Giovanni Comunello 1, Marco Ciolli 1, Paolo Cherubini 2

In this work we present a method for the evaluation of avalanche risk (Comunello, 2000). The study site (Tòf Larch) is an avalanche track located in Val di Pejo, in Trentino (Italy).

GIS and dendrochronological techniques provide useful information. We compared field observations with the geographical model. At the study site it was difficult to obtain complete information regarding the avalanche track, because of the lack of significant historical records.In order to obtain more data, we analyzed the spatial distribution of tree vegetation and made a dendrochronological preliminary study of the larch trees (Larix decidua Mill.) growing within the avalanche path.

With the aid of a numerical model elaborated by means of the GIS G.R.A.S.S. (Geographical Resources Analysis Support System) forest areas exposed to the avalanche hazard were mapped. We individuated two different kinds of risk:

• morphological risk, based on topographical features that lead to the detachment of snowpack, i.e. slope, aspect and presence of morphological discontinuities;
• potential risk, as a function of two parameters of the stand structure, i.e. number of stems per ha and canopy density (Salm, 1978; Meyer-Grass, 1985).

Using these models, the protective function of the forest may be assessed and evaluated, analysing the so called ‘global risk areas’, where the risk parameters are simultaneously presented.

We compared the results of the GIS study with field observations. The presence of clear-cut trimlines (Burrows and Burrows, 1986) suggested the occurrence of past avalanche events. A dendrochronological study was carried out in order to reconstruct and map the spatial extent of past avalanches. Three sampling zones were selected along the main trimline and within the track. The occurrence of dendroecological indicators and the observation of various kind of damage on trees indicated the more active avalanche path, in agreement with the topographical output produced by the GIS.

This multidisciplinary approach revealed to be useful. The comparative analysis was effective to deduce information about the past occurrence of avalanches, especially where historical records and direct observations of phenomena are uncomplete.