Holocene vegetation and climate variability as recorded in high-altitude mires (western Italian Alps)

Climate variability during the Holocene (the last 11700 calendar yr b2k) is relatively subtle whether compared with the higher amplitude characterizing the last glacial period. Nonetheless, several proxy data (e.g. pollen, plant macrofossils, glacier length variations), indicate detectable changes in temperature and/or moisture during this period. Retreating glaciers may uncover peat sequences that were once ice-buried. Palynological analysis of those organic deposits provides information on plant communities colonization in proglacial areas as well as long-distance vegetations signal. The aim of this study is to investigate the potential of high-altitude pollen records for vegetation history and climate reconstruction during the Holocene in the south-western Italian Alps. The study sites are located in the Rutor Glacier area (La Thuile Valley). Within this thesis, modern (a) and past (b) vegetation and climate conditions have been analyzed using different methodologies; a) Modern pollen rain, vegetation, climate and terrain parameters have been collected at 27 sampling sites placed along an altitudinal gradient from the village of Morgex (983 m asl) to the Rutor Glacier forefield (2668 m asl). This altitudinal training set has been designed (i) to provide a robust modern reference for reliable palaeoenvironmental and quantitative interpretations of past changes in vegetation composition; (ii) to integrate the newly-obtained pollen spectra into a large, continental dataset of modern pollen samples (EMPD - European Modern Pollen Database) for quantitative climate reconstructions. b) New paleoecological, stratigraphical and geochronological data have been obtained from peat deposits (Lac dans la Roche peat bog, 2594 m asl) and integrated with the well-known buried peat sequences exposed at 2510 m asl by the '80es Rutor Glacier retreat. The result is a composite paleoecological record covering most of the Holocene. At the base of the composite sequence, meltwater glacier sediments testify to the early Holocene development of a proglacial lake, before Abies alba expansion in the western Italian Alps. These sediments bear the palynological evidence of a primary plant succession testifying to an ecological mechanism of colonization on deglaciated terrains. Moreover, macrofossil analysis highlighted the local presence of alpine dwarf - shrubland species (e.g. Salix cfr. foetida). Pinus cembra occurs rather densely in the alpine belt from ca. 8000 yrs cal. BP, representing the beginning of the Holocene thermal maximum in the high Alps. Between 8000 - 4000 yrs cal. BP timberline was higher than today. A radiocarbon age obtained from a Pinus cembra wood fragment suggests the occurrence of pines as high as ca. 2600 m asl at around 5650 yrs cal BP. The dated wood fragment cannot have been long-distance transported, thus it indicates the presence of this species in situ. Moreover, paleobotanical insights and comparison with climate threshold of modern treeline (ca. 9,3° C at 2400 m asl), suggest a positive treeline shift of almost 150 m. Preliminary pollen-inferred Tjul reconstructions show higher values (up to 2 - 3°C) than today for this interval. At around 4000 yrs cal. BP Picea abies and Alnus viridis started to expand. A progressive natural/climatic driven decline of Pinus cembra took place. Pollen inferred Tjul shows a decreasing trend between 2900 – 2000 yrs cal BP and possibly a short-lived cold event starting at ca. 1140 yrs cal. BP, pre-dating the onset of the LIA (Little Ice Age). A subsequent arolla pine forests reduction can be attributed to LIA glacier advance (max occurred between 1751 – 1864 AD), which is testified in the upper part of the sequence by fluvioglacial deposits.