Forests are biodiversity hotspots and provide a multitude of ecosystem services. An important characteristic of forests is their microclimate beneath the canopy layer, which buffers temperatures. The forest microclimate provides favourable conditions for understorey species, which are sensitive to climate warming. Some ecosystem functions are strongly driven by temperature, e.g. litter decomposition. Urban areas are characterised by increased temperatures compared to rural areas, a phenomenon referred to as the urban heat island effect. How this heat island affects the forest microclimate, biodiversity and ecosystem functioning in urban forests is unclear, whereas it provides a unique opportunity to gain insight in the effects of global warming. In this research project, I will first assess the microclimatic buffering in urban forests. Secondly, I will investigate the impact of the urban heat island on understorey plant communities and, thirdly, on litter decomposition. These three topics will be studied in an innovative design along three spatial climate scales: (1) a European latitudinal gradient, (2) a regional urbanisation gradient, (3) a local forest structure gradient. The results of this study will provide novel insights on the impact of the urban heat island and climate change on forest microclimates, biodiversity and ecosystem functioning, being highly relevant for forest managers, conservationists, as well as urban planners and policymakers.