Description du sujet de thèse

Impact of old plantations on forest biodiversity

Contexte de travail

In the face of climate change and forest dieback, one of the strategies currently favored by forest managers is planting. The impact of plantations, both old and new, on forest biodiversity, however, has been the subject of very few studies, both internationally and nationally. The only meta-analysis currently available was carried out by Chinese researchers and published in Global Ecology and Biogeography in 2022 (Wang et al. 2022). It concludes that several factors have a direct influence on biodiversity: tillage intensity at planting, diversity of species planted, origin of species (native or exotic), management methods, age of plantation and site conditions. This meta-analysis covered 138 sites worldwide, but none in France. It also covered a short period (1980-2000). Other studies around the world have sought to establish the link between plantations, particularly exotic ones, and certain ecosystem compartments (soil, fauna, flora), but no study has been carried out in France on the influence of long-term plantations in lowland and mountain forests (Andrès, Ojeda, 2022; Bremer, Farley, 2010; Brockerhof et al., 2008, Wang et al. 2019).
In this context, the FORESTT research program (PEPR 2024-2030) has been set up, focusing on forest resilience and the socio-ecological transition of forest systems (https://www.pepr-forestt.org/), funded by the France 2030 recovery plan. The first targeted project (PC1) of this PEPR, entitled Régé-Adapt, aims to improve understanding of forest regeneration phases, and in particular the effects of artificial regeneration on forest resistance, resilience and transformation. As part of this project, the PEPR FORESTT is funding half of the thesis (funding already secured), as well as the full cost of missions and analysis fees. It is being carried out in parallel with another proposed thesis on the impact of recent plantations.
The subject of this research is the impact of old plantations on current forest biodiversity. The study sites are located in beech and oak-beech forests in the Hauts de France (notably in the forests of Compiègne, Villers-Cotterêts and Chantilly), in Atlantic oak forests (notably in the forest of Orléans), and in mountain forests (Isère). The term “old-growth plantations” refers to plantations planted around 200 years ago or more, i.e. those planted between the middle of the 18th century and the middle of the following century.
The first step will be to use a geo-historical approach to document in as much detail as possible, at plot level, the phases of planting, the types of tillage (ploughing, crocheting, for example), the species and their provenances, and the subsequent silvicultural itineraries. This information will be obtained by processing and analyzing old management documents (management documents, hammering reports and logs, etc.), old planimetry and comparing them with Lidar images. The second step will be to select plots combining, on the same type of forest site and at close spacing, hardwood stands resulting from natural regeneration and hardwood or softwood stands resulting from an old plantation, sometimes renewed in the 20th century. Funding has been obtained to carry out a study of around sixty plots. In a third phase, field surveys and laboratory analyses will be carried out to assess current biodiversity using a multi-taxonomic approach: epigenetic floristic and fungal biodiversity, soil macrofauna and microfauna, soil microbial communities, environmental DNA.