grass, like many many life forms, exists in narrow climatic environments, which for grass is governed (mainly)by there bieng insufficient rain for trees, and in some special cases, too much wind for trees. much of modern pasture is artificial in that tree's have been eliminated and mowing and other continued maintenance kills off tree seedlings, which if stopped results in spontainious re-forestation. grass dies off when it gets too hot and dry, or becomes weak and prone to damage from live stock or other mechanical damage and then errodes. in any case climate is most definitly what creates different ecological nitches, forest, grasland,desert, rainforest, tundra, muskeg, etc ,etc