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Les espèces exotiques envahissantes représentent une menace pour la biodiversité à l’échelle mondiale. Parmi les plantes problématiques en France et en Europe, les renouées asiatiques (Reynoutria spp.) posent un véritable défi, leur gestion étant coûteuse et d’efficacité aléatoire. Premièrement, les gestionnaires doivent choisir d’agir ou de ne pas intervenir. Pour décider, il leur faut mieux connaître les conséquences (y compris de la non-intervention) sur le fonctionnement de l’écosystème. Ensuite, ils doivent décider de la technique de gestion. Les techniques issues du génie végétal basées sur la réintroduction d’espèces végétales indigènes compétitives permettent de réduire la dominance de l’espèce exotique envahissante tout en réintroduisant de la diversité végétale. Si de nombreuses opérations de génie écologique ont été menées, leur efficacité pour restaurer le fonctionnement de l’écosystème n’a jamais été évaluée, encore moins sur la diversité et l’activité des organismes du sol. Cet article synthétise les résultats d’une étude portant sur les effets des renouées asiatiques et de la restauration des berges envahies par génie végétal sur la communauté végétale et le sol. Il s’appuie notamment sur une analyse fine de la chimie et de la faune du sol dans deux bassins versants (en Isère et dans le Rhône).

Soil processes such as decomposition are mainly performed by soil biota. Although soils worldwide are extremely biodiverse, the relationship between decomposers (fauna and microorganisms), and ecosystem function is poorly understood. Collembola are abundant and ubiquitous microarthropods that are found in terrestrial ecosystems. They can affect the amount of biomass and the activity of microbial communities, either directly through selectively feeding on fungi and bacteria, or indirectly by dissemination of microbial propagules, and the alteration of nutrient availability. However, despite the functional role they play in belowground food webs, the interactions between natural assemblages of soil microbes and Collembola receive little attention. This study, conducted in microcosm conditions, examines the effects of two distinct natural assemblages of functional groups of Collembola (ep- and euedaphic) upon microbial communities using PLFA markers and their associated soil functions (e.g., enzymatic activities and C mineralization rate) over a 2-month period. Our principal objective was to determine whether different functional groups of Collembola had varying effects on microbial soil community abundance, structure and activity, resulting in potentially important effects on ecosystem processes. Our findings show that the interactions of the functional groups of Collembola with microbial communities vary significantly whether they are alone or combined. A distinct response in the composition of the microbial communities was found at the end of the 2-month period. The communities were significantly different from each other in terms of PLFA marker composition. We found that the epedaphic species were related to and promoted Gram+ bacteria whereas euedaphic species were related to Gram- bacterial markers. This had further repercussions on soil function, such as nutrient recycling. Combining both functional groups did not lead to a complementary effect on soil microbial properties, with a drastically different outcome between the first and the second month of the experiment. Additional research dealing with the interactions between decomposers using natural assemblages will help to predict the functional outcomes of soil biota structure and composition.

Fencing and planting: two restoration strategies for the improvement of vegetation, soil fertility and soil surface properties in Algerian arid rangelands. Description of the subject. The struggle against desertification and the implementation of consecutive steppic ecosystem protection plans have become a priority in Algeria over the last four decades. Among the restoration techniques set up to manage or even restore sensitive steppic rangelands have been (i) fencing and (ii) fencing and the planting of exotic shrubs. Objectives. This study aims at describing the effect of these two restoration modes with control plots after four years on soil physico-chemical and surface state parameters and phyto-ecological properties. Method. Three hundred twelve phyto-ecological and pedological relevés were performed both on the restored plots and on controls four years after the initial implementation of management strategies. Phyto-ecological, soil surface and soil chemical parameters were measured for each modality. Results. Numerous parameters differed between the restored plots and the controls, including levels of total nitrogen and soil organic matter. However, textural parameters did not differ between the restored and the control plots. Conclusions. Our study shows that the implementation of both the restoration techniques in an arid region led within a relatively short space of time (four years) to an improvement in soil physico-chemical properties. We recommend the measurement of the percentage of coarse elements on the soil surface as an indicator of the system restoration state, and the use of the results as an important decision-making tool in reopening restored areas to grazing.

Persistence of former land-uses and conservation management of calcareous grasslands in France. In Europe, a lot of chalk grasslands are managed for the conservation of their great biodiversity. However, traditional agricultural practices are at the origin of their existence and maintained them. Although these practices are becoming well documented, very few studies are carried out on the links between traditional agricultural practices and long-term vegetation dynamics. Most of these practices have been abandoned during the last centuries. The aims of our study were to identify the traditional agricultural practices and their impacts on the composition, structure and dynamics of chalk grassland vegetation in interaction with the influence of new management systems created for their biological conservation. Three study sites were chosen in a gradient from North-West France to South-East France. In the chalk grasslands of Upper-Normandy (NW), our results showed that the species which disappeared or are about to disappear from the above-ground vegetation are arable weed species of the 19th century cultivated plots and not typical species of nowadays chalk grasslands. In the centre of France, our results showed that,whatever the present disturbance regimes, old calcareous grasslands always have a higher species richness than grasslands close to former cultivated plots. In South-Eastern France, our results showed that formerly cultivated plots have a very different floristic composition than grazed grasslands. Also, their species-richness is always significantly lower than that of grazed grasslands even after 35 years of abandonment and with the same present disturbance regime. Dry grasslands are not resilient to punctual disturbances taking place within centennial disturbance regimes. Managers must take into account these parameters to set their management priorities. Concurrently to ecological diagnosis and biodiversity inventories, they must carry out historical ecological studies to understand more clearly which variables are responsible for the structure and dynamics of these ecosystems.