Worth noting is Canadian military figure Richard Rohmer's "Mid-Canada Development Corridor" of Cold War vintage which called for massive infrastructure investments in the Subarctic to create new urban areas and best exploit available resources. Presumably, farming and ranching in those areas would get a big boost thanks to this infrastructure investment and the availability of nearby markets.
Map:
I'm of the opinion that if Canada had not been an Anglo country bordering another Anglo country, there could be a much greater sense that Canada needed to spread out to avoid perceived threats from the Anglo menace south of the border. French Canada, or going back even further, a Vinland (if not settled until the 15th century or so), may be the way to get a more spread out Canada rather than having the majority of the population living within a hundred miles of the US border.
In general, earlier colonisation of North America--and from both ends--may accomplish the settlement of these marginal lands. If we have settlement of northern America (by the English and French) around the same time the Spanish are destroying the Mesoamericans, we have many extra years for the population to grow. Maybe we could have even earlier settlement in the late 15th century where Didrik Pining or another North Atlantic explorer reaches "Vinland" while sailing for Denmark and soon Newfoundland and Nova Scotia have Scandinavian colonies. On the other end (with a still earlier PoD), Japan pushes north to Hokkaido and Karafuto and tempted by the gold and other resources they find there end up in Kamchatka and eventually Alaska, where they'll find yet more gold. A lot of Japanese peasants lived off marginal, rocky land, so a cold place like the Kenai Peninsula or Mat-Su valley or even the Alaska Peninsula might not be too harsh for growing buckwheat and millet and supplementing it with imported food and whatever can be hunted or fished. Point is, this gives a large, expanding agricultural population which by the early 1800s or so will have settled a lot of marginal land. Of course, by the mid-late 19th century many, many of them will find their farms failing or will simply abandon their land to move to expanding cities. But many of these cities may well be located near this marginal land, leading to a much heavier settlement of the Subarctic and Great Basin.
A native agriculture PoD would too, likely along the lines of alternate domesticated plants. The PNW has potential domesticates which could do well in much of the Subarctic (
Sagittaria cuneata is recorded as far north as the Yukon and NWT and according to one source grows in the Mackenzie Delta). OTL Eastern Agricultural Complex plants are better suited for the High Plains and Prairie Provinces than the Mesoamerican Three Sisters agriculture which supplanted it.
Also as a side-note, I'd add Anticosti Island to this. It's about 50% larger than Prince Edward Island yet has a population of 240 people. It could easily have several times that.
I don't know if supply chains would matter much, since that area almost universally won't be competitive with grain grown in the rest of the country. Although more investment in icebreakers/earlier climate change may do the job since you could open ports in the area (and strengthen Churchill, MT) to ship using the Northwest/Northeast Passage. It won't survive without a lot of government subsidy (even OTL Alaskan agriculture in the best spots runs into this problem), but for an independent Alaska or maybe a very nationalist or communist Canada it could be a viable option.
