Terrain's steepness governs sensitivity of urban oak forests to climate variability

Urban forests, vital for providing ecosystem services in cities, face challenges from climatic factors, particularly in heterogeneous urban environments. Kyiv, one of Europe’s largest and most populated cities, boasts a varied topography, with about 55 % of its area covered by forests, where Quercus robur predominates. In this study we investigate the sensitivity of Q. robur trees to climatic factors within Kyiv's urban forest, utilizing tree ring widths as a proxy. Employing dendrochronological approaches, we built nine site-mean ring-width chronologies and identified the growth pointer years and their frequency. Using moving windows in daily correlation analysis, we examined Q. robur response to climatic variations over the last 135 years. Additionally, linear mixed-effects models (LMMs) were applied to the site-specific characteristics such as slope steepness, altitude a.s.l., soil type, soil mechanical composition, soil pH, forest type and coefficient of soil saturated hydraulic conductivity, to identify which of them exert a significant influence on studied trees’ sensitivity to precipitation and temperature. Our results showed that the frequency of negative or positive pointer years in Q. robur growth coincides with extremely dry or wet years. The precipitation amount falling from the dormant period to the early growing season emerges as the key factor for Q. robur trees' radial growth. The trees' sensitivity to this factor was found to be significantly influenced by the slope steepness. Our findings contribute valuable insights into the complex interplay between urban heterogeneity, climatic factors, and the climatic sensitivity of Q. robur in urban areas. This research contributes to a deeper understanding of urban forests’ dynamics, proposing management strategies for slope stability, soil water retention, and natural tree regeneration.