Trends and patterns in annually burned forest areas and fire weather across the European boreal zone in the 20th and early 21st centuries

Fire remains the main natural disturbance factor in the European boreal zone (EBZ), which exhibits strong gradients in climate conditions, modern and historical patterns of forest use, and the modern human infrastructure density. Understanding climatic forcing on fire activity is important for projecting effects of climate change on multiple ecosystem services in this region. Here we analyzed available records of annually burned areas (ABA) in 16 administrative regions of EBZ (countries or sub-country units) and fire weather variability to test for their spatio-temporal patterns over 1901-2017. To define sub-regions of EBZ with similar fire activity we compiled 30-60 year long ABA chronologies and clustered them in Euclidian space to identify regions of EBZ with temporally synchronous fire activity. We then reconstructed 100-year long ABA chronologies for each cluster, capitalizing on its member with the highest correlation between observational fire record and climatological fire weather proxy (MDC, monthly drought code). The 100-year chronologies helped identified large fire years (LFY), i.e. years with the ABA being above 10% of its long-term distribution. The climatic forcing of these events was tested in superposed epoch analysis operated with gridded 500 hPa pressure fields. Finally, we tested trends in (a) synchrony of LFY's across clusters, (b) MDC values over the EBZ, and (c) spatial variability in July MDC over the EBZ geographic domain during 1901-2017.

EBZ exhibits large variability in forest fire activity with the fire cycles varying from ~10⁴ (Scandinavia) to 3*10² years (Russian Republic of Komi). Clustering of administrative units in respect to their ABA suggested the presence of sub-regions with synchronous dynamics of ABA, located  along W-E and S-N gradients. LFYs in each of the cluster was associated with the development of the high pressure cell over the regions in question in July, indicating climatic forcing of LFYs. Contingency analysis indicated no long-term trend in the synchrony of LFYs observed simultaneously in several administrative units. We observed a trend towards higher values of MDC for the months of April and May in the western section of EBZ (April) and southern-eastern sections of the Baltic sea region and North sections of EBZ in Russia (May). Trends in MDC during the summer months were largely absent. We discuss teleconnections of fire activity in the EBZ with Atlantic SST.