How large is the land area covered? Are we talking the size of s country (like, say, the size of France) or the size of a continent (like Westeros). That will determine the cause of the different biomes in your world. If it all exists in a temperate zone surrounded by ocean, then Rho will be an inland basin made into a desert through a rain shadow effect. But if we are talking an area the size of a continent, then we could attribute the different biomes to different latitude.