The current natural ecological disturbance baseline of fire activity in the southern European Alps – climate or humans?

Fire is a crucial disturbance agent in many ecosystems, such as in the Southern European Alps, where a considerable increase in fire activity is projected under anthropogenic climate change. However, currently it remains unclear whether natural or human factors mainly control the regional fire regime, since a current natural ecological disturbance baseline of regional fire activity is still missing. Therefore, we analyzed polycyclic aromatic hydrocarbons (PAHs) as a novel paleofire proxy on a well-dated ∼8 ka sediment record from Lake Pusiano in northern Italy. Fire activity was low during the Mid Holocene (ΣPAH of <1.8 ng g −1 yr −1 ) and increased since the Bronze Age (∼4 cal ka BP; average ΣPAH of 4.1 ng g −1 yr −1 ), being maximal during the Roman period (∼1.7 cal ka BP; ΣPAH of 10.8 ng g −1 yr −1 ). This agrees with multiple regional charcoal-based paleofire records, making the reconstruction of the regional paleofire history robust. The observed paleofire pattern contrasts with past climate changes and is instead highly consistent with human activities, indicating a human-driven fire regime. Therefore, without human influence, there should be little to no notable regional fire activity in the Southern Alpine ecosystems, which thus constitutes the current regional natural ecological disturbance baseline. During recent climate change, however, observed droughts increasingly exceed Holocene levels. Therefore, the naturally not fire-prone southern Alpine ecosystems may already be shifting towards a drought-driven fire regime. These findings offer an original perspective that may reshape the view on fire risk and management also for other sub-Mediterranean regions.