The three subgenera of Magnolia are not longer accepted:

Major clades and a revised classification of Magnolia and Magnoliaceae based on whole plastid genome sequences via genome skimming: https://onlinelibrary.wiley.com/doi/10.1111/jse.12588

Plastid and nuclear phylogenomic incongruences and biogeographic implications of Magnolia s.l. (Magnoliaceae): https://onlinelibrary.wiley.com/doi/ftr/10.1111/jse.12727

https://species.wikimedia.org/wiki/Magnolia