In pre-settlement Wisconsin, oak savanna was one of the state's most characteristic ecological communities, an environment between forest and prairie with between one and twenty trees per acre. Wisconsin had four plant communities that exhibited savanna traits: oak opening or oak savanna, scrub oak barren, jack pine barren, and cedar glade. Oak savanna covered twenty percent of the state's land area, and fires sweeping over the prairie created these distinctive areas. Bur oak was the most common tree in the oak savanna community, and its thick, corky bark allowed it to survive fires when other growth was burned away. Grasses and wildflowers were characteristic of oak savanna, supporting a uniquely diverse plant and animal environment. After settlement, fires were stopped, prairies were converted to cropland, and less fire-resistant trees flourished in oak savanna, so the once open prairie stretches dotted with oaks quickly became closed oak forests. Before settlement, Wisconsin had 5.5 million acres of savanna, but today less than .01% remain.