The historical demise of Pinus nigra forests in the Northern Iberian Plateau (south-western Europe)

1.Pinus nigra Arn. forests dominated over extensive areas of the Northern Iberian Plateau (Spain) during the Holocene, but a strong decline during the historical period (c. 1300-700 cal. BP) led to the present fragmented populations. This demise has been generally attributed to land-use changes or climate, but the specific roles of disturbance regimes such as fire variability and grazing on the long-term are not fully understood yet. 2.We combine multi-proxy palaeoecological data (fossil pollen, spores, conifer stomata, microscopic and macroscopic charcoal) together with quantitative analyses (ordination and peak detection) from a high-resolution sedimentary sequence (Tubilla del Lago, 900 m a.s.l.) to assess the causes of pine forests demise. A new microscopic charcoal record from an additional sequence (Espinosa de Cerrato, 885 m a.s.l.) is used to assess burning and pine decline at a more regional (100 km radius) scale. 3.Pinus nigra forests could cope with drought and fire regime variability (FRI = 110-500 years), with forest recovery taking c. 100-200 years after fires. Only at 1300-1200 cal. BP a long-lasting irrecoverable demise of P. nigra forests occurred when human-induced fires together with arable and pastoral farming became widespread in the area. Subsequently, Quercus woodlands expanded in the remnant patchy pinewoods. This vegetation shift was primarily caused by three particularly important fire episodes in less than a century (c. 1300-1200 cal. BP). 4.Synthesis. Pinus nigra forests have shown a millennial resilience to the natural fire regime of the Northern Iberian Plateau, that was characterized by relatively frequent small-moderate fires and rare high-intensity fires. However, frequent human-caused crown fires and the onset of intensive farming caused their demise over an extensive area. Ongoing land-use abandonment in the Iberian mountains could promote the occurrence of high-intensity, severe fires due to the rapid build-up of high fuel loads. Forest management could mimic the natural fire regime by periodically reducing fuel loads for a transitional period until natural disturbance variability is fully restored, thus preserving these relict native plant communities