Servizi ecosistemici e disturbi naturali: cronosequenza valanghiva in una pecceta montana della Valle d'Aosta

The literature has addressed the interactions between natural disturbances and ecosystem functions, generally considering them one at a time (biodiversity, carbon, habitat, etc.). However, the effects of avalanche disturbance on the multiple functions of a forest has never been addressed before. This research aims to analyze the changes imposed on the functions of a mountain spruce forest of Valle d'Aosta following disturbance by an avalanche with different return intervals. The study area is the Crammont avalanche, in central Valle d'Aosta, at elevations between 1030 m and 2500 m a.s.l. I analyzed disturbance effects in three treatments where the avalanche occurred more than 100 years ago (control area), about 50 years ago (last known event: 1959, stand now dominated by early-seral broadleaves) and 1-2 years ago (i.e., the avalanche channel and its banks). The survey, conducted on 29 plots arranged according to a systematic sampling design, was based on measurement of dendrochronological and dendrometric variables, floristic composition and diversity (Shannon and evenness indices), presence / impact of wildlife (browsing pressure on the herbaceous and shrub layers) and on the quantification of carbon stocks (aboveground and belowground tree carbon, shrub and herbaceous layer, deadwood and soil C). In particular, soil C was determined from the topsoil (5 cm) by sieving to 0.5 mm, computing bulk density and %C by using an elemental analyzer. All variables were subjected to analysis of variance (ANOVA) to determine significant differences between the treatments. Some output variables were also modeled using generalized linear models (GLM) as a function of site, structure and compositional predictors. The results showed large differences between the areas occasionally disturbed and those regularly affected by the avalanche. In the latter, basal area, stand density, mean diameter and height, and crown cover were much lower than in the other two treatments, where time had allowed the recovery of pre-disturbance structures. Floristic diversity (number of herbaceous and shrub species and their relative abundances) and habitat for wild ungulates were, however, similar between treatments. Regarding the other ecosystem services, we found a greater amount of total carbon stocked and a better protection from rockfall in the area dominated by broadleaves (50 years since last disturbance), where stand density and structure were more suitable for there purposes. In conclusion, the avalanche disturbance (especially with medium-long return interval) can produce positive effects on ecosystem functions, e.g., improving protection from natural hazards, and increasing biodiversity.