Causes of co-existence of cool-temperate Fagus and warm-loving evergreen Quercus forests in central Italy during the Holocene thermal maximum

Mediterranean forest communities are particularly diverse but at risk due to their sensitivity to global warming. Understanding the long-term vulnerability of Mediterranean vegetation to climate change is crucial for conservation and management purposes. Studies on past changes of forest communities in response to climate change at ecologically meaningful resolutions (i.e., decadal time scales) are therefore essential, but still very rare. The Holocene thermal maximum (HTM; ca. 10,000–5,000 ​cal ​years before the present (BP)) may be used to study species and community responses to warmer conditions than during recent decades. We performed high-resolution multiproxy palaeoecological analyses on sediments from crater Lake Mezzano in central Italy to reconstruct vegetation, diversity, and fire dynamics between 8,450 and 7,050 ​cal ​years BP. Ordination, cross-correlation, and species-response analyses were used to investigate the response of Mediterranean forest communities to HTM climate warming, human impact, and fire. Vegetational changes prior to 7,450 ​cal ​years BP were driven by climate. Fagus sylvatica spread into mixed deciduous oak forests during the Early Holocene in response to declining seasonality (cooler summers and warmer winters). Subsequently, Fagus sylvatica declined and evergreen Quercus ilex expanded after 8,200 ​cal ​years BP when the climate became warmer. Although reduced, Fagus sylvatica remained important together with deciduous oaks. The co-existence of Fagus sylvatica and evergreen Quercus forests is extremely rare today. Human impact significantly affected forest vegetation after 7,450 ​cal ​years BP, when Neolithic agricultural activities became important, ultimately extirpating these special communities but fostering the overall biodiversity. However, their past occurrence in several central Italian calderas during the HTM suggests that these environments provided habitats that permitted the thriving of cool-temperate forests of Fagus sylvatica under mesomediterranean conditions, with summers ca. 1–2 ​°C warmer than today. Cool and moist calderas may thus become increasingly important for maintaining Mediterranean mesophilous forest species under global warming conditions.