I'm aware of this problem, though it's good that you reminded me. The next version will have a stockpile of extra resources in the dwarf town. You'll get to have them when you conquer the dwarves.
For the future, I've been thinking about renewable resources, kind of like mines in other RTS games that produce a fixed amount per turn or so. It should create a more interesting economy. But I don't know yet how to implement that within the game. Ideas welcome.