Vegetation History

Postglacial Vegetation History of Southwestern Wisconsin

Detailed descriptions of the postglacial vegetation history of the Midwest and Wisconsin are presented in Pielou (1991) and Cochrane & Iltis (2000), and the following summary was compiled primarily from these sources. Analysis of fossilized pollen occurring in sediment cores taken from ancient lake beds provides evidence that at least some grassland vegetation existed in what is now southwestern Wisconsin prior to the Quaternary Ice Age and also during the interglacial periods when the climate warmed slightly and glaciers temporarily receded (Pielou 1991).

USGS probe extracts sediment cores from the Preserve

USGS probe extracts sediment cores from the Preserve

During the Quaternary Period, southwestern Wisconsin’s climate became more arctic and many of the existing grassland species were compelled to migrate to the south, east, and west of the advancing ice. The majority of these species took refuge in clearings in the eastern forests of the Allegheny highlands, in the Great Plains region of the west, in the Ozark highlands, or in the arid southwest and Texas. Often, some of the more widely distributed species migrated to more than one of these areas. Genetically isolated from other populations, these species underwent evolutionary divergence into separate ecotypic races, varieties, and subspecies, and even into new species. After returning to southwest Wisconsin during the Holocene, they occupied habitats similar to those they had migrated from and were once again sympatric with their cousin populations.

When the climate became milder and drier, the glaciers retreated and these species were able to migrate back into their former ranges, although these migrations were by no means uniform and were probably still occurring up to the time of Euro‐American settlement. Additional migrations from eastern coastal areas and arctic and cordilleran (subarctic) Canada have also contributed species to the flora of southwestern Wisconsin. Post‐settlement land‐use patterns have fragmented the present landscape to such an extent that natural long‐distance migrations probably no longer occur.

Plant species indigenous to the Preserve and surrounding landscape that arrived from the Alleghenian‐Ozarkian Floristic Element (migrated from glacial refugia in the interior and Midwest) include cream gentian (Gentiana alba), round‐headed bush clover (Lespedeza capitata), bottlebrush grass (Hystrix patula), and purple milkweed (Asclepias purpurescens). Many of the species occurring in the dry and dry‐mesic prairie remnants of the Preserve migrated   from the Northern Great Plains and Arid Southwest Elements, and include prairie turnip (Pediomelum esculentum), downy yellow painted cup (Castilleja sessiliflora), silky aster (Aster sericeus), hoary vervain (Verbena stricta), and the dropseeds (Sporobolus heterolepis and S. cryptandrus). These species had  become adapted to the dry conditions of the Great Plains, and took residence in similar habitats in southwestern Wisconsin. The dry calcareous southern and western slopes of the driftless area possess microclimates similar to the Great Plains, as do the St. Peter Sandstone caps. Species that migrated from the Southeastern Floristic Element include the compass plants (Silphium terebinthinaceum, S. integrifolium, S. laciniatum, and S. perfoliatum),

The iconic compass plant (Silphium laciniatum)

glade mallow (Napaea dioica), and white and cream indigo bushes (Baptisia leucantha and B. leucophaea). Species that migrated from the Eastern Deciduous Forest Element include New England aster (Aster novae‐anglae), early buttercup (Ranunculus fascicularis),

culver’s root (Veronicastrum virginicum), bottle gentian (Gentiana andressii), prairie phlox (Phlox pilosa), Canada tick‐trefoil (Desmodium canadense), and also several woodland species including wood betony   (Pedicularis canadensis), wild geranium (Geranium maculatum), and wild columbine (Aquilegia canadensis). Plant species that migrated to southwestern Wisconsin from Southwestern Floristic Elements include leadplant (Amorpha canadensis), switchgrass (Panicum virgatum), and the bluestems (Andropogon gerardii and Schizachyrium [Andropogon] scoparium).  Several species of the Preserve’s dry mesic prairie remnants migrated from the Madro‐Tertiary Floristic Element, with origins in the Sierra Madre Mountains. These include white and purple prairie clovers (Dalea candida and D. purpurea), side oats grama (Bouteloua curtipendula), and whorled milkweed (Asclepias verticillata). In addition, several species with transcontinental distributions migrated from northern Arcto‐Tertiary Floristic Elements, among them pasque flower (Pulsatilla [Anenome] patens), Canada anemone (Anenome canadensis), dwarf two‐flowered Cynthia (Krigia biflora), Canada wild rye (Elymus canadensis), smooth aster (Aster laevis), meadow rue (Thalictrum dasycarpum), prairie cinquefoil (Potentilla arguta), and white prairie sage (Artemesia ludoviciana).

The present composition of the vegetation of southwestern Wisconsin is thus a composite mixture of species that migrated from southern, western, eastern, and northern glacial refugia into the bareground areas left behind by the retreating glaciers. As the glaciers receded, the bareground they exposed was initially colonized by conifer forests (Anderson 1998). Fossil pollen analyses indicate that grassland and savanna communities first appeared in the area approximately 8,000 years ago (Anderson 1998). During the interval between the replacement of conifer forests with the grassland/savanna complex and Euro-American settlement (8,000 years ago until ca. 1830), southwestern Wisconsin was vegetated by a shifting mosaic of grassland, savanna, oak woodland, and southern mesic forest vegetation, each community expanding and contracting in response to changes in wildfire regimes brought on by short‐term climatic variations (Curtis 1959; Anderson 1998; Cochrane and Iltis 2000). Grassland and savanna in southwestern Wisconsin reached its greatest extent during the Hypsithermal Interval between 8,000 and 3,500 years ago, a warm and dry period conducive to increased frequency of wildfire. 3,500 years ago, southwest Wisconsin’s climate became cooler and moister, and the prairie‐oak savanna vegetation complex began to be replaced by mesic forest (Anderson 1998). Continued use of wildfire as a management tool by indigenous populations stabilized the range of prairie‐savanna communities, and was the first large‐scale example of human alteration of nature in North America.

 Presettlement Vegetation of the Preserve

Township 8 North, Range 7 East, surveyor notes used to determine vegetative history at the SL Preserve

Township 8 North, Range 7 East, surveyor notes used to determine vegetative history at the SL Preserve

The Preserve is located within Wisconsin’s Prairie‐Southern Forest Province (Curtis 1959). At the time of settlement, approximately one‐third of the Driftless Area consisted of a shifting mosaic of prairie, oak savanna, and oak woodland (Curtis 1959). Original surveyor’s notes dating from October 1832 (available through the University of Wisconsin‐Madison Map Library) described Section 33 of Township 8 North Range 7 East as consisting of bur and white oak savanna interspersed with prairie and “marsh” (probably sedge meadow and/or emergent aquatic vegetation). The surveyor (John H. Mullett) also noted the existence of several stands of “timber oak” which may indicate oak woodlands dominated by white oak, since bur oak is often unsuitable for framing and finish lumber. From these records, we can infer that the presettlement vegetation of the Preserve consisted of oak savanna interspersed with oak woodland, prairie, and sedge meadow. Despite 180 years of Euro‐American occupancy (with associated disturbances related to changes in land‐use and wildfire suppression), a shadow of this presettlement condition has persisted on this landscape. Restoration and rehabilitation of these remnant communities to their presettlement condition is possible given diligent management, adequate funding, and time.