An increasing number of studies have reported on forest declines and vegetation shifts triggered by drought. In the Swiss Rhone valley (Valais), one of the driest inner-Alpine regions, the species composition in low elevation forests is changing: The sub-boreal Scots pine (Pinus sylvestris L.) dominating the dry forests is showing high mortality rates. Concurrently the sub-Mediterranean pubescent oak (Quercus pubescens Willd.) has locally increased in abundance. However, it remains unclear whether this local change in species composition is part of a larger-scale vegetation shift. To study variability in mortality and regeneration in these dry forests we analysed data from the Swiss national forest inventory (NFI) on a regular grid between 1983 and 2003, and combined it with annual mortality data from a monitoring site. Pine mortality was found to be highest at low elevation (below 1000 m a.s.l.). Annual variation in pine mortality was correlated with a drought index computed for the summer months prior to observed tree death. A generalized linear mixed-effects model indicated for the NFI data increased pine mortality on dryer sites with high stand competition, particularly for small-diameter trees. Pine regeneration was low in comparison to its occurrence in the overstorey, whereas oak regeneration was comparably abundant. Although both species regenerated well at dry sites, pine regeneration was favoured at cooler sites at higher altitude and oak regeneration was more frequent at warmer sites, indicating a higher adaptation potential of oaks under future warming. Our results thus suggest that an extended shift in species composition is actually occurring in the pine forests in the Valais. The main driving factors are found to be climatic variability, particularly drought, and variability in stand structure and topography. Thus, pine forests at low elevations are developing into oak forests with unknown consequences for these ecosystems and their goods and services.

-We assessed the effect of chemical soil properties and acidifying depositions (sulphur and nitrogen) on the occurrence of storm damage during the storms "Lothar" and "Martin" (December 1999). Data from 969 sites in France, southern Germany and Switzerland was analysed with multiple logistic regression models. Variables found to be significantly related to storm damage, which was mainly scattered damage in our study, were "country", "soil pH", "proportion of coniferous trees", "slope", "humus type", "stand height", and "altitude". Wind speed was not significantly related to storm damage in the global model, but only in the model for France. Soil pH was one of the most significant factors with a lower pH on damaged plots. Atmospheric deposition rates were significantly associated with soil pH, but not directly with storm damage. Even though the mechanisms involved in the relationship between soil acidity and storm damage are still poorly understood, soil acidity should be considered a significant risk factor. Moreover, this large-scale study confirms that increasing the proportion of deciduous trees would reduce the susceptibility of forests to storm damage. deposition / logistic regression / soil pH / wind damage / wind speed Résumé -Les forêts au sol acide sont plus souvent endommagées par les tempêtes. Nous avons étudié l'effet des propriétés chimiques des sols et des dépôts acidifiants (soufre et azote) sur les dommages dus aux tempêtes durant les passages de « Lothar » et de « Martin » en décembre 1999. Les données de 969 sites en France, au sud de l'Allemagne et en Suisse ont été analysées à l'aide de modèles de régression logistique multiple. Les variables liées de manière significative aux dommages dus aux tempêtes étaient les suivantes : le pays, le pH du sol, la proportion de conifères, la déclivité du terrain, le type d'humus, la hauteur des arbres et l'altitude. Dans la plupart des sites, les dommages n'étaient que partiels. La vitesse du vent n'était pas liée de manière significative aux dommages dans le modèle global, mais dans un modèle utilisant uniquement les données de France. Le pH du sol, qui s'avère être l'un des principaux facteurs, était plus bas dans les forêts endommagées. Les taux de dépôts atmosphériques étaient étroitement liés à l'acidité des sols, mais pas directement aux dommages dus à la tempête. Même si les mécanismes provoquant l'interdépendance de l'acidité du sol et des dommages dus aux tempêtes ne sont pas clairement élucidés, l'acidité du sol devrait être considérée comme un facteur risque de grande importance. En outre, cette étude réalisée à large échelle confirme qu'une plus grande proportion d'arbres à feuilles caduques réduirait la sensibilité des forêts aux dommages dus aux tempêtes. dépôts atmosphériques / régression logistique / pH du sol / dommages dus aux tempêtes / vitesse du vent

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In restoration ecology the term biodiversity is widely used to define endpoints, to assess restoration needs, and to monitor restoration success. However, the term is far from clear because different uses of it result in ambiguities and misunderstandings.