if you read kaesar "journal" during his campaigns in gaul there was a draught that caused food shortage and he was surprised with the amount of grain available in britain

as for the luigi they lived in a mountenous region overbearing the polish great plains one of europes most fertile land i was in those mountains during winter sports season in a place called zakopane so yes it´s easy to practice transumance (pastoralism) during harsher periods its actually less affected since it´s less land intensive

and weater or cooler periods are positive for agriculture lack of water is far worse and combined with the "taming" of the keltic culture the evolving of agricultural practices in the mediterranean particulary the carthaginians wich was later transfered to the rest of the mediterranean by the romans at the same time in britain and the keltic world the appearance of farming estates begun to develop so agriculture practices was on the rise and expanding to more territories wich until then had only had pastoralist practices mainly iberia and gaul wich begun allowing the acumulation of wealth and the apearance of yet more trade particulary exotic and luxury goods such as wine amber and so forth

ofc that some regions mostly mountain regions farming is harder and breading goats and sheeps is just easyer to the luigii who lived in a mountenais regions same for the germans who lived in an heavily forested region but they still wanted the luxury goods of the perfumed civilized people so if they couldn´t get it by trade they got it by weapons just like the kelts did for hundreads of years until the appearance of a landed nobility wich discovered it was easyer to produce and trade then to risk their lifes in batle so more wealth concentration more creation of farm steads more wealth more slaves to work the lands means higher productivity smaller farmers are forced to sell their land so more concentration of wealth

the grachi roman revolution wasn´t exclusive to the romans the growth of agriculture was clear and it had clear effects on how society was changing

hope this helped sometimes i loose focus and go off topic