The substitution of fossil fuels with biofuels to mitigate climate change has caused increased interest in enhancing forest biomass production through fertilisation. We investigated the effects of different fertilisation frequencies on the diversity of understory vegetation in young stands of Picea abies on five sites distributed in regions in the middle and south of Sweden. The treatments included fertilisation conducted annually, every second year or every third year, as well as an unfertilised control. A lower number of vascular plant species was observed on fertilised plots than on control plots, whereas the number of bryophyte species remained unchanged. Fertilised plots also showed a lower variance in species composition and a lower Shannon’s diversity index than unfertilised plots. Fertilised plots were more similar to each other than unfertilised plots were to each other over the geographical range. The two most intensive fertilisation treatments had similar effects on the vegetation, whereas the effects of fertilisation conducted every third year were not as substantial. However, the treatment in which fertilisation occurred every third year implies a lower stem-wood production, and there is little knowledge of the longterm differences between the treatments. We conclude that fertilisation of young stands will lead to long-term changes in understory vegetation at the stand scale, whereas the effects at the landscape level are still largely unknown.