In the forest-steppe zone, the plant species composition under the forest, especially birch, canopy can be quite similar to that of zonal grasslands. Here we synthesize, for the first time, plant compositional data for forests and grasslands across the entire forest-steppe zone with the aim to address the questions how different are local species pools across different parts of the Asian forest-steppe zone and to what extent is the species composition of forests and grasslands similar and whether the degree of a difference between forests and grasslands is consistent along a large longitudinal gradient. To answer these questions, a transect of about 5,000 km was stretched from the west to the east, involving nine key areas. Based on 2,000 vegetation-plot records, we calculated Bray-Curtis Coefficients between forests and grasslands and between the individual areas. NMDS was used to compare species pools of forests and grasslands in the individual key areas in a gradient space. The forest and grassland floras differed in species composition in the individual areas, but the forests and grasslands of the same area displayed similar positions along the principal NMDS axis. We observed rather steep compositional changes along the longitudinal gradient, but quite a large set of species was shared between the key areas from the Ural Mountains to the Yenisei River. The North Asian forest-steppe of Western and Central Siberia can hence be considered a single entity in terms of species composition at the species pool level. Eastward from the Yenisei river, the species composition of both grasslands and forests changed suddenly, mirroring an important biogeographical boundary. Our results suggest that the forest-steppe zone should be viewed as a single biome that is characterized by an alteration of two compositionally and structurally distinct habitats, forest and grasslands, which, however, show similar patterns in terms of species pool size and local diversity.