Arctic and alpine tundra ecosystems are important for global carbon sequestration, as they store more than half of the global soil carbon (C), and these systems are predicted to have the greatest soil C loss following climate warming. Deciduous shrub cover has increased in tundra ecosystems during recent decades, but herbivory can counteract this. An open question is how shrub expansion will affect the carbon balance of these ecosystems? Where some studies predict increased ecosystem C storage with shrub expansion because of more vegetation biomass, a growing body of literature indicate that shrubs release soil C through below-ground processes. The overall aim of this thesis was to investigate how deciduous shrub expansion affects the growing season ecosystem CO2 fluxes and C pools in an Empetrum-dominated heath, an herb-and cryptogam-dominated meadow, and a Salix shrub-dominated community in Dovrefjell, Central Norway. Additionally, experimental treatment effects of exclusion of small and large herbivores together with planting of small Salix plants was measured, and abiotic and biotic drivers of C dynamics identified. The results from this thesis indicate how shrub expansion into low statured alpine tundra communities may influence the summer C cycling differently depending on plant community. Also, soil C pools might decrease due to below-ground differences and processes happening outside of the growing season. In this way, shrub expansion into alpine ecosystems might contribute to increased atmospheric CO2 through a net CO2 release from below-ground pools.