Soils evolution and treeline fluctuations under late Holocene climaticchanges: an integrated approach from Valle d’Aosta (Western EuropeanAlps, Italy)

International audienceThe treeline ecotone, defined as the transition belt in mountain vegetation between the closed forest (timberline)and the alpine grasslands, is one of the most distinctive features of mountain environments and it is widelyconsidered as a climatic boundary. Treeline altitudinal fluctuations may be considered to assess past and ongoingclimatic and environmental changes. Although the ecological dynamics of the alpine treeline ecotone is mainlyinfluenced by climate, especially by soil temperature, climatic parameters are not the only factors that influencethe treeline position. In fact, the treeline altitude may be locally influenced by environmental factors, geomorphological processes, soil development, and human activities.This study aims at the reconstruction of late Holocene soil evolution and environmental changes at the treeline onthe SW slope of the Becca di Viou Mountain in Valle d’Aosta (Western Italian Alps).First, we performed a detailed reconstruction of the treeline altitudinal dynamics. In addition, field (includingair and soil temperatures) and laboratory (of both mineral and organic compounds) characterizations have beenperformed along two transects of seven soil profiles developing at an altitude ranging from 2100 m a.s.l. (closedforest) to 2400 m a.s.l. (treeline ecotone), in order to understand the relationships between colonization by treesand soil development under the ongoing climate change.The upward shift of the treeline was assessed analyzing tree age distribution along the slope by means of atree-ring based approach. The reconstruction of the treeline altitudinal dynamics (based on years at which the treesreached 2 m in height) at the study site reveals an upward shift of 115 m over the period 1901–2000, reachingthe altitude of 2515 m a.s.l. in 2008. The recent treeline shift and the acceleration of tree colonization rates in thealpine belt can be mainly attributed to a climatic input, and particularly to an increasing temperature.The investigated soils show a decreasing development with increasing altitude. Indeed, in the forest area (about2100 m a.s.l.) soils are well developed (i.e. Podzol), but at higher altitude, they are less developed (i.e. Ranker). Inthe treeline ecotone, possible traces of Paleosols are also observed.However, future treeline upward shifts in the study area might be severely limited by the geomorphic processes:even if temperature will continue to increase, at higher altitudes, the treeline will meet harsher geomorphicenvironments characterized by high-energy gravity processes and rock faces that impede soil evolution and treecolonization.The integrated analysis of geopedological, dendrochronological and climate data will provide high resolutioninformation about the responses of high-altitude biological and abiological systems through the Holocene and tothe ongoing climate change