[usrusr]

> Regardless, I find it difficult to believe that already-struggling tribes would further starve themselves just in case this or that opponent tried to invade.

Allow me to apologize for my lack of precision, I never intend to imply that they deliberately avoided "invader-friendly" crops. Wheat just had not spread that far, due to climate (not yet sufficiently adapted by breeding?) and/or cultural reasons: nomadism (well, more effect than cause I guess) and the fact that large scale forest clearances are the type of project that can only happen in presence of big organizations that are stable enough to enable such long term investments. Many German settlements still carry the name of the medieval nobleman who commissioned the original clearance (names ending in -rode, -roda, -reuth and probably some more regional variations), which implies that before the clearance, there was only wilderness.