Past growth suppressions as proxies of fire incidence in relict Mediterranean black pine forests

Global warming and land use changes, contributing to landscape level fuel increments, could threaten Mediterranean pine forest resilience to wildfire disturbances. Reconstructions of historical fire regimes allow for the disentanglement of these two drivers by comparing the influence of climatic and anthropogenic variables on fire. Here we combine three sources of historical data: charcoal accumulation rates from a peat bog, detailed historical records of fire incidence and tree-ring width data from five relict black pine (Pinus nigra) forests with fire-scarred trees located in Sierra de Gredos (central Spain). We found growth suppression in 1893 and 1894 in all the sites which coincided with a peak of fire incidence in historical records and an increase in charcoal accumulation rates. The occurrence of these three synchronous events suggests increased wildfire incidence in the area which shaped the current stand structure of relict black pine forests. These late 19th century developments, we argue, can be mainly attributed to anthropogenic factors and contributing climatic drivers. We argue that the dissolution of the “Mesta”, the biggest transhumance livestock organization in Europe lasting from the 13th to the 19th centuries, led to more extensive grazing and uncontrolled use of forests and grasslands which likely contributed to increased wildfire incidence. Additionally, 1893 was characterized by anomalously warm spring temperatures which may have facilitated vegetation flammability. Our approach couples human and climate systems as drivers of historical fire incidence in Mediterranean pine forests.