Further observations on sudden diebacks of Scots pine in the European Alps

Since the beginning of the 1990s, significant mortality of Scots pine trees has been observed in inner valleys of the Alps. The objectives of this work were to investigate and describe the aetiology of a Scots pine dieback in the Aosta Valley (western Italian Alps) in 2005. Surveys were conducted in three forest stands. Crown transparency was assessed to evaluate the incidence and severity of dieback symptoms. Phytosanitary surveys were also performed. The time series of the major climatic parameters was analyzed in order to identify common climatic anomalies between the dieback of 2005 and similar dieback episodes in other periods and in other areas of the western Alps. Dendrochronological analyses were performed to assess the relationship between tree-ring widths and climatic parameters. The lack of primary biotic agents and the low frequency of secondary pathogens suggest an abiotic origin of diebacks. The time series analysis shows that two consecutive years with a value of summer dryness index below 1.5 preceded the diebacks. Tree-ring width and summer dryness index were strongly and significantly associated.