What would happen to the prairies if we stopped farming them?

The prairies would likely become grassland if they were no longer farmed, despite the fact that millions of trees have been planted as so called 'shelter belts'
Abandoned prairie farms would likely revert to grassland, much like this. (B.Gjetvaj / branimirphoto.ca)

 Dr. Colin Laroque, a Professor in the Department of Soil Science at The University of Saskatchewan in Saskatoon feels that the prairies would become grasslands if they were to be abandoned by farmers.  This is based on observations of the many millions of boreal forest species that were planted as part of a 'shelter belt' program beginning in the early 1900's.  The idea of shelter belt trees was to protect the prairies from erosion, and also provide a build up of snow for soil hydration.  Species such as white spruce - which were part of the program - have not done very well in the more southern prairie environments.  This suggests that grasslands and not forest would take over in the absence of farming.