In 1994, when I was told that Timberhill was “a rare oak savanna” I had no idea what that meant. In the years since as I watched the land recover from years of neglect I have learned what that means. For those embarking on their own restoration, I thought more detail about what oak savanna means here in south central Iowa might be of interest. That is not to say that this restoration should be used as a model. Each site is unique; if restoration is limited to thinning and burning it will respond with its own assemblage of plants and animals.
The diversity of the 200 acre Timberhill landscape with dry, open ridgetops, steeply wooded slopes, riparian creek borders, mesic prairie openings, hillside seeps and wetlands is founded in Decatur County geologic history. In the Southern Iowa Drift Plain, Decatur County is on high land almost midway between the Mississippi and Missouri Rivers. Erosional processes have cut the glacial plain into high narrow ridgetops, dissected by narrow ravines. The major streams flow north-south; their tributaries don’t deviate much east or west. Travelling east to west one must cross a series of alternating ridges and valleys which early settlers described as the ‘devils washboard’. North-south one can usually find a ridge road (Howell and Smith 1915). For example, in this photo showing Timberhill topography a trail used by migrating Indians and early settlers followed the central ridge north then turned northeast along a secondary ridge down to Brush Creek. Presettlement the creek was on the same level as the adjacent ground and easily navigable on a horse or horse and buggy. (I didn’t believe that this was actually the route followed by Indians until I found a perfectly shaped arrowhead on the trail.)
The soil surface is old, dating back to the last glaciation, 500,000 years ago. It comprises a thin mantle of loess-like top soil over fine to coarse sand with some gravel below. Under this layer is gumbotil, a dark sticky clay which locals call ‘hard pan’. Unable to penetrate the hard pan, ground water flows laterally through the upper soil layers until it exits in seeps where the soil surface and water vein intersect. The seeping hill slopes are the perfect habitat for conservative plants such as lady’s slipper orchids.
Because of its sharply dissected landscape, much of Decatur County has survived the plow . Although they may have been heavily grazed or are badly overstocked, the unplowed woodlands are highly restorable. The biggest problem is controlling erosion. However, native plants and seeds retained in the understory will respond to restoration. As the graminoids and perennial forms are restored they absorb nearly all precipitation into the soil. All that is needed is thinning of the overstocked woodland and prescribed fire. In this photo one can see how restoring the native plant cover is controlling erosion.
(I’m going on spring break. The next post will be on May 7.)
H.F. Bain, “Geology of Decatur County” in Howell and Smith. 1915. History of Decatur County.
Jean C. Prior, 1991. Landforms of Iowa. Iowa City: University of Iowa Press.