Holocene bison
Wood bison
Plains bison
The great bison belt is a tract of rich grassland that ran from Alaska to the Gulf of Mexico from around 9000 BC. The great bison belt was supported by spring and early summer rainfall that allowed short grasses to grow. These grasses retain their moisture at the roots which allowed for grazing ungulates such as bison to find high-quality nutritious food in autumn.
These grasses are what allowed the bison population to thrive, as they were able to receive all of their nutrients from the short grasses, unlike other Ice Age animals which expanded in the postglacial period. This area was important to the Plains Paleo-Indians, who around 8500 BC turned to bison hunting instead of hunting a broader range of food.