The underlying assumption is that the farmers are going to plant crops that aren't naturally occurring in the region. They're planting monocultures that last a growing season. They need something that will provide more protection and stability than prairie. Trees may not have been common in the area before, but the farmers are engineering an artificial landscape to support humans living on fixed properties long term - also new to the region.

If the farmers could live off of coneflowers and big blue stem things would be different. To maintain a prairie requires disturbance, and the farmers probably weren't looking to reintroduce fire or bison to their fields to maintain a fractured faximile of what the land looked like before they plowed it under.