HEAT TOLERANCE, ACCLIMATION AND STRESS IN TREE SPECIES COMMONLY USED IN SWEDISH CITIES

Abstract

The investigation of heat tolerance in Nordic urban trees represents an underexplored domain, yet it is a critical area of study due to escalating global temperatures, particularly in urban settings. This study examined heat tolerance, heat acclimation and the connection between tree traits and leaf temperature. The primary methodology employed in this study revolved around chlorophyll fluorescence for plant stress assessment, but measurements of leaf size, stomatal conductance, and leaf angles were also conducted. The four tree species included were Acer platanoides, Betula pendula, Prunus avium, and Tilia cordata, all native to southern Sweden. The findings indicated a few differences in heat tolerance levels among the examined species, with Tilia cordata being slightly better than Betula and Prunus. The study further revealed a lack of significant heat acclimation among the experimental tree species since there were no treatment differences after heatwave 1 and heatwave 2, and also since the stress was the same in the control and heatwave groups after heatwave 3. Several significant regressions between temperature and tree traits (stomatal conductance, characteristic leaf dimension, and leaf angles) were identified, though the significance of these regressions varied across different heatwaves. Substantial disparities in thermal safety margins were also observed among the species, with Tilia having the largest thermal safety margin and Pav having the lowest one after the third heatwave. This study offers valuable insights into select Swedish urban tree species' heat tolerance dynamics, bridging knowledge gaps about heat tolerance and acclimation mechanisms where data has remained scarce. The findings have significant implications for urban planners and foresters, providing guidance for managing urban heat challenges and enhancing the resilience of urban greenery. Future research should focus on long-term monitoring and interdisciplinary efforts to further understand the mechanisms regulating urban tree responses to temperature stress and to develop strategies for enhancing urban tree resilience in the face of climate change.