Tallgrass Prairie Oak Savannah

Oak Savanna at Rock Island Preserve near Cedar Rapids, Iowa.  This savanna is composed of  Red and Bur Oaks with the prairie grasses Little Bluestem, Big Bluestem and Indian Grass.

Transitioning between the Tallgrass Prairies of the Midwest and the Eastern Deciduous forests is the Oak Savanna.  These are not Simba's "Lion King" African tropical savanna, though they are similar in that they contain grasslands with widely interspersed trees.  And instead of Wildebeests, historically these savannas had large bison herds grazing them. Today the bison are gone except in a few limited protected areas, and due to the relatively rich soil for farming, fire suppression, and abundant grasses for grazing, this ecosystem is one of the most endangered in North America.  Unsurprisingly, Oak Savannas are a mixture of woodland and prairie species.  The fewer trees the more prairie like or the more trees the more woodland like the savanna becomes.  The one most important thing in preventing woodlands from invading the grasslands is fire.  Healthy  savannas require fires about every two years.  Any more than that the trees will be all burned away, any less the woodlands will take over.  Trees such as Bur and Red Oaks are highly adapted to fire with thick fire resistant bark and the ability colonize recently burned areas.  Other tree species are easily killed by fire and therefore woodlands don't invade actively burned grasslands.  When fire is suppressed Hickory, Elm, Basswood, and Maple invade the grassland, converting it into a woodland (At least in Eastern Iowa).

Red and Bur oak savanna at Wickiup Hill Natural Area near Cedar Rapids, Iowa.

Bur and White Oak savanna/woodland where the trees are so dense woodland forbes dominate and prairie species are almost completely absent.