1993 Proceedings of the Midwest Oak Savanna Conferences
CHARACTERIZATION OF MIDWESTERN OAK SAVANNAS
Oak savannas were a common feature in the transition zone in Minnesota prior to European settlement. They occurred on a wide variety of surface geologic units and soil types in areas that were influenced by fire. The Minnesota Natural Heritage Program recognizes two oak savanna natural community types, classified according to soil moisture: mesic and dry oak savanna. Within the dry oak savanna type, 3 subtypes are recognized, classified according to soil texture and landform: barrens, hill, and sand-gravel subtypes. These communities are all considered to be endangered or critically endangered in the state. The Minnesota County Biological Survey (DNR) has identified several significant oak savanna natural areas and has mapped them using the ARC/INFO computerized Geographic Information System. A GIS map was prepared to show the relationship between soil type and existing natural communities, including oak savanna, in a portion of the Whitewater River valley in southeastern Minnesota.
The Minnesota Natural Heritage Program (NHP) has identified oak savanna as one of the most imperilled natural communities in the state. As part of the Minnesota County Biological Survey (MCBS), a systematic county by county inventory of natural communities and rare species, the NHP has documented several important occurrences of oak savanna. During the survey, difficulties arose in classification, inventory and mapping of savannas. For example, the distinctions between savanna and woodland/brushland, and between savanna and prairie, was often difficult to make; it is essential that criteria be developed to make these decisions in the processes of field inventory, data entry, and mapping. This paper documents the methods adopted by MCBS, summarizes the known status of oak savanna in the state, and presents a digitized map of one of Minnesota's most important oak savanna natural areas to illustrate the relationship between soil type and the presence of oak savanna.
Many definitions and classification systems of Midwest oak savanna exist (Nuzzo, 1986). The dynamic nature of the fire influenced transition zone in which savanna occurs makes classification and description of this community particularly challenging. The Minnesota Natural Heritage Program (NHP) has attempted to meet this challenge with a recently revised savanna and woodland-brushland classification for the state, developed as part of a key and descriptive guide to Minnesota natural communities (Minnesota Natural Heritage Program, 1993).
Minnesota's classification scheme distinguishes between oak savanna and oak woodland-brushland. The canopies of both are described as broken to scattered, with 10-70% total cover. However, the matrix surrounding the trees in woodland-brushland is described as dense tall brush cover and less than 30% open grassland or primary communities, while the matrix in savanna has sparse tall brush cover and greater than 30% open grassland or primary communities.
Woodland-brushland is thought to have occurred on the presettlement landscape in areas in which landscape features favored tree and tall brush growth more than in areas that supported savanna. These landscape features, including heavy soils, rough topography, and proximity to water bodies, may have encouraged woody growth by reducing fire frequency and/or intensity. Many occurrences of woodland-brushland today, however, are disturbance communities that formed recently from grazing or selective logging of deciduous forests, or from fire suppression in former savannas or prairies.
Minnesota oak savannas are divided into two natural community types: mesic oak savanna and dry oak savanna. Mesic savanna occurs on level to rolling topography, in mesic habitat, on clay loams to loamy fine sands that are somewhat poorly to somewhat excessively drained. Characteristic trees are bur oak and northern pin oak, with quaking aspen becoming common northward. Dry savanna occurs on undulating to rough topography, in dry-mesic to dry habitats, on sandy loam to sand on gentle slopes, or on any soil texture on steep slopes, with well to excessively drained soils. The principal trees are bur oak and northern pin oak, with black oak becoming common in the southeast, and quaking aspen becoming common northwards.
Dry oak savannas are further subdivided into 3 subtypes, classified by soil texture and landform. The hill subtype occurs on unsorted glacial till, on loamy soil with a well-developed profile. The barrens subtype occurs on aeolian sand, alluvium, or sandy colluvium, on fine to medium sand with generally little to no profile development. The sand-gravel subtype occurs on alluvium, on sandy loams to loamy sands with a gravel fraction >10%, generally with some profile development.
To reflect the geographic variation in all types and subtypes across Minnesota, four "sections" of savanna are defined: northwest, central, southwest and southeast (Minnesota Natural Heritage Program, 1993) (figure 1).
Prior to European settlement, oak savanna was common in a long, narrow diagonal zone northwest to southeast across Minnnesota (Marschner, 1974). This area, forming a transition zone between prairie to the west and conifer-hardwood forest to the northeast, was heavily influenced by fire, and contained a mix of woodland, brushland and savanna not easily distinguishable from the surveyors' notes that formed the basis for the presettlement vegetation map (Marschner, 1974).
Oak savanna is now classified as critically endangered to endangered (depending on subtype and section) in the state. On richer soils, nearly all mesic savannas have been converted to cropland or pasture, or have succeeded to woodland or forest in the absence of fire and grazing by bison and elk. There are several remaining high-quality examples of sand-gravel and barrens subtypes of dry oak savanna in the northwest, central and southeast sections of the state, where the sites have been too dry or infertile to support sustained agricultural use, and tree and shrub growth have generally been limited by edaphic conditions.
There are 101 known occurrences of oak savanna in Minnesota. Of those in which quality was assessed, 35% were considered to be good to excellent, although many of these sites were somewhat overgrown or small. Sites ranged in size from 3 to 450 acres. The breakdown by type, subtype and section is as follows:
Areas where many of the remaining dry savannas occur include the Anoka Sand Plain, a broad area of sandy outwash covering a large expanse just north of the Twin Cities; areas of outwash in the valleys of the Root and Whitewater rivers in southeastern Minnesota; and old beach ridges of Glacial Lake Agassiz in northwestern Minnesota. The absence of records in the southwest can be partly explained by the heavy agricultural use of that region and partly by the fact that savanna was uncommon there even in presettlement times; additionally, little survey work has been completed there.
All MCBS data are digitized using the ARC/INFO Geographic Information System. This mapping provides a visual representation of remaining natural communities and rare species, and facilitates examination of the relationship between these occurrences and other geographic data such as geologic formation, soil type, location of water bodies, etc.
A map was created for a portion of the Whitewater River watershed to show the relationship between the occurrence of Plainfield sand and the presence of dry oak savanna (barrens subtype) and related natural communities (Fig. 2). This area is a highly dissected portion of the Paleozoic Plateau, characterized by steep bluffs and deeply carved valleys. The portion of the watershed mapped contains outwash-derived sand on terraces east of theWhitewater river, with bluffs supporting bedrock bluff prairies and oak forest and woodland rising dramatically above the terraces (Fig. 2).
The GIS map was studied together with topographic maps to make preliminary determinations regarding reasons for the presence or absence of oak savanna.
All of the oak savanna in the watershed occurs on Plainfield sand (Fig. 2). Most of the sand supports oak savanna, except 1) on moist north to east facing slopes, 2) in several small areas adjacent to wetlands, and 3) in areas that have been farmed or planted to pine plantations. The areas on moist slopes and next to wetlands support oak woodland and oak forest communities. These latter areas can be explained by either 1) reduced fire frequency caused by topographic and hydrologic barriers, or 2) more rapid and denser woody growth resulting from more soil moisture.
This information leads to speculation regarding fire frequency on Plainfield sand in the Whitewater River valley during presettlement time. It is widely believed that fires, including those caused by lightning and those set by Indians, played an important role in maintaining dry oak savanna (Curtis, 1959). It has been suggested, however, that in some sites, fire may be less important than edaphic factors in maintaining oak savanna on coarse well-drained soils (eg Whitford and Whitford 1971). The dissected nature of the Whitewater valley would have slowed the spread of fire in presettlement time; the east-west trending valleys that support oak savanna are separated from each other by steep bluffs and sometimes by streams and floodplain forests (Fig. 2). The flat, loess-covered uplands above the valley supported oak openings and prairie during the time of the GLO Survey (Marschner, 1974). Historic accounts indicate these uplands burned on a frequent basis with wide-ranging impressive fires (Curtiss-Wedge, 1913). No specific mention of fire history in the Whitewater valley was found in the literature.
It is suggested that fire frequency on Plainfield sand in the Whitewater river valley was less frequent than in the surrounding uplands, because of rough topography and of coarse sandy soils that supported less fuel than more mesic soils. In the future, investigations of fire history in the area will be made by examining fire scars and more thoroughly reviewing historical accounts to better understand the probable fire history in the area.
Cynthia Lane provided assistance in locating sand prairie and savanna areas in the Whitewater valley. Al Epp, Scott Freberg and Karl Bardon did much of the work on production of the digitized map. Barbara Delaney and Robert Dana were primary authors of the woodland-brushland and savanna classification.
Curtis, J.T. 1959. The vegetation of Wisconsin: an ordination of plant communities. University of Wisconsin Press, Madison, Wisconsin. 656 pp.
Curtiss-Wedge, F. 1913. History of Winona County Minnesota, Vol. 2. Cooper Jr. & Co., Chicago, IL.
Marschner, F.J. 1974. The original vegetation of Minnesota (map). United States Forest Service, North Central Forest Experiment Station, St. Paul, Minnesota.
Minnesota Natural Heritage Program. 1993. Minnesota's native vegetation: a key to natural communities. Minnesota Department of Natural Resources, St. Paul, MN. 110 pp.
Nuzzo, V. 1986. Extent and status of midwest oak savanna: presettlement and 1985. Natural Areas Journal 6(2):6-36.
Whitford, P.B. and K. Whitford. 1971. Savanna in central Wisconsin, U.S.A. Vegetation 23:77-87.
Figure 1. The Southeast, Southwest, Central, and Northwest Sections of Mesic Oak Savanna and Dry Oak Savanna
Figure 2. Dry Oak Savanna (Barrens Subtype), Associated Natural Communities, and Plainfield Sand in a Portion of the Whitewater River Watershed