Oak Savanna

Unit Designation Size
2 LIP Savanna Remnant 10 acres
3 East Savanna Remnant 12 acres
4 Brooks’ Savanna 4.5 acres
6 IAT Savanna Remnant 5 acres
11 Goth Savanna Remnant 3 acres
12 WHIP Savanna Remnant 7 acres
33 KP Savanna Remnant ½ acre

Total Area:                 37.5 acres

Southwestern Wisconsin’s oak savanna lies at the ecotonal boundary between the prairies of the western Great Plains and the eastern deciduous forests  (Cottam 1949). Transeau (1935) referred to this boundary as “The Prairie Peninsula”. Due to its position in the regional landscape, oak savanna contains elements of both prairie and forest. Oak savanna is a fire‐maintained community dominated by open grown oak trees. Curtis (1959) divided Wisconsin’s oak savanna into three community types: 1) Oak Barrens, which occur in the central sands region of Wisconsin, are dominated by black oak (Quercus velutina) and Hill’s oak (Q. ellipsoidalis) on sandy soils, with an understory composition similar to dry prairie or sand barrens, 2) Riparian Savannas, which occur on wet soils bordering rivers and streams, are dominated by swamp white oak (Q. bicolor), with an understory similar to wet prairie or sedge meadow, and 3) Oak Openings, which occur in mesic alfisols on loamy upland sites, are dominated by bur oak (Q. macrocarpa), with white oak (Q. alba), black oak, shagbark hickory (Carya ovata), and hackberry (Celtis occidentalis) as frequent subdominant components. Although the presettlement composition of oak opening understory is not precisely known (Henderson 1995), several authors who have surveyed the literature on this subject (e.g., Cottam 1949; Curtis 1959; Bray 1960; Anderson 1998; Leach and Givnish 1998; Cochrane and Iltis 2000) indicated that the understory of oak openings was composed of a changing mixture of prairie species, true savanna species, and fire‐tolerant mesic forest species. Cottam (1949), citing earlier authors, described “hazel” (probably Corylus americana), New Jersey tea (Ceoanthus americanus), prairie grasses, and grubs of oak and hickory formed by “almost annual” wildfires. Bray (1960) described prairie species occurring in a lower grass/forb ratio (i.e., they were more forb‐rich) than open prairie grasslands. Leach and Givnish (1998) recorded the presence of rare or conservation concern species such as cream gentian (Gentiana alba), wild hyacinth (Camassia scilloides), lady’s tresses (Spiranthes spp.), and autumn coral root (Corallorhiza odontorhiza) in southern Wisconsin savanna remnants. Relic populations of two of these species exist in the oak savanna remnants of the Preserve and Natural Heritage Inventory (NHI) records exist for the presence of autumn coral root within Berry Township. Unfortunately, the understory composition was not recorded in the original surveyor’s notes for section 33 of Berry Township.

Park Elementary school in front of oak savanna

Traditionally, land managers have defined oak openings according to the canopy coverage of oak trees, and conservation programs were designed and implemented with an endpoint of greater than one tree per acre and less than 50% total canopy cover. This definition was first presented by Curtis (1959), who was compelled to use arbitrary cut‐off points to classify the vegetation of Wisconsin into distinct community types. Using this definition, Curtis (1959) estimated that oak savanna covered 5.5 million acres (or 42% of the land surface south of the tension zone) of Wisconsin prior to Euro‐American settlement. Using the same definition of oak savanna, the Wisconsin DNR estimated that less than 500 acres of this community remain in remnant condition in Wisconsin(possessing both open grown oak trees and an intact understory species composition) less than 0.01% of the original acreage (Henderson 1995). However, Leach and Givnish (1998) argued that conservationists have been employing a definition of oak savanna that is too narrow.  Their assertion was based on three premises: 1) Percent canopy can be an inaccurate and misleading measure of direct sunlight reaching the understory canopy,

2) Historically, savanna trees were not evenly distributed on the landscape, but were arranged in clusters, groves,  and peninsulas (Cottam (1949) made this same observation), and these heterogeneous associations skew the meaning of percent canopy when averaged over a specified area, and 3) Percent canopy describes the present condition of a savanna but not its past conditions.  Anderson (1998) and Cochrane & Iltis (2000) argued that oak savanna had already been disturbed by human activity for more than 100 years before they were described scientifically, and that a more realistic definition of oak savanna canopy might range from between 10 and 80 percent canopy cover.  Leach and Givnish (1998) proposed three criteria for identifying highly restorable oak savanna remnants in southern Wisconsin: 1) The presence  of open grown oak trees, 2) The presence of a ground layer dominated by both shade‐tolerant and sun‐loving species, and 3) A history of fire within the previous ten years. The savannas of the Preserve meet all three of these criteria, qualifying them as retrievable oak savanna remnants. However, the understory compositions of management units 2, 3, and 4 have been severely degraded by decades of competition by invasive shrubs, principally buckthorn (Rhamnus cathartica). The understory compositions of management units 5, 12, and 24 are in better condition yet are still in danger of replacement by buckthorn encroachment.

Savanna understory

In an effort to restore understory structure to remnant savanna recently cleared of brush and fire‐intolerant tree species, management unit 2 and the northern portion of management unit 3 were planted with a mixture of 95 understory species on 11 November 2008 (Appendix A‐8). Species with a variety of shade tolerances were selected for the planting. Some species were collected from adjacent remnants at the preserve, some were donated by the Dane County Parks Commission and Ice Age Park and Trail Foundation, and some were purchased from local nurseries. The site was seeded at a rate of 6 lbs/acre with a graminoid/forb ratio of 1:2. Prior to planting, the site was burned to remove leaf litter and any senescent herbaceous vegetation. The site was prepared for planting by raking and piling any remaining slash and debris to expose the soil surface. The planting surface was prepared by making sets of perpendicular passes on an all‐terrain vehicle equipped with a drag with two‐inch teeth. Seed was hand broadcast onto the exposed soil surface. Seed was mixed with vermiculite to ensure uniform coverage. One‐half inch of precipitation fell onto the site during planting, which helped the seed stick to the soil. Two inches of snow fell on the site the evening after planting.

Planted grasses and forbs in oak savanna.

In 2008, four Casey’s lady’s tresses (Spiranthes casei var. casei), a species considered to be of Conservation Concern in the recently published Flora of North America (volume 26), were observed in management unit 2, as was a deme of the Threatened giant yellow hyssop (Agastache nepetoides). An additional yellow hyssop deme occurs in management unit 3. Protective exclosures were placed around all of these plants to curtail deer browsing. All savanna management units support populations of the Threatened red‐headed woodpecker  (Melanerpes erythrocephalus). Management unit 12 is also utilized by brown thrasher (Taxostoma rufum) and woodcock (Scolopax minor), two species of Special Concern in Wisconsin.