Factors related to natural disturbances in mountain Norway spruce (Picea abies) forests in the Julian Alps

An analysis of natural disturbances in the Julian Alps was carried out in the Jelovica-Pokljuka region (9627 ha) for the period 1979-2006. Data from salvage cuttings were used to examine the occurrence of windthrow, snow break, and insect attack disturbances in relation to site and stand characteristics, previous disturbances, and forest management. Forest stands at the site were characterized by a high growing stock (400 m(3).ha(-1) on average) and a large proportion of Norway spruce (87%). Salvage cutting due to windthrow, snow break and insect attacks was strongly related to altitude, aspect, topographic position, diameter structure, developmental phase of the stands, and forest landscape heterogeneity. Pole stands were more susceptible to snow breakage and less susceptible to windthrow and insect attack. A more heterogeneous forest landscape increased the risk of windthrow and insect attack occurrence, while the severity of all natural disturbances was lower in more heterogeneous forest landscapes. Natural disturbances and previous cuttings also significantly influenced the likelihood of occurrence and the severity of subsequent disturbances. To improve stand resistance, forest management measures considering the silviculture system and thinning are discussed.