Urban and peri-urban monumental trees as biodiversity hubs: an international study across Grenoble (France), Helsinki -Espoo (Finland), and he Veneto region (Italy)

Urban and peri-urban trees and forests are increasingly recognized as valuable components of landscape settings for biodiversity conservation. Among them, monumental trees stand out as significant elements in urban settings. Monumental trees, characterized by distinct size, shape, and age, stand out for their significant ecological, landscape and cultural values. Therefore, monumental trees require targeted management and planning measures to protect their unique contributions. In this study, we aim to identify, analyze, and describe monumental tree communities, focusing on their biodiversity and ecological roles. We compared three study areas: the Veneto region (Italy), the city of Grenoble (France), and the cities of Espoo and Helsinki (Finland). Through the survey of tree-related microhabitats (TreMs) on 499 monumental trees, we investigated the biodiversity potential provided by monumental trees in urban areas across three European countries along an environmental gradient. We found a large variety of TreMs types across the analyzed monumental tree community. On average (±s.d.), this community hosts a TreMs richness of 5.5±3.7 with regional variation (Veneto: 5.5±3.6; Grenoble: 4.8±3.3; Espoo: 7.8±3.4; Helsinki: 4.9±4.7). Among them, crown microsoil and bryophytes stood out as the most prevalent TreMs, found in 56% and 51% of monumental trees, respectively. This project seeks to improve understanding of urban monumental tree communities by highlighting their ecological roles, thereby advancing conservation efforts for these exceptional individuals. In this context, surveying tree-related microhabitats is a promising approach for studying the multifaced biodiversity support provided by monumental trees.