Holocene climate and human impacts on vegetation dynamics in the Swiss Alps highlight biodiversity conservation challenges

Abstract

The record of Lake Seewli (2027 m a.s.l.) in the Northern Alps covers 11,000 years of vegetation dynamics. Pioneer vegetation characterized Seewli at the beginning of the Holocene, followed by the expansion of treeline conditions at 9200 cal. BP during the Holocene Thermal Maximum (HTM), when Pinus cembra occurred locally. Abies alba established at the site from ca. 8400 cal. BP onward, forming open stone pine-silver fir forests that are very rare today. The forest composition changed significantly with Picea abies expanding around 6900 cal. BP, favored by increasing fire activity and a decline in fire-sensitive Abies alba and Pinus cembra. Densest forest and therefore highest timberline elevation was reconstructed during ca. 6300–5200 cal. BP. We observe anthropogenic impact from ca. 6100 cal. BP onward. Subsequently, intensifying land use replaced climate as the primary driver of vegetation dynamics during the Late-Holocene. Agricultural production and pastoralism increased during the Bronze and Iron Age. This trend was interrupted during the Late Roman/Migration Period when land abandonment led to rapid forest expansion. Land-use intensity peaked during the Middle Ages before declining during the past two centuries. Our study suggests a rapid afforestation of highly diverse meadows and an upward shift of treeline under future climate change and socioeconomically-driven land abandonment. Conversely, low-intensity agropastoralism has the potential to mitigate climate-driven upward shifts of mountain forests and can therefore maintain high levels of biodiversity and ensure important ecosystem services.