Population dynamics of Arbutus in its Mediterranean-Atlantic range over the past 30 ka: a longitudinal trend

The genus Arbutus (Ericaceae) is present with four species in the Old World, ranging from the Canary Islands to Crimea and from Ireland to the Levant, being an emblematic element of the Mediterranean broadleaved evergreen biota. We investigated how its complex distribution has developed since the last glacial period, focusing on the location and extent of refuge areas, long-term persistence, postglacial dynamics in relation to dispersal capacity, responses to climate fluctuations and human pressure, and potential vulnerability. Understanding these issues is essential for effective conservation measures. We analysed the fossil record and compared it with the present-day distribution. A total of over 1100 fossil sites, including pollen and macrofossils, were collected from the literature and databases. Pollen grains of the genus Arbutus are distinct from other Ericaceae, although not identifiable at the species level. Range maps of past distribution were produced for the last 30 ka at 1000-year intervals. In addition, modern pollen data were gathered from both databases and literature, to link fossil pollen records to the modern distribution. The dynamics of the Mediterranean-Atlantic distribution of Arbutus reveal two clear spatiotemporal patterns. In the Northern and Western Iberian Peninsula, Arbutus is consistently represented throughout the last 30 ka, with pollen and macrofossils records even during the Last Glacial Maximum. In the Western and Central Mediterranean Basin, it progressively appeared at the beginning of the Holocene, showing up along the coasts of the Black Sea and in the Levant during the Middle to Late Holocene transition. The general feature that emerges from these progressive appearances is an original longitudinal pattern, west-to-east oriented. The population dynamics of Arbutus over the last 30 ka appears consistent with the biogeographical divides known for other plant species, namely the east-west Iberian divide, the Rhone delta, the Amphi-Adriatic divergence, and the Intra-Anatolian Suture Zone.