The American Midland Naturalist Published Quarterly by The University of Notre Dame, Notre Dame, Indiana Vol. 152 July 2004 No. 1 Am. Midl. Nat. 152:1–11 Soil Properties and Soil Nitrogen Dynamics of Prairie-like Forest Openings and Surrounding Forests in Kentucky’s Knobs Region 1 2 3 C. C. RHOADES, S. P. MILLER AND M. M. SHEA Department of Forestry, University of Kentucky, Lexington 40546 ABSTRACT.—Herbaceous communities located within forest openings increase plant species diversity of forests in the Knobs Region of Kentucky. Although these grass-dominated communities are protected and managed for rare plant species conservation, it is unclear how soil conditions may delineate the grassland-forest boundary. We compared soil chemical and physical properties and soil nitrogen pools and transformations of three forest openings with those in the surrounding forest and the wooded edge separating the grassland and forest communities. Soil pH, extractable cations, bulk density and silt content were higher in openings compared to forest soils and extractable phosphorus was lower. In forest-opening grasslands, total soil nitrogen and the availability and production of plant-available N were 25 to 50% of levels in adjacent forest. Soils of the woody edge were similar to the surrounding forest. Our results suggest that the presence of these calcareous glade openings is highly correlated with edaphic conditions, so that efforts to expand rare plant habitat may be constrained by the distinct soils of these forest openings. INTRODUCTION Isolated herbaceous communities situated within forest openings are an important component of plant diversity in the eastern deciduous forests of North America (Quarterman et al., 1993; Anderson et al., 1999). These openings have various possible origins. They can be stable, spatially-discrete communities whose extent is controlled by edaphic conditions, successional communities originating from natural disturbance or human land use practices or communities resulting from the combination of preexisting edaphic conditions that are further modified by canopy or soil disturbance (Baskin et al., 1994). Within Kentucky, forest-opening communities are a conservation concern because they contain locally rare plant species threatened by land conversion, land degradation and woody plant encroachment. Statewide, 51 species of state or federally threatened plant 1 Corresponding author present address: U.S. Forest Service, Rocky Mountain Research Station, Fort Collins, Colorado 80526; Telephone (970) 498-1250; FAX (970) 498-1212; e-mail: [email protected] 2 Present address: Department of Biological Sciences, Georgetown College, Georgetown, Kentucky 40324 3 Present address: Bernheim Arboretum and Research Forest, Clermont, Kentucky 40110 1 2THE AMERICAN MIDLAND NATURALIST 152(1) species are found within such openings (Kentucky State Nature Preserves Commission, 1996). Although unique soil conditions may control the extent and composition of some forest- opening communities (Anderson et al., 1999), the focus of most studies in these communities has been floristic (Baskin and Baskin, 1973, 1975, 1978). Major difference in soils of the opening and the forest could have a strong influence on the extent and successional trajectory of forest opening communities. If edaphic conditions control the location and extent of forest openings, the border between the herbaceous and forest community should be relatively stable, corresponding to a relatively abrupt change in parent material type or profile depth and associated changes in soil chemical and physical properties (Kalisz and Boettcher, 1990). In contrast, where the openings are formed by forest canopy disturbance, woody vegetation should readily reinvade herbaceous commu- nities following fire, drought, wind damage or other perturbations. In such cases, biotically mediated soil processes (i.e., nitrogen turnover, nutrient uptake and organic matter stabilization) will shift with species composition and litter inputs across the forest to grassland ecotone ( Jenny, 1980; Daniels et al., 1983; Rhoades and Coleman, 1999; Rhoades et al., 2000). Where fire suppression or other land conversion has resulted in woody plant encroachment into forest openings, forest-opening communities may expand relatively rapidly following mechanical felling of trees growing at the periphery of these openings. In contrast, where the boundary between herb-dominated forest openings and the adjacent forest corresponds to abrupt parent material transition (Kalisz and Boettcher, 1990; Boettcher and Kalisz, 1991), forest clearing may not be sufficient to promote expansion of the herbaceous species associations. In either case, without consideration of the linkage between vegetation communities and chemical or physical soil factors, efforts intended to favor expansion of unique plant species into newly cleared or expanded openings may fail. The overall objective of this study was to compare soil chemical and physical properties and soil nitrogen dynamics across the ecotone between forest openings and adjacent forest. Large differences in soil chemical conditions between the openings and surrounding forest may provide insight regarding the historic extent of the herbaceous communities and, thus, help delineate vegetation management activities aimed at maintaining or expanding the glade openings. METHODS Site description.—We studied a group of three forest openings located in Kentucky’s Knobs Region on the western border of the Bluegrass section of the Interior Low Plateaus Physiographic Province (Quarterman et al., 1993; Fig. 1). The study site is located at the Bernheim Arboretum and Research Forest, in the southern extreme of Bullitt County (378529N; 858359W) near its border with Nelson County. The openings are on south-facing slopes associated with calcareous shale interbedded with dolomite of the Silurian lower Laurel formation (Kepferle, 1969). The series of openings corresponds to a band of soils mapped as Caneyville-Rock outcrop or Caneyville-Beasley-Rock outcrop complexes and classified as fine, mixed mesic Typic Hapludalfs (Soil Conservation Service Staff, 1986). Together these soil groups cover about 5% of Bullitt County. In Kentucky’s Knobs Region on the ‘‘driest situations, where limestone outcrops exist,’’ Braun (1950) described ‘‘xerophytic red cedar communities and occasional typical prairie patches’’ amidst the surrounding deciduous forest. Such openings are currently classified as either limestone glades (Evans, 1991) or xeric limestone prairies (Baskin et al., 1994). At the Bernheim forest, ‘‘glade’’ openings were noted on land deed boundaries surveyed in the 2004 RHOADES ET AL.: FOREST SOIL PROPERTIES 3 FIG. 1.—Study site location at the Bernheim Research Forest and Arboretum within Kentucky’s Knobs Physiographic Region 1780s (Dickson and Kollath, 2001). The three forest openings were visible on 1928 aerial photographs. Presently, the 0.25 to 0.30 ha openings are the only herbaceous-plant- dominated hillside communities known within the 6500 ha Bernheim Research Forest. The openings are dominated by the grasses Schizachyrium scoparium (Michx.) Nash and Sporobolus vaginiflorus Torr. and to a lesser extent by Panicum capillare L. and Danthonia spicata L. Common forbs include Scutellaria parvula Michx., Lithospermum canescens Michx., Croton monanthogynus Michx., Hypericum dolabriforme Vent. and Echinacea simulata McGregor. The openings also contain the state-listed threatened plants Viola septemloba Le Conte var. egglestonii (Brainerd) McKinney and Spiranthes magnicamporum Sheviak (Kentucky State Nature Preserves Commission, 1996). Juniperus virginiana L. is the most abundant tree scattered within the forest openings and within both the edge and forest communities (Table 1). In addition to J. virginiana, the overstory community of the forest surrounding the openings is codominated by Fraxinus americana Marshall, Acer saccharum Marshall, Quercus muehlenbergii Engelm. and Pinus virginiana Miller. Cercis canadensis L. and Rhamnus caroliniana Walter. are common midstory species within all three community types. Surface features differ between the forest openings and the surrounding habitats. Large exposures of rock are absent from the openings, whereas gravel, cobble or bare soil cover about 60% of the openings. Exposed rock and soil are absent from forest sites and cover only 13% of the wooded edge. Study design.—Each of three sites was partitioned into the following habitat types arranged in concentric zones: (1) opening, (2) woody edge and (3) surrounding forest. The openings were round to elliptical; distance from the center of the openings to the woody edges ranged from 15 to 50 m. The woody edge formed a 5- to 10 m-wide band. Two transects originating at the opening center and radiating through the edge and into surrounding forest were located at each opening site (n ¼ 6 transects). In each opening, transects were stratified into opposing halves of the opening then oriented along randomly selected radii (i.e., 1 to 1808 or 181 to 3608). Along each transect, sample plots were located (1) midway between the opening center and the woody edge, (2) centered within the wooded edge and (3) 20 m beyond the edge within the surrounding forest (n ¼ 6 plots per vegetation zone). Chemical and physical soil properties.—The chemical and physical properties of opening and surrounding forest soil profiles were sampled and analyzed by depth increments extending 4THE AMERICAN MIDLAND NATURALIST 152(1) TABLE 1.—Density,