An oak savanna is a community of scattered oak trees (Quercus spp.) above a layer of prairie grasses and forbs. The trees are spread out enough so that there is no closed canopy and the grasses and forbs receive plentiful amounts of sunlight. The savanna is a transition ecosystem between the tallgrass prairie and woodland environments, so it is an important habitat for both woodland and prairie animals and insect species.
Once common in Minnesota, the oak savanna is now a rare ecosystem. Before European settlement, oak savanna covered roughly 10% of the state, and now there is only a fraction of that left. What happened? Well, a savanna relies on periodic disturbances such as fire, grazing and drought to flourish. Such disturbances prevent other trees from establishing themselves and turning the habitat into a forest community. Oak trees and prairie grasses are resilient to fire while the trees of a woodland community are not. Oak trees have extremely thick bark that protects them from fire, and prairie grasses have evolved to thrive after a fire. Therefore, fire allows the oaks and grasses a competitive advantage over other trees that may try to invade the savanna. Without fire, tree saplings begin to grow in the savanna and are able to take over, shading out and eliminating the grass and forb species. Soon, where there used to be an oak savanna, there is now a woodland habitat. Oak savannas have become practically extinct because European settlers suppressed natural fire cycles and the fires set by Native Americans. Farming and development has also helped obliterate the oak savanna ecosystem.
Oak savannas are important because they are beautiful, dynamic environments with diverse plants and animals that have evolved complex relationships over time. Since savannas are transitions from prairie to woodland, they have extremely high diversity in flora and fauna. Diversity is a measure of health and stability, so it is important that habitats with diverse native plants and animals exist.