es ▁the ▁under story . ▁Pra irie ▁and ▁widely ▁spac ed ▁o ak ▁trees ▁evol ved ▁to ▁co exist ▁in ▁the ▁o ak ▁sav anna ▁e cos ystem . ▁ ▁F ert ility ▁ ▁In ▁spite ▁of ▁long ▁rec urrent ▁dr ought s ▁and ▁occas ional ▁tor r ential ▁ra ins , ▁the ▁grass lands ▁of ▁the ▁Great ▁Pla ins ▁were ▁not ▁subject ▁to ▁great ▁soil ▁er os ion . ▁The ▁root ▁systems ▁of ▁native ▁pra irie ▁grass es ▁firm ly ▁held ▁the ▁soil ▁in ▁place ▁to ▁prevent ▁run - off ▁of ▁soil . ▁When ▁the ▁plant ▁died , ▁the ▁fung i , ▁b acter ia ▁returned ▁its ▁nut ri ents ▁to ▁the ▁soil . ▁These ▁deep ▁roots ▁also ▁help ▁native ▁pra irie ▁plants ▁reach ▁water ▁in ▁even ▁the ▁dri est ▁conditions . ▁Native ▁grass es ▁suffer ▁much ▁less ▁damage ▁from ▁dry ▁conditions ▁than ▁many ▁farm ▁cro ps ▁currently ▁grown . ▁ ▁Ge ographical ▁regions ▁ ▁Pra irie ▁in ▁North ▁America ▁is ▁usually ▁split ▁into ▁three ▁groups : ▁wet , ▁mes ic , ▁and ▁dry . ▁They ▁are ▁generally ▁character ized ▁by ▁tall gr ass ▁pra irie , ▁mixed , ▁or ▁short gr ass ▁pra irie , ▁depending ▁on ▁the ▁quality ▁of ▁soil ▁and ▁ra inf all . ▁ ▁W et ▁In ▁wet ▁pra i ries , ▁the ▁soil ▁is ▁usually ▁very ▁mo ist , ▁including ▁during ▁most ▁of ▁the ▁growing ▁season , ▁because ▁of ▁poor ▁water ▁d rain age . ▁The ▁resulting ▁st agn ant ▁water ▁is ▁condu ci ve ▁to ▁the