Christophe Bouget

Current Near‐to‐Nature Forest Management Effects on Functional Trait Composition of Saproxylic Beetles in Beech Forests

With the aim of wood production with negligible negative effects on biodiversity and ecosystem processes, a silvicultural practice of selective logging with natural regeneration has been implemented in European beech forests (Fagus sylvatica) during the last decades. Despite this near-to-nature strategy, species richness of various taxa is lower in these forests than in unmanaged forests. To develop guidelines to minimize the fundamental weaknesses in the current practice, we linked functional traits of saproxylic beetle species to ecosystem characteristics. We used continental-scale data from 8 European countries and regional-scale data from a large forest in southern Germany and forest-stand variables that represented a gradient of intensity of forest use to evaluate the effect of current near-to-nature management strategies on the functional diversity of saproxylic beetles. Forest-stand variables did not have a statistically significant effect on overall functional diversity, but they did significantly affect community mean and diversity of single functional traits. As the amount of dead wood increased the composition of assemblages shifted toward dominance of larger species and species preferring dead wood of large diameter and in advanced stages of decay. The mean amount of dead wood across plots in which most species occurred was from 20 to 60 m(3) /ha. Species occurring in plots with mean dead wood >60 m(3) /ha were consistently those inhabiting dead wood of large diameter and in advanced stages of decay. On the basis of our results, to make current wood-production practices in beech forests throughout Europe more conservation oriented (i.e., promoting biodiversity and ecosystem functioning), we recommend increasing the amount of dead wood to >20 m(3) /ha; not removing dead wood of large diameter (50 cm) and allowing more dead wood in advanced stages of decomposition to develop; and designating strict forest reserves, with their exceptionally high amounts of dead wood, that would serve as refuges for and sources of saproxylic habitat specialists.

In search of the best local habitat drivers for saproxylic beetle diversity in temperate deciduous forests

Deadwood-associated species are increasingly targeted in forest biodiversity conservation. In order to improve structural biodiversity indicators and sustainable management guidelines, we need to elucidate ecological and anthropogenic drivers of saproxylic diversity. Herein we aim to disentangle the effects of local habitat attributes which presumably drive saproxylic beetle communities in temperate lowland deciduous forests. We collected data on saproxylic beetles in 104 oak and 49 beech stands in seven French lowland forests and used deadwood, microhabitat and stand features (large trees, openness) as predictor variables to describe local forest conditions. Deadwood diversity and stand openness were consistent key habitat features for species richness and composition in deciduous forests. Large downed deadwood volume was a significant predictor of beetle species richness in oak forests only. In addition, the density of cavity- and fungus-bearing trees had weak but significant effects. We recommend that forest managers favor the local diversification of deadwood types, especially the number of combinations of deadwood positions and tree species, the retention of large downed deadwood and microhabitat-bearing trees in order to maximize the saproxylic beetle diversity at the stand scale in deciduous forests. To improve our understanding of deadwood-biodiversity relationships, further research should be based on targeted surveys on species-microhabitat relationships and should investigate the role of landscape-scale deadwood resources and of historical gaps in continuity of key features availability at the local scale.

Evaluation of window flight traps for effectiveness at monitoring dead wood-associated beetles: the effect of ethanol lure under contrasting environmental conditions.

1 Subsequent to the diversity of saproxylic beetles being proposed as a management tool in forestry, more explicit knowledge about the efficiency and selective properties of beetle sampling methods is needed.

Implications from large‐scale spatial diversity patterns of saproxylic beetles for the conservation of European Beech forests

Abstract.  1. European Beech (Fagus sylvatica) is the natural dominant tree species in many forests across Europe. Despite Europe’s global responsibility for these forests, the correct conservation strategies are still a matter of debate. In particular, it remains controversial whether high conservation efforts should be directed towards beech forests, owing to the small number of insects that are Fagus specialists, and at what spatial scale conservation should take place.

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

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.

Are stumps important for the conservation of saproxylic beetles in managed forests? – Insights from a comparison of assemblages on logs and stumps in oak-dominated forests and pine plantations

Abstract.  1. The expanding interest in harvesting stumps for bioenergy may represent a potential threat to forest biodiversity. Whereas stumps are common in managed stands, knowledge of their associated saproxylic assemblages is still incomplete.

Are biodiversity patterns of saproxylic beetles shaped by habitat limitation or dispersal limitation? A case study in unfragmented montane forests

Understanding the processes that shape biodiversity patterns is essential for ecosystem management and conservation. Local environmental conditions are often good predictors of species distribution and variations in habitat quality usually positively correlate to species richness. However, beside habitat limitation, species presence-absence may be constrained by dispersal limitation. We tested the relative importance of both limitations on saproxylic beetle diversity, using forest continuity as a surrogate for dispersal limitation and stand maturity as a surrogate for habitat limitation. Forest continuity relies on the maintenance of a forest cover over time, while stand maturity results in the presence of old-growth habitat features. Forty montane beech-fir forests in the French pre-Alps were sampled, under a balanced sampling design in which forest continuity and stand maturity were crossed. A total of 307 saproxylic beetle species were captured using flight-interception traps and Winkler–Berlese extractors. We explored the response of low- versus high-dispersal species groups to forest continuity and stand maturity. Saproxylic beetle diversity increased significantly with stand maturity and was mostly influenced by variables related to deadwood diversity at the stand scale and suitable habitat availability at the landscape scale. Surprisingly, no evidence of dispersal limitation was found, as diversity patterns were not influenced by forest continuity and associated variables, even for low-dispersal species. Our study demonstrates that in an unfragmented forest landscape, saproxylic beetles are able to colonize recent forests, as long as local deadwood resources are sufficiently diversified (e.g. tree species, position, diameter and/or decay stage).

Effect of deadwood position on saproxylic beetles in temperate forests and conservation interest of oak snags

1. Intensive forestry has drastically reduced the volume of standing deadwood, even more than total deadwood volume. It has been demonstrated in boreal forests that snags have a high conservation value for many saproxylic species and that the lying or standing aspect of the deadwood makes a difference. The conservation of saproxylic beetles in the changing deadwood landscape of temperate managed forests depends on a deeper understanding of the ecological value of forest stand legacies.

Influence of sampling effort on saproxylic beetle diversity assessment: Implications for insect monitoring studies in European temperate forests

Saproxylic beetle diversity monitoring provides a tool for estimating the efficiency of forest conservation measures. Flight interception traps are commonly employed to monitor beetle assemblages, although little explicit knowledge of the efficiency of this trapping method is available. The present study investigated how slight changes in sampling effort can influence species richness and species composition of assemblages in data sets from standard window‐flight traps. At both trap and plot levels, an additional year or an additional trap provided a 50% increase in the number of species detected (a 75% increase for rare species) and resulted in a different estimated composition of the assemblages. Adding 2 or 3 years of sampling gave twice as many species and resulted in assemblages that were 50% dissimilar. Increases in the detection of species and the dissimilarity of assemblages were similarly affected along a gradient of forest conditions, suggesting that changes in sampling effort were not affected by forest condition. At the forest level, year or trap replication provided smaller increases in species richness (31% and 25%, respectively). Within sites, distance measures in species composition between traps did not differ significantly when based on 1 or 2 years of data. Using two traps per plot compared with one trap influenced comparisons between stand types, based on species richness, in 25% of the cases. Species detection was similarly increased by either year replication or trap replication. The results of the present study highlight the significant role played by finescale patterns of habitat structure and inter‐annual variation with respect to determining catch size and assemblages of saproxylic species.

Can rove beetles (Staphylinidae) be excluded in studies focusing on saproxylic beetles in central European beech forests

Monitoring saproxylic beetle diversity, though challenging, can help identifying relevant conservation sites or key drivers of forest biodiversity, and assessing the impact of forestry practices on biodiversity. Unfortunately, monitoring species assemblages is costly, mainly due to the time spent on identification. Excluding families which are rich in specimens and species but are difficult to identify is a frequent procedure used in ecological entomology to reduce the identification cost. The Staphylinidae (rove beetle) family is both one of the most frequently excluded and one of the most species-rich saproxylic beetle families. Using a large-scale beetle and environmental dataset from 238 beech stands across Europe, we evaluated the effects of staphylinid exclusion on results in ecological forest studies. Simplified staphylinid-excluded assemblages were found to be relevant surrogates for whole assemblages. The species richness and composition of saproxylic beetle assemblages both with and without staphylinids responded congruently to landscape, climatic and stand gradients, even when the assemblages included a high proportion of staphylinid species. At both local and regional scales, the species richness as well as the species composition of staphylinid-included and staphylinid-excluded assemblages were highly positively correlated. Ranking of sites according to their biodiversity level, which either included or excluded Staphylinidae in species richness, also gave congruent results. From our results, species assemblages omitting staphylinids can be taken as efficient surrogates for complete assemblages in large scale biodiversity monitoring studies.