This dataset is associated with an article researching how functional traits of mountain birch (Betula pubescens subsp. czerepanovii) seedlings are influenced by warming in the context of different soil properties and presence of a neighbour, Empetrum nigrum subsp. hermaphroditum. All functional traits (shoot height, root/shoot ratio, total leaf area, specific leaf area, leaf dry matter content, N content per leaf area, C/N ratio in leaves, stems and roots, root/shoot ratio, fine root proportion in the root system, main root length, CO₂ assimilation rate and mycorrhizal colonisation of roots) were measured on mountain birch seedlings grown in a pot experiment for 12 weeks. The majority of the values were calculated as means from three sub-replicates (physical pots), with some exceptions (please see the article, including its supporting information, for more details on the used methods and design of the experiment). The results of the study revealed that warming significantly increased seedling growth, and growth rates were positively associated with several above- and belowground traits indicating resource acquisition. Growth rates were not correlated with leaf traits such as specific leaf area and leaf dry matter content. Soil origin and the presence of neighbour did not affect the growth rate responses to warming, although seedlings grown in high-elevation soils generally grew faster and had higher CO2 assimilation rates. These seedlings also expressed traits commonly associated with conservative growth strategy.