Abstract
Forests in the upper continental montane zone are important ecotones between lowland and subalpine forest ecosystems. A thorough understanding of the past vegetation dynamics at mid elevation is crucial to assess past and future altitudinal range shifts of tree species in response to climate change. Lake sediments from Lac de Champex (1,467 m a.s.l.), a small lake in the Canton Valais in the Central Swiss Alps were analysed to reconstruct the vegetation, land use and fire history for the last 14,500 years, using pollen, macrofossils, non-pollen palynomorphs and charcoal. The record indicates that the tree line had already reached the Champex area during the Allerød (14,000 cal bp) but dropped below the lake’s catchment during the Younger Dryas cooling (12,750–11,550 cal bp). Reforestation started again with Betula and Pinus sylvestris in the Early Holocene at 11,500 cal bp in response to rapid climate warming. Temperate tree species (Ulmus, Tilia, Quercus, Acer) may have reached the altitude of the lake during the Holocene Thermal Maximum (ca. 10,000–5,000 cal bp). Mixed forests with mesophilous Abies alba were dominant between 7,500 and 5,000 cal bp. The mass expansions of Picea abies after 5,000 cal bp and Alnus viridis thickets after 4,500 cal bp were directly linked to increasing human disturbance. High values of coprophilous Sporormiella fungal spores and cereal pollen suggest pastoral and arable farming at the site from the Late Neolithic/Early Bronze Age onwards (5,000 cal bp). Our data imply that vegetation at intermediate elevation was less affected by human activities than at higher or lower elevations but that these areas served as important stations between the permanent settlements in the valleys and the seasonally occupied alpine huts at higher elevations. We argue that future climate warming will lead to drastic reorganizations of mountain ecosystems.
Highlights
Mountain ecosystems are considered to be prone to environmental changes since distances for vertical migration are short and ecological gradients are generally steep (IPCC 2014)
13,900–12,700 cal bp, Ammann et al 2013; Rach et al 2014), which is confirmed by macrofossils from P. sylvestris and Betula at Lac de Champex and Pinus stomata at Zeneggen-Hellelen (Welten 1982b)
Our new palaeoecological record from Lac de Champex provides the first continuous and well-dated documentation of vegetation dynamics and land use from the upper continental montane zone of the Central Alps during the last 14,500 years. It suggests that first trees (Betula and P. sylvestris) were growing around the site at 1,500 m a.s.l. already during the Allerød (14,000 cal bp), which is in agreement with previous studies from the Central and Southern Alps
Summary
Mountain ecosystems are considered to be prone to environmental changes since distances for vertical migration are short and ecological gradients are generally steep (IPCC 2014). To predict future changes in mountain ecosystems, it is essential to understand species responses to climate and human impacts over long. Palaeoecological approaches (e.g. sedimentary pollen and plant macrofossil analyses) are best suited to study long-term changes in terrestrial ecosystems and to constrain responses of key tree species to climatic and land use change. Only a limited number cover the mid altitudes (Welten 1982a, b; Rey et al 2013) and some of the older studies do not have a reliable radiocarbon chronology (Welten 1982a, b), which is crucial for site-to-site comparisons at centennial time scales (Rey et al 2019b). The consideration of plant macrofossil assemblages is important for tree line reconstructions to unambiguously confirm the local presence of tree species (Birks 2001). Only a few well-dated, multiproxy reconstructions in the Alps are available (Gobet et al 2003; Tinner and Theurillat 2003; Wick et al 2003; Genries et al 2009; Blarquez et al 2010; Berthel et al 2012; Rey et al 2013; Schwörer et al 2014; Thöle et al 2016; Finsinger et al 2021) and modern studies from the upper continental montane zone in the Central Alps (ca. 1,300–1,850 m a.s.l., Landolt et al 2015) are currently lacking
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