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agroPastoral Activities and effects on Landscapes and ErOsion dynAmics in the alps: a new insight from Geological appRoaches and lake sedIment DNA

Periodic Reporting for period 1 - PALEO-AGRI (agroPastoral Activities and effects on Landscapes and ErOsion dynAmics in the alps:a new insight from Geological appRoaches and lake sedIment DNA)

Reporting period: 2015-06-01 to 2017-05-31

Due to their unique biological and geological resources, mountains have attracted humans since Prehistory, despite their sensitivity to environmental and climatic changes. In the current context of unprecedented speed and amplitude of global changes (climate, pollution, biodiversity decline), the investigation of past human-climate-environment interactions in such significant areas is imperative. Such studies can provide “lessons from the past” when engaging with present and future scenarios: tracing past environmental changes allows us to better understand current conditions as they are, in part, inherited from past successions of climatic change and land use. Few studies, however, have investigated the detailed spatiotemporal dynamic of alpine agrarian activities (including their precise nature) or their impacts on plant cover and erosion. This is largely due to the lack of suitable proxies for human activities in these environments. The PALEO-AGRI project addressed this challenge through the optimisation of an emerging tool, the analysis of extracellular DNA archived in lake sediments (a continuous archive of the past) and soils in archaeological contexts. The design of our project comprised the study of several sites distributed across elevation and latitudinal gradients in the French Alps (Fig. 1).

Despite the increasing popularity of sediment DNA studies, almost nothing is known about the taphonomic processes (DNA source, transfer and deposit), which might affect and bias DNA records - WP1 of PALEO-AGRI aimed to address this issue. WP2 focused on the study of archaeological sites in the Ecrins Massif, where appropriate lakes for DNA analyses are missing. The goals of this WP were: 1) to provide information about the past use of the structures that yield very little material culture due to poor preservation conditions, and 2) to assess the legacy of past human activities on the current ecosystem. WP3 focussed on the reconstruction of the spatiotemporal dynamic of agro-pastoral activities and their effects on soil erosion and plant cover.
The implementation of the project was based on the coring of several lakes to provide material required for WPs 1 and 3. DNA Metabarcoding analyses and qPCR focusing on plants and mammals were performed and used to trace agrarian and plant-cover histories. Some analyses are still in progress. Sedimentological and geochemical analyses (lithology, grain size, LOI, XRF core scanner) helped study the effect of sediment composition on DNA records and were used to reconstruct the erosion dynamics. For WP2, soil samples were collected from the archaeological sites. To study the past use of these structures, we analysed the elemental (XRF, C, N, bioavailable P) and isotopic (δ15N, δ13C) compositions of soils. These analyses and botanical surveys undertaken during the fieldwork also provide a picture of the legacy on the current ecosystem state. To complete this picture, modern DNA analyses focusing on several groups of organisms living in soils are in progress.
WP1: Our data suggest that extracellular DNA from the catchment area is more efficiently archived in lakes with high detrital inputs. The source of eroded material is also important: the erosion of deep soil horizons and inputs of glacial materials trigger dilutions of DNA quantity archived in sediments. Moreover, specific chemical conditions affecting lake sediment DNA preservation and/or analysis success can explain the non- or low-detection of DNA. Factors that influence the DNA record in lake sediments can change over time and thus significantly affect the quality of reconstructions of plant cover and livestock farming/pasturing histories.
WP2: Significant lead concentrations were detected in all medieval structures as well as in the modern enclosure, which suggests these sites were associated with mining activities. Interestingly, the beginning of this period corresponds to the most significant development of pastoral activities in the area. To understand the dynamic of pastoral activities over time, we should thus also consider the history of the development of mining activities in high-altitude areas and their economic importance.
The disturbance of the environment directly related to human activities (via the archaeological structures and the immediate environment) can still be detected through the soil geochemical signature and plant communities at least several centuries after disturbance.
WP3: In the Northern lakes (Anterne, La Thuile), the earliest pastoral activities might have developed during the Late Neolithic. However, our indications remain debatable and need to be reconsidered in light of new analyses, which are in progress. During the Late Iron Age and the Roman period, pastoral activities increased substantially, with sheep and cattle at high altitude. This intensive use of pastures led to soil erosion. Another phase of intensive grazing activity took place during the Medieval Period, from 1000 AD. However, there is no/little erosion, which might reflect a new land-use management strategy. Especially at the mid-altitude site (La Thuile), the development of fruit trees (Pyrus sp., Juglandaceae sp. and Prunus sp.) might have limited soil loss. From the 13th-14th centuries, it is apparent that cattle become more important than sheep, which might indicate an alpine economic “revolution”. Between 1200 and 1450 AD, the detection of Vitaceae sp. suggests vine culture. The decline of this culture coincides with a significant decrease in temperature during the Little Ice Age; a possible response by the Alpine community to climate change. To the south (Lake Muzelle), pastoral activities seem to develop later. In fact, over the last 1600 years, plants often associated to pastoral activities (Rumex sp. and Mentheae sp.) are only sparsely detected from 1500 AD and continuously from 1800 AD. This different temporal pattern highlights the complex history of agrarian activities in the French Alps.
"WP1, provided new knowledge on taphonomic processes affecting DNA from the catchment. This scientific advance is significant, as it will enhance the robustness of the interpretations of lake sediment DNA records. Consequently, we can now provide criteria for the choice of lakes that record DNA from the catchment, thus facilitating future studies. Results from WP2 improve understanding of the resilience of subalpine ecosystems relative to human occupation and activities. Our research result is highly original; up until now, such studies were rare, especially in this type of environment. Due to the public interest in Alpine landscapes, the synthesis of the results of this project will be presented at public lectures in the National Park interpretation centres and integrated with the results of the archaeological research. Moreover, team members are involved in a network called “Grande Histoire des Alpages"", which includes shepherds, cheese and meat producers from the Alps. Active shepherds have already demonstrated an interest in our earlier research. The results of PALEO-AGRI will also be of interest to this network."
Figure 1