In my last play through I have noticed that dropping a district on a tile with something chopable (usually woods or rainforest) removes those things and gives no benefit. I started using a build from my builders to clear the tile before I start the district. It gives me a nice boost in production and food with the rainforest. This production is either used on the current project or will be applied to the district if the production queue was open. Anyway, I had missed out on this for my first few games and figured I would share.