Urban areas account for a large share of global energy use and emissions and face increasing climate- and heat-related challenges. These conditions drive the development of district-scale strategies such as Positive Energy Districts. Urban Building Energy Modelling (UBEM) enables the systematic assessment of urban energy performance, but vegetation effects are often neglected or require complex model coupling. This paper presents an approach to integrate simplified 3D tree geometries into a CityGML-based UBEM framework using the energy simulation software SimStadt. Experiments are conducted in two neighbourhoods in Rotterdam, The Netherlands, using current and future climate datasets. Results show that cooling demand increases under future climate conditions, while heating demand decreases. Moreover, trees reduce cooling demand but introduce heating penalties due to winter shading, with a limited net energy effect and a seasonal imbalance between cooling and heating demand.