Multi‐century reconstruction of fire activity in Northern European boreal forest suggests differences in regional fire regimes and their sensitivity to climate

Abstract

Summary 1.Forest fires are one of the main disturbance agents in boreal and temperate ecosystems. To decipher large-scale temporal and spatial patterns of past fire activity in Scandinavia, we analysed the synchronicity of dendrochoronologically reconstructed fire events in a large network of sites (n = 62; 3296 samples, 392 individual fire years) covering a wide geographical gradient (56.5-67. 0°N and 9.3-20.5°E) over AD 1400-1900. We identified large fire years (LFY) as years with regionally increased forest fire activity and located the geographical centres of climatic anomalies associated with synchronous LFY occurrence across the region, termed LFY centroids. 2. The spatial pattern of LFY centroids indicated the presence of two regions with climatically mediated synchronicity of fire occurrence, located south and north from 60°N. The return intervals of LFYs in Scandinavia followed a Weibull distribution in both regions. Intervals, however, differed: a period of 40 years would carry a 0.93 probability of LFY occurrence in the southern region, but only a 0.48 probability of LFY occurrence in the northern region. 3. Over 1420-1759, the northern region was characterized by significantly higher temporal variability in LFY occurrence than the southern region. Temporal correlation of LFYs with reconstructed average summer temperature and total precipitation was evident mainly for the northern region. LFYs in this region were associated with positive temperature and negative precipitation anomalies over Scandinavia and with colder and wetter conditions in more southern parts of the European subcontinent. 4. Synthesis. Historical patterns of the occurrence of large fire years (LFY) in Scandinavia point towards the presence of two well-defined zones with characteristic fire activity, with the geographical division at approximately 60°N. The northern and mid-boreal forests, although exhibiting lower LFY frequencies, appeared to be more sensitive to past summer climate, as compared to the southern boreal forests. This would imply that fire regimes across Scandinavia may show an asynchronous response to future climate changes.

Multi-variable bias correction: application of forest fire risk in present and future climate in Sweden

“…The results in Drobyshev et al (2014) pointed towards the presence of two well-defined zones with characteristic fire activity, geographically divided at approximately 60 • N. Such division was also reflected in Dai et al (2012) who applied the self-calibrated Palmer drought severity index to study the global aridity in present and future climate. The calculated indices indicated drier conditions in southern Sweden than in the northern part under the present climate.…”

“…extremely low relative humidity, strong wind speed and extreme high temperature), are also one of the causes that make fuels conductive to ignition and spread (Fendell and Wolff, 2001;Ryan, 2002). Dendrochronological fire studies have indicated a large temporal and spatial variability in fire activity in Sweden during the last 500 years (Niklasson and Granström, 2000;Drobyshev et al, 2014). A recent study by Drobyshev et al (2014) reveals that a geographical division between one northern and one southern region with different characteristic fire activity could be found around 60 • N.…”

Growth dynamics of tree-line and lake-shore Scots pine (Pinus sylvestris L.) in the central Scandinavian Mountains during the Medieval Climate Anomaly and the early Little Ice Age

“…1300 and 1450 (Young et al, 2012;Rosqvist et al, 2013), while moisture sensitive tree-ring data from Finland indicate that this was quite a dry period (Helama et al, 2009a). Moreover, glacial expansion in Norway (Dahl and Nesje, 1994), an extended period of positive NAO (Trouet et al, 2009), increasing lake levels in Finland (Nevalainen et al, 2013), as well as high return intervals of forest fires in Sweden (Drobyshev et al, 2014) all indicate an increase in precipitation during this particular period. The effect of this change in climate was most evident in the lakeshore material.…”

Tree diversity patterns along the latitudinal gradient in the northwestern Russia

“…3c). Moreover, the lower density of fires in the north taiga, compared to the middle taiga, is due to shorter fire danger season and increased proportion of swampy areas (Drobyshev et al 2014).…”