Research on the cooling effect of trees at public squares in Germany

The increasing frequency of heat waves due to global warming, coupled with the urban heat island effect (UHI), poses significant risks to human health in cities, particularly in highly frequented areas such as public squares. As urbanization continues and temperatures rise, effective heat mitigation strategies are essential. Trees, with their cooling effects through shading and evapotranspiration, offer a key solution by reducing air and surface temperatures, thereby improving thermal comfort in urban environments.

This study investigates the cooling potential of trees in public spaces in Karlsruhe, Germany, a region in the heat-prone Upper Rhine Valley. It examines how tree characteristics - such as trunk height, diameter at breast height, and crown volume - and site factors - such as sky view factor, tree view factor, and leaf area index - influence the heat index, which measures thermal comfort. An essential aspect of the study was to assess the correlation between surface temperature and heat index, allowing the prediction of heat index from satellite-derived land surface temperatures. The novelty of this research lies in its integrative approach, combining tree characteristics and site factors and focusing on an under-researched region.

Field measurements were taken at eight public squares with varying tree cover and size during July and August 2024. Data collected included surface temperatures, tree-level variables, and site metrics, which were statistically analyzed with the heat index using correlations and simple linear regressions.

The results showed that squares with higher tree cover had significantly lower heat index values, indicating improved thermal comfort. Larger trees with higher trunk heights were particularly effective in reducing heat stress. The study also found that a lower sky view factor and a higher tree view factor correlated with reduced heat stress, highlighting the critical role of tree canopies in cooling public squares through shading. In addition, surface temperature was strongly correlated with heat index, suggesting that satellite-derived temperature data could be used to estimate thermal comfort in urban squares.

In conclusion, this research highlights the critical role of trees in mitigating the UHI effect in public squares, where heat stress can significantly impact public health. The results provide valuable insights for urban planning, demonstrating that targeted greening strategies, such as maintaining large trees, increasing canopy cover and frequency of large trees, can improve thermal comfort in public squares. In the future, cities can use satellite-derived land surface temperatures to accurately model and predict heat index, enabling more efficient and cost-effective planning to address heat-related challenges and create more sustainable, liveable public spaces.