Lichens, renowned for their resilience in extreme environments, serve as valuable bio-indicators of environmental conditions. Despite this recognition, environmental influences on lichen ecophysiology are not well understood in urban environments. In this study, we explore the use of functional traits in analyzing the impact of the urban heat island (UHI) on epiphytic chlorolichens. Lichen material was collected from 12 sites across an UHI gradient in Amsterdam and Leiden, the Netherlands. For each lichen specimen, the specific thallus mass (STM) and water-holding capacity (WHC) were calculated. The relationship between the UHI and STM/WHC was assessed using linear mixed models and ANOVA. Our study provides functional trait values (STM and WHC) for 18 species for which no prior data were available. Furthermore, our findings reveal a significant correlation between the UHI and the STM, which suggests STM as a potential indicator for the UHI.
