The idea that it's easier to tax grains than cassava or other roots and tubers is not ours. The contribution of our paper is to say that the surplus argument is wrong. And so it's a very simple question, not subtle. But let me just because I want to be really accurate and not to take credit to myself, which is unjust.
Since at least Adam Smith, the common wisdom has been that the transition from hunter-gathering to farming allowed the creation of the State. Farming, so went the theory, led to agricultural surplus, and that surplus is the prerequisite for taxation and a State. But economist Omer Moav of the University of Warwick and Reichman University argues that it wasn't farming but the farming of a particular kind of crop (but not others) that led to hierarchy and the State. Moav explains to EconTalk host Russ Roberts storability is the key dimension that allows for taxation and a State. The conversation includes a discussion of why it's important to understand the past and the challenges of confirming or refuting theories about history.