in Europe there are a lot of country with great potential too (Austria, Hungary, Bulgaria, Romania.. with mountains etc) ->
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For example in France nice scot pines are found @ aprox. 1000 meters high. Higher you can find european spruce, larix, mugo pines, etc etc
The most important thing, IMO, is that you have in your garden Bonsai that "make your day"
The wild areas in the U.S. and the rest of North America (hello Canada and Mexico) are still far greater in total than in Europe. Many Europeans simply don't get the scale of the U.S. or North America. Don't know if you've ever been to the Western U.S., but it's pretty thinly settled and the spaces between people are vast. The Bureau of Land Management (BLM), the custodian of public lands that don't include national parks, are primary sources for bonsai collectors. It's where Randy Knight and other collectors pull many spectacular trees from. BLM is responsible for managing 247 million acres (1 million square kilometers) of territory, most of it centered in the 12 Western states. That's almost double the size of France in collectible territory, not just territory.
Additionally, the climate zones in the U.S. are very funky. We run the entire range of climates on the continental U.S., from temperate rain forests in the Pacific NW, to alpine regions along the continental divide, to high desert in South Dakota to great plains in Texas, and lowland and mountainous hardwood forests in Appalachia, to tropical swamps along the Gulf Coast. Alaska and Hawaii add more diversity. And yes, there are many microclimate zones within each zone. For instance, specific areas in the SW deserts have ALL OF THOSE ZONES IN A SINGLE MOUNTAIN. "Sky Island"mountains that range above 10,000 feet in the deserts of Arizona and New Mexico have alpine, temperate, subtropical and desert climates all in one place. That kind of climate diversity leads to a lot of diversity in plants, which is reflected in the number of North American tree species.
Look I could do this all day. For the most part, Japan, with its limited geography, was played out of good yamadori material a hundred years ago. There isn't much left there. Europe has been thickly settled for centuries. There is very little wild land left there. Yeah, there is some, but it's not Montana.
And yes, it is important to have bonsai in your garden that make your day. That's what's going on with the sale, I think. Most of those trees would make anyone's day.