The aim of this study was to assess how the plant community and its species diversity are affected by establishment methods and warming. Priority effects, i.e., the order of species arrival in plant establishment, are an important subject when invasive species become more common, and the biodiversity decreases as the global temperature continues to increase. A field experiment was carried out to investigate the effect of establishment methods and temperature on plant communities by using four different establishment methods. The plants were divided into two groups (fast and slow germinating) and sown at two occasions, one earlier than the other. Half of the study plots were then treated with OTC (Open Top Chambers) and the other half were used as control. There was a difference in temperature between the OTC blocks and the control blocks. OTC had no effect on the number of species or the Shannon diversity index but affected the proportion fast germinating, slow germinating, and naturally established species. There was a significant effect of the establishment events on the plant community. There was also a significant interaction between OTC treatment and establishment, only present for the proportion of fast germinating species. The temperature differences can provide information on plant responses to global warming and the results can mostly be explained by priority effects such as numerical superiority and competition in combination with germination rate are. The results of this study can be used to better understand how warming and priority effects may affect plant communities in the future.