Mortality rates of urban trees in relation to tree species and size development

Abstract

Urban trees provide vital benefits such as carbon storage and urban cooling. However, limited knowledge on urban tree mortality hinders effective management of sustainable urban tree stock. Utilizing cadaster datasets, this study investigates urban tree mortality across 959,466 trees in four German cities from 2016 to 2022 and identified key factors affecting tree mortality. While assessing annual mortality rates, we found an average annual mortality rate of 1.3 %, with a peak in 2019. Street trees exhibited a 0.3 % higher mortality rate than non-street trees, underscoring their vulnerability to urban conditions. Additionally, annual mortality rates were higher for coniferous species (2.2 %) compared to broad-leaved species (1.3 %). Using multivariable logistic regression approach, significant correlations were found between mortality probability, drought tolerance indices, and local climate determined by de Martonne Index (dMI). Our findings showed that younger and drought-intolerant street trees are likely more vulnerable to urban environmental stressors than matured, drought-tolerant ones. The study successfully assessed cadaster data, demonstrating their utility for urban tree mortality monitoring but highlighted challenges such as difficulty in accessing data, inconsistent data collection intervals, various data formats and less frequent monitoring of non-street trees. This research underscores the need for long-term monitoring and highlights the importance of species-specific and environmental factors in urban forestry management.