Where does “берёзка” come from?

берёзка (Russian) comes from Russian берёза, from Old East Slavic береза, from Proto-Slavic bèrza, from Proto-Balto-Slavic berźas, from Proto-Indo-European bʰerHǵos — birch.

берёзка (Russian): (small) birch(-tree); young birch(-tree)