Diversity of plant assemblages in managed temperate forests: a case study in Normandy (France)

We investigate the patterns of variation of plant community diversity along a silvicultural cycle in a managed beech forest. A statistical model of the silvicultural cycle is constructed in order to identify: (i) the relationships between species and environmental factors, and (ii) species assemblages occurring along the successional gradient. We measure six diversity indices accounting for the structural (species richness (SR), Shannon index, evenness index), the functional (factorial diversity, FD) and the compositional dimension (similarity index within and between records) of biodiversity. We compare their patterns of variation along the succession in order to understand the different information provided by each index. The model reveals that species assemblages occurring in regeneration stands have the highest SR along the cycle. Species coexistence in these assemblages appears to be based on non-equilibrium mechanisms. On the other hand, mature stands with closed canopy seem to host the more organised communities reflecting true coexistence based on equilibrium mechanisms. We discuss the indicative values of the different diversity measures and their interest for the development of sustainable forest management. We advocate that conservation priorities should thus be defined in reference to the processes controlling biodiversity and to the taxonomic groups responding to these processes.