Changes in the fire resilience of black pine in response to Mediterranean climate variability over the past 300 years

Elucidating the resilience of trees to wildfires remains a critical issue for forest conservation. The potential changes in tree resilience over time in response to the combined changes in fire regime and climate remain unknown. We hypothesised that tree resilience was influenced by mean fire-return intervals (MFI) and by their interactions with atmospheric conditions over the past 300 years, particularly during drought and hot years. This hypothesis was tested by analyzing tree-ring widths following past fires, using growth during fire-free periods as baseline, and atmospheric values (annual and seasonal temperature and precipitation, drought index and CO 2 concentration). The study focused on black pine ( Pinus nigra ) as a model species to investigate long-term ecological responses to fires in natural forests of Corsica, a western Mediterranean island. The study is based on 180 cross-dated black pines from four Corsican mountain sites. 54 fires were recorded between 1719 CE and 2012 CE. Four contrasted periods, defined according to MFI and atmospheric records are used to assess the black pine responses to fire events, MFI period, and climate. The fire resistance and resilience of trees have gradually declined since about 1850, independently of the MFI and increased atmospheric CO₂ levels. Post-fire radial growth recovery lengthened over time, indicating a decline in resistance. Concurrently, tree-ring responded more positively to precipitation and negatively to drought intensity. In isolation, fire does not compromise tree resilience. But the combined effects of drought and wildfires thus threaten black pines by shrinking their resilience mainly since 1971, which underscores the increasing vulnerability of this Mediterranean tree to fire in an increasingly dry climate.