influence of microtopography on soil chemistry and understory riparian

INFLUENCE OF MICROTOPOGRAPHY ON SOIL CHEMISTRY AND UNDERSTORY RIPARIAN VEGETATION Irene M. Unger and Rose-Marie Muzika1

Abstract.—The success of riparian forest restoration efforts depends in part on an understanding of the relationship between soil characteristics and vegetation patterns and how these change with site conditions. To examine these relationships for floodplains in northern Missouri, we chose three unchannelized streams as study areas. A sampling grid was established at two plots within each site for characterization of understory vegetation and collection of soil samples for chemical analysis. Microtopographical characterization was conducted using standard survey equipment. Relationships between microtopography and soil chemistry were weak but significant. Slope and elevation explained 15 percent of the variation observed in nitrate and 41 percent of the variation observed in both total C and total N. Vegetational components of the understory (herbaceous species and woody seedlings) were analyzed separately for association with soil chemistry and microtopography. Total N content of the soil explained 28 percent of the variation observed in total herbaceous cover and 41 percent of the variation observed in woody stems. When we considered microtopography, a multiple regression of flow accumulation and slope best predicted vegetational observations. These two variables explained 38 percent of the variation in herbaceous cover and 40 percent of the variation observed in woody stems.

INTRODUCTION With the growing awareness of the importance of wetland services, preservation and restoration of wetlands has become an increasingly important focus in ecology. If these efforts are to be successful, however, understanding abiotic influences, such as topography and soil characteristics, on vegetational patterns is essential. While the influence of site on vegetation is more easily understood in upland forests, flooding can obscure patterns in riparian systems. Flood frequency, intensity, and duration will vary according to stream order and watershed size. Within a watershed, position on the landscape is important; as distance from river increases, particularly when moving from bottomlands to upslope positions, the frequency, intensity, and duration of flooding decrease. Differences in soil and vegetation would be expected with these gradients. Studies of riparian systems indicate plant species distributions correlate with soil morphological and microtopographical variations (Beatty 1984, Grell and others 2005, Holmes and others 2005, Lyon and Gross 2005, Nilsson and others 1994, Ohmann and Spies 1998). Beatty (1984) found that forest floor microrelief could explain distributions of the most common herbs and woody seedlings in maplebeech forests of eastern New York. Grell and others (2005) concluded that differences in vegetation were primarily the result of subtle elevational variation within bottomland old-growth Pinus taeda forests of southern Arkansas. Holmes and others (2005) observed differences in ground-flora composition between floodplain and upland landforms in north-central Ohio. However, Becker (1999) found that environmental variables affecting vegetation distribution differed depending on the creek of interest. Lower-order Ozark streams(1st and 2nd order) in this study were influenced by bedrock, parent material, pH of the A horizon

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Graduate Research Assistant (IMU), and Associate Professor (RMM), Department of Forestry, 203 Anheuser-Busch Natural Resources Building, University of Missouri, Columbia, MO 65211. IMU is corresponding author: to contact, call (573)884-3435 or email at [email protected]. Proceedings of the 16th Central Hardwoods Forest Conference

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and waterway position, while higher-order Ozark streams (3rd - 5th) were influenced by landform features, pH of the A horizon, and soil texture characteristics (Becker 1999). In addition, several studies found that species associations shift along key environmental gradients, but different forest layers do not necessarily shift in concert. Site heterogeneity was a likely contributing factor to this differential response (Lyon and Gross 2005, Lyon and Sagers 1998, Sagers and Lyon 1997). Collectively, these results illustrate the need to study multiple watersheds within a region and the need to include multiple layers and especially herbaceous species in all characterizations of riparian forests.

OBJECTIVES Riparian forests in the river hills of north-central Missouri have not been investigated for these relationships. The river hills exhibit different environments, topographies, and soils from the Ozarks, where these relationships have been previously investigated (Becker 1999, Lyon and Sagers 1998, Lyon and Sagers 2002, Sagers and Lyon 1997). Topographic dissimilarities between the river hills and the Ozarks are expected to result in unique relationships between flooding, soil chemistry, and riparian forest understory composition. Specific objectives of this research include: 1) to determine how microtopographical relief affects soil characteristics such as inorganic-N and total soluble polyphenolic content; and 2) to determine how understory vegetation (herbaceous and shrub layers) changes with microtopographical relief and soil characteristics.

STUDY AREAS This study was conducted in the Central Dissected Till Plains of Missouri; this ecoregion covers almost all of Missouri north of the Missouri River (Nigh and Schroeder 2002). Floodplains along three streams in the northwest section of this ecoregion were selected: Locust Creek (Pershing State Park), Yellow Creek (Yellow Creek Conservation Area), and Thompson River (Crowder State Park) (Fig. 1). These are all second-order streams and tributaries of the Grand River. Study plots were positioned along forested unchannelized sections of each stream.

METHODS

Sampling Plots At each stream, two plots, 100 m apart (the second downstream from the first), were established. Plots were 30 m x 30 m and consisted of a grid of eight transect lines: four transects oriented parallel to the stream and four transects oriented perpendicular to the stream. The transect closest to the stream was at 20 m from the water’s edge and all transects were 10 m apart. Transect lines perpendicular to the stream were likewise 10 m apart. A 1-m2 understory sampling quadrat was established at each intersection of the transect lines, yielding a total of 16 quadrats per plot at each of the floodplain sites. Floodplain Characteristics Topographical maps and preliminary observations revealed little relief in the areas chosen for this study. In order to determine site microrelief, survey equipment was used to measure changes in ground-level (i.e., elevation) as compared to a reference location (reference height = 100 cm). Elevation measurements were taken at each quadrat, at locations mid-way between quadrats and at locations 5 m outside the quadrats (i.e., around the perimeter of the study plot), for a total of 81 measurements per plot. These measurements were then used to calculate planform, flow accumulation (FA), and slope for each quadrat using ArcGIS (Environmental Research Systems, Redlands, CA). Planform is a measure of the change in slope perpendicular to the slope direction. A negative planform value indicates that the surface is concave

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Figure 1.—Study sites were located in the Central Dissected Till Plains of northwest Missouri (shaded area on state map). Floodplains along three creeks were investigated: Locust Creek in Pershing State Park, Yellow Creek in Yellow Creek Conservation Area, and Thompson River in Crowder State Park. All are second-order streams and tributaries of the Grand River.

at that cell, while a positive value indicates that the surface is convex at that cell (Gallant and Wilson 2000). Since planform describes relative convexity and concavity (Gallant and Wilson 2000), it could be used to describe areas of water accumulation in a landscape. Flow accumulation is a measurement of the volume of water flowing into (or collected by) the quadrat; the higher the value, the greater the water flow into the cell (Sharma and others 2006). Slope (in degree) is a measure of the change of elevation between each quadrat and its neighbors (Sharma and others 2006). Slope affects the velocity of water movement and thus would have an impact on the transport of sediment and dissolved substances. A greater velocity results in increased transport of substances. Together these measures provide an indication of places in the landscape where water might be expected to accumulate and thus have an impact on soil nutrient cycling and vegetative growth. Soil Sampling Composite surface soil samples were collected at each quadrat in early June 2006. The top 20 cm of soil was collected from the center and the four corners of each 1-m2 quadrat with a push-probe; these samples were mixed together to create a single sample per quadrat. Samples were placed in a cooler in the field Proceedings of the 16th Central Hardwoods Forest Conference

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and then frozen upon return to the lab. Samples were later freeze-dried, ground, and passed through a 2-mm sieve. Soils were analyzed for: nitrate (NO3-N), ammonium (NH4-N), total N (TN), total C (TC), C:N ratio, and total soluble polyphenolics (TSP). Soil inorganic N content was determined using Lachet QuickChem Methods. A LECO TruSpec CN analyzer (LECO Corp., St. Joseph, MI) was used to determine TC, TN and the C:N ratio. Total soluble polyphenolic content was determined using the FolinCiocalteau technique (Suominen and others 2003). For this analysis, soil extracts (2 ml) were mixed with 2 ml distilled water, 5 ml Na2CO3 (20 percent) and 1 ml Folin-Ciocalteau reagent (Sigma-Aldrich); this mixture was incubated for 30 minutes to allow for color development. Tannic acid was used as the standard and absorbance was read at 735 nm. Vegetation Sampling Vegetation sampling occurred in July 2006; a mid-season date was chosen to allow the capture of the greatest understory richness with a single sampling event. At each 1-m2 quadrat, all herbaceous vegetation rooted within the quadrat was identified to species where possible and percent cover was estimated. Woody vegetation < 1 m in height within the quadrat was identified to species and number of stems tallied. Data Analysis Correlation analysis was used to determine associations between the soil chemical characteristics (NO3-N, NH4-N, TN, TC, C:N ratio, and TSP), the understory vegetation characteristics (herbaceous percent cover or number of woody stems) and the topographical characteristics (planform, FA, slope, and elevation). Regression analysis was used to determine if any of the soil chemical parameters could explain the variation observed in understory vegetation characteristics. Likewise, regression analysis was used to determine if topographical characteristics could explain the variation in soil chemical parameters or in understory vegetation characteristics. Log transformations of NO3-N, NH4-N, flow accumulation, and slope were used in these analyses due to non-normal distributions of these data parameters. All statistical analyses were performed using SAS 9.1 (SAS Institute Inc., Cary, NC, 2002-2003).

RESULTS Floodplain Characteristics Yellow Creek Topographical evaluation of the Yellow Creek plots revealed a flat terrain. At plot 1, the elevation gradually decreased as distance from the river increased; the change in elevation across the plot was a total of 0.35 cm (Fig. 2A). Change in elevation at plot 2 was more subtle, with a difference of only 0.15 cm across the plot. This plot did not exhibit the same trend in elevation observed at other plots, as elevation increased as distance from river increased (Fig. 2B). Planform analysis for plot 1 revealed concave areas associated with the 30-m parallel transect line (quadrats B2, C2, and D2) and parts of the 50-m parallel transect line, particularly near quadrats A4 and D4. Planform features for plot 2 closely mimicked elevation changes. In plot 2, the 20-m parallel transect showed a concave planform; this area of concavity extended diagonally across the plot, and corresponded to areas designated as having lower elevations, such as quadrats C2 and C3. One small area, near quadrat A2, showed a convex character. In plot 1, the greatest FA was associated with quadrat A4 (FA = 169.0). Other quadrats with high FA included B3 (FA = 41.8), B2 (FA = 28.0), and C2 (FA = 24.9). Slope was minimal across the plot, with the


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Figure 2.—Elevation maps of Yellow Creek Plots: A) Yellow Creek Plot 1, B) Yellow Creek Plot 2. Elevations for each quadrat were taken with standard survey equipment using a reference point of 100 cm. All transect lines are 10 m apart. Parallel transects numbers correspond to position relative to stream bank; i.e. parallel transect 1 is closest to the stream (20 m distance from stream bank) while parallel transect 4 is farthest from the stream (50 m distance from stream bank).


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highest slopes observed at quadrats B3 and C1 (slope = 0.023° for both). In plot 2, FA varied little across the landscape. Quadrats C2 and D4 each had an FA of 10.0; all other quadrats had an FA of 5.0. Again, slope was minimal with the highest slopes recorded in quadrats A4, C4, and D4 (slope = 0.04 degrees for each). Locust Creek As with the Yellow Creek plots, elevation differences across the floodplains were minimal for the Locust Creek sites. Locust Creek plot 1 showed a similar pattern to Yellow Creek plot 1, with elevation gradually decreasing in a diagonal fashion across the plot from a “high” at quadrat D1 to a “low” at quadrat A4 (Fig. 3A). Similarly, Locust Creek plot 2 had highest elevation at the 20-m parallel transect line, and a gradual decrease in elevation with increased distance from the river (Fig. 3B). The total elevation gradients for plots 1 and 2 were 0.30 cm and 0.20 cm, respectively. Planform analysis for Locust Creek plot 1 revealed a gently rolling topography. Concave features were found at or near quadrats B1, C3, and A4, while convex features were found at or near quadrats D1, C2, and D4. Locust Creek plot 2 was notable in its lack of variation; one convex feature of note was located near quadrat A4. Two potential concave features were observed near quadrats C2 and C4. Flow accumulation in plot 1 was greatest in quadrat B2 (FA = 79.7). Other quadrats with high FA included B3 (FA = 43.6) and B4 (FA = 30.1). In plot 1, slope was greatest in quadrats A3 (slope = 0.95 degrees) and C2 (slope = 0.92 degrees). Quadrats D3, C1, and A2 also had high slopes (slope = 0.86 degrees, 0.74 degrees, and 0.73 degrees respectively). Flow accumulation was generally lower in plot 2 than in plot 1, with the highest FA observed in quadrat C4 (FA = 43.7) and the second highest FA observed in quadrat C2 (FA = 22.05). The highest slope in plot 2 was observed in quadrat C4 (slope = 1.3 degrees). Slope was also high in quadrats A3 (slope = 0.9 degrees), C3 (slope = 0.8 degrees) and B3 (slope = 0.7 degrees). Thompson River Elevation patterns at Thompson River plot 1 resembled those at Yellow Creek plot 1 and Locust Creek plot 1, in that elevation gradually decreased as distance from river increased (Fig. 4A). Again, the gradient was minimal, with a 0.30-cm change in elevation across the floodplain. Thompson River plot 2 showed little variation in elevation, having only a 0.15-cm change in elevation across the floodplain and relatively large areas with no changes in elevation (Fig. 4B). Planform analysis for Thompson River plot 1 indicated that this plot was centered on a relatively featureless portion of the floodplain. Two small convex areas were apparent at quadrats A3 and D3 of this plot, while concave areas occurred at quadrats B1 and C1; the landscape changed little over the rest of the plot. Plot 2 seems to have more variation in landforms. Two small concave areas were observed at quadrats C2 and D3. Convex features were observed at quadrats C1, D2, and D4, with an almost ridge-like feature extending from quadrat B2 to B3. The Thompson River plots showed little variation in FA. In plot 1, the greatest FA was recorded in quadrats B4 (FA = 33.5), B3 (FA = 13.5), and A4 (FA = 8.1). In plot 2, quadrats C3 (FA = 12.3) and B4 (FA = 10) had the greatest FA. All other quadrats in both plots had FA = 5.0. The highest slopes overall were recorded in Thompson River plot 1. Quadrats A3 (slope = 2.4 degrees), C2 (slope = 2.2 degrees), and A4 (slope = 2.1 degrees) were the steepest. Other quadrats in plot 1 with high slopes were B3 (slope = 1.8


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Figure 3.—Elevation maps of Locust Creek plots: A) Locust Creek plot 1; B) Locust Creek plot 2. Elevations for each quadrat were taken with standard survey equipment using a reference point of 100 cm. All transect lines are 10 m apart. Parallel transect numbers correspond to position relative to stream bank; parallel transect 1 is closest to the stream (20 m from stream bank) while parallel transect 4 is farthest from the stream (50 m from stream bank).


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Figure 4.—Elevation maps of Thompson River plots: A) Thompson River plot 1; B) Thompson River plot 2. Elevations for each quadrat were taken with standard survey equipment using a reference point of 100 cm. All transect lines are 10 m apart. Parallel transect numbers correspond to position relative to stream bank; parallel transect 1 is closest to the stream (20 m from stream bank) while parallel transect 4 is farthest from the stream (50 m from stream bank).


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degrees) and C3 (slope = 1.7 degrees). In plot 2, quadrat D4 had the greatest slope (slope = 1.1 degrees), followed by quadrat C3 (slope = 1.0 degrees). Other quadrats with high slopes included C1, A3, and B1 (slopes = 0.8 degrees, 0.8 degrees, and 0.7 degrees, respectively). Soil Chemistry and Vegetation Soil Chemistry Differences between sites and plots within sites were observed for NO3-N, TC, TN, and TSP. Overall, Locust Creek soils had the highest levels of NO3-N and the lowest levels of TC, TN, and TSP. Yellow Creek soils had the highest levels of TC and TN while Thompson River had the highest levels of TSP (Appendix 1). Plot-level differences in NO3-N were observed at the Thompson River sites and within-plot differences were observed at Yellow Creek plot 2 and Locust Creek plot 2. Plot-level differences in TC and TN were observed at Locust Creek, with plot 2 having lower levels of both variables. Yellow Creek plot 2 showed increasing TN with distance from river; no other within-plot differences were observed for TC and TN. There were no plotlevel differences observed for TSP levels; however, within-plot differences were observed at Thompson River plot 2. Finally, differences in NH4-N and C:N were not detected at any level of comparison. Understory Characteristics Understory vegetation was measured in two ways: percent cover of herbaceous species and number of stems of woody shrubs and vines. The three study sites differed for both of these characterizations. Overall, Locust Creek had significantly greater percent cover of herbaceous vegetation than Thompson River. (Yellow Creek had intermediate herbaceous coverage.) On the other hand, the shrub layer at Locust Creek was nearly absent while the Yellow Creek shrub layer density was so great as to impede movement through the stand. No trends between understory vegetation and distance from river were observed. Herbaceous coverage did not increase, nor were any differences in number of woody stems observed as distance from river increased although both parameters varied from line to line at each plot. See Appendix 2 for a list of the most common understory species in each plot. Herbaceous species differences from detected at the creek level and for all plots except the Yellow Creek plots. These differences can be attributed primarily to Laportea canadensis and Parthenocissus quinquefolia, which had the highest overall cover values and were significantly different than most other species. Species differences were also observed in the shrub layer; Toxicodendron radicans had the highest number of stems and was significantly different from most other species. Toxicodendron radicans was particularly dense at Yellow Creek and contributed to the high number of woody stems observed at those plots. Overstory Characteristics The overstory vegetation was not significantly different among sites when compared in terms of basal area (BA), relative dominance, relative density, or importance values. However, a disproportionate dominance by a single species was observed in most plots (Appendix 2). Yellow Creek plot 1 was dominated by Acer saccharinum, while Yellow Creek plot 2 was dominated by Quercus macrocarpa. Acer saccharinum dominated both plots at Locust Creek. The dominant species at Thompson River plot 1 was Celtis occidentalis, while at Thompson River plot 2, dominance was shared between Platanus occidentalis and Ulmus americana. Thompson River plot 2 had a high number of small-diameter Ulmus americana, a notable difference from other plots. Correlation of Soil, Vegetation, and Microtopography Correlation analysis of soil parameters indicated positive correlations between TC and TN (r = 0.96, P