Tree rings as ecological indicator of geomorphic activity in geoheritage studies

Abstract

Mountain areas are characterized by geomorphic processes, especially mass wasting and snow avalanches, which may impact the landscape affecting also the biological component, trees included. If sites colonized by trees are characterized by geomorphic features with a high Global and Scientific Value, including Representativeness of geomorphological processes, Educational Exemplarity, and Integrity, they can be considered geomorphosites. In the framework of assessment of the Scientific Value of geomorphosites, Ecological Support Role is of great importance. Hence, tree rings derived information can be used as indicators to refine the Scientific Value of the sites and also to propose multidisciplinary approaches to understand landscape dynamics. In fact, trees colonizing sites of geomorphological interest are used for detecting past and present events and tree rings may be considered ecological indicators under different points of view. Arboreal vegetation can register growth disturbances in terms of morphological features, at macro- (particular morphologies of trunks) and micro-scale (annual growth rings, stress indicators like compression wood, traumatic resin ducts), becoming a powerful indicator of the geomorphic activity affecting the landscape. In some cases, combined with other techniques like climate data analysis, they may allow refining the often lacunose historical records of geomorphic events impacting different territories. The integrated analysis carried out in the Loana Valley (Sesia Val Grande UNESCO Geopark, Western Italian Alps), considering a selection of geomorphosites affected by mass wasting processes and snow avalanches and located along a touristic trail, allowed to detect which meteorological thresholds favour hydrogeological instability (i.e. overcome of Mean Annual Rainfall of 6–10%). Tree rings data coming from the investigated sites provided information on the recurrence of geomorphic activity, allowing filling gaps within the historical archives, by individuating years during which hydrogeological or snow-related events probably occurred and that were missed (i.e. 1986, 1989, 2001, 2007), and adding details on sites for which temporal constraints had not been found before (i.e. Pizzo Stagno Complex System). Finally, investigated sites demonstrated to differently record the history of instability affecting the area and this difference is mirrored in the sites values that are adopted in the framework of geoheritage analysis (Scientific Value, Ecological Support Role and Educational Exemplarity). The proposed multidisciplinary approach, including geomorphology, dendrogeomorphology and climatology, represents, hence, a useful tool in geoheritage valorisation and management strategies.