Long-term trajectories of mountain agro-ecosystems in the North-Western Alps

Abstract

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Human-triggered magnification of erosion rates in European Alps since the Bronze Age

“…This had two effects: (i) increasing the surface area covered by glaciers 21 , 52 , (ii) and enhancing erosion in the glacial region through the combined effect of increased precipitation and the advance of the glaciers. These two periods are also known to have seen a marked decline in human activity, linked to invasions during the Migration Period 19 , 24 and to the reduction in agricultural practices 53 , 54 probably induced, for both periods by the shortening of summer 24 . These combined effects explain the strong decrease observed in the erosion excess signal for these two periods (Fig.…”

“…1 ). We hypothesize that the two to three sites selected by altitudinal range illustrate the intensity of pastoral and/or agricultural activities in these areas 24 , 27 , 55 . This assumption is considered as robust as the history of activities recorded at the sites in a given altitudinal range are very similar (Supplementary Fig.…”

“…The Alps presents currently the highest erosion rates over the entire European continent 3 , 9 , making it a hotspot of soil loss 18 . In addition, as one of the best documented places on Earth in terms of climate fluctuations 19 – 23 during the Holocene and of human activities from the Neolithic period 24 – 27 , this region represents an ideal environment to quantify the impact of both climate and human activities on erosion rates. Lake Bourget drains one of the largest alpine catchments (4976 km 2 ) which is particularly well suited for such a study as it hosts a diversity of landscapes in terms of land use, geomorphology, and lithology 28 (Supplementary Fig.…”

“…The catchment culminates at 4808 m at the top of the Mont Blanc massif, where the presence of glaciers precludes any land use, such that erosion there is largely controlled by climate 29 , 30 . Moreover, most of this glacier-covered part of the catchment (158.1 km 2 situated between 2300 and 4808 m) is underlain by granites and metamorphic rocks 31 with a geochemical signature distinct from those of the sedimentary formations underlying the lower elevation area 32 – 34 (3803.9 km 2 excluding limestone formations; 232–3057 m), where agro-pastoral activities have developed over the last 7000 years to become a major human activity in the region 24 , 27 , 35 . This configuration can be leveraged to quantify the role of various driving factors on erosion in the Lake Bourget catchments, given the following assumptions: (i) erosion in the glacier-covered part of the catchment responds only to climate (temperature and precipitation), whereas (ii) erosion in lowermost, non-glaciated regions depends both on climate and land use; and finally (iii) if climate shifts were the sole factor of changes in erosion, they would affect equally any part of the catchment area.…”

“…This index is blind to other human activities such as deforestation and mining. Because each altitudinal range presents its own environmental trajectory over the Holocene 24 , these indexes were then binned by altitudinal ranges, that are named foothills (250–850 m), mid-altitude (850–2000 m) and high altitudes (>2000 m). Our source-to-sink approach involves the collection and measurement of 29 samples of fine river sediments from the flood plain of the Rhône and the Arve rivers, the main tributaries of Lake Bourget, and their tributaries (Methods).…”