Northeastern Naturalist: Vegetation composition and structure of Eversgerd Post Oak Flatwoods, Clint

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Northeastern Naturalist: Vegetation composition and structure of Eversgerd Post Oak Flatwoods, Clint

ABSTRACT - Eversgerd Post Oak Flatwoods is a 16 ha woodlot that is part of an extensive band of timber associated with the broad Kaskaskia River valley in the Southern Till Plain Natural Division of Illinois. This flatwoods was studied to determine its woody composition and structure, and the effects of fire suppression on canopy closure and plant succession. Tree density within the site averaged 266 trees/ha with a basal area of 24.8 m^sup 2^/ha. Quercus stellata Wang, (post oak) dominated the overstory and woody understory with an importance value of 171 (200 possible). Quercus bicolor Willd. (swamp white oak) and Q. palustris Muenchh. (pin oak) were common species in a shallow depression, accounting for their second and third in importance value ranking. The forest has an open, park-like appearance. Small woody saplings averaged 820 stems/ha and large saplings averaged 221 stems/ha. Grasses and sedges dominated the herbaceous layer; Danthonia spicata (L.) Roem. & Schultes and Carex cephaloplwra Willd. were the dominant species. The large number of post oaks with low branches and branch scars indicate that this forest was more open in the past.

INTRODUCTION

At the time of European settlement forests dominated by Quercus stellata Wang. (post oak) were common throughout much of the Midwest from Ohio to Missouri, especially on the Illinoian glacial till plain (Braun 1950, Telford 1926). Post oak forests occurred on barrens with thin soil and exposed bedrock (Ebinger et al. 1994), as well as on flats with heavy clay soils and with a claypan at or near the surface (Coates et al. 1992, Taft et al. 1995). Standing water was common on these flats during parts of the growing season. In shallow depressions of these flatwoods a Quercus palustris-Quercus bicolor seasonally flooded forest alliance persisted and was surrounded by the post oak flatwoods (Drake and Faber-Langendoen 1997, Tecic et al. 2001). In areas with improved drainage, due to topographic relief, post oak was usually associated with Q. velutina Lam. (black oak) and various species of hickory (Carya spp.) (McClain and Ebinger 2003).

Studies indicate that post oak forests were relatively open (Anderson and Anderson 1975, Ebinger and McClain 1991). At the time of European settlement these open canopy forests were maintained by periodic fire (Abrams and Seischab 1997, Blake and Schuette 2000, Davies 1994, Dolan 1994, McClain and Elzinga 1994, Williams 1989). With the cessation of landscape fires, woody plants encroached and the canopy closed (Ebinger 1986, Ebinger and McClain 1991, Reich et al. 1990). The study was undertaken to determine the forest structure and floristic composition of Eversgerd post oak flatwood forests that is undergoing canopy closure, and how fire suppression is altering oak dominance.

STUDY AREA

The study area is a small section of an extensive block of timber located along Shoal Creek near where it enters the Kaskaskia River. Here the Kaskaskia River valley is extremely broad, heavy clay soils are common, and many of the soils have a dense claypan. This woodlot was the best remaining example of the post oak forest that once was common. Protected by the owner Franklin Eversgerd, no cutting, grazing, or other disturbances have taken place within the past 50 years, though it is very probable that the woods has been subjected to occasional ground fires. This woodlot was considered a grade B southern flatwoods by the Illinois Natural Areas Inventory (White and Madany 1978).

Eversgerd Post Oak Flatwoods is located in Germantown Township about 5 km south of Germantown, Clinton County, Illinois in the Effingham Plain Section of the Southern Till Plain Natural Division (Schwegman 1973). The woodlot is a mesic to wet-mesic post oak woods of about 16 ha (SW1/4 NW1/4 Sec. 28 T1N R4W). The overall relief does not exceed 1 m. The soils are Wynoose and Bluford silt loams which are poorly drained soils that occur on broad, loess-covered till plains. The climate is continental, with humid, hot summers and cold winters. Average annual precipitation is 101 cm. Of this total, about 53.5 cm falls as rain during April through September. The highest temperature on record is 41 [degrees]C for July 15, 1980, the lowest is -24 [degrees]C on December 22, 1989. June, July and August are the warmest months, with average daily maximum temperatures greater than 29.4 [degrees]C. December, January, and February are the coldest when average daily maximum temperature is 5.8 [degrees]C. The frost-free growing period averages 180 days (Sabata et al. 1998).

METHODS

During the spring of 2001, a 10.5 ha section of the woodlot was divided into quadrats 25 m on a side (168 quadrats). In each quadrat all living and dead-standing woody individuals 10.0 cm dbh (diameter at breast height) and above were identified to species and their diameters recorded. Living-stem density (stems/ha), basal area (m^sup 2^/ha), relative density, relative dominance, importance value (IV), and average diameter (cm) were calculated for each species. As used here, the IV is the sum of the relative density and relative dominance (total of 200). Dead-standing stem density (stems/ha), basal area (m^sup 2^/ha), and average diameter were also determined for each species.

Woody understory composition and density (stems/ha) were determined using 1, 10, and 100 m^sup 2^ nested circular plots located at 20 m intervals along randomly located line transects scattered throughout the study area; four additional 1 m^sup 2^ plots were located 7 m north, east, south, and west of each center. In the 1 m^sup 2^plot, seedlings ( 40 cm tall and

The flatwoods was visited 16 times throughout the growing season in 2000 and 2001. During each trip, voucher specimens were collected from in and around the flatwoods, including disturbed and open areas along the woodland edge. The specimens were identified and deposited in the Stover-Ebinger Herbarium (EIU) of Eastern Illinois University, Charleston, Illinois. Nomenclature follows Mohlenbrock 1986).

RESULTS

Tree density in the woodlot averaged 265.8 stems/ha with a basal area of 24.8 m^sup 2^/ha (Table 1). Of the 15 species of trees, post oak ranked first with an IV of 171.1. This taxon averaged 209.5 stems/ha, and accounted for more than 90% of the total basal area (22.8 m^sup 2^/ha). Most of the large post oaks had an open-grown appearance with low branches or branch scars and broad open crowns. Even-aged and uniform-sized post oaks were common with most individuals in the 20 to 50 cm diameter classes (Table 1). The largest tree found was a post oak of 102 cm dbh.

Swamp white oak and pin oak were common species in shallow depressions, accounting for their second and third ranking in IV (Table 1). This Quercus palustris-Quercus bicolor seasonally flooded forest alliance (Drake and Faber-Langendoen 1997) is occasionally found along the Kaskaskia River drainage in southern Illinois (Tecic et al. 2001). A shallow depression of about 0.9 ha occurs in the study area in which mature pin oak/swamp white oak community exists. This shallow depression is very irregular in outline, the forest community gradually changing from a post oak to a pin oak/swamp white oak dominated overstory.

The remaining woody species were not common and none had an IV greater than 4.0, or a density greater than 8 stems/ha (Table 1 ). Of these, four were oaks, with Quercus marilandica Muenchh. (blackjack oak) and Q. velutina (black oak) occurring on drier sites, while Q. imbricaria Michx. (shingle oak) was scattered throughout the woodlot. Two hickory species were found, Carya ovata (Mill.) K. Koch (shagbark hickory) was fourth in IV, while C. tomentosa (Poir.) Nutt. (mockernut hickory) was rare.

The understory was relatively open with few woody seedlings and saplings (Table 2). Woody seedlings averaged only 2,520 stems/ha, small saplings averaged 820 stems/ha, and large saplings 221 stems/ha. Post oak dominated all categories, but Sassafras albidum (Nutt.) Nees (sassafras) and pin oak were also relatively common (Table 2).

Tree mortality averaged 31.9 stems/ha with a basal area of 2.1 m^sup 2^/ha. Post oak had the highest mortality with 27.5 stems/ha followed by blackjack oak with 2.4 stems/ha. The average diameter of dead-standing individuals of post oak was 26.8 cm with the largest individual at nearly 100 cm dbh. Though less common in the woodlot (7.7 total living and dead stems/ha), nearly 30% of the blackjack oaks stems were dead. Individuals of this species were generally smaller than mature post oaks so this species was more affected by canopy closure (Table 1).

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Species diversity in the flatwoods was not high; only 169 taxa in 55 families were found. Of these, two were ferns, 41 were monocots in six families, and 134 were dicots in 47 families. Woody species accounted for 39 of these, while 14 exotic taxa were also found. Of the exotic species, none was common except Lonicera japonica Thunb. (Japanese honeysuckle), which occurred in a few open areas.

The majority of the herbaceous species were rare, being confined to disturbed sites in and at the edge of the woods, open areas where edaphic factors limited tree growth, or infrequent though widely scattered throughout the woods. The majority of the herbaceous layer was composed of various species of grasses and sedges. Danthonia spicata (L.) Roem. & Schultes and Panicum lanuginosum Ell. were the dominant grasses, whereas Carex cephalophora Muhl. was the dominant sedge. In wetter areas Carex meadii Dewey, C. annetens Bickn., and Eleocharis verrucosa (Svens.) Harms. dominated.

DISCUSSION

Flatwood communities dominated by post oak are characterized by open-grown trees, the general absence of a well developed shrub layer, and a grassy ground layer. In southern Illinois most of these flatwoods were grazed and were subjected to periodic fires into the late 1950s. Changes in land use patterns, however, particularly fire suppression and the elimination of grazing, resulted in woody plant encroachment into some flatwoods, while others lacked woody encroachment and maintained an open understory. The reason for these differences is not clear, but probably the time since the woodlot was last grazed, and the time and intensity of past fires or other disturbances, were critical (Guyerre and Cutter 1991, Taft et al. 1995).

Only a few post oak flatwoods in Illinois have been examined in detail. Fralish (1988) studied a small post oak flatwoods on a wet terrace in Washington County, Coates et al. ( 1992) inventoried a flatwoods on an upland site in Effingham County, and Taft. et al (1995) examined six flatwoods on both upland and terrace sites in the Southern Till Plain Natural Division of Illinois. At all sites post oak was by far the leading dominant whereas blackjack oak and Quercus velutina Lam (black oak) ranked a low second and third in IV. At the sites studied by Taft et al. (1995) the IV of post oak ranged from 83.9 to 148.4 (averaged 114.8), while density averaged 195.2 stems/ha, and basal area averaged 16.5 m^sup 2^/ha. In these flatwoods the IV of blackjack oak averaged 17.3, while the IV of black oak averaged 14.9 (Taft et al. 1995). In Eversgerd flatwoods, in contrast, post oak had a higher IV (171), while blackjack and black oak were not common with IVs of 2.5 and 1.4 respectively (Table 1). In Eversgerd flatwoods these two species were replaced by swamp white oak and pin oak, probably the result of wetter condition. Also, Taft et al. (1995) reported overall tree densities of 284 to 533 stems/ha compared with the 209 stems/ha at Eversgerd flatwoods (Table 1). The woody understory was also dense in most of the flatwood studied by Taft et al. (1995), saplings averaged 824 to 9,673 on their sites while total saplings averaged 1,041 stems/ha at Eversgerd flatwoods (Table 2). The only flatwoods with a lower sapling density was at Lake Sara where management had involved yearly burns (Taft et al. 1995).

In Eversgerd Post Oak Flatwoods the canopy was relatively open with post oak the dominant of a cathedral-like overstory; the understory had a low density of seedlings, saplings, and shrubs; and the ground layer was open and dominated by grasses and sedges. This flatwoods may have been grazed later than most flatwoods of the region, or may have been subjected to occasional ground fires. No fire scars were found in the few dead-standing stems cut, and no information was available concerning past fires in the woods. There was evidence, however, that some of the adjacent woods did have occasional ground fires, including charred snags, and basal fire-scars. Based on the open nature of the woods, and the open understory, it is probable that low intensity ground fires had occurred relatively recently in this flatwoods. Taft et al. (1995) found that vegetation trends associated with fire suppression included an increased diversity of woody species and a decreased importance of post oak in six southern Illinois flatwoods. Also, ground cover species diversity and abundance declined with fire absence, but was enhanced with prescribed fire. Dolan (1994) found that fire contributes to the unusual species composition and community structure of post oak flatwoods in southern Indiana.

Presently post oak dominates the woodlot, and its importance will continue for the near future. The relatively small number of post oaks in the 10-19 cm diameter class, however, suggests that conditions may no longer favor the long-term dominance of this species. Occasional fires would probably be sufficient to allow the continued regeneration of post oak. Fire suppression, however, will result in canopy closure, and over the long term, a change in canopy composition. This decrease in oak regeneration is occurring throughout the Midwest, probably due to fire suppression (Ebinger and McClain 1991, McClain and Elzinga 1994). The resulting canopy closure favors the growth of shade-tolerant, fire-sensitive species that take advantage of canopy openings as older trees die. Presently it is not clear which tree species will replace the post oaks in the canopy, but in other post oak forests various species of hickory, particularly shagbark and mockernut hickories, are common in the understory and lower tree strata (McClain and Ebinger 2003).

ACKNOWLEDGMENTS

The authors thank Mr. Franklin Eversgerd for access to the study site, the Illinois Department of Natural Resources for a grant to complete the study, and Dr. Gordon Tucker, Eastern Illinois University, for help in the identification of Cyperaceae.

LITERATURE CITED

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Sabata, L.R., W.M. McCauley, K.D. Brinkman, and M.R. Gajewski. 1998. Soil Survey of Washington County, Illinois. United States Department of Agriculture, Natural Resources Conservation Service, and Illinois Agriculture Experiment Station, United States Government Printing Office, Washington, DC. 202 pp. + maps.

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Tecic, D., W.E. McClain, and J.E. Ebinger. 2001. Structure and composition of pin oak/swamp white oak flatwood forests in Illinois. Transactions of the Illinois State Academy of Science 94:19-26.

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BOB EDGIN1, WILLIAM E. MCCLAIN1, BOB GILLESPIE1, AND JOHN E. EBINGER2,*

1Illinois Nature Preserves Commission, 524 South Second Street, Springfield, IL 62701. 2Emeritus Professor of Botany, Eastern Illinois University, Charleston, IL 61920. *Corresponding author - cfjee@eiu.edu.