There is a swath across the map of Minnesota that runs northwest to southeast across the state, separating the major biomes of the eastern two thirds of the country, and for complicated reasons. North, it is colder, south warmer. Much of the moister in the region, especially in the summer, comes from the Gulf of Mexico, directly to the south, whence air masses move north and swerve east. So, there is a west to east gradient of increased rainfall, and a south to north gradient of decreased rainfall. However, the cooler conditions to the north mean that what rain does fall counts for more, as there is less evaporation. There are other factors. Ultimately, the complex interaction between continental westerly, southerly gulf air masses with their storms, moisture gradients, and temperature gradients means that to the west of a certain line prairie (grasslands) is more likely to thrive, while woodland and forest should predominate to the east. And, north of a certain line, evergreen forest is more likely to thrive than deciduous. The exact mix of trees that thrive in each area depends on historical contingency, so each interglacial can have a different dominant species. In the present era, we see white pine in the north and oak with hickory in the south, but that changes even as we speak and various species grow back to replace the wanton, out of control lumbering of the 19^th and early 20^th century.
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