I would like to know how much climate influences agricultural output - as we know, places with temperate climate (such as my country - Poland) have a growing season lasting roughly from April to October but places located in the tropics or the equator have a year-round growing season. How much does it influence the productivity of agriculture and, hence, population density?

Seeing that, potentially, Africa and areas around the equator have the "best" climate, yet usually don't have the same output, i 'd say climate is mostly a restraining factor;

people can grow agricultural produce inside an area that has the minimal required climate, whether they do and how well depends on factors as economics, culture, technology.

 

When i was little there i had an atlas that showed a "wheat border" but i can't find it online.