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Urban trees are a source of ecosystem services and contribute to increase quality of life for many communities and their residents. At the same time they could also determine disservices, such as damages to structures and risk to human safety due to critical combination of tree defects and environmental factors. Although knowledge about tree structure and host–pathogen interactions has grown, the assessment of risk has not been fully explored yet. In particular, the multidimensionality of tree risk assessment as well as the concern about weighting techniques in the construction of indices to measure it need to be addressed. 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. [ABSTRACT FROM AUTHOR]

Since adequate information on the distribution of biodiversity is hardly achievable, biodiversity indicators are necessary to support the management of ecosystems. These surrogates assume that either some habitat features, or the biodiversity patterns observed in a well-known taxon, can be used as a proxy of the diversity of one or more target taxa. Nevertheless, at least for certain taxa, the validity of this assumption has not yet been sufficiently demonstrated. We investigated the effectiveness of both a habitat- and a taxa-based surrogate in six European beech forests in the Apennines. Particularly, we tested: (1) whether the stand structural complexity and the herb-layer species richness were good predictors of the fine-scale patterns of species richness of five groups of forest-dwelling organisms (beetles, saproxylic and epigeous fungi, birds and epiphytic lichens); and (2) the cross-taxon congruence in species complementarity and composition between herb-layer plants and the target taxa. We used Generalized Linear Mixed Models (GLMMs), accumulation curves and Procrustes analysis to evaluate the effectiveness of these surrogates when species richness, complementarity and composition were considered, respectively. Our results provided a limited support to the hypothesis that the herb-layer plants and the stand structural complexity were good surrogates of the target taxa. Although the richness of the herb-layer plants received a stronger support from the data than structural complexity as a predictor for the general patterns of species richness, the overall magnitude of this effect was weak and distinct taxa responded differently. For instance, for increasing levels of herb-layer richness, the richness of lichens showed a marked increase, while the richness of saproxylic fungi decreased. We also found significantly similar complementarity patterns between the herb-layer plants and beetles, as well as a significant congruence in species composition between herb-layer plants and saproxylic fungi. Finally, when different stand structural attributes were considered singularly, only the total amount of deadwood received support from the data as a predictor of the overall species richness. At the fine scale of this study, herb-layer plants and stand structural complexity did not prove to be effective surrogates of multi-taxon biodiversity in well-preserved southern European beech forests. Rather than on weak surrogates, these results suggest that sound conservation decisions should be supported by the information provided by comprehensive multi-taxonomic assessments of forest biodiversity. [ABSTRACT FROM AUTHOR]