Montreal Urban Observatory: research platform to monitor urban forest ecosystems for global change adaptation and health

As urban populations grow, cities are increasingly seen not only as drivers of climate change but also as critical arenas for implementing mitigation and adaptation strategies. Urban forests and green infrastructure play a vital role in shaping environmental quality and human health, yet access to these benefits remains inequitable. The Montreal Urban Observatory was designed to investigate the complex relationships among biodiversity, ecosystem functioning, and human health. Twenty-five permanent plots were established across the Island of Montreal, strategically located along gradients in vegetation cover, population density, and mean household income. Interdisciplinary research within the Observatory focuses on themes including urban forest structure and function, abiotic conditions, biodiversity, ecological interactions, natures contributions to people, related human health outcomes, and developing novel methodological approaches. The Observatory supports long-term, cross-scale monitoring and fosters collaborative research on urban social-ecological systems, thus contributing to global initiatives to enhance urban sustainability and equity through nature-based solutions. Early results confirm the relevance of the main gradients for several organisms and response variables, from microbes to trees and abiotic factors. For example, sampling all trees, public and private, within a radius of 200m centered on each plot revealed significant differences in the diversity and structure of private and public trees, including an overwhelming dominance of Thuya occidentalis not captured in commonly used public tree inventories.