I wonder how much this is a factor of time. After significantly more than 1500 years of intensive human management, the ecology of Japan's countryside will have evolved for those conditions. America's ecology has not had a similar amount of time to adapt to the loss of the giant bison herds, et cetera.

I would probably agree Japans country side was populated and farmed to a larger degree than in the US.

While there definitely was some groups of people/tribes who farmed the land historically. In the US definitely was a smaller amount compared to Japan during that stretch of time before even the USA was around.