This may be a stupid, or too complex question, I know nothing on the argument, in fact.
Assuming a low-tech setting and a temperate climate, which crops would grow better in a particularly wet region as compared with one which has rivers and irrigation but significantly less rain?
Boy, my farmgirl roots are coming in handy today :)
Actually, it depends on more than just the area...
Even in a low tech situation, *most* crops can be grown in nearly any climate *as long as there's water*.
Take any random crop - even a made up one - let's call it Garglesnaps.
If I live in a COOL/mildly warm climate, Garglesnaps need about 5-7 inches of rainfall per month. (Keep in mind that that's 7 inches of water over ALL the soil)
Obviously, if there's no rain, you need to PROVIDE that much water in other ways, like irrigation. In the same TEMPERATURES and conditions, it doesn't matter if the water is from rain or irrigation, as long as they get the right amount.
However, if you move somewhere hot/dry, the Garglesnaps will need MORE water.
That being said... Certain plants will die in extremes of heat or cold. And some plants thrive *better* in certain climates, making them more logical/profitable.
Greens and Root vegetables (lettuce, spinach, carrots, potatoes) do well in extreme heat - but they need TONS of water.
So places like the southern USA are good, hot and humid.
Corn and beans and similar crops do well with middling temperatures. They need less water, but still quite a bit.
Tomatoes, peppers, opuntia (prickly pears): These types of plants can grow with less water than most... they do well almost anywhere.
Grains: For rice.. you need VERY wet, marshy plots of land. For wheat... it's kind of weird. Different TYPES of wheat do better in different temperatures - which is why you have winter wheat and Spring wheat... but there are also different types of wheat best suited for breads, or for pastas.... You could probably grow SOME type of wheat almost anywhere. Oats (and Barley) do best with cooler, dry climes.
Most trees (apple, pear, plum) will grow anywhere, as long as you have sufficient water. Some do best in hot, wet, humid climes, (Bananas, mangoes, oranges). Very few (olives, figs) will grow well in desert-like temps.
I think I'll make garglesnap the staple crop of my world.
Besides that, to better explain why I am asking the question: I have a "main" region which is crossed by many rivers, including a great one, but due to its inland nature has relatively low rainfall. I also have another small costal region (without any good natural ports), at the same latitude, without many rivers, but which gets much more rainfall from the sea. To make this region somehow relevant, I was thinking that it may be able to support some crop that does not grow in the other larger one, and I was thinking whether rainfall vs. irrigation might be a good way of justifying the difference. If I interpret correctly your answer, that is not the case. Though I think that the river region would still be dryer than the coastal one, since irrigation techniques are not so advanced in this low-tech world.
I think Jalyha did a wonderful job covering
this. I'll just point out that moist-climate
crops, like corn, were extremely important
to the Mexican natives. However, the
Spanish conquistadors that came and took
over had to invent new words in the
language to cover the new fruits and foods
So, while you probably could develop an
efficient irrigation system, there's also the
matter of what plants are native to the area.
Eh... "Garglesnaps" just popped into my head, which means I read it somewhere - it could have a weird meaning, I'd check before using it :p
And... you both sort of got it...
Crops are just plants. Long ago, before there were ever any people (and probably pre-anyotherlife) there were plants.
However people arrived on your planet, the plants were there (and if they were terraforming, then whatever home planet they originally came from had plants too).
Some of those plants were edible - some weren't.
Since people have an annoying habit of taking things they like and messing with them, the people of your world tried to take the edible plants and replicate (as best they could) the growing conditions (right food, covering them in winter, whatever.) If they were successful, they harvested the edible bits and called them crops.
This is true on any world. (except conworlds, but those would still hold true for the planet of origination)
SO.... uninhabited island. Same basic temperatures on the whole island, BUT... is it a bit cooler along the coast than in the jungle? Of course it is. Because of the WIND. So it's cooler. Some of the water (in the air, at least) is salt water, not fresh water.
So the vegetation - the *plants* that grow along the coast will already be different than elsewhere on the island. Including the edible ones your first explorers find. (future crops).
ALSO, as foremost hinted at... people adapt things they like to their own purposes. If I like rice and I don't have the right conditions to grow it naturally, well then, by all the gods of Naos, I will MAKE the right conditions. Even if it means building a basement, or a greenhouse, or...
I'm digressing. The point is, people will take crops from their native climate, and bring them home to a different climate. If conditions are similar (temperature, amount of water, type of soil) then it usually doesn't change much.
But... if conditions are different... even with ONE significant change (like that salt air or cool wind) then you could come up with a different variety of the same plant.
People also (further back than recorded history) have a habit of hybridizing things. (What's a mule? It's mentioned in the earliest stories, lol, there are even documented cases of mules giving birth (BAD OMEN!!) as early as 500 BC.) We do the same with plants, especially crops. That changes them, on a fundamental level, and makes it possible to grow new varieties in new places as well.
Finally, since we aren't talking about modern times...
If you're building a new city, you need a way to make money. A way to *trade* with other cities (villages, towns, forbidding sorcerer's towers, whatever).
If the city of Hockaloogee produces enough grain to feed the entire population, put some aside, and still has grain left over - well, they don't WANT your grain. They don't need it.
So maybe you try to grow enough grain for yourself, but you don't plant more than necessary. But you find these little pink fruits growing on the trees, and they're really sweet. First you make sure they aren't poisoned... then you GROW them. You plant the seeds to grow more and more because people in Hockaloogee will pay a fortune for something new.
Tomatoes grow in Lurgee, but not well. You brought some tomatoes with you and you find it grows well here. You plant tomatoes because people in lurgee will pay for tomatoes.
If you have to *justify* why different plants -- I mean crops -- grow in a different area, you can use anything you want to justify it. Commerce, air conditions, different native vegetation, deliberate hybridization, the difference in temperature from the wind, or even more POTABLE water through rivers and irrigation, vs. Seawater and the occasional freshwater pool ;)
It's not really the sort of thing that needs justification - it's a pretty normal/natural occurence. It just FEELS wrong because in today's society, we've found ways around it.