Birch-sedge communities, forest withdrawal and flooding at the beginning of Heinrich Stadial 3 at the southern Alpine foreland

The southern Alpine foreland, facing windward to moist southern airmasses, is claimed to have supported forest vegetation throughout the Last Glaciation. Here we present a multiproxy paleoecological record from a compressed peat, uncovered at Casaletto Ceredano, N-Italy, spanning the interval from 33 to 30.5 kyr cal BP. Stratigraphically, it underlies a fluvioglacial belt attributed to the Last Glacial Maximum. The peat records a floodplain swamp community with tree birch and tall sedges, pine woodlands in upland areas, and only limited patches of open vegetation. Plant macrofossils – bark, charcoal, wood and fruits – establish the predominant role of Betula pubescens group (downy birch) in the anoxic wetland, thanks to its ability to enhance gas exchange through a distinctive type of bark lenticels. A fire-induced birch-to-pine cycle is repeated twice along the 2500 years-time span covered by the peat layer. The climate reconstructed from modern pollen analogs compares to northern boreal zone, with Tjuly < 15 °C, excluding warm-temperate trees. A major flood sealing the swamp with minerogenic silt is precisely dated to 30,497 ± 594 yr cal BP (2σ uncertainty). Here, the pollen record shows a substantial forest withdrawal, and development of grasslands and semideserts, pointing to co-factorial action of increased climate continentality and of river dynamics. According to teleconnections with the Atlantic and Arctic framework of the stadial–interstadial climate variability, the age and pattern of this event are consistent with the onset of Heinrich Stadial 3, causing a lockdown of moist westerlies and of their Rossby waves in the W-Mediterranean.