Eastern Morocco is mostly desert. That is a broad term, though, and most desert is not what you might think. The picturesque, sandy desert with dunes (like Erg Chebbi) is fairly rare. Your average Moroccan desert is a bleak expanse of blackish-brown gravel, like a half-finished road paving project.
Trees, when you see them, are mostly acacias. Driving around Morocco I would sometimes forget that we were technically in Africa, but seeing those acacias with their splayed branches and flat crowns made me want to bust out that song from The Lion King. You know the one.