Open access publications in the SLU publication database

Abstract

Forest fires are one of the main disturbance agents in boreal and temperate ecosystems. To deci- pher 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 region- ally increased forest fire activity and located the geographical centres of climatic anomalies associ- ated 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 med- iated 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 aver- age 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 Scan- dinavia 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 geographi- cal 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 south- ern boreal forests. This would imply that fire regimes across Scandinavia may show an asynchro- nous response to future climate changes.