50 Years of Change in Illinois Hill Prairies

by Kenneth R. Robertson, Mark W. Schwartz, Jeffrey W. Olson, Brian K. Dunphy, and H. David Clarke

Center for Biodiversity
Illinois Natural History Survey
607 East Peabody Drive
Champaign, IL 61820 USA

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NOTES: Illinois Native Plant Society
Yearly membership in the Illinois Native Plant Society includes a subscription to Erigenia and the quarterly newsletter, The Harbinger. For more information, write:

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This paper was originally published in Erigenia: Journal of the Illinois Native Plant Society Number 14, pages 41-52. November 1995 [published January 1996].

Current address for MWS: Section of Plant Biology, Robbins Hall, University of California-Davis, Davis, CA 95616.

Current address for BKD: Department of Botany, University of Georgia, Athens, GA 30602-7271.

Current address for HDC: Department of Plant Biology, University of Illinois, Urbana, IL 61801.


Abstract

Hill prairies are island-like patches of prairie vegetation occurring on otherwise wooded steep slopes that face south or south-west. In Illinois, hill prairies appear intermittently along most of the western border of the state formed by the Mississippi River and along the Illinois River from north of Peoria south to its junction with the Mississippi, with a few in east-central Illinois and other scattered localities. Illinois has four types of hill prairies: loess (by far the most abundant), glacial drift, gravel, and sand. The floras found on hill prairies are combinations of species that also occur in other types of prairies-- dry, black soil, sand, and gravel-- with only a few species largely restricted to hill prairies. Hill prairies appear to be decreasing in size from the encroachment of woody species. Rates of area change for nine hill prairies were examined using a series of aerial photographs from approximately 1940 to 1988. On average, these hill prairies lost 63% of their area, became fragmented into smaller units, and increased the ratio of edge to center. Field studies showed that sites losing a larger portion of area also had lower species richness.

Introduction

For nearly 30 years, Dr. Robert A. Evers of the Illinois Natural History Survey studied the hill prairies of Illinois. His landmark treatise Hill Prairies of Illinois was published in 1955. During the early 1970s, Dr. Evers revisited many of his earlier sites and took extensive notes with the aim to update the 1955 publication. However, the update was never published, although the notes remain in files at the Natural History Survey. The senior author of the current paper was fortunate to accompany Dr. Evers during the late 1970s on several trips to Bland Hill Prairie in Green County and Cap au Gris Hill Prairie in Calhoun County, which first sparked his interest in hill prairies.

Hill prairies are island-like prairie openings occurring on steep slopes that are (or were) otherwise forested. This prairie vegetation usually occurs only on the slopes, not on the tops of hills, where a combination of factors result in droughty conditions, such as the south to west-facing slope aspect, steep slope angle, dry prevailing winds, and well drained soil. Measurements made of slopes at six hill prairies in Jersey County Illinois showed average percent slopes of 17.3% to 56.3% (Kilburn and Warren 1963).

According to Evers (1955), the term "hill prairies" was first used by A. G. Vestal in 1943 during his ecology classes and seminars at the University of Illinois; they have also been called bluff prairies, goat prairies, and prairie openings. Prior to European settlement, hill prairies likely never formed large continuous belts in Illinois, but were fragmented by forested ravines that dissect the river bluffs and slopes.


Methods

As part of a project to investigate critical trends in the changing Illinois environment (Robertson and Schwartz 1994), we used aerial photos from 1938 to 1988 for nine loess hill prairies in Illinois to ascertain the loss of hill prairies during the last 50 years (Table 1, Figure 3). We digitized the aerial extent of each hill prairie at three to five time periods into a Geographical Information System (GIS - ARC/INFO version 6.0, 1992, as implemented on SUN workstations). Aerial photographs were obtained from the archives of the University of Illinois Map Library. Photographs prior to 1988 were taken at a 1:20,000 scale by the US Department of Agriculture Soil Conservation Service. The 1988 photographs had lower resolution than previous series, but the poorer resolution did not systematically bias our results. The Bobtown Hill Prairie photo coverage for 1988 was partially obscured by late afternoon shadows making habitat identification very difficult; we eliminated this photo from our analysis.

To assess the relative amount of edge through which woody species may encroach upon each prairie, we used the perimeter to area (p/a) ratio. This value should be related to how fast areas may be invaded, other factors being equal. A simple geometric relationship is expected in the p/a ratio over time if the area is decreasing. To determine if the p/a ratio is increasing at rates faster than expected, we use a p/a ratio index, which is a measure of the p/a ratio relative to a circle of equal size. Since a circle has the minimum p/a ratio of any two-dimensional shape, this index will always be greater than 1.0. There is no maximum to the p/a ratio index; higher values indicate a more complex shape with increasing amounts of edge for a given area.

Each of the nine sites were also sampled for vegetation characteristics. Between 1 and 10 permanent sampling transects were established at each site to sample for prairie species richness and woody species invasion (Table 2). Transect ends were located at random intervals along the crests of hill tops, with each transect set out downslope from the crest of ridges at the steepest angle possible. Sampling stations were established at 5-meter intervals along transects from the crest to the lower edge of hill prairies. At each sampling station a 25m2 plot was sampled for woody species. All woody stems within this plot were identified to species and counted. At the center of each 25m2 plot, a 1m2 quadrat was sampled for cover of all vegetation. Cover was estimated to the nearest percent. Summary data for sites were generated by the mean of all plots sampled at each hill prairie.


Distribution of Hill Prairies

Hill prairies are found along certain river systems in the central United States. The largest number of hill prairies occur along the Mississippi River, or tributaries near their junctions with the Mississippi, from near St. Paul, Minnesota and adjacent Wisconsin southward to Iowa and southern Illinois. In Minnesota, hill prairies occur in Filmore, Goodhue, Houston, Wabasha, and Winona counties, south of Minneapolis. They are found mostly along tributaries that flow into the Mississippi River from the west, and they can be up to 8 miles from the Mississippi (Olson 1989). In most instances, hill prairies in Minnesota occur on southwest-facing slopes in thin soil atop bluffs that have a northwest to southeast orientation.

In Wisconsin, hill prairies are mostly found on steep south to west-facing slopes above limestone, sandstone, or dolomite bluffs on the eastern side of the Mississippi River from Pope County southward to Grant County. There are Wisconsin State Natural Areas for hill prairies in Crawford, Grant, Pierce, and Vernon counties. Five-Mile Bluff Prairie Natural Area occurs on slopes above bluff tops in Pepin County along the Chippewa River approximately 3 miles from its junction with the Mississippi River (personal communication from Kelly Kearns and Eric Epstein, Wisconsin Department of Natural Resources). See Shimek (1924) for early photographs of hill prairies near Prairie du Chien, Wisconsin.

Hill prairies in Iowa can be found in both the eastern and western parts of the state (Cooper and Hunt 1982, White and Glenn-Lewin 1984, Rosburg et al. 1994). There are only a few in the northeastern, "Driftless Area" of the state (Shimek 1910, 1924). More than a dozen hill prairie remnants have been catalogued as "element occurrences" by the Iowa Department of Natural Resources (personal communication to J. Olson) in Allamakee, Dubuque, Clayton, Fayette, and Jackson counties; these are called "alkaline high prairies: Midwestern type." Some of these occur along the Mississippi River, while others are along the Iowa River. At least one is found in Fayette County above a creek that flows into the Volga River.

Hill prairies also occur in the "Loess Hills" region along the Missouri River in northwestern Missouri, western Iowa, and southeastern South Dakota (Novacek 1985). In Missouri, Steyermark (1963) says that these prairies are best developed in Atchison and Holt counties, but extend southward into Jackson County, below Kansas City. Floristically, these prairies are quite different from hill prairies along the Mississippi River and nearby tributaries, with many plant species from the Great Plains reaching their eastern-most limits of distribution.

In Illinois, hill prairies are scattered along the Mississippi River from near the Wisconsin border southward to the southern part of the state (Evers 1955); those currently listed on the Illinois Natural Areas Inventory (INAI) are shown in Figure 1. In southern Illinois, there is some gradation from hill prairies to barrens and glades; see Heikens and Robertson (1994), Heikens, West, and Robertson (1994), and Robertson and Heikens (1994) for discussions of barrens in Illinois. Hill prairies also occur along the Illinois River from north of Peoria southwards its junction with the Mississippi River; additional hill prairies appear along the Sangamon River, a tributary of the Illinois River (Evers 1955). A few small hill prairies can be found in east-central Illinois along the Embarras River in Coles County; these were described by Vestal (1918), Reeves et al. (1978), and Ebinger (1981).

Field work conducted for the INAI during 1976--1977 located 446 hill prairies; only 127, many less than 1 acre in size, were relatively undisturbed by grazing (Nyboer 1981) (Figure 2). A total of 534.4 acres of Grades A and B hill prairies were included in the Inventory, representing four types of hill prairies: loess (463 acres), glacial drift (51.5 acres), gravel (14.7 acres), and sand (5.2 acres). Because of their steep slopes and relative inaccessibility, many hill prairies have not been plowed and converted to row crops. It is likely that a higher proportion of original hill prairie remains in Illinois than other prairie types. Thus, hill prairies represent some of the last living windows into the ecology of the prairie biome that dominated Illinois for 8,000 years.


Types of Hill Prairies

In Illinois, there are four basic types of hill prairies recognized by the INAI (White, 1978): loess hill prairies, sand hill prairies, glacial drift hill prairies, and gravel hill prairies. The first, loess hill prairies, are the most abundant type in the state and occur primarily along the Mississippi and Illinois rivers. They are named for their characteristic wind-blown loam soil, which was deposited as the glaciers receeded (Pielou 1991). Two subclasses of loess hill prairies can be recognized-- one occurring on loess deposited above bluffs and the other where the loess is deposited just above the floodplains of rivers, mostly on top of mounds of glacial till. Two examples of loess hill prairies above bluffs in the Illinois Nature Preserves system include: Fults Hill Prairie and Pere Marquette (only one small hill prairie is actually in the latter Nature Preserve, but larger hill prairies are in the nearby Pere Marquette State Park). Examples of loess hill prairie nature preserves over glacial till include: Meredosia Hill Prairie, Revis Hill Prairie, and Witter's Bobtown Hill Prairie. Large amounts of the latter type of loess hill prairie, most severely grazed, also occur at Site M in Cass County, recently purchased by the Illinois Department of Conservation.

Varying amounts of wind-blown sand can be mixed in with the loess, and pockets of sand can also be deposited on top of loess. Areas with extensive amounts of sand that support some characteristic sand prairie plants are considered to be sand hill prairies. Some examples include: Hanover Bluff Nature Preserve, the Principia College Prairies (see Kilburn and Ford 1963, Kilburn and Warren 1963, Bland and Kilburn 1966, Ranft and Kilburn 1969), and French Woods Hill Prairie. Sand is also found to varying degrees at loess hill prairies, such as Revis Hill Prairie.

Glacial drift hill prairies occur on steeply sloping glacial till that does not have a mantle of loess. These are widely scattered in Illinois. Several occur along the Illinois River from just south of Peoria north to Putnam, LaSalle, and Grundy counties. Examples in the Nature Preserve system include: Crevecoeur, Ridgetop Hill Prairie, Robinson Park Hill Prairies, and Wier Hill Prairie. A few glacial drift hill prairies also occur in the extreme eastern part of Illinois. Windfall Prairie Nature Preserve in Vermilion County occurs on steep southwest-facing slopes above the Middle Fork of the Vermilion River. Calcareous seeps occur on the lower slopes of the prairie. Several small hill prairies are found in Coles County. These were first described by Vestal (1918) and subsequently studied by Reeves et al. (1978) and Ebinger (1981). Recently, Behnke and Ebinger (1989) described the invasion of one of these hill prairies by the non-native Euonymus alatus (winged wahoo, burning bush). According to J. E. Ebinger (personal communication), some plants frequently seen on these Coles County hill prairies, but not mentioned on the lists given below, are: Calystegia spithamaea (dwarf bindweed), Galium circaezans var. hypomalacum (wild licorice), Helianthus divaricatus (woodland sunflower), Lespedeza virginica (slender bush clover), Monarda bradburiana (Bradbury beebalm), Silphium terebinthinaceum (prairie-dock), and Thaspium barbinode (hairy meadow parsnip). Melilotus alba (white sweet clover) is a common non-native species. None of these Coles County hill prairies is dedicated as an Illinois Nature Preserve; the poorest quality one is located in a city park and the others are privately owned.

There are only a few gravel hill prairies in Illinois. There is some intergradation between "gravel hill prairies" and "gravel prairies" as recognized by the INAI. These two Nature Preserves are considered gravel hill prairies by the Inventory. Beach Cemetery Prairie in Ogle County is located on a gravel kame. Manito Prairie in Tazewell County is located on a gravel and sand terrace above the floodplain of the Illinois River. Numerous other dry prairies occur on gravel kames and eskers in northern Illinois.


Hill Prairie Plants

On 36 mostly loess Illinois hill prairies, Evers (1955) reported 252 taxa (248 species) of vascular plants. Most of these are native prairie species, but these figures also include a few native forest species and non-native species found in 18 or more sites. Of these 252 taxa, 123 occurred in 3 or fewer sites, while only 53 were found in 12 or more prairies. This relatively large number of species is due to the considerable ecological diversity that occurs on hill prairies in Illinois and the latitudinal difference of nearly 400 miles from the northernmost to the southernmost locations.

The most frequently occurring vascular plant species in hill prairies include: Schizachyrium scoparium (little bluestem), Bouteloua curtipendula (side-oats grama), and Erigeron strigosus (daisy fleabane) (Anderson 1972, Evers 1955, Kilburn and Ford 1963). At Fults Hill Prairie, Sorghastrum nutans (Indian grass) and side-oats grama had the highest percent of cover (Dziadyk 1978). The INAI lists the first two species plus Indian Grass as dominant plants on loess hill prairies, with the following listed as characteristic plants: Asclepias viridiflora (green milkweed), Kuhnia eupatorioides (false boneset), Linum sulcatum (wild flax), Lithospermum incisum (fringed puccoon), Penstemon pallidus (pale beardtongue), Psoralea tenuiflora (scurf-pea), Sisyrinchium campestre (blue-eyed-grass), and Spiranthes magnicamporum (scented ladies' tresses) (White 1978). Data are presented above (Table 2) of our own sampling of nine loess hill prairies, and the three most frequently occurring species were little blue stem, side-oats grama, and Petalostemum purpureum (purple prairie clover), while the three species with the largest percent cover were little bluestem, big bluestem, and Rhus glabra (smooth sumac).

Non-native plant species are generally less of a problem in hill prairies than in other types of prairies in Illinois. Melilotus alba (white sweet clover) is a serious problem in many hill prairies, as is Robinia pseudoacacia (black locust). Bush honeysuckles (Lonicera maackii, members of the L. tatartica complex, and L. x bella (bush honeysuckles) are found on some hill prairies and have the potential to become serious woody invaders. Belamcanda chinensis (blackberry-lily), native to China and Japan, is a fairly frequent herbaceous plant on hill prairies, as are Lespedeza stipulacea (Korean clover) and Verbascum thapsus (woolly mullein). Some non-native grasses frequently observed on hill prairies include Poa pratensis (Kentucky bluegrass), Poa compressa (Canadian bluegrass), Bromus tectorum (downy brome), B. inermis (smooth brome), and Festuca elatior (tall fescue). Woody non-native species sometimes seen on hill prairies are Rosa multiflora (multiflora rosa), Elaeagnus umbellata (autumn olive), Rhamnus cathartica (common buckthorn), and Maclura pomifera (Osage orange). Behnke and Ebinger (1989 ) suggest that Euonymus alatus (burning bush) could be a major woody invader of hill prairies when a seed source occurs in the immediate area.

The floras of these island-like xeric habitats are combinations of plant species that also occur in other types of dry prairies, black soil prairies, sand prairies, and gravel prairies, with only a few species largely restricted to hill prairies. It is always difficult to place individual species into categories, as there are nearly always exceptions to generalizations. The following groupings of plant species found on loess hill prairies are based on the author' experiences in Illinois, taking into account discussions with John E. Ebinger, William E. McClain, Loy R. Phillippe, and John B. Taft (i.e., errors are ours, not theirs). We also compared species lists from Evers (1955) with species lists in Betz and Lamp (1989, 1992) and several unpublished species lists in the files of the Illinois Natural History Survey. The senior author would greatly appreciate any additions, corrections, or other comments from readers. Information on endangered and threatened species is from Illinois Endangered Species Protection Board (1994).

Common on loess hill prairies, also frequent in various dry and mesic prairies:

Amorpha canescens (leadplant)
Andropogon gerardii (big bluestem)
Anemone cylindrica (thimbleweed)
Asclepias verticillata (horsetail milkweed)
Aster azureus (sky-blue aster)
Aster ericoides (heath aster)
Bouteloua curtipendula (side-oats grama)
Cassia fasciculata (partridge pea)
Ceanothus americanus (New Jersey tea)
Comandra umbellata (false toadflax)
Coreopsis palmata (prairie coreopsis)
Coreopsis tripteris (tall tickseed)
Echinacea pallida (pale coneflower)
Elymus canadensis (nodding wild rye)
Erigeron strigosus (daisy fleabane)
Eupatorium altissimum (tall boneset)
Euphorbia corollata (flowering spurge)
Heuchera richardsonii (prairie alumroot)
Koeleria macrantha (June grass)
Kuhnia eupatorioides (false boneset)
Lespedeza capitata (round-headed bush clover)
Liatris aspera (rough blazing-star)
Linum sulcatum (wild flax)
Lithospermum canescens (hoary puccoon)
Lobelia spicata (spiked lobelia)
Monarda fistulosa (wild bergamont)
Oxalis violacea (purple oxalis)
Penstemon pallidus (pale beardtongue)
Petalostemum candidum (white prairie clover)
Petalostemum purpureum (purple prairie clover)
Physostegia virginiana (false dragonhead)
Pycnanthemum pilosum (hairy mountain mint)
Rosa carolina (pasture rose)
Ruellia humilis (wild petunia)
Schizachyrium scoparium (little bluestem)
Scutellaria parvula (small skullcap)
Silphium integrifolium (rosinweed)
Solidago nemoralis (field goldenrod)
Solidago rigida (rigid goldenrod)
Solidago speciosa (showy goldenrod)
Sorghastrum nutans (Indian grass)
Tradescantia ohiensis (spiderwort)
Verbena stricta (hoary vervain).

[Conspicuous by their general absence on hill prairies are these mesic prairie species: Asclepias sullivantii (Sullivant's milkweed), A. tuberosa (butterfly weed), Baptisia leucantha (white wild indigo), B. leucophaea (cream wild indigo), Desmodium canadense (showy tick trefoil), Eryngium yuccifolium (rattlesnake master), Hypoxis hirsuta (yellow star-grass), Liatris pycnostachya (prairie blazing star), Parthenium integrifolium (wild quinine), Prenanthes aspera (rough white lettuce), Viola pedatifida (prairie violet), and Zizia aurea (golden Alexanders). Three common mesic prairie grasses, Panicum virgatum (switch grass), Sporobolus heterolepis (prairie dropseed), and Stipa spartea (needle grass), rarely occur on hill prairies.]

Mostly found on loess hill prairies and sand prairies:

Agalinis skinneriana (pale false foxglove)--State Threatened
Anemone caroliniana (Carolina anemone)
Asclepias amplexicaulis (sand milkweed)
Asclepias viridiflora (green milkweed), also gravel prairies
Aster oblongifolius (aromatic aster)
Aster sericeus (silky aster), also gravel prairies
Coreopsis lanceolata (tickseed coreopsis)
Desmodium sessilifolium (sessile-leaved tick trefoil)
Draba reptans (whitlow grass), also gravel prairies
Helianthus occidentalis (western sunflower), also gravel prairies
Heterotheca camporum (golden aster)
Liatris cylindracea (blazing-star), also gravel prairies
Opuntia macrorhiza (prickly-pear)
Panicum villosissimum (hairy panic grass)
Phlox bifida (cleft phlox)
Polygala incarnata (pink milkwort), also glades--State Endangered
Polygala verticillata var. isocycla (whorled milkwort)
Sisyrinchium campestre (blue-eyed grass)
Solidago ptarmicoides (white goldenrod).

[A few other typical sand prairie plants can occasionally be found in sand pockets on hill prairies, such as Aster linariifolius (flax-leaved aster), Helianthemum bicknellii (frostweed), Monarda punctata (horsemint), Plantago purshii (salt-and-pepper-plant), Selaginella rupestris (rock spikemoss), Talinum rugospermum (flower-of-an-hour), Tephrosia virginiana (goat's-rue), and Viola pedata (bird's-foot violet).]

Mostly found on loess hill prairies and gravel prairies:

Lithospermum incisum (fringed puccoon)
Microseris cuspidata (prairie dandelion)--State Endangered.

Largely restricted to loess hill prairies in Illinois, although they can occur in different habitats in other states.

Asclepias stenophylla (narrow-leaved green milkweed)--State Threatened
Buchnera americana (blue hearts)
Hedyotis nigricans (narrow-leaved bluets), also glades
Onosmodium occidentale (marbleseed)
Psoralea tenuiflora (scurf-pea)
Rudbeckia missouriensis (Missouri coneflower), also glades--State Endangered
Solidago radula (rough goldenrod), also glades
Spiranthes magnicamporum (scented lady's tresses).

Loss of Area

People have observed that hill prairies in Illinois have been decreasing in area, primarily due to the encroachment of woody species (Anderson 1972 and 1991, Behnke and Ebinger 1989, Ebinger 1981, Kilburn and Warren 1963, McClain and Anderson 1990, Reeves et al. 1978, Werner 1994, White 1978). This observation was quantified at Pere Marquette State Park by McClain (1983), who found from an analysis of aerial photographs that five hill prairies had been reduced in size by an average of 62% between 1937 and 1974.

Robert A. Evers resurveyed most of the sites described in his 1955 publication Hill Prairies of Illinois during the early 1970s. In his unpublished notes in files at the Illinois Natural History Survey, Evers frequently commented that hill prairies seen in the 1950s were being invaded by woody species by the 1970s. For example, in 1955, Evers noted that prairie vegetation dominated Seehorn Cemetery Hill Prairie in Adams County. Notes made by Evers on 9 June 1970 said "On this visit, prairie no longer occupied the cemetery. The cemetery has some large red cedars (6dm dbh), a few other large trees, and a host of saplings of Ulmus, Quercus muhlenbergii, Carya sp., Morus rubra, Fraxinus americana etc. The herb layer was also practically all forest species."

Several points can be observed by comparing Figure 1 of Evers (1955), a map of the 100 hill prairies described or observed in the early 1950s, with Figure 1 in the present publication, a map showing the locations of the 91 hill prairies currently on the Illinois Natural Areas Inventory. (1) Many of Evers' sites are still extant and are now on the Illinois Natural Areas Inventory. (2) There are a number of sites now on the INAI that were not recorded by Evers. Loess hill prairies were the primary focus of Evers, and he did not visit many glacial till hill prairies. Also, some hill prairies have been discovered since Evers' work. (3) Many sites on Evers map are not on the INAI. Some have been completely filled by forest species, as in the example of Seehorn Cemetery given above. Evers described nine hill prairies in Adams County, but none from this county are currently on the INAI. A few sites have been destroyed by quarrying. Other Evers' sites are probably still extant but are too small to include on the INAI. Lastly, Evers mentions that many of the hill prairies he visited were grazed, and even if those were still extant, they may not be of high enough quality to include on the INAI.

It is a matter for conjecture why hill prairies have not completely disappeared in the past 100 years. Prior to European settlement, Native Americans likely burned the hill prairies, perhaps to function as lookout points. Being high above the surrounding terrain, lightning is frequent on hill prairies and probably started natural wildfires. The first generations of European settlers probably burned hill prairies periodically; at any rate they did not stop wildfires. However, with the advent of the "Smokey the Bear" philosophy beginning in the 1930s, hill prairies were no longer burned, and woody vegetation began encroaching upon the prairies.

Although the steep slopes of hill prairies precluded their conversion to row crops, hill prairies were likely used extensively for grazing domestic livestock. Both Evers (1955) and Nyboer (1981) commented that grazing has deleterious effects on species composition in hill prairies. However, grazing probably kept many hill prairies open that would have otherwise reverted to forest. Hill prairies that have been grazed, such as Revis Hill Prairie Nature Preserve, have shown remarkable resiliency in eventually recovering much of their original quality. Perhaps 50 or more acres of hill prairies at Site M in Cass County, recently purchased by the Illinois Department of Conservation, have the potential of increasing in natural quality with proper management.


Results

Over the approximately 50-year study interval, hill prairies were reduced in size by an average of 63.0% (Table 1, Figure 4), ranging from 36.7% at Clendenny to 82.5% at Phegley. A careful examination of Figure 4 shows that the rate of loss has not been uniform over time. Fults has been managed as a nature preserve with regular burning since the early 1970s, yet it still lost 33.1% of its 1940 area between 1962 and 1988. Revis, the largest unit in our study, has a history of intermittent fall burns since the early 1970s (Schwegman and McClain 1985), yet it decreased in size from 32.51 hectares in 1969 to 17.37 in 1988. Perimeter maps showing the loss of area over the study period for Bland and Phegley hill prairies are shown in Figure 5. While our sites do not evenly sample sites of varying size, there was no indication of a relationship between the size of a site at the beginning of the study interval and rate of habitat loss.

In addition to losing significant area, our results show that hill prairies are becoming more fragmented with two-thirds of the sites increasing the number of units or patches of hill prairie at some point during the study interval (Table 1); this is graphically demonstrated for Phegley in Figure 5. In the case of New Canton and Phegley, the number of patches is currently decreasing as small isolated units are completely filled in by woody vegetation (Table 1).

The perimeter to area ratio shows an average increase of over 100% during the study interval ( Figure 6). Not only are our hill prairies declining from woody invasion at an alarming rate, but the propensity for this woody invasion is accelerating because of an increased ratio of edge to center of the habitat. As any two-dimensional object shrinks in size, the ratio of edge to center will increase. The p/a ratio index assesses the amount of edge present in a hill prairie patch relative to a circle of equal area. This p/a ratio index does not show dramatic increases over the study interval at most sites ( Figure 7), although the p/a ratio index was higher in 1988 than at the beginning of the series in seven of the nine sites (Table 1). Clendenny, one of the two sites where the p/a ratio index ended lower than it began, is an exception in that it lost numerous patches in 1988. Prior to 1988, this site also had an increasing p/a ratio index.

In our field sampling of these nine hill prairies, we found little bluestem and side-oats grama to be by far the most frequent in occurrence (Table 2), as in previous studies (Anderson 1972; Evers 1955, Kilburn and Ford 1963). Other species occurring in at least 25% of the sites are purple prairie clover, panic grass (Panicum species), sky-blue aster (Aster azureus), big bluestem, and scurf-pea. A total of 22 species were found in more than 10% of the meter square plots (Table 2). The most frequent woody species were Cornus drummondii (rough-leaved dogwood), Juniperus virginiana (red cedar), and smooth sumac. These woody species, however, are not necessarily indicative of hill prairie loss. These species persist in low abundance and low stature in many hill prairie sites. The woody species rank much higher in percent of total area (Table 2), with smooth sumac ranking second, red cedar fourth, and rough-leaved dogwood fifth. The only non-native species observed in more than 10% of our plots was Melilotus alba (white sweet clover).

Our sampling data also shows that the higher the proportion of 1940 area remaining in 1988, the larger the number of species observed in our m2 sample plots ( Figure 8). Conversely, the lower the proportion of 1940 area remaining in 1988, the larger the number of sample plots that contained woody species ( Figure 9).


Discussion

The results of this study suggest that hill prairies have, on average, been more than halved in size since 1940. This estimate includes only those sites that have managed to remain hill prairies during this period, and is thus a conservative estimate of the total rate of habitat loss. Much of this hill prairie loss is from woody encroachment around the borders of sites, especially by red cedar, smooth sumac, and rough-leaved dogwood. There is a natural propensity for habitat patches or units to increase in edge perimeter as the total size declines. Hill prairies are shown to be no exception to this rule as most sites increased the amount of edge relative to a uniform shape of the size during each sample period, indicating increasing complexity of habitat patch shape. This increasing amount of edge is an important trend as woody invasion takes place mostly along edges, and thus the rates of conversion of hill prairie to woody vegetation also increase. It is likely that what we observed at the nine sample hill prairies represents what is happening at hill prairies throughout Illinois, and thus it appears that many of the hill prairies in Illinois could disappear in the next 10 to 20 years.

The results of this study point to the single clear trend toward loss of the already rare hill prairie habitats in Illinois. The Illinois Department of Conservation and the Illinois Nature Preserves Commission are currently taking measures to protect hill prairie sites through active fire management. These measures, however, may be inadequate. Of the nine sites studied, only Revis and Fults have a history of fire management since the 1970s. While these two sites have relatively high species diversity values ( Figure 8), they do not have particularly low levels of woody species invasions ( Figure 9). In addition, sites acquired by the state of Illinois and managed by fire, such as Revis and Fults, have still declined in size since the 1970s. This may be a result of long intervals between fire treatments on these habitats. There is much debate on the natural fire frequency for hill prairies-- estimates range from 5 to 30 years. When combating advanced woody encroachment, managed fire frequencies may need to exceed natural levels until woody invasion is suppressed. In addition, it is necessary in some cases to manually cut and remove the woody plants.


Acknowledgments

The authors wish to thank the following for sharing their knowledge of hill prairies: John E. Ebinger, Don Gardner, William E. McClain, John Schwegman, John Taft and Stan Tyson. Celine D'Onofrio and Liane Suloway provided us with data from the Illinois Natural Areas Inventory database. Katherine J. Hunter prepared Figure 2, Jennifer Tate obtained literature, and Ann Marie Trame and Ann Kirts helped compile data from species lists. Loy R. Phillippe, John K. Bouseman, William E. McClain, and Gerould Wilhelm provided critical reviews. Careful readings by Scott C. McKay, Amanda S. Robertson, and Charles Warwick greatly improved readability. Funding was provided by the Illinois Department of Energy and Natural Resources.

Literature Cited

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