[Bears Armed]

Regarding Laeral's and High Fells' soil issues, I think that I may have made a mistake in explaining the scope of the soil issues in Laeral. Yes, the regions of Laeral away from the Riverlands are somewhat less fertile than the Riverlands themselves. However, this doesn't mean that areas not in the Riverlands are harsh deserts or completely unproductive. It's more that the majority of Laeral's crops are sourced from the Riverlands, and that the soil in non-Riverlands areas can't support certain crops, and isn't quite as fertile as it could be. The disparity between the Riverlands and other regions is more of a social issue than anything else, with the Riverlands being more developed and having a higher population and average income than other regions (in fact, around one-third of Laeral's population lives in the Riverlands provinces). So I think the 'shield' explanation works if we note that there aren't any of the climatic problems that prevent agriculture in say, the Australian outback. I just need an explanation for how some regions of Laeral would be more fertile than others. So wheat/barley can be grown in most parts of Laeral and High Fells. High Fells, meanwhile, has a much smaller population than Laeral (around ten million) and their diet features more meat than the Laeralian diet does, because there is more animal grazing there.

Your explanation about the silkworm works well, and given that the non-Riverlands soil is more fertile than you've been assuming, we can say that mulberry trees can, in fact, grow in most of Laeral, not just the Riverlands. Of course, the prevalence of mulberry trees could be an explanation for how Laeral can't yet provide all its own food. It's possible that farmers in places that could support silk production or agriculture choose to produce silk rather than food because silk commands higher prices- with the end result that Laeral has a lot of silk but not as much food as is desired.

That makes sense... and maybe the differentiation between food-producing Riverlands and silk-producing lands outside of those was made more extreme during the period of French rule, because Silk production was increased for export to France?
Given the historical tradition of population and the "traditional' crops in the Riverlands, is it possible that the upland areas -- especially those further from the main river, and perhaps especially in High Fells -- are inhabited to some extent by peoples that already differed to some extent in their basic culture from the Riverlanders (whether they were descended from the same original stock of East Asian origins as those, or belonged to an older 'Native Iduvian' stock instead) before French colonisation and that there are still differences today? This would parallel the situations in RL southern China, where Chinese expanding south from the Yellow River's basin took over the lowlands at quite early dates but parts of the highlands are still inhabited by non-Chinese people today, and in RL Vietnam where the uplands are largely inhabited not by 'Vietnamese' as such but by various other people (whom the French colonial authorities referred to, collectively, as 'Montagnards') instead...