In the early 2000s, beech forests in Western Europe suffered from a so far unexplained burst of mortality. Necroses, ambrosia‐beetle and fungal attacks were observed on the trunks. The symptoms were similar to previous events reported throughout the 20th Century. One current hypothesis is that these phenomena were related to early frost events for which the trees were physiologically unprepared and which made them vulnerable to biotic attacks. In the present study, we aimed to test this hypothesis further, by retrospective meteorological analyses and also by an experimental approach. Our meteorological analyses highlighted the occurrence of cold waves a year before the beech declines were reported in 1929, 1942 and 1998. In our experimental approach, frost injuries were inflicted to mature trees in a beech stand using dry ice. The treated trees were more attractive to insects than untreated controls. Insect attacks were observed in the treated zones on the trees but colonization was not very successful. The galleries had aborted most of the time with only a few larval chambers. Very few insects were caught in emergence traps. The results of these two approaches support and strengthen the hypothesis that frost induced beech dieback. Frost injuries increased tree attraction to ambrosia beetles to the point of inducing attacks. However, the overall success of these attacks was much lower than that observed in the 2000s. These differences might reflect limitations in our experimental approach, where frost wounding was applied locally to the trees.

© iForest -Biogeosciences and Forestry IntroductionSoil bioengineering uses sound engineering practices in conjunction with integrated eco logical principles, using living vegetation and other materials to construct slopes (hill slopes, riverbanks, and lake/shorelines), sta bilize slopes, control erosion, protect wildlife habitats and enhance the functioning of eco systems (Donald & Robbin 1996, Gray & Sotir 1996. Successes of ecological engi neering make it an increasingly attractive al ternative to traditional engineering approa ches, which are often much more expensive to construct and sustain (Li & Eddleman 2002). Vegetation can affect the stability of slopes by modifying the hydrological regime in the soil. Species often used in bioenginee ring applications include willow, poplar, grasses and native shrub species anyway usually fast-growth species.In the last years wood has been widely used as a suitable natural material to be used in soil bioengineering works (Sauli et al. 2002). In particular, among wood characte ristics, its mechanical resistance and its dura tion over time is not always necessary, being its role that of allowing plants colonization in its early stages and so these characteristics are important only in specific situations. Among the various environmental factors in fluencing the performance of the woodenwork, soil plays an extremely important role. Its properties in fact not only determine wood decay rate, but are in turn influenced by wood decomposition which can deeply affect soil physico-chemical and biochemical features. Another important aspect of soil bioengineering is thus emphasized: soil is an element either active in wood decay or pas sive, being itself influenced by wood decay. Soil nutrients cycling and availability are in deed deeply affected by wood decay so that decomposing logs have been referred as "slow-release fertilizers" (Carey 1980). Dead wood is no longer considered as me rely debris, and wood decomposition is wi dely recognized as a key ecological process (Franklin et al. 1987). Decaying wood also plays an important role in soil development because the residues from the degradation of wood components (especially lignin) are one of the substrates for humus formation (Ste venson 1982). Additionally, leaching of dis solved organic matter from decaying wood contributes to the soil organic matter pools and, providing easily available organic sub strates, may fuel soil microbial biomass, thus enhancing its size and/or activity (Yavitt & Fahey 1985, Spears et al. 2003.However, regarding the effect of decaying wood on the physical, chemical and bioche mical properties of the underlying soil, con trasting results were reported and, to our knowledge, not specifically on soil bioengi neering works. Some studies found higher C (carbon) and N (nitrogen) percentages in soil under decaying wood, but did not observe changes in the C:N ratios (Kayahara et al. 1996), while Hafner et al. (2005) reported hi gher C:N ratio beneath decaying logs.Further approaches, aimed to infer effe...

Abstract• The aim of this study is to assess the impact of drought on insect-tree relationships. Survival and feeding performances of leaf-beetles, Chrysomela populi put in cages with leaves of host-plants submitted to different levels of water stress were compared.• Ninety 1-year-old poplar (Populus × euramericana) cuttings were grown in a greenhouse, and distributed into 3 groups submitted to different water treatments during six weeks. Adult C. populi were then encaged for four days with these cuttings, together with the five terminal leaves of a twig. Beetle survival and consumed leaf areas were then determined.• Predawn leaf water potential values indicated that the drought treatments induced different levels of plant water stress. Plant height growth and total leaf numbers were reduced by plant stress. Beetle survival and leaf areas eaten were also reduced proportionally to plant water deficit.• In conclusion, this experiment suggests that drought has a negative impact on C. populi survival and feeding. Mots-clés :déficit hydrique / relations plantes-insectes / herbivores / sécheresse / populus Résumé -Étude de l'influence d'un stress hydrique appliqué à des boutures de peuplier sur la survie et l'alimentation de chrysomèles phyllophages.• L'objectif de ce travail est d'évaluer les effets d'une sécheresse sur les relations plantes-insectes. La survie et l'alimentation de chrysomèles phyllophages, Chrysomela populi, exposées à un hôte subissant divers niveaux de stress hydrique sont comparées.• Quatre-vingt dix boutures de peuplier (Populus × euramericana) âgées d'un an ont été cultivées en serres, et réparties en trois groupes soumis à des traitements hydriques différents. Après un stress hydrique continu de six semaines, les boutures ont été exposées à des chrysomèles encagées avec les cinq dernières feuilles de rameaux, pour quatre jours. La survie et les surfaces foliaires consommées par les chrysomèles ont ensuite été déterminées.• Les valeurs de potentiel hydrique foliaire de base indiquent que les traitements hydriques ont induit chez les plantes différents niveaux de stress hydriques. La croissance en hauteur des plantes ainsi que le nombre de feuilles ont été réduits par le stress hydrique. La survie des insectes et les surfaces foliaires consommées par ceux-ci ont également été réduites proportionnellement au déficit hydrique.• En conclusion, cette expérience suggère que la sécheresse a un impact négatif sur la survie et l'alimentation de C. populi.