Historically speaking, most of it was trees. Lebanon? Trees. Scotland? Trees. Ireland? Trees. Norway? Trees. Greenland? Trees. Israel? Trees. Australia? Trees. That's the great thing about trees - they're pretty much the steady-state of the plant world. If trees will grow there, trees were what the original ecosystem was. You can see this from the shelterbelt - if trees were supposed to be there, they do well. If they weren't, they need tending.

I agree with you mostly, but a lot of the land loss to trees has been converted to farmland or other private land use. One of the things people talk about a lot is how to get private landowners to plant more trees, of which there are tons of programs out there experimenting on how to do just that, such as tax abatements and land use credits. But in a lot of places, where once stood trees now stand soybeans or cows or houses and there's economic and practical issues that prevent that land from being converted back. Hence my original comment talking about agroforestry and urban forestry as two different ways to alleviate some of that struggle.

If trees will grow there, trees were what the original ecosystem was.

But if the new ecosystem is different now, and trees no longer grow there, we need to ask questions such as "why is the ecosystem now different and is that different good or bad" and "would planting trees here be beneficial or detrimental?" The choices and reasons behind them aren't binary, but protecting biodiversity is very important, so questions are often asked like "what plants and animals that are here now, would have difficulty thriving if trees are introduced" or "what trees, if introduced, would create environments for destructive animals and diseases to thrive?"

The shelterbelt link is interesting, just because some of that area looks to be near the great plains. The reason I find that interesting is because when we lost all of the bison, trees started to sprout up because there weren't millions of giant herbivores around anymore to keep them from growing. So while they're not abundant, they're more common than they used to be, and there are unintended results from that. It's been suggested that these few extra trees here and there were enough to give the barred owl a bridge from the Eastern States to the Western States where it is now establishing itself and is starting to crowd out native western species like the spotted owl.