VEGETATION ANALYSIS OF A RESTORED SOUTHERN ILLINOIS
R. Todd Bittner
K. Andrew West
According to original public land survey notes and accounts from settlers and travelers, Illinois terrestrial communities were forest, prairie, or barrens. However, the State's original landscape has been dramatically changed from human disturbances, including fire suppression, habitat destruction, and herbivore extirpation. Natural forest openings such as barrens have become overgrown and are subsequently degraded. Barrens communities, once the most common type of natural forest opening, are now rare. For example, in Illinois only 2.4 ha of shale-associated natural forest openings are believed to remain (White 1984) and this rare community type has been ranked G2 (rare) by The Nature Conservancy (Ambrose et al. 1994).
Brown Shale Barrens, located in the Ozark Hills of southern Illinois, is the only shale barrens in the Illinois Nature Preserve system (McFall 1991). The nature preserve on which the barrens is located totals 11.5 ha. The shale bedrock of the steep, south-facing slope is highly erodible and often close to or exposed at the surface. An assemblage of old, open-grown post (Quercus stellata ) and white oak (Quercus alba) and other native species adapted to xeric environment is present. Aerial photographs from 1938 reveal widely spaced "wolf trees" throughout an evidently open landscape. In contrast, 1988 aerial photographs of the same area show an increase in forest cover. The barrens tree and shrub canopy apparently increased since successful fire suppression programs were enacted in the 1930s. The flora has shifted to a more mesophytic association of canopy-producing species.
Restoration efforts were initiated to promote the canopy conditions more favorable to barren-community plants (Bittner et al. 1994). This report evaluates the response of the flora to the restoration efforts (canopy opening). Tree ring analyses also were conducted to define the fire history.
Restoration began in 1990, when Illinois Department of Conservation staff conducted a prescribed spring burn within the most open part of the barrens. In 1991, mesophytic woody invaders were cut from a 0.25 ha section within the burned section and the cut stumps treated with a herbicide (Garlon 4).
In September, 1993, eight permanent 50 m2 plots initially established by Heikens in 1988, were resampled in the restored (cleared and burned) section of the barrens, and eight new 50 m2 plots were sampled in the unrestored area (not cleared, but burned). Ground cover of each species was estimated within each quarter plot (12.5 m2 ) using the modified Daubenmire scale (Abrams and Hulbert 1987). A one-way ANOVA was performed on log transformed cover per quarter plot of each species in the restored and unrestored areas. Detrended Canonical Correspondence Analysis (ter Braak 1988) was performed on the 50 m2 plots using species cover to determine the relationships between plots and species.
Clearing of mesophytic woody species <1 5 cm DBH continued in September, 1993, in a 1.0 ha section of the barrens. Trunk sections were collected, their circumference measured, and the species identified. The number of tree rings and rings with fire scars were recorded and dates calculated.
Thirty five species (5 woody and 30 herbaceous) showed a significant difference (P < 0.05) in cover between the restored and unrestored areas. Fifteen species had significantly lower mean cover in the restored plots compared to the unrestored plots while 19 species had significantly higher cover. The twenty herbaceous species with highly significant (P < 0.01) differences in means between treatments are listed in Table 1. Two barrens indicator species, Schizachyrium scoparium and Muhlenbergia capillaris, were more than ten times as abundant within the restored area than in the unrestored area. Acalypha gracilens, Aster patens, Croton monanthogynus, and Dichanthelium acuminatum also had significantly higher cover in the restored area. Three species had lower cover in the restored area; i.e., Asplenium platyneuron, Galium circaezans, and Muhlenbergia sobolifera.
The DCA ordination accounted for 39% of the variance with the first and second axes. There was a distinct separation of the restored and unrestored plots into two groupings (Fig. 1). The species biplot shows that the restored area was characterized by grasses, while the unrestored area consists of trees and Helianthus divaricatus, a woodland forb.
Ring counts of 98 trees cut during the 1993 restored area indicated 12 fire events from 1915 to the present (Fig. 2). Three times, four trees had fire scars for the same year. Five other times only a single tree showed evidence of fire. The number of trees from the sample was not even through the period studied. From 1948 through 1990, the mean interval between fires was 5.4 years (SE = 1.9); however, there was a 31 year period (1916 - 1947) where where no fire scars were found.
The experimental clearing of woody species from a small portion of the barrens has significantly changed the composition of the flora in three seasons. Restoration has enabled the barrens to revert to a grass-dominated community. These results were similar to those reported by Anderson and Schwegman (1991) from the Burke Branch Barrens. They found an increase in herbaceous species richness and abundance following prescribed fires, but found increases in woody species and woodland herbs 15 years after the final prescribed burn. They concluded that fire frequency played an important role in maintaining the barrens community when it occurred often enough to encourage the prairie species but not so often that the forest species are eliminated. The Brown Barrens community seems to be in the grass and forb (prairie) dominated stage following the disturbance, but the community will naturally succeed into slightly more woody species before the next disturbance. The degree to which each community type (woodland and prairie) is represented at any one point in a barrens is determined by a complex suite of factors including but not limited to fire frequency, intensity, and history, drought and other climactic factors, and management activities.
Due to a lack of published literature related to barrens (especially shale), it is difficult to define the original vegetational composition of the typical shale barrens community. Thus, the success of the restoration can not be empirically documented. However, the major transformation towards more xerophytic species and increased grasses, sedges, and forbs is due to the management work on the barrens. We believe that the xerophytic species now dominating the restored barrens are those which occurred on the more extensive barrens of 50 years ago.
The low moisture and high light of the restored barrens enables species adapted to these conditions to flourish from seed within the seed bank and individuals existing as stunted suppressed plants.
Tree ring and fire scar analyses provided some insight into local fire history, frequency, and intensity. Although specimens exhibiting a record from the first half of this century were few, some valuable information was obtained. The analysis indicated only 12 fire events in the past 80 years (with a noticeable hiatus from 1915-1948). However, only 15 cm and smaller diameter trees were removed during the restoration. This possibly biases these data by limiting the number of older trees. The majority of the trees examined were small and 74% were less than 50 years old. The large number of young trees coincides with the changes observed from the 1938 to 1988 aerial photographs. The barrens contained an uneven stand age distribution that was skewed with high numbers of small trees.
An interesting aspect of the analysis is the low number of trees with fire scars during each identified fire year. The low number seems to indicate infrequent, low intensity fires. These fires were not severe enough to kill the trees of the sizes sampled. High fire intensity seems characteristic of a duff-based fuel rather than grass. Apparently, the low fire frequency and intensity (combined with a seemingly 32 year period when fire was all but absent) contributed to the progressive invasion of woody plants.
In 1989 and 1990 (prior to and following the prescribed burn), Heikens et al. (1994) sampled the vegetation at Brown Barrens and concluded that fire alone is not sufficient to restore the barrens communities since low fuel loads would not promote large, tree damaging fires. In addition, frequent fires would reduce or destroy the soil organic layer. They recommended that fire should not be the main restoration tool for barrens. This recommendation prompted the Illinois Department of Conservation to employ other restoration techniques.
Woody species removal can significantly alter the floral composition of a barrens in a matter of a few years; fire alone will not produce totally satisfactory results. Fire, used sparingly, in conjunction with canopy removal seems to be a successful method for barrens restoration. Additional studies at Brown Shale Barrens will include seed bank analysis and resampling of the previously restored and newly restored parts of the nature preserve.
The authors gratefully acknowledge the Illinois Department of Conservation for permitting the study to be done within Brown Barrens Nature Preserve. Thanks also to the Illinois Native Plant Society and the SIUC, Dept. of Plant Biology Grassland Ecology class for assisting with the restoration and data collection.
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Table 1. Mean cover, SE, and confidence level (P < 0.01) of herbaceous species in restored and unrestored areas of Brown Shale Barrens. Nomenclature according to Mohlenbrock (1986).