Interactions between climate and vegetation during the Lateglacial period as recorded by lake and mire sediment archives in Northern Italy and Southern Switzerland
Résumé
We reconstruct the vegetational history of the southern side of the Alps at 18,000–10,000 cal yr BP using previous and new AMS-dated stratigraphic records of pollen, stomata, and macrofossils. To address potential effects of climatic change on vegetation, we compare our results with independent paleoclimatic series (e.g. isotope and chironomid records from the Alps and the Alpine forelands). The period before 16,000 cal yr BP is documented only at the lowland sites. The previous studies used for comparison with our new Palughetto record, however, shows that Alpine deglaciation must have started before 18,000–17,500 cal yr BP south of the Alps and that deglaciated sites were colonized by open woods and shrublands (Juniperus, tree Betula, Larix, Pinus cembra) at ca 17,500 cal yr BP. The vegetational history of a new site (Palughetto, 1040 m a.s.l.) is consistent with that of previous investigations in the study region. Our results show three conspicuous vegetational shifts delimited by statistically significant pollen zones, at ca 14,800–14,400, 13,300–12,800 and 11,600–11,200 cal yr BP. At sites situated above 1000 m a.s.l. (e.g. Palughetto, Pian di Gembro) forests expanded in alpine environments at ca 14,500 cal yr BP (onset of Bølling period, GI-1 in the Greenland ice record). At the same time, rather closed treeline communities of the lowlands were replaced by dense stands of Pinus sylvestris and Betula. These early forests and shrublands consisted of Larix, P. cembra, Juniperus, P. sylvestris, Pinus mugo, and Betula, and had become established at ca 16,000 cal yr BP, probably in response to a temperature increase. If combined with other records from the Southern Alps, our data suggest that treeline ascended by ca 800–1000 m in a few centuries at most, probably as a consequence of climatic warming at the beginning of the Bølling period. At 13,100–12,800 cal yr BP the onset of a long-lasting decline of P. sylvestris was accompanied by the expansion of Quercus and other thermophilous tree taxa below ca 600 m a.s.l. This vegetational change was probably induced by a shift to warmer climatic conditions before the onset of the Younger Dryas, as indicated by independent paleoclimatic records. Only a few centuries later, at ca 12,700–12,500 cal yr BP, an expansion of herbaceous taxa occurred in the lowlands as well as at higher altitudes, documenting an opening of forested habitats. This change coincided with the beginning of the Younger Dryas cooling (GS-1), which according to the paleoclimatic series (e.g. oxygen isotope series), started at 12,700–12,600 cal yr BP and lasted for about 1000 years. Environments south of the Alps responded markedly to climatic warming at the onset of the Holocene (11,600–11,500 cal yr BP). Thermophilous trees that had declined during the Younger Dryas re-expanded very rapidly in the lowlands and reached the high altitude sites below ca 1500 m a.s.l. within a few centuries at most. Our study implies that the synchronous vegetational changes observed over wide areas were probably a consequence of abrupt climatic shifts at the end of the Last Glacial Maximum (LGM) and during the Lateglacial. We emphasize that important vegetational changes such as the expansion of forests occurred millennia before the onset of similar processes in northwestern and central Europe.