Michaël Aubert

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

Abstract 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.

Effects of tree canopy composition on earthworms and other macro-invertebrates in beech forests of Upper Normandy (France)

Summary The effect of canopy composition on soil macro-invertebrate communities in two deciduous temperate forests: a pure beech (Fagus sylvatica L.) and a mixed beech-hornbeam (Carpinus betulus L.) stand was investigated. The initial hypothesis was that heterogeneity of trophic resources within the mixed stand might increase diversity and heterogeneity of soil macro-invertebrate communities at stand level. The macrofauna was sampled in autumn and spring by hand sorting 121 (25 × 25 × 30 cm) soil monoliths regularly distributed within each stand. Earthworms were identified to species level while the remaining macro-invertebrates were identified to family level. Results were analysed by univariate and multivariate (PCA) statistical tools as well as a geostatistical tool. Few differences were observed when comparing the total macro-invertebrate density between sites and dates. In contrast, significant patterns were found for several taxonomic or trophic groups taken separately (e.g. Lumbricidae, detritivore and predator densities were significantly higher in the pure beech stand (PS). The first and the third PCA axes respectively revealed a site and a season effect while the second axis revealed a spatial segregation within the detritivore group as it distinguished high densities of Lumbricidae from those of Isopoda and Diplopoda. The variance of record scores on this axis measured for each site and date revealed that spatial variability of soil macrofauna communities was greater for PS on both dates than for MS. Semi-variance analysis performed on record coordinates on the second axis of the PCA revealed that only macro-invertebrate communities sampled under the pure stand in autumn were spatially structured (autocorrelation range about 32 m). These results do not support the general agreement that resource diversity and patchiness increases soil fauna biodiversity and heterogeneity.

Soil detritivore macro-invertebrate assemblages throughout a managed beech rotation

This work addresses the driving factors responsible for patterns in the detritivore macrofaunal communities of a managed beechwood chronosequence (28 to 197 years old, Normandy, France). We investigated the variation patterns of density, biomass and diversities of detritivore macrofauna throughout this rotation. Multivariate analyses were carried out to identify the main covariation patterns between species and some properties of their physical environment, and to describe the main ecological gradients constraining the macro-invertebrate community assembly. A total of 6 earthworm, 6 woodlouse and 7 millipede species were found in the whole data set. Density, biomass and diversity were profoundly influenced by forest ageing, mainly because of variation in humic epipedon spatial variability. Three groups of species were identified according to their environmental requirements. Some hypotheses regarding the external (related to management practices) or internal (related to inter-specific interactions) assembly rules behind species assemblages are proposed, an approach which has rarely been used in soil ecology. Finally, the impact of forestry practices on soil functioning through their impact on detritivore macro-invertebrate communities is discussed.RésuméCe travail a pour but d’identifier les facteurs responsables des schémas de variation des communautés de la macrofaune detritivore d’une chronoséquence (28 à 197 ans) de futaie régulière de hêtre (Normandie, France). Les modèles de variation de la densité, la biomasse et la diversité ont été recherchés. Les modèles de covariation entre les espèces et certaines propriétés physiques du milieu ainsi que les gradients écologiques qui contraignent les assemblages de macro-détritivores ont été décrits à l’aide d’analyses multivariées. En tout, 6 espèces de vers de terre, 6 espèces d’isopodes et 7 espèces de diplopodes ont été identifiées. La maturation du peuplement de hêtre, principalement par les modifications de l’épisolum humifère, influence fortement les densité, biomasse et diversité. Trois groupes d’espèces sont identifiés sur la base de leurs exigences environnementales. Quelques hypothèses sont proposées quant aux règles externes (liées aux pratiques sylvicoles) et internes (liées aux relations interspéciques) qui contraignent la composition des assemblages d’espèces, cette approche ayant jusqu’à présent été peu utilisée en écologie du sol. Enfin, l’impact des pratiques sylvicoles sur le fonctionnement du sol, au travers de leur impact sur les communautés de macro-détritivores, est discuté.

Forest management adaptation to climate change: a Cornelian dilemma between drought resistance and soil macro-detritivore functional diversity

Global warming induces new constraints on forest ecosystems and requires forest management adaptation. The reduction in stand density is currently debated as a potential tool to face increasing summer drought risk by improving forest resistance to climate change-induced tree mortality. However, few studies have yet assessed the impacts of this management change on soil biodiversity. We conducted a large-scale, multi-site assessment of the response of soil macro-detritivore assemblages and soil functioning to experimental manipulations of stand density. A total of 33 stands were studied covering a wide gradient of stand density, that is stand basal area from 25 to 437m(2)ha(-1), stand age, that is 18-171years old, and local abiotic context. We observed contrasting responses as a function of both taxonomic and functional groupings. Exploratory analysis using causal diagrams, that is path analysis, highlights that these changes were mainly related to alterations in understorey vegetation, microclimatic and soil pH conditions. The response of soil macro-detritivore assemblages to stand density manipulation was consistent over the gradient of stand ages. Among the litter-dwelling macro-detritivores, millipede abundance and diversity decreased with stand density reduction, while woodlice and epigeic earthworms were unaffected. Further, a shift in soil-dwelling earthworm community composition was observed in mull stands. Endogeic earthworm abundance showed a sharp increase with stand density reduction, which translated into an increase in soil respiration. In contrast, anecic earthworm abundance decreased and was strongly associated with a decline of the rate of forest floor turnover.Synthesis and applications. Our study provides strong evidence that reductions of stand density will have substantial impacts on soil macro-detritivore assemblages and cascading effects on soil functioning, particularly in mull stands. Managing stand density of oak forests at an intermediate level, that is 25m(2)ha(-1), appears to be best to optimize the trade-off between improving forest resistance to climate change and ensuring the conservation of functional diversity to preserve forest ecosystem functioning and stability. Our study provides strong evidence that reductions of stand density will have substantial impacts on soil macro-detritivore assemblages and cascading effects on soil functioning, particularly in mull stands. Managing stand density of oak forests at an intermediate level, that is 25m(2)ha(-1), appears to be best to optimize the trade-off between improving forest resistance to climate change and ensuring the conservation of functional diversity to preserve forest ecosystem functioning and stability.

Tree species richness induces strong intraspecific variability of beech (Fagus sylvatica) leaf traits and alleviates edaphic stress

Manipulating stand composition is an important management tool that foresters can use to affect the nature of forests and ecosystem processes. In mixed stands, interspecific interactions among trees can cause changes in tree performances. Nevertheless, these interactions are context dependent (cf. stress-gradient hypothesis, SGH). We thus investigated how intraspecific functional changes in leaf trait (19 traits) of European beech (Fagus sylvatica) were influenced by stand composition. We compared pure beech stands with four mixed stands containing from one to three additional tree species along a gradient of edaphic stress (gradient of soil water-holding capacity and rooting depth). First, we demonstrated that stand composition induced strong intraspecific leaf trait variation in beech for LDMC, LMA, phenolic compounds, leaf pH and magnesium concentration, suggesting higher nutrient acquisition by more diverse stands. Nevertheless, these results were modulated by edaphic stress. Mixed stands only conferred an advantage in relatively stressed sites (luvisol and leptosol). Besides, the addition of oak to beech stands had unexpected negative effects in sites with less severe stress (cambisol) as indicated by the null or positive LogRR of LMA, LDMC and phenolics. This study found that stand composition is an important though often-overlooked driver of intraspecific variability in leaf quality, and potentially reflects changes in beech tree physiology and productivity. Our results also suggest that positive interactions prevail in sites with stressful conditions. Such validation of the SGH is rare in natural or managed mature forests. Lastly, we strongly recommend that forest managers consider stand composition and abiotic factors when implementing forest growth models to improve their yield predictions.