# Evaluation of Dynamic Properties of Trees Subjected to Induced Vibrations
## 1 Introduction  
Trees in urban areas are an important component of the landscape, contributing to improving the quality of life of the inhabitants. On the other hand, depending on their dimensions, architecture (size and configuration of trunk and branches), and position in the context of urban zones (streets, parks, squares, etc. ), falling trees can also represent a dangerous situation for the safety of inhabitants, the integrity of surrounding structures, and the temporary interruption of common activities. At the same time, in the case of historical sites, ancient trees represent an integral part of the history of the site (a beautiful example is represented by the plane trees along the Tiber River in Rome), and the falling of these trees could affect the integrity of the cultural heritage of the site itself. The attention of the scientific community toward the trees in urban areas is certainly testified by the relevant number of studies available in the literature ([1, 2, 3, 4, 5, 6, 7]) and also by the development of specific procedures based on both visual inspections and instrumental measurements. In particular, modern instruments were employed for monitoring the complex dynamic behavior of trees under wind actions [8], for measuring parameters specifically concerning the trees' geometry, and to investigate the internal structure of tree trunks [9]. In recent years, particular attention has also been devoted to the development of numerical models, similar to the ones employed for engineering structures, able to simulate the dynamic response of trees. Among these, of particular relevance are the models based on single- and multi-degree-of-freedom systems [10] and finite element models [11]. Both monitoring activity and numerical models are essential supports for performing the dynamic identification of trees [12]. Recent studies indeed focused the attention on the extension of dynamic identification approaches employed for engineering structures to trees. This certainly represents a possible important improvement to the current state of the art concerning the health monitoring and safeguarding of trees. The use of the approaches developed for engineering structures (in particular building structures [13; 14; 15; 16; 17; 18]) for studying the dynamic behavior of trees directly relies on the equivalence among the structural members (roots-foundations, trunk and branches-elevation structure, canopy-nonstructural elements) and their role within the structural system. Moreover, in terms of damage, a substantial similarity between trees and building structures can be observed. Indeed, damage influencing the structural response of buildings generally consists of defects due to the construction process, degradation of materials due to age or aggressive agents present in the environment, foundation movements due to phenomena at the level of the soil foundation, and local failures (cracks) induced by the occurrence of moderate/severe events during the life of a building. At the same time, in the case of trees, the process of growing could be affected by wood defects in both the trunk and branches, diseases or fungi leading to voids and wood degradation, variations in soil conditions leading to a loss of the anchoring function of roots with a consequent movement of the tree, and moderate/severe wind events inducing damage to the trunk or branches leading to changes in the level of vulnerability of trees toward the occurrence of subsequent wind events. On the basis of this equivalence between trees and building structures, and taking into account the important differences between nature-made and man-made structures, the identification of dynamic properties of trees during their life (in particular before and after extreme events, or in the case where visual inspections underline specific signals on vegetation) could be of particular relevance for deriving information about damages affecting the safety level of trees toward future possible events.