1993 Proceedings of the Midwest Oak Savanna Conferences

VEGETATIONAL CHANGES IN A SOUTHERN ILLINOIS BARRENS

In 1968 we began a study to determine the response of a southern Illinois barren to fire. After two prescribed burns the frequency of prairie plants and annual legumes increased and tree basal area and density declined. A few woody species were eliminated by burning but two species of shrubs, silky dogwood and prairie willow, increased in density. During a fifteen year period without prescribed burns on the site, the frequency of prairie plants and the density of the two shrubs declined, while the frequency of woodland herbs and density of tree seedlings and saplings increased. In 1990, prescribed burning was again applied to the site. After three burns, prairie plants are increasing in abundance, while the abundance of woodland plants is declining. Nevertheless, our data suggests that continued burning may not return the site to those conditions which existed when we began our study.

A precise definition of barrens, acceptable to the majority of ecologists and naturalists, is difficult to derive. In the 1800s, Government Land Office (GLO) surveyors and diarists used the term barrens to describe areas with restricted tree growth. Barrens were apparently the result of, among other causes, dry substrate conditions and/or periodic anthropogenic fires that limited tree growth. Herbaceous species, many of which are associated with prairies and open woodlands (Vestal 1936, Anderson and Schwegman 1991), were abundant.

In 1968, we began a study of a small barrens area (0.2 ha) that is a U.S. Forest Service Research Natural Area within the Shawnee National Forest. The site is located adjacent to Burke Branch Creek in Pope County in extreme southeastern Illinois. It is about 56 km south of the historic distribution of prairies in Illinois (Anderson and Schwegman 1991) in a region that was largely forested prior to extensive settlement by nonnative Americans. Barrens were a minor vegetation type in this area and occupied some of the broader stream valleys and gently sloping ridge tops. This is indicated by remnant vegetation patterns, GLO survey records, and the occurrence of place names such as Barrens Creek, which Burke Branch Creek joins about 3 km downstream from our study site. Thus, the Burke Branch site meets the description of barrens given by White and Madney (1978) as being local inclusions in forested land of prairie flora mixed with forest.

We established permanent sampling locations containing a mixture of prairie and forest species on the site. Prescribed burning was used to enhance the growth of prairie plants and depress the growth of woody species. Woody species readily invade the site; its open character was historically maintained by periodic fire. The exact fire history of the site is unknown, but it probably burned on a regular basis before the 1930's, prior to the establishment of the Shawnee National Forest. Areas dominated by prairie grasses such as big bluestem (Andropogon gerardii) and Indian grass (Sorghastrum nutans), and a few prairie forbs such as tall tickseed (Coreopsis tripteris) and partridge pea (Cassia fasiculata) presently occur approximately 1.7 km downstream from the study area on Burke Branch Creek. Other areas closer to the study site contained these species when we began our work, but no longer support them. Currently, only forest species are present.

Vegetation samples were obtained pre-burn in 1968, and after prescribed burns in 1969, 1970, and 1971. The site was burned in 1972 and 1973, and sampled in 1983 and 1988. Prairie grasses and forbs responded positively to the fires. The number of shrub species and tree density declined with burning. Two shrubs, prairie willow (Salix humulis) and silky dogwood (Cornus amomum), had large increases in stem density following burning as a result of vigorous resprouting (Anderson and Schwegman 1991).

Fire cessation reversed presettlement patterns, and during the 15 years without prescribed burns, there was an influx of some woodland herbs, wood rush (Luzula multiflora), Christmas fern (Polystichium acrostichoides), clear weed (Pilea pumila), and spotted touchmenot (Impatiens capensis). In the 1988 sample, we did not record two prairie species, little bluestem (Schizachyrium scoparium) and oldfield goldenrod (Solidago nemoralis), which had relatively high quadrat frequencies in 1970. Apparently they were eliminated from the site (Table 1). Prairie willow and silky dogwood experienced marked declines in stem numbers following fire cessation.

In 1990, to reverse the pattern of increasing dominance by woodland plants on the site, woody plants were cut and treesized individuals on the site were girdled. The site was burned in 1990, 1991, and 1992, and the vegetation was sampled in the same years. Some prairie species (big bluestem, partridge pea and Indian grass) responded positively to this management regime. Surprisingly, so did a few of the forest herbs, such as the annuals, clear weed and spotted touchmenot. We expect the abundance of these species will decline as competition of prairie plants increases in response to continued burning.

Our results suggest that barrens are fire dependent communities containing a mixture of prairie and woodland plants. Increased fire frequency favors the growth of prairie plants and some species of shrubs in this community, whereas decreased fire frequency encourages the invasion of woodland plants. Shrub species, such as prairie willow and silky dogwood, are apparently natural components of barrens communities. Management of this vegetation type should include maintenance procedures to retain these woody species. Historically, barrens may have burned at irregular intervals that allowed fire dependent and somewhat fire sensitive species to coexist.

LITERATURE CITED

Anderson, R. C. and J. E. Schwegman. 1991. Twenty years of vegetational change on a southern Illinois barren. Natural Areas Journal 11:100107.

Nuzzo, V. A. 1986. Extent and status of midwest oak savanna: presettlement and 1985. Natural Areas Journal 6:636.

Vestal, A. G. 1936. Barrens Vegetation in Illinois. Transactions of the Illinois Academy of Science 29:7980.

White, J. and M. Madney. 1978. Classification of natural communities in Illinois. Pp. 309405. In J. White, ed., Illinois Natural Areas Inventory Technical. Vol. 2, Survey methods and results. Illinois Natural Areas Inventory, Urbana.

Table 1. Changes in quadrat frequency of selected herbaceous species on the Burke Branch Barrens (1) after two prescribed burns (1970), (2) following 15 years without burning (1988), and (3) after woody plant removal and reintroduction of prescribed burning (1990 and 1992).