Can composite indices explain multidimensionality of tree risk assessment? A case study in an historical monumental complex

The aim of this paper is to develop a composite tree risk index (R) useful for the management of amenity trees, with a two-fold perspective. First, the multidimensionality issue is explored by considering three dimensions of the tree risk: the Hazard, that represents the likelihood of failure of the tree, the Contact factor, i.e. the nature and the value of the target, and the Damage factor, i.e. the potential for injury or damage. Each dimension is assessed as a function of a set of observed variables related to dendrometric attributes, health status, proximity to buildings and artifacts, tree esthetic value, derived both from field data and spatial analysis performed under GIS environment. Second, in the aggregation of variables into specific-dimension indicators we verify if and to what extent various positive and normative weighting techniques influence the construction of composite index. The R index, obtained as the product of the specific-dimension indicators, was computed for 130 trees located within the formal garden and the park of Villa Lante in Bagnaia, Viterbo (Central Italy), a well-known Renaissance monumental complex, where the needs of cultural heritage conservation and human safety have to be balanced with recreational uses. Sensitivity analyses were carried out in order to test the robustness of R with respect to the considered techniques.