1993 Proceedings of the Midwest Oak Savanna Conferences
USE OF SOIL COLOR TO ASSESS THE ABSENCE OF WATER TABLE CHANGES WITHIN NATIVE WOODED ECOSYSTEMS SINCE SETTLEMENT
Richard Hootman and Patrick Kelsey
Water tables in the soil can be measured directly using observation wells, or, in the absence of the water table, by using soil color. In the glacially derived soils of northeast Illinois, the yellowish and brownish colors result from iron from the soil parent materials. When soils with temperatures above 5oC are saturated for prolonged periods, iron becomes reduced. Because the reduced form of iron is more soluble, it is more easily leached from the soil profile. The resultant loss of iron leaves gray soil colors which reflect historic saturation elevations within the soil profile. Water table elevations (a reflection of soil saturation) were measured weekly in wooded ecosystems at the Morton Arboretum (1990-1992) and West Chicago Prairie (1991-1992) in northeastern Illinois. Observation wells set 1.5 m deep were established along soil catenas within topographic and vegetation gradients. The soil at each well site was examined and soil colors were recorded. Soil colors were compared with existing high water table elevation data to determine the degree of water table change since settlement of the area in the early 19th century. Seasonal high water tables were measured at or above elevations of gray soil colors associated with saturation, indicating that soil water tables are at levels at which they have been historically. Thus, precipitation infiltration and subsequent percolation through the soil have not changed since settlement.
Most northeastern Illinois soils are derived from 12,000 to 14,000 year-old glacial deposits (Willman, 1971). Since that time, this region has experienced climatic fluctuations and associated shifting of plant communities reflecting the "forest-prairie tension zone" of the Prairie Peninsula (Davis, 1977; Geis and Boggess, 1968; King, 1981; Transeau, 1935). Such shifts complicate interpretation of soil development, historical vegetation change, and the definition of presettlement plant communities. Land surveys of the early to mid-nineteenth century recorded only one generation of the presettlement vegetation scenario in Illinois.
Soils provide a wealth of information for interpreting conditions prior to settlement. Historic drainage conditions and dominant vegetation cover can be determined from relict conditions within the modern soil profile. The use of soils information, therefore, is becoming more commonplace in understanding presettlement and modern ecological conditions in the Midwest (Kelsey and Hootman, 1992; Steiger, 1981; Whitney and Steiger, 1985).
One of the most important uses of soil color is for interpreting drainage class, which reflects many chemical, physical, and biological soil factors. Oxidized or ferric iron (Fe III) minerals provide the red, brown, and yellow coloration in subsoil horizons. Individual sand, silt, and clay particles in soil are coated with these abundant oxides; without the coatings, the soil particles would be gray. During periods of saturation, ferric iron is chemically reduced to ferrous iron (Fe II). Ferrous iron is more soluble than Fe III and more mobile. It moves easily with soil water and can be leached from the soil profile. Removal of iron through this process leaves the soil with gray colors called redoximorphic (redox) features (Soil Survey Staff, 1992).
Soil color variables (hue, value, and chroma) are measured using a Munsell color chart (Soil Survey Staff, 1975). For example, with the color 10YR 5/2, 10YR is the hue, 5 is the value, and 2 is the chroma. The oxidation state of iron is expressed most strongly in the chroma color, therefore, reducing conditions are most typically measured with chroma colors. To reflect reducing conditions, gray redoximorphic features must have a color value of 4 or more and a color chroma of 2 or less (Soil Survey Staff, 1992). Color values less than 4 have likely been influenced by the addition of organic matter.
The abundance of redoximorphic features is related to the duration of reducing conditions (Vepraskas, 1992). The 2-chroma colors may dominate the soil matrix or may appear as mottles within the matrix. Greater coverage of 2-chroma redox features within a soil horizon indicates longerperiods of reduction.
Soil temperatures must be above 5oC for reducing conditions to exist. Above 5oC, the anaerobic bacteria essential in the reduction process are most active (Soil Survey Staff, 1975). This temperature also corresponds roughly to the start and end of the growing season.
Elevations of redox features within the soil profile correspond well to measured water table elevations within the same profile (Evans and Franzmeier, 1986; Franzmeier et al., 1983; Simonson and Boersma, 1972; Zobeck and Ritchie, 1984). Periods of seasonal high water table elevations, therefore, reflect the highest levels of soil saturation and, during the growing season, the maximum elevation at which reducing conditions occur. In the absence of a water table at the time of sampling a soil, redox features with a chroma of 2 or less can be used to verify the elevation of the seasonal high water table, and can reflect historic saturation and drainage characteristics within the soil.
Our study on woodlands at the Morton Arboretum and on the West Chicago Prairie was established to 1) classify soils in these two ecosystems and identify elevations of historic or relict redoximorphic features within them, 2) define extant seasonal high water table (soil saturation) levels in these wooded ecosystems, 3) correlate the redoximorphic features with existing high water table elevations, and 4) determine the extent of soil disturbance imposed since settlement and whether this has impacted surface and subsurface soil water movement.
The 607 ha Morton Arboretum in DuPage County, Illinois, approximately 40 km west of Chicago, contains more than 250 ha of native woodland communities. The three Arboretum sites presented in this study represent a soil catena within a Quercus alba and Q. rubra closed canopy woodland. Soils developed in approximately 50 cm of loess over silty clay loam glacial till. The Typic Hapludalf in the study area is moderately well drained and the Aquic Hapludalf is somewhat poorly drained. Each is an Alfisol or forest-derived soil. The Typic Epiaquoll is a poorly drained Mollisol or prairie-derived soil located along a drainageway within the woodland. Local relief in the study area is 3.35 m.
The 120 ha West Chicago Prairie Forest Preserve, 50 km west of Chicago near West Chicago, Illinois, is managed by the Forest Preserve District of DuPage County. The study area within West Chicago Prairie presented in this paper is an oak savanna - predominantly Quercus macrocarpa and Q. velutina in the overstory - which developed on 50 to 100 cm of silty materials over sand and gravel glacial outwash. Three of the seven water table monitoring sites established on the savanna are presented in this study. These sites represent three Alfisols: a Typic Hapludalf (moderately well drained), a Mollic Endoaqualf (somewhat poorly drained), and a Typic Endoaqualf (somewhat poorly drained) (Soil Survey Staff, 1992). Local relief in the study area is 1.04 m.
Water table monitoring wells were established to a depth of 1.5 m at each site. The wells consisted of 3 cm-diameter solid wall PVC pipe. Water tables were measured weekly from spring thaw until winter freeze-up in 1990-1992 at the Arboretum and 1991-1992 at West Chicago Prairie.
Soils were described at each study site to a depth of 1.5 m and classified using Keys to Soil Taxonomy (Soil Survey Staff, 1992). Soil color variables (hue, value, and chroma) were determined using Munsell color charts.
Precipitation levels in northeast Illinois in 1990 were above normal throughout the year. Spring and summer of 1991 were very dry, but autumn was wetter than average. Spring drought occurred again in 1992, but rainfall returned to near average by July. Water tables are a response to precipitation, and water tables were higher throughout 1990 compared to growing seasons of the other study years at the Arboretum (Fig. 1). At depths of 50 cm and higher, soil temperatures above 5oC were reached on April 18 in 1990, April 9 in 1991, and April 8 in 1992 (Morton Arboretum, unpublished data) allowing reducing conditions to develop in saturated soils for those years. After November 6, 1990 and 1991, and November 12, 1992, soil temperatures fell below 5oC at some depth above 50 cm (Morton Arboretum, unpublished data), curtailing reducing conditions.
Soils at both study areas have been relatively undisturbed physically and chemically. Grazing likely occurred on a portion of each site. Nevertheless, surface soil structure was normal within each profile examined suggesting that if grazing occurred, it did not create a lasting impact on the soil (Tables 1 and 2). No significant sheet or rill erosion has been noted in the study areas. Soil pH was 5.5 to 6.5 in the upper 55 cm at the West Chicago sites and 4.5 to 6.0 above 60 cm within the Arboretum Alfisols, implying long-term development beneath a forest canopy without prairie, and no anthropogenic chemical alteration (Tables 1 and 2). The Mollisol in the study also appeared undisturbed.
Depths of 2-chroma redox features and measured water table elevations in soils at the Morton Arboretum are presented (Fig. 1 and Table 1) and those at West Chicago Prairie (Fig. 2 and Table 2) indicate that the highest water tables occurred from late autumn until mid- to late spring. The Typic Hapludalfs at both study areas had water table elevations well above the 2-chroma elevations for several weeks early in each of the growing seasons (Figures 1 and 2). Franzmeier et al. (1983), Simonson and Boersma (1972), and Zobeck and Ritchie (1984) noted water tables often were above levels of 2-chroma redox features. Reasons for this may be interpreted from Franzmeier et al. (1983) who found that 3-chroma colors represented saturated soils with lesser periods of reduction in Indiana, often within Hapludalfs. The same appears true in this study. The 3-chroma colors on the Arboretum Typic Hapludalf occur at a depth of 20 cm (233.48 m elevation) and on West Chicago they occur at a depth of 23 cm (231.45 m elevation), elevations that are higher than the 2-chroma elevations and correspond much better to the seasonal high water tables (Figs. 1 and 2). Evans and Franzmeier (1986) and Franzmeier et al. (1983) indicate that reduction is virtually nonexistent until soil temperatures rise above 10oC. Indeed, soil temperatures rise above 10oC around May 1 in northern Illinois (Morton Arboretum, unpublished data), about which time water tables drop below 2-chroma features in the Typic Hapludalf of each study site.
The two Endoaqualfs at West Chicago show a good correlation to 2-chroma redox features (Fig. 2). The Mollic Endoaqualf water table was at or above the 2-chroma elevation for one to two weeks early each growing season. The Typic Endoaqualf water table was at or above the 2-chroma elevation, and the soil surface, for several weeks each season. Redox features may exist within the A horizon of this soil, but the dark surface color may mask them.
This same trend is noted in the more poorly drained Arboretum woodland soils (Fig. 1). Most of the high water tables measured within these soils occurred above the zone of redox features. The color of the E horizon of the Aquic Hapludalf and the black A horizon of the Typic Epiaquoll probably mask redox features higher in these profiles.
Wilhelm (1991) suggests the soils of Arboretum woodlands are eroding because of excessive runoff and the inability of the soils to absorb and "hold" moisture. Subsequent lack of rainfall infiltration and percolation within the soils is resulting in a concomitant lowering of the water table (Wilhelm, 1991). These suggested changes are blamed on modification of the vegetation, primarily removal of the herbaceous cover. On the contrary, water table data collected on these study sites show existing high water tables to be as high as what redox features indicate existed within these soils historically. The abundance of redox features within the soils studied appears to adequately reflect the amounts of saturation and reduction now occurring within them. Soils with longer periods ofreduction during the growing season had more abundant redox features.
Although spring drought was significant in 1991 and 1992, water table elevations in the Morton Arboretum woodlands were as high or higher than those in 1990. West Chicago Prairie data also show high water tables during the spring droughts. This is an indication of these soils' ability to maintain moisture levels, and that water table depletion within the soil is a reflection of uptake by plants during the growing season and not water loss due to surface runoff. Whether or not the woody or herbaceous cover has changed over the last 150 years, the existing canopy during the growing season and litter layer during winter have maintained the hydrological, physical, and chemical integrity of these soils.
We would like to acknowledge Wayne Lampa and the Forest Preserve District of DuPage County for establishing the monitoring wells at West Chicago Prairie Forest Preserve and allowing us to collect soil and water table information, and Christopher Whelan and Marlin Bowles of the Arboretum for reviewing this manuscript.
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