From short +‎ grass.

shortgrass (usually uncountable, plural shortgrasses)

1. The set of short, tough grasses that thrive in arid plains; often specifically the native grasses of the North American Great Plains.
   • 1988 May 20, Jerry Sullivan, “Field & Street”, in Chicago Reader‎^[1]:

     The original centers of abundance of upland sandpipers were the shortgrass and mixed-grass prairies of the Great Plains.

• (grasses): tallgrass