Assessing the vitality of Norway maple trees in an urban setting near streets within pits and strips using morphological and ecophysiological methods

Climate change-related stressors are leading to early tree deaths in cities worldwide, often before they reach their expected sizes. This study assessed the health of Norway maples (Acer platanoides L.), the most common urban tree in central Europe, in Karlsruhe, Germany. We combined observational and experimental methods at several street sites, comparing tree vitality in different site types (pits vs. strips) near streets. We also explored how various biotic and abiotic factors impacted tree health. During the 2022 growing season, we collected data on morphology, eco-physiology, and environmental conditions from 235 randomly chosen trees across two site types. For each tree, we calculated leaf area index (LAI), building index (reflecting neighboring building competition), and Hegyi’s competition index (indicating competition from nearby trees). Using generalized linear models and linear mixed-effects models, we analyzed the influence of factors such as pit vs. strip location, crown height, distance to roads, light exposure, vegetation cover, competition indices, crown volume, pruning, and others on traits including leaf area, crown projection, crown openness, crown loss, dieback, discoloration, sun scald, epicormic shoots, stomatal conductance, and electron transport rate. Trees in strips had, on average, five times more open surface area than those in pits. Site types (pits vs. strips) had a statistically significant impact on variables such as crown dieback, discoloration, and sun scald. Dieback, leaf discoloration, and crown openness were notably higher in pits. Neighborhood tree competition reduced crown projection and density, increased crown loss, and prevented sun scalding. Vegetation cover reduced crown loss and stomatal conductance. High light exposure negatively impacted most measured variables. Overall, the study highlights the need for a comprehensive arboricultural approach to understand and manage urban trees. It seeks to balance canopy size and density to optimize cooling and shading benefits while maintaining tree health.