In French oak high forests, current silvicultural trends include two seemingly opposed practices: shortening the forestry cycle, which contributes to forestry intensification, and lengthening rotations in temporary set-aside stands, called ‘ageing islands’, to favour biodiversity and high-quality large-diameter tree production.
To derive the potential effects of these two trends, we studied habitat structure and saproxylic beetles biodiversity along an age gradient in a French oak high forest. Four age classes were surveyed: premature (i) 160/180 years and (ii) 180/200 years, (iii) mature 200/220 years, and (iv) overmature 300-year-old stands. Structural features were noted: deadwood volumes, density of large trees with or without microhabitats, number of cavities, presence of dead large canopy branches, sap droppings and sporophores of saproxylic fungi.
Results showed that beetle species richness was positively related to stand age. Globally, overmature stands differed significantly from younger premature and mature forests in species composition and structure. Younger stands tended to show both fewer structural features and lower levels of saproxylic biodiversity. As the forest aged, the overall structural complexity and saproxylic biodiversity increased. However, no individual stand characteristic influenced preferentially biodiversity, and stand age was the best explaining factor.
In conclusion, we discuss how (i) shorter rotations in the high forest production cycle and (ii) temporary set-aside forest islands affect forest structure and deadwood-associated assemblages. Shortening rotation length in oak high forests may negatively impact saproxylic biodiversity, whereas temporary set-asides may play a key role for biodiversity conservation in a managed forest matrix.