Studio integrato fito-geomorfologico di un sistema di rocce montonate di natura serpentinitica (Campagneda, Alpi Retiche, Sondrio)

The aim of this work is to identify repeatable linkages between landforms and diagnostic features of the vegetation, as a preliminary step in the development of phytoindicators for geomorphological forms and processes. The study area is located in the upper boreal belt of the Malenco Valley (Sondrio Province, Rhaetian Alps). The roches moutonnées consist of Mesozoic serpentinitic substratum and are surrounded by a sequence of glacial deposits. This sequence represents the expression of three little terminal morainic ridges, belonging to the Late Glacial Age. SYN-TAX 2000 was used to analyse 19 phytosociological relevés; a cluster analysis (group average, similarity ratio) and a metrical ordination (PCoA, Chord distance), applied to all the relevés, pointed out the presence of 5 distinct groups. Richness and diversity indexes (Shannon-Weaver index: H = -Σ i pi ln pi), dominance (Simpson index: λ = Σ i pi 2) and evenness (Pielou index: J = H / Hmax, where Hmax = ln n° individuals) have been calculated for all the plant communities, to look into their ecological stability and structural composition. We estimated the age of the glacial deposits, that surround the roches moutonnées, by using the raising of the equilibrium line altitude. This allowed to attribute more than 13.000 years B.P. to the substratum of the roches moutonnées. The distribution and variability of the phytocoenoses are linked to the existence of different environmental conditions, depending on geomorphological processes and forms of relief, that influence the water availability in the substratum, the snow cover and the wind effects. The extremely severe climatic and edaphic conditions stop the dynamic of the both plant communities of summits and interposed depressions. Nevertheless processes of edaphic rejuvenation promote regressive events in the vegetation cover; this is probably related to intense meteorological processes or depends on deep mechanical stresses caused by cryo- and/or termoclastic processes, that affect the substrata. We can, however, suppose that the potential expression of the vegetation could lead, in the long term, to acidocline alpenrose heaths (Rhododendretum ferruginei). Because of the mosaic distribution of plant communities, related to the repetitive presence of microhabitat and potentially linked by catenal and seral relationships, the roches moutonnées system can be considered a geosigmetum.