Tile yields? What's behind them?
Enjoying the Fallen Enchantress Beta intensely, but one thing has bugged me so far. What actually causes the yields of tiles? Most tiles have no yield, and then suddenly there is a small patch somewhere with several tiles with varying yields. But, for the life of me, I can't figure what's so special about those locations. They seem to be near forests or rivers, but then again, I've seen many forested areas near rivers with only empty tile-yields.
Is there some logic behind the numerical values of tile-yields (like in Civ, where you could calculate them manually knowing the specifics of the soil-type, resources, etc.), or is it completely random and without connection to the tile-contents like forests, resources, rivers, etc.?