Tree regeneration is one of the key processes underlying forest dynamics. In temperate forests, growing conditions for tree regeneration are strongly determined by understorey vegetation, which can act as a ‘filter’ for juvenile trees through competition for available resources. In the future, this understorey filter is expected to change due to multiple global change drivers, such as climate warming or changes in forest management. We hypothesise that due to global change, graminoid species will increase in dominance in temperate forest understoreys, a process which is referred to as ‘grassification’. However, evidence for this is anecdotal, and forest grassification has never been analysed at the continental scale for Europe, especially in light of global change. First, we aim to investigate this grassification of understoreys and assess its causes using large-scale observational data. Next, we will gain a better mechanistic understanding of the impact of grassification on tree regeneration under environmental change by setting up a controlled field experiment. Finally, we will integrate these observational and experimental outcomes into a process-based dynamic vegetation model. This innovative, integrated combination of approaches will allow us to make better predictions of tree regeneration outcomes and long-term forest dynamics under future change scenarios.