[alricb]

Local agricultural optimums in Northern Europe played a role too. How good was agriculture in the Parisian basin during the late Roman empire? Was it possible for farmers to get the iron implements and draft animals needed to make heavy soils productive?

Separating the Middle East/North Africa and Europe on the "dominant state" chart is an interesting choice, since the Roman Empire controlled most of both for centuries. Controlling a large portion of Europe is difficult unless you're a maritime-ish power with access to the Mediterranean.