Oak Savanna - Mississippi National River and Recreation Area (U.S. National Park Service)
The oak savanna is a transitional ecosystem, between prairie and woods. Compared to oak savanna, forest was not good hunting country, nor relationship between their fire practices and the desirable savanna landscape. In contrast to a forest, which has a closed canopy, the oak savanna canopy prairie species, such as big and little bluestem grass, and many. Thus, the upper limit between savanna and forest is generally considered to be a tree canopy with 50% Thus, oak savannas and prairies are closely linked ecologically as well as topographically. . Link to Map of Midwestern Savannas.
Elevation relative to site was used as an approximation of depth to water table or an indirect measurement of soil moisture, which fluctuates seasonally with floods, rainfall, and evaporation.
The sixty sampling quadrats at each site were surveyed to determine relative elevation using trigonometic leveling and Theodolite equipment. High river levels flooded Avoca Prairie in the spring. Chiwaukee was not flooded, but depressions were inundated by onsite spring runoff. By midsummer, standing water remained in depressions at both sites, but slightly elevated areas became dry.
The groundlayer was composed of both terrestrial and semiaquatic plants, and species diversity was high. Individual species favored specific areas along the light and soil moisture gradients.
The soils at the Avoca Prairie sites were strongly to moderately acid with pH values ranging from 5. The soils at Chiwaukee Prairie, in contrast, were neutral with a mean pH of 6. Swamp white oak were found to be most common at the west Avoca site, while a few river birch Betulanigra were also present. Swamp white oak also dominated the east Avoca site, but bur oak and river birch were observed as well. Chiwaukee Prairie supported mostly bur oak, but white oak Quercus alba and willow Salix sp.
- Oak savanna
Groundlayer Composition and Diversity The groundlayer at all three sites was diverse. At the two Avoca sites, 55 to 83 species were identified, while species were observed at the Chiwaukee site. The three sites contained species in aggregate. Mean a diversity was higher under oak canopies than in open areas, but showed a closer relationship with distance from tree Fig.
Light and Species Distributions Several species showed affinity for a specific light regime. Wood nettle Laportea canadensis and lanceleaved violet Viola lanceolata or arrowleaved violet Viola sagittata favored shade Fig.
Several other species also showed affinity for shade Table 2. Meadow anemone Anemone canadensis and sensitive fern Onoclea sensibilis favored moderate shade Fig. Field mint Mentha arvensiscord grass Spartina pectinataand other species favored sun Fig. Cardinal flower Lobelia cardinalisbottle gentian Gentiana andrewsiiand downy gentian Gentiana puberula occurred outside of sampling quadrats, and were chosen as the species whose light environments would be quantified with additional canopy photographs.
Soil Moisture and Species Distributions Several species also showed affinity for a specific regions along the soil moisture elevational gradient. Carex bicknelii and lanceleaved violet Viola lanceolata or arrowleaved violet Viola sagittata favored wet microsites Fig. Several other species favored wet microsites as well Table 3. Grassleaved goldenrod Euthamia graminifolia and blueeyed grass Sisyrinchium campestre favored wetmesic microsites Fig. Wild rose Rosa sp. Correlation tests showed that light and soil moisture were not significantly related across quadrats within each site, indicating that they were independent environmental variables.
Several species showed affinities for the microhabitats created by the overlapping of the light and soil moisture gradients Table 4.
Oak savanna | MNopedia
Many species that showed an affinity for one gradient did not show an affinity for the other, and thus dropped out of this analysis. The acidic soils of Avoca Prairie lie over Cambrium sandstone, while neutral soils of Chiwaukee Prairie were deposited over glacial drift and Silurian dolomite Martin Bray and Curtis note that swamp white oak was the major tree of lowland savannas, but do not list the nondominant tree species Curtis The presence of fireintolerant trees Betula nigra, Salix sp.
Standing water in lowland savannas probably blocks and retards wildfire. The groundlayer composition of lowland savanna remnants did include a large number of species identified by Curtis as modal and prevalent in wetmesic prairies, but they also contained a significant number of species associated with other communities.
Remnants also contained species indicative of floodplain forests, sedge meadows, and wet prairies. These preliminary results suggest that lowland savanna groundlayers are not actually analogous to those of wetmesic prairies. Sedges were an important component of the groundlayer.
They accounted for a significant number of the common graminoids. Three of the common sedgesCarex conoidea, Carex haydenii, Carex laeviconicawere not modal in any of the communities defined by Curtis. Though Carex laeviconica was abundant at one site, it is generally rare in Wisconsin, as here it reaches the northern limit of its range. This suggests that at least these three sedges are unique to lowland savanna communities.
Other uncommon sedges may be located in high quality lowland savanna remnants throughout the Midwest. Alpha diversity may be closely associated with distance from trees rather than light availability because of the associated factors of leaf litter, precipitation, and vapor pressure.
Litter accumulation may result in higher soil organic matter and nutrients in canopycovered soils than in adjacent open areas Zinke Leaf litter also effects soil moisture Lodhi and JohnsonKuceraas do precipitation and vapor pressure KittredgeKucera In lowland savannas, however, the influence of these factors on soil moisture is likely insignificant compared to that of site hydrology.
A significant number of species did show affinities for specific points along the independent gradients of light and soil moisture, which overlap to form a complex matrix of microsites. The variety of microsites within lowland savannas may account for the overall species richness of their groundlayers.
Theodore Cochrane kindly assisted me in identifying sedges, and Robert Kowal helped with other taxa. This research was supported by a grant from The Nature Conservancy. The savanna vegetation of Wisconsin and an application of the concepts of order and complexity to the field of ecology.
The distribution of savanna species in relation to light intensity.
Canadian Journal of Botany Photographic estimations of photosynthetically active radiation: Elevation of a computerized technique. University of Wisconsin Press, Madison. Water resources of Racine and Kenosha Counties, southeastern Wisconsin.
An ecological study of a hardwood forest area in central Iowa. In addition, they suppressed the fire cycle. Thus surviving pockets of savanna typically became less like savannas and more like forests or thickets. Many oak savanna plant and animal species became extinct or rare. Prescribed burn; Wisconsin bur oak savanna Restoration[ edit ] With the rise in interest in environmental conservationrestoration and preservation of surviving areas of oak savanna began.
Low intensity, spring prescribed burns have been used since at the Cedar Creek Ecosystem Science Reserve in Minnesota in an attempt to restore the area to an oak savanna.
Restoration work began in the s in Illinois, followed by work in Wisconsin, Indiana, Iowa, Missouri, and Minnesota. After Europeans arrived, fire suppression and settlement diminished the oak savannas to a fraction of their former expanse, which currently exist in many fragmented pockets throughout its native range. Many sites are protected and maintained by government bodies or non-profit organizations such as The Nature Conservancythe Wisconsin and Minnesota Departments of Natural Resources, and the Iowa Natural Heritage Foundation.
A disappearing ecosystem: Minnesota’s oak savannas
Ecology and Management of North American Savannas. University of Arizona Press, Tucson. Extent and status of Midwest oak savanna: Natural Areas Journal 6: Just a few oddball species: