A multi-century perspective of the Sala mega-fire: understanding risks for future large fires in Sweden

In the summer of 2014, the Sala municipality in central Sweden registered the largest fire in modern Swedish history. The circumstances under which the fire took place highlight the complex interactions between the ignition as well as the spread of fire, and human activities along with weather. Although the role of human activities and climate are of critical importance in shaping modern fire activity, their joint effects remain largely unstudied in Northern Europe. The main aim of this thesis was to investigate the impact of landscape properties (natural and human-related) and climate patterns on forest fires at different spatial and temporal scales. Dendrochronological study (Paper II) suggested that in the past the lack of major firebreaks, homogenization of forests due to its long-term management, and a long period without fires might have contributed to the occurrence of the exceptionally large 2014 mega-fire in Sala. A study of modern fire activity (Paper I) showed that a combination of human-related ignitions, weather conditions controlling fire spread, and vegetation composition are the main drivers of fire activity in Sweden. At the scale of the European boreal zone (Paper IV), forest fire activity remains strongly connected to the annual climate variability. Predictions of the future area burned in Sweden (Paper III) indicated that changes in climate would lead to an increase in area burned, with changes in vegetation leading either to further increase or mitigation of this increase to a certain degree.