Abrupt regime shifts in post-fire resilience of Mediterranean mountain pinewoods are fuelled by land use

Abstract

Post-fire forest resilience must be quantified in a long-term perspective considering changes in land-use related to fire dynamics. Historical land-use changes leading to increased wildfire severity may produce no analogue regime shifts including a loss in post-fire growth recovery. Here we reconstruct the historical fire dynamics by combining paleoecological proxies, historical fire records and tree-ring width data of relict Pinus nigra subsp. salzmannii forests in the Sierra de Gredos (central Spain). A high incidence of historical fires was recorded in the 1890s, coinciding with a peak in charcoal accumulation rates and a sharp decrease in pollen of P. nigra/Pinus sylvestris with a rapid increase of pollen of more flammable Pinus pinaster and shrubs. The shift observed in pollen assemblages, coupled with a peak in charcoal influx, support the occurrence of high-severity fires during the 1890s, when abrupt growth suppressions were observed. Trees took 2 years to recover to their pre-fire growth rates. Lasting growth-recovery periods or no growth suppression were observed in the 1920s and 1980s, when fire frequency was also high but the study sites were fragmented or protected. We documented an abrupt regime shift in the fire record during the 1890s affecting pine forests, which rapidly recovered pre-fire growth rates.