1994 Proceedings
North American Conference on Savannas and Barrens

Barrens communities have been variously interpreted as dry grasslands on thin soil (Bartgis 1993) to extremely dry, open forests with an understory of grasses and prairie forbs (Anderson and Schwegman 1991). In Illinois, upland, open forest communities dominated by Quercus stellata Wangh. (post oak) and Q. marilandica Muenchh. (blackjack oak) with an understory of grasses and sedges and some prairie forbs are common in Saline and Hardin counties of southern Illinois. Hall and Ingall (1910) mention these open forests on southern, drier slopes of Hardin County, while Vestal (1936) considered that these very open stands of post oak with a grassy ground cover may have been the "barrens" of the early Illinois literature. White and Madaney (1978) described dry barrens as areas of shallow soil over bedrock with a tree layer of stunted xerophytic oaks, and a sparse grass layer shorter than 1 m tall. The present study was undertaken to compare two post oak barren communities in Saline County, Illinois, one at Cave Hill that has been subjected to recent burns, and one at Dennison Hollow that has not been recently burned.

STUDY AREA

The study areas are located at Cave Hill (NE1/4 Sec 3 T10S R7E) and Dennsion Hollow (NE1/4 Sec 16 T10S R7E), located about 16 km southeast of Harrisburg, along the north flank of the Eagle Creek syncline in the southeast corner of Saline County, Illinois. These sites, within 3 km of each other, contain dry, upland, open forest on extremely thin, sandy soil, underlain with sandstone of the Abbott Formation. The soils are silt loams, averaging 35% sand, 55 % silt, and 10% clay, and the pH averaging 4.7. Both are in the Shawnee National Forest, and are located at the northeastern corner of the Greater Shawnee Hills Section of the Shawnee Hills Division (Schwegman 1973).

METHODS

The barrens were surveyed during the summer of 1993. At each site a 1/2 ha transect was placed parallel to the slope. These transects (100 m x 50 m) were divided into quadrats 25 m on a side. The number, size and species of all living and dead standing stems (above 10 cm DBH) were recorded for each of the eight quadrats. The relative dominance, relative density, and importance value (IV200) were then recorded for each species in each of the study areas (McIntosh 1957; Boggess 1964). The average diameter (cm), density (number of stems/ha) in broad diameter classes, and basal area (m2/ha) were also calculated for each species. The percent cover of each study site was determined by photographing the canopy from below (16 points) and calculating the percent of area covered using a 100 dot matrix. Seedlings (< 40 cm tall) were tallied in 0.0001 ha plots; small saplings (>40 cm tall but < 2.5 cm dbh) were recorded in 0.001 ha plots; and large saplings (2.5-10.0 cm DBH) were recorded in 0.01 ha plots, two randomly located in each of the 25 m x 25 m quadrats. During late summer, four transects 25 m long were located randomly, perpendicular to the slope, two toward the top of the slope and two toward the bottom. Along each transect, 1/4 m2 quadrats were randomly located at 1 m intervals. The frequency and cover index (Daubenmire 1959) of all herbaceous plants were determined (Mohlenbrock 1986). Due to the larger size of the Cave Hill barrens, the herbaceous understory was examined at three different areas: Area 1 where the overstory was studied, and areas 2 and 3, about 200 and 400 m to the west.

RESULT AND DISCUSSION

At both the Cave Hill and Dennison Hollow barrens, the dominant overstory species was Quercus stellata which accounted for an IV exceeding 120, more than 50 % of the individuals present, and about 65 % of the basal area. Subdominants were Carya texana Buckl. (black hickory) and Q. marilandica, with small numbers of a few other species (Table 1). Seedlings and saplings of post oak and hickory were also common, indicating that recruitment into the overstory is continuing. Besides the overstory species listed in Table 1, a few other tree species were encountered, mostly as seedlings and saplings. For the most part these were mesophytic species, and included Sassafras albidum (Nutt.) Nees., Quercus rubra L., Prunus serotina Ehrh., Celtis occidentalis L. and Diospyros virginiana L. Understory tree species found at both sites included a few seedlings and saplings of Amelanchier arborea (Michx. f.) Fern., Juniperus virginiana L. and Cornus florida L.

At Cave Hill, tree density averaged 526 stems/ha with a basal area of 15.4 m2/ha, and an overstory cover of 52 %, while at Dennison Hollow, tree density averaged 768 stems/ha, basal area exceeded 20.8 m2/ha, and overstory cover was 77 %. Most of the increase at Dennison Hollow was related to the large numbers of Q. stellata and Q. marilandica in the 10-19 cm diameter class (Table 1). Recruitment into this diameter class at Cave Hill is not high since fire has removed many of the saplings.

Though both areas had a relatively open understory, many shrubs were encountered. Blackberries and dewberries (Rubus spp.) were common, particularly in places were the canopy was open, and accounted for the majority of the shrubs. Vaccinium arboreum Marsh. (farkleberry) was also common at both sites, usually forming thickets, with many individuals reaching 3 m. Rhus copallina L. (dwarf sumac) was fairly common at Cave Hill, with most being root sprouts from recently burned individuals.

The herbaceous layer was dominated by a mixture of forest and prairie species with grasses and sedges being the most common (Table 2). Dichanthelium depauperatum and Danthonia spicata were the common grasses, while Carex pensylvanica ranked third. Common dicots included Solidago nemoralis, Cunila origanoides and Helianthus divaricatus. Prairie grasses and forbs, though not common, were found throughout the areas. Overall, the herbaceous layer at Cave Hill was more diverse and had a higher cover than Dennison Hollow (Table2).

Recent studies suggest that fire is necessary to maintain the prairie component of the barrens in southern Illinois, and in the absence of fire they would become, over time, closed forests. Fire was commonly used to "clear out the understory" in these forests until the creation of the Shawnee National Forest in the 1930s (Miller 1920; Anderson and Schwegman 1991). It is likely that the site studied at Dennison Hollow has not burned since being included in the national forest, while the forest at Cave Hill has been occasionally burned. During the past few years the understory of the Cave Hill forest has been burned by the U. S. Forest Service (Beth Shimp -personal communication). The high density of smaller diameter post and blackjack oaks, the overstory cover of 77 %, the large number of saplings, the low cover class and the absence of prairie grasses and forbs at the Dennison Hollow site suggests that this area is undergoing succession to a closed forest. The use of fire, however, could maintain an open canopy at Cave Hill by preventing recruitment of saplings into the lower diameter class, and removing some of the canopy trees.

LITERATURE CITED

Anderson, R. C. and J. E. Schwegman. 1991. Twenty years of vegetational change on a southern Illinois barren. Natural Area Journal 11:100-107.

Bartgis, R. L. 1993. The limestone glades and barrens of West Virginia. Castanea 58:69-89.

Daubenmire, R. 1959. A canopy coverage method of vegetation analysis. Northwest Science 33:43-64.

Boggess, E. R. 1964. Trelease Woods, Champaign County, Illinois: woody vegetation and stand composition. Transactions of the Illinois State Academy of Science 57:261-271.

Hall, R. C. and O. D. Ingall. 1910. Forest conditions in Illinois. Bulletin of the Illinois State Laboratory of Natural History 9:175-253.

McIntosh, R. P. 1957. The York Woods. A case history of forest succession in southern Wisconsin. Ecology 38:29-37.

Miller, R. B. 1920. Fire prevention in Illinois. Forestry Circular No. 2. Illinois Natural History Survey,

Mohlenbrock, R. L. 1986. Guide to the vascular flora of Illinois. Southern Illinois University Press, Carbondale, IL.

Schwegman, J. E. 1973. Comprehensive plan for the Illinois nature preserves system. Part 2. The natural divisions of Illinois. Illinois Nature Preserves Commission, Rockford, IL. 32pp.

Vestal, A. G. 1936. Barrens vegetation in Illinois. Transactions of the Illinois State Academy of Science 29:79-80.

White, J. and M. Madaney. 1978. Classification of natural communities in Illinois. In: J. White, ed. Illinois Natural Areas Inventory Technical Report. Vol 2, Survey methods and results. Illinois Natural Areas Inventory, Urbana, IL.

Table 1. Densities (#/ha), diameter classes, basal areas (m2/ha), relative values, importance values and average diameters of the woody species in post oak barrens, Saline County, Illinois.