Why is taiga the dominant natural zone of Eastern Siberia?

1) The harsh continental climate with severe winters makes it difficult for any trees to grow, except for conifers. Therefore, there is no broad-leaved forest zone here. At the same time, the background radiation and summer temperatures are high enough for tundra vegetation to be replaced by forest.
2) The elevation of the territory takes place with the formation of different spectra of altitudinal zonality, but pushes the territory to the north – to the kingdom of the taiga.