Numerous challenges – from population increase to climate change – threaten the sustainable development of cities and call for a fundamental change of urban development and green-blue resource management. Urban forests are vital in this transition, as they provide various ecosystem services and allow to re-shape and re-think cities. Based on a Europe-wide community effort with diverse experts centered around urban forests and urban greening, we propose five key research fields to generate the knowledge required to unlock fundamental changes in urban development and green-blue resource management: circular bioeconomy, climate resilience, governance, social and human environment, and biodiversity. To support the design of greener, cooler, more inclusive and resilient cities, all these research fields require inter- and transdisciplinary collaboration, engaging stakeholders in transforming urban engagement and functioning. We summarise main inter-, trans- and multi-disciplinary research paths for each field and the cross-cutting knowledge areas that can help to address the challenges many cities face (e.g., modelling and assessment of the urban microclimate). For transforming cities further knowledge is needed on e.g., urban innovation, transition, participation, and more. Finally, we address how the identified research gaps can be implemented (e.g., international coordinated research effort, interdisciplinary networks).