An integrative urban tree risk index as a novel framework for risk assessment: a case study of Montreal, Canada

Urban trees provide essential ecosystem services in cities, but are increasingly at risk from climate extremes, urban stressors, and pests and diseases. Assessing and managing these risks is vital to maintaining the benefits urban forests offer to communities. These challenges highlight the need for robust, integrative tools to assess species-specific vulnerability and support evidence-based urban forest management. Here, we introduce the Urban Tree Risk Index (UTRI), a novel integrative framework designed to evaluate the vulnerability of urban tree species. The UTRI incorporates five components: climatic safety margins (reflecting exposure to climates outside the species’ tolerance limits), species abundance (affecting risk of disease spread), tree size and age (both affecting risk of mortality), and four key ecophysiological traits (associated with tolerance or resistance to stressors: leaf nitrogen content, seed mass, specific leaf area, and wood density). We assessed the urban forest of Montreal, Canada, and our results suggest substantial variability in safety margins across species, with many already experiencing conditions near or beyond their climatic thresholds. Notably, the five most abundant species, including Acer platanoides and A. saccharinum, constitute nearly half of Montreal's urban forest and rank among the most vulnerable species according to UTRI, due to their high abundance, low climate resilience, and high climate vulnerability. By analysing with combined climate, demographic and ecophysiological trait data, the UTRI provides an integrative risk assessment, highlighting vulnerable species and locations, and supporting strategic urban forest management to enhance resilience under future climate challenges.