The effects of forest age on saproxylic beetle biodiversity: implications of shortened and extended rotation lengths in a French oak high forest

Abstract

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.

Specialisation and diversity of multiple trophic groups are promoted by different forest features

“…However, stand age effects are thought to operate through greater heterogeneity and dead wood availability in older stands (Lassauce et al . ) and, as it is likely that these effect were captured by our composition and heterogeneity measures, stand age per se may not a major driver of forest communities.…”

“…It could be because we did not measure very old stands, which are rare in Central European production forests, and which would have higher levels of deadwood and tree cavities due to senescent trees, typical of primeval forests (Christensen et al 2005). However, stand age effects are thought to operate through greater heterogeneity and dead wood availability in older stands (Lassauce et al 2013) and, as it is likely that these effect were captured by our composition and heterogeneity measures, stand age per se may not a major driver of forest communities.…”

Protection against rockfall along a maturity gradient in mountain forests

“…Studies comparing unmanaged versus managed forests (see, for example, Bouget et al, 2014;Burrascano et al, 2013;Marage and Lempérière, 2005;Pernot et al, 2013;Whitman and Hagan, 2007) or focusing on time since logging abandonment gradients (Lassauce et al, 2012(Lassauce et al, , 2013Pernot et al, 2013;Sitzia et al, 2012;Vanderkhove et al, 2009) highlight the following major structural changes during the maturation process: stands become richer in very large trees, large snags and large pieces of lying deadwood in various degradation stages increase, and tree microhabitats develop. Some studies also report that canopy gaps become more frequent (Rugani et al, 2013).…”

Past levels of canopy closure affect the occurrence of veteran trees and flagship saproxylic beetles

“…This problem, however, is rarely considered by conservation practice and even science. Numerous studies, for example, compared the biodiversity of managed and unmanaged forests mostly concluding that minimal intervention management is important to sustain forest biodiversity (Lassauce, Larrieu, Paillet, Lieutier, & Bouget, ; Martikainen, Siitonen, Punttila, Kaila, & Rauh, ; Paillet et al., ). In fact, most of the “unmanaged” forests in Europe have now been unmanaged for only a few decades.…”