Climate-driven tree failures: how extreme rainfall threatens the survival of monumental Araucaria angustifolia trees

This study presents a case-based forensic analysis of the stability and failure of monumental Araucaria angustifolia trees following extreme rainfall events in southern Brazil. A. angustifolia is a keystone species of the Araucaria Forest, notable for its ecological significance, unique morphology, and longevity, but is increasingly threatened by extreme weather events. Recent years have seen an increase in the frequency and intensity of extreme rainfall events, leading to soil waterlogging, reduced root anchorage, and a higher incidence of large-old tree failure. The research focuses on the René Frey Ecological Park, an urban forest in Fraiburgo, Santa Catarina, where several large Araucaria trees fell during extreme rainfall events in 2023, while other monumental individuals survived. Through a multidisciplinary approach, the study integrates morphological analysis, biometric data, pedological, and climatological assessments to evaluate the factors contributing to tree instability and uprooting. Results highlight that prolonged soil saturation led to a marked reduction in soil shear strength, approaching critical thresholds for clayey soils, thereby weakening root anchorage. When combined with sheer weight and broad canopies of old-growth Araucaria specimens increase the risk of toppling. The study also considers landscape positioning and individual tree characteristics to explain varying outcomes among trees during the same climatic events. Findings demonstrate that the interplay of morphological vulnerabilities and altered soil physical-mechanical properties underpins the susceptibility of these monumental trees to climate-driven disturbances. This research underscores the urgent need for targeted conservation strategies and proactive monitoring of rare, large Araucaria individuals. These actions are particularly important in poorly drained landscapes vulnerable to extreme rainfall. The loss of these trees represents not only an ecological impact but also a threat to the cultural and natural heritage of the species across its native distribution. The findings further highlight direct implications for urban and peri-urban tree management under extreme rainfall scenarios.