now the obvious answer would be architecture, but many subjects have their merits when you apply the knowledge to your craft - and env art isn't just only architecture.
But having a good understanding of how colors and light work will help you with texturing and lighting and creating materials. Composition helps you seeing the big picture and how all the elements you create relate to each other.
Or just go exploring with a camera - discover things you haven't looked at before and think how to present them best (after all, presenting a game, is what our art does!) - look at buildings, textures, trees, environments in the real world and discover what is it that makes them look real and how you can simulate this in your own work for more realism.
If you want to stay on the theoretical side - I always enjoyed art history. It gives you a lot of pointers where to look for new inspiration - different cultures, art movements, periods...