A cross-continental analysis of tree-related microhabitats, stress symptoms, and biodiversity assessed through eDNA in Acer platanoides L. and Quercus rubra L. trees located in urban parks and on streets

As urban forestry becomes increasingly vital as a nature-based solution to mitigate the risks of climate change and urbanization, a thorough understanding of the significance of tree-related microhabitats (TreMs) in trees growing in parks and along streets is essential. Previous studies highlight the importance of tree size, species origin, and management intensity in creating TreMs; however, the specific influence of species origin—whether native or introduced—on microhabitat provision and urban forest biodiversity remains unclear. To investigate this, we assessed 320 trees in the cities of New Haven (USA) and Karlsruhe (Germany) using TreM and stress-symptom evaluations, and conducted environmental DNA (eDNA) analysis on a subset of 24 trees. Comparing Red oak and Norway maple in two urban environments, we provide initial insights into how species origin, size, location, and tree vitality influence TreMs, and whether eDNA can reveal biodiversity patterns between New Haven and Karlsruhe. Generalized linear models that included both cities indicated that stress symptoms, species identity, and tree size were better predictors of TreM provision than site or species origin. Overall, Red oaks had lower TreM richness and abundance across our study cities than Norway maples, while our eDNA analysis revealed that Red oaks supported greater taxonomic diversity than Norway maples. The eDNA results demonstrated a distinct community of shared biodiversity across Germany and the USA, including vertebrates, invertebrates, fungi, and bacteria. These findings may be linked to Red oak’s role as a long-lived, late-successional species in forest ecosystems and its larger size compared to Norway maple. We conclude that cities should retain larger trees to support habitat diversity and that additional research is needed to link biodiversity, TreMs, tree species identity, and arboricultural practices.