accompanied by passive fire suppression policies, were possibly the main drivers of the decline in fire activity. Superposed epoch analysis (SEA) did not show significant departures in the drought proxy during the ten years with the largest area burned between 1480 and1690. It is unclear whether the result is due to the relatively small area sampled or an indication that human controls of fires dominated during that period. However, significant departures during the following period with low fire activity (1700–1756), which just preceded the last fire-free period, suggested that the climate became an increasingly important driver of fire during the onset of the suppression period. We speculate that the lack of major firebreaks, the homogenization of forests, and the lack of burned areas with low fuel loads might contribute to the occurrence of the exceptionally large 2014 fire in Sala.
Highlights
Forest fires are the main drivers of boreal ecosystem dynamics (Johnson 1992; Johnstone and Chapin 2006) and forest carbon emissions (Conard and Ivanova 1997; Amiro et al 2001)
We identified the seasonality in only 48 fire scars (~ 15% of the total number of dated scars) dated to 30 different fire years
The multi-century fire history reconstruction in Sala revealed a period with frequent fires between 1480
Summary
Forest fires are the main drivers of boreal ecosystem dynamics (Johnson 1992; Johnstone and Chapin 2006) and forest carbon emissions (Conard and Ivanova 1997; Amiro et al 2001). Forest fires are mostly driven by regional climate conditions with landscape properties, primarily vegetation and topography, affecting their frequency, spread, and severity (Hellberg et al 2004; Pinto et al 2020). Landscape-scale fire-scar-based reconstructions in Sweden and Norway have shown a pattern with few, large climate-driven fires dominating the fire regime up to the early 1600s and small but more frequent fires during most of the 1600s and 1700s (Niklasson and Granstrom 2000; Niklasson et al 2010). The fire cycle in the Northern European boreal forests prior to the 1700s varied between 50 and 300 years (Niklasson and Granstrom 2000; Drobyshev et al 2012; Ryzhkova et al 2020)
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