Historical ecology and biodiversity conservation of ancient forests in protected mountain areas : a multi-taxon approach

In France, the forest area decreased continuously until the middle of the 19th century, then increased rapidly following the industrial revolution. This forest minimum marks the limit between pre-existing "ancient forests" and the "recent forests" that were established later, because it makes it possible to assume a much longer forest continuity and to estimate precisely the historic forest areas thanks to the existence of complete historical maps (Napoleonic cadastre and Ordnance Survey maps among others). This temporal continuity of ancient forests induces a stability likely to favor the presence of particular species with low dispersal capacity and sensitive to disturbances.If the French forest area has almost doubled since 1850, it is estimated that 15 % of the forests present in 1840 has disappeared today at a national scale. Despite IUCN recommendations to protect ancient forests, conservation actions are still poorly defined and fall under the precautionary principle. This thesis project is part of a partnership with five French national parks located in mountain areas (Pyrenees, Cevennes, Mercantour, Ecrins and Vanoise) which have already vectorised and identified ancient forests on their territory. It aims to answer three main research questions :1- What is the role of ancient forests for the conservation of threatened species?2- Does the type of ancient land use influence the biodiversity of recent forests?3- Does forest management impact the biodiversity of ancient forests?The first question was addressed through statistical analysis of naturalist data collected by different networks of observers in the areas of adhesion of the five national parks in our study area. This work showed that threatened spermaphytes, pteridophytes, bryophytes, and forest beetles were more responsive to historical forest area than to current forest area, highlighting a 150-year lag in response to landscape change. The second part of the study was based on botanical surveys carried out in the forests of the Vanoise national park on four types of ancient use: forest, pasture, hay meadow and crop. This study showed that there was no difference in edaphic conditions between ancient forests and recent forests developed on former pastures and hay meadows, whereas forests located on former cropland had richer soils. On the other hand, differences in taxonomic and functional composition of understory plant communities were also smaller between ancient forests and former pastures than between ancient forests and former cropland. Finally, the third question was treated using metabarcoding surveys of fungal communities in the public forests of Vanoise with a gradient of time elapsed since the last harvest from one to 75 years. This study showed that the low-intensity silvicultural management of the Vanoise forests had little impact on soil fungal communities and confirmed the weak long-term traces left by grazing in recent forests.This work therefore highlights the importance of old-growth forests for the conservation of biodiversity, particularly forest species, but also emphasizes that the type of ancient land use is important to consider in recent forests. Former pastures in particular, which represent an important part of recent forests in mountain areas, have a lower impact than other land uses.