Midwestern Oak Savanna
On a large scale, ecotones describe transitional landscapes that exist where two or more distinct landscapes converge. Forests that dominate the eastern United States, a land of abundant rainfall and humidity, must somehow become the arid Great Plains. Walking from Denver to D.C. you would encounter innumerable changes in the landscape, minor transitional places. About halfway through the journey you would cross though a much more significant transition zone wending from Minnesota south though Texas; a relatively thin band of scattered trees, predominantly oaks (Quercus sp.), outliers of the forest to the east, with an understory that mostly resembles the prairie to the immediate west. It is a landscape of borrowed parts combined to create something altogether different. This is the Midwestern oak savanna, a melding of prairie and forest.
Exact boundaries of grassland and forest are established and maintained by fires. Started naturally by lightning or artificially by humans, occasional fire stops the encroaching forest by killing shrubs and young trees before they can become established. Oaks, however, have adapted to survive under these conditions. Bur oaks (Quercus macrocarpa) are the most frequent arboreal occupant of Midwestern oak savannas, though a handful of other oak species occur and dominate in areas. These oak species possess a range of growing condition preferences and their presence is dictated by topography and hydrology within the savannas. What they all have in common though is remarkably thick bark that allows them to survive the fires that occasionally sweep quickly but intensely through desiccated grasslands. While trees with thinner bark perish in the flames, oaks at even a young age are able to withstand the blaze and continue growing. After a fire, released nutrients and exposed soil allows the herbaceous plants to quickly regrow from their deep roots and seed bank. The oak savanna is sustained.
There are particular things about this landscape that appeal to me. They showcase the wonderful diversity of prairie plants and add to that diversity with intermingled woodland species capable of venturing out of the deep forest under the sprawling boughs of oaks. They offer expansive views that you don’t get to the east and they break up sightlines that can go on too long to the west. They give a vertical element to a westerner and a horizontal element to an easterner. They can showcase to humans the complexities of ecosystem functions in a way that is meaningful and not incomprehensible; a potential lesson for us if we want to learn to practice wise thought and action as newly come rulers of this planet.
The unfortunate reality is that this special landscape has been almost entirely destroyed over the past 200 years. The climactic transitional zone remains, and you can sense the landscape changing as you pass gnarled old oaks that still dot the land but the diverse assortment of herbaceous plants are mostly gone having fallen to the plow in rural areas and to shrubs established as a result of fire suppression near settlement. Without these plants, oak savanna habitat does not exist. This altered understory not only excludes the amazing variety of grasses and wildflowers, but also inhibits future generations of oak. Saplings cannot take root in a field that is tilled annually, nor in the dense shade of thick shrubbery. We can be thankful that pockets of protected and restored oak savanna exist but traveling through this area, it is easy to see the extent of the damage and imagine how extraordinary the land must have been as the first acre was painstakingly turned under.
And yet I am glad that I have experienced this: to be in the shade at the base of a massive oak on a hot summer day watching the colorful prairie dance in the wind while the leaves rustle overhead.