Fire frequency and intensity associated with functional traits of dominant forest type in the Balkans during the Holocene

Disturbances by fire are among the most important processes that shape forest dynamics and diversity. However, the long-term variability of fire disturbance regimes in many European forests and specifically in the mountains of the Balkan Peninsula is not well understood. Here, we present the first Holocene record of fire regimes based on macrocharcoal morphologies in combination with pollen-based reconstruction of forest dynamics and fire-related strategies of prevailing mountain forests in the Rila Mountains, Bulgaria. While biomass burning followed the main trends in climate, the frequency and intensity of fire were strongly linked to fire-related coping strategies of dominant tree taxa (resisters, avoiders or invaders). Frequent fires of low intensity between 12,000 and 9000 cal year BP were concurrent with the dominance of invaders (Betula, herbs, ferns). Intermittent occurrence of low- and high-intensity surface and crown fires with longer return intervals between 9000 and 4000 cal year BP was associated with codominance of resister (Pinus sylvestris, Pinus peuce, Pinus mugo) and avoider (Abies alba and Picea abies) forest types, whereas a lengthening of the fire return interval over the past 4000 years was linked to increased abundance of P. abies. As a rising number of fire episodes may drive land cover towards more fire-adapted plant communities and towards less intense fire events, we expect increased dominance of invaders (resprouters that rapidly reach maturity stage) as well as resisters (properties protecting from fire damage) under future warmer and drier climate. This study also shows the potential of combining charcoal morphologies with pollen records to track variability in fire intensity and plant functional attributes over long timescales that are also relevant to forest management stakeholders.