The interacting ecological effects of large-scale disturbances and salvage logging on montane spruce forest regeneration in the western European Alps

Insect outbreaks and wind storms are the most common natural disturbances affecting the northern slopes of montane Norway spruce (Picea abies (L.) H. Karst.) forests in the European Alps. The combined effects on stand dynamics, including tree regeneration, of these natural disturbances and an anthropogenic (i.e., salvage logging) disturbance were studied to assess the role of post-disturbance management practices. The study areas were in two adjacent inner valleys of the Aosta Valley Autonomous Region (western Italian Alps). Les Combes study area experienced an insect (Lymantria monacha L.) outbreak that affected 91. ha in 1984-1990, while a wind storm impacted 70. ha in the other valley (Parriod study area) in 1990. Salvage logging took place in both study areas 2. years after the natural disturbance occurrence (i.e., in 1991-1992 in Les Combes, and in 1992-1993 in Parriod), and only dead trees were salvaged. Site characteristics, overstory and understory trees, and coarse woody debris (CWD) data were collected during the summer of 2010 in 90. plots randomly distributed within disturbed portions and in control stands of the two study areas. The variability of natural regeneration structure and composition in relation to environment, forest structure, type of disturbance, and salvage logging were analyzed by univariate (i.e., ANOVA and correlations) and multivariate (i.e., MRPP, PCA, and RDA) analyses. Regeneration structure and composition were clearly different for the two disturbance types. Individuals originated mainly from seed after the insect outbreak, while an important role was played by advance regeneration in the windthrow area. Regeneration was more abundant and rich in shade tolerant species (i.e., Abies alba Mill.) after the insect outbreak, while Larix decidua Mill. and light demanding broadleaves were dominant after the windthrow. The two major factors influencing post-disturbance regeneration were the amount of the residual overstory and the amount and distribution of CWD. The presence of residual forest patches facilitated regeneration establishment by reducing herb and shrub competition, and regulating the light that penetrates to the ground. Woodpiles had a negative influence on regeneration in terms of abundance and also influenced composition. On the contrary, scattered CWD facilitated the establishment of new individuals by reducing competition with herbs and shrubs, reducing the impact of browsing, and improving substrate moisture, especially for Norway spruce.