During the 20th century, high mortality rates of Scots pine (Pinus silvestris L.) have been observed over large areas in the Rhô ne valley (Valais, Switzerland) and in other dry valleys of the European Alps. In this study, we evaluated drought as a possible inciting factor of Scots pine decline in the Valais. Averaged tree-ring widths, standardized tree-ring series, and estimated annual mortality risks were related to a drought index. Correlations between drought indices and standardized tree-ring series from 11 sites showed a moderate association. Several drought years and drought periods could be detected since 1864 that coincided with decreased growth. Although single, extreme drought years had generally a short-term, reversible effect on tree growth, multi-year drought initiated prolonged growth decreases that increased a tree's long-term risk of death. Tree death occurred generally several years or even decades after the drought. In conclusion, drought has a limiting effect on tree growth and acts as a bottleneck event in triggering Scots pine decline in the Valais.

We review the literature dealing with mediterranean climate, vegetation, phenology and ecophysiology relevant to the understanding of tree-ring formation in mediterranean regions. Tree rings have been used extensively in temperate regions to reconstruct responses of forests to past environmental changes. In mediterranean regions, studies of tree rings are scarce, despite their potential for understanding and predicting the effects of global change on important ecological processes such as desertification. In mediterranean regions, due to the great spatio-temporal variability of mediterranean environmental conditions, tree rings are sometimes not formed. Often, clear seasonality is lacking, and vegetation activity is not always associated with regular dormancy periods. We present examples of tree-ring morphology of five species (Arbutus unedo, Fraxinus ornus, Quercus cerris, Q. ilex, Q. pubescens) sampled in Tuscany, Italy, focusing on the difficulties we encountered during the dating. We present an interpretation of anomalies found in the wood structure and, more generally, of cambial activity in such environments. Furthermore, we propose a classification of tree-ring formation in mediterranean environments. Mediterranean tree rings can be dated and used for dendrochronological purposes, but great care should be taken in selecting sampling sites, species and sample trees.

Cores from conifers growing in four different climatic regions were analyzed using radiodensitometric methods. All the sites are located along altitudinal gradients between the subalpine timberline and low elevations, in some regions the arid tree line. At each of the elevation levels considered, the soil conditions range from as shallow and dry as possible to deep and moist. Various dendroecological methods were applied to determine the relationships between tree-ring growth, climatic conditions, site elevation, and site characteristics. The dominant mode of linear response between ring growth and the monthly temperature and precipitation variables was identified using response functions. A new graphical method permitted a year for year description of the complex and often nonlinear responses of tree-ring growth to the limiting factors. This is a departure from different statistical approaches that measure the most dominant relationships for a population of many years. The described single-year method is suited to analyze time-dependent growth phenomena. Typical cases of single-year responses are illustrated and the dynamics of the overlapping environmental influences are discussed. The broad spectrum of sites was used to identify ring parameters and site types most suitable for reconstructions of temperature or precipitation records.

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