The answer is very simple. Farms and not Farms.
Early on the limitations for trees were REAL prairie grass which is 7+ feet tall and extremely thick at the base. Everything else was forest where possible. It's practically impossible for trees to set root into that stuff. Once settlers arrived the land that buffalo "managed" (stomping seedlings and eating grass and pooping nutrient dense fertilizer) was change forever. this phenomenon vanished and much of the land was converted to farm land. What you'll notice in the low plains is that trees hug rivers, not just because there's more water there, but because it wasn't tillable or wasn't tilled. If you draw a line from Edmonton Canada, to Madison Wisconsin, to New Orleans, You can imagine virtually everything east and north of that line being forest, and west of that mainly prairie or transitional. Much of that forest was managed by native peoples in some way before Europeans arrived, They actively culled low vegetation---they had to, that's how they kept warm and how they built stuff. Pretty much all of this at one point or another was forested by about 1890. Where land was flattest, it was converted to farmland.
For Example, Zoom in on Illinois. This is right at the historical transition from prairie to forest. You'll notice that today, anything that is farmable is treeless---Except by major rivers and in towns. Same goes for Minnesota, Iowa, Missouri and eastern OK/KS. etc. Trees grow just as prolific in those areas regardless of species.
Michigan on the other hand has a different story. (one it shares with Wisconsin and Minnesota) In the 1800's all of the flat land was converted for farm use. However, there's a point where both wheat and corn are very difficult to grow (historically) and that's in Zone 4 or lower. That line runs right through the center of Wisconsin and Michigan. This is roughly where total 80 degree days start to number less than about 90 in a year. At that point there is less focus on farming--and as a result the land was allowed to rebound to a forest state...And then later, became state forest. Simultanously, since around 1930 or so, family farming gave way to more or less industrial farming. Homesteads became vacation properties and subdivisions...or were just left to fallow.
In my particular spot, I'm in a very flat treeless area, about 15 sqaure miles with rich dark dirt from floodplains. Surrounding that is state land, game preserves, recreation areas, lakes, cottages, etc...and far fewer farms. Those areas have been allowed to return to forest for the last 50-75 years. They are THICK with forest.
One interesting thing is, that we actually can get 80 degree dews! In late July, corn in Iowa, Indiana and Illinois, combined with a SW wind turn Michigan into a sauna. Nearly all of that land, 4 larger states worth was converted from shady dark leafed forest with a cool floor----or tall prairie grass with low transpiration rates----to corn stalks. Ever tried to sleep in 75 degree weather with 100% humidity? You will experience that soon enough. Every corn stalk can transpire up to 1 gallon of water per day at it's peak in late July. Not surprisingly that corresponds with those insane pulse storms we get.
That's climate change if there's any---directly affecting a large area of planet by changing how the surface absorbs, reflects, and radiates heat and water---not just by tailpipes and trace weakling greenhouse gasses.
Edit: I did a little more research and fact checking myself. There's some additional factors I neglected...Droughts occur more frequently in the very areas where there are less trees, and also, historic prairies experience frequent fires in those drought years with few natural fire breaks. That helped beat the trees down too, while the grasses themselves absolutely loved fire for soil nutrients. So basically we have a historic reason for trees versus few, and a modern factor (farming) that roughly mimics that natural boundry.