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A comparison of vegetation and seed bank community structure in a Sand Prairie in Illinois, U.S.A.

INTRODUCTION

In grassland communities, differences in species composition and community structure between the above-ground vegetation and below-ground seed bank arise from both ecological processes and species traits. Competition may exclude some species from the vegetation but not the soil seed bank (hereafter 'seed bank') (Brown and Venable, 1986). Disturbance events may increase recruitment from the seed bank (Kirkman and Sharitz, 1994) and/or alter seed banks through pulsed input of persistent seeds (Renne and Tracy, 2007). Absence of regeneration microsites may prohibit recruitment from the seed bank (Pakeman and Small, 2005). Species traits that contribute to differences between the vegetation and seed bank include dependence on seed for reproduction (predicting high abundance in seed bank, e.g., annual life history) versus clonal growth (reducing plant dependence on seed, e.g., perennial grasses) (Willms and Quinton, 1995). These processes and species traits result in differing inputs and losses that are reflected in low similarity of species composition and community structure between the vegetation and seed bank. Observational studies that compare the vegetation and seed bank provide a more complete picture of the community structure of a site. They also contribute to our understanding of species potential for storage and regeneration, i.e., which species of the vegetation have a stored source for regeneration, and which species of the seed bank are expressed in the vegetation.

Most grassland studies in North American prairies show low similarity of species composition between the vegetation and seed bank (except see Henderson et al., 1988), especially when relative abundances and other measures of community structure are considered in combination with species composition (Rabinowitz, 1981; Abrams, 1988). In addition, European studies indicate that seed banks in grasslands do not provide the necessary species pool for regeneration of above-ground community structure (Bekker et al., 1997; Bossuyt and Honnay, 2008). Nevertheless, similarity indices for grassland vegetation and seed banks may be higher than other community types (Hopfensperger, 2007).

One grassland-specific factor that may contribute to the discord between the vegetation and seed bank is the presence of competitively dominant perennial grasses (Wedin and Tilman, 1993). They do not create a persistent seed bank (Coffin and Lauenroth, 1989; Kinucan and Smeins, 1992) and may suppress recruitment from the seed bank through direct competition and abundance of litter (Foster and Gross, 1997). Therefore, recruitment from the seed bank may be restricted to small- or large-scale disturbances (Platt, 1975; Rogers and Hartnett, 2001), which, in turn, bring opportunities to replenish the seed bank (Pakeman and Small, 2005).

Seed banks in grasslands, as in other communities, reflect historic and/or current input from the vegetation and serve as a repository for species with transient and persistent seed banks (Thomson and Grime, 1979). Transient-seeded species, e.g., some perennial grasses, must be present in the vegetation (or have a nearby source) to provide seed sources for recruitment. Persistent-seeded species, including species with hard seed coats or other enforced dormancy, remain as dormant individuals until exposed to the proper germination cues (Baskin and Baskin, 1989). Thus, low incorporation of some species into the seed bank and low recruitment of persistent-seeded species may decrease similarity above- and below-ground.

Differences in the vegetation and seed bank may also reflect traits such as species location of origin (i.e., native or introduced), life histories and functional guilds. Native prairie vegetation is subject to invasion by introduced species (Spyreas et al., 2004) that often create persistent seed banks (Gibson et al., 2002; Clark and Wilson, 2003; Mason et al., 2007). Annuals and biennials may be abundant in the seed bank because of high seed production and/or high likelihood for persistent seeds, but their dependence on appropriate germination and establishment conditions reduces their abundance in the vegetation (Facelli et al., 2005). In contrast, perennials with capacity for vegetative spread may not be as well represented in the seed bank (Willms and Quinton, 1995). Many perennial grasses have transient seeds; perennial forbs are frequently underrepresented in the seed bank (Abrams, 1988), and many sedges (Leck and Schutz, 2005) and legumes (Rice, 1989) have persistent seeds.

The vegetation of central North American prairies has been well documented (Samson and Knopf, 1996), including the sand prairie used in the current study (Bowles et al., 2003; Ebinger et al., 2006). However, although species composition and soil properties are known to affect seed storage in the soil (Coffin and Lauenroth, 1989), seed bank studies of many North American prairie types, including sand prairies, are not adequately represented in the literature. This study adds to our knowledge of sand sites by comparing the vegetation and seed bank of a grass-stabilized site, and complements the information existing for the vegetation and seed bank in successional sand dunes (Leicht-Young et al., 2009) and sedge-dominated sandhill sites (Perez et al., 1998). In addition, historic grazing on the study site may provide insight into sand prairies with a similar history, as general effects of grazing on vegetation composition and disturbance regimes have potential to influence input to and/or recruitment from the seed bank.

In this study of a sand prairie, species composition and community structure of the vegetation and seed bank were compared using standard measures of diversity, floristic quality (Taft et al., 1997) and similarity. We also analyzed species richness, abundance and frequency in the vegetation and seed bank separately, and examined differences in species location of origin (native vs. introduced), life history and perennial functional guild.

METHODS

STUDY SITE

The study site was located in and adjacent to Thomson-Fulton Sand Prairie (41[degrees]55'N, -90[degrees]07'W), an Illinois Nature Preserve in Whiteside County, Illinois, U.S.A. The 86-ha site is a mosaic of dry and dry-mesic sand prairie; the dry-mesic area used in this study is dominated by the perennial bunchgrass Schizachyrium scoparium (Michx.) Nash (little bluestem). The site was formed when sand was deposited as glacial outwash along the historic floodplain of the Mississippi River (Willman and Frye, 1970). As with other sand areas, these types of inland sand deposits have low organic content (Symstad, 2004) and are subject to wind erosion when vegetation cover is reduced (Gleason, 1910; Curtis, 1959). From Jun.-Aug. 2005, average max/min temperatures were 28.2 C/16.2 C and total precipitation was 24.3 cm, 7.2 cm below average (National Weather Service records from Dubuque, Iowa) (www.crh.noaa.gov/dvn/).

The characteristic low fertility and high potential for erosion of sand prairies led them to be maintained as pasture and not converted to row crop agriculture. Thus, they were often 'conserved' in some state of their original form. The study site was grazed by cattle for an unknown period until its dedication as a nature preserve in 1970, at which time grazing ceased and burns (prescribed and wildfire) occurred sporadically, approximately one per decade (pers. comm., Illinois Dept of Natural Resources). Only portions of the site were burned each time, with the last burns occurring in the mid- and late-1990s. The site is in an agriculture-dominated landscape, with row-crop fields, pine plantations and degraded prairie adjacent to the study site.

VEGETATION SURVEYS

Vegetation surveys of representative dry-mesic sand prairie were conducted along six 50-m transects within a 1-km radius. Four transects were placed within the preserve and two were located immediately adjacent to the preserve in a railroad right-of-way. Transect locations were selected to maximize the distance between transects, avoid known past disturbances and avoid areas with shifting sand (i.e., selected areas were grass-stabilized).

Surveys of species composition and cover were completed in both Jun. and Aug. 2005. At 10-m intervals along each 50-m transect, a permanent 1 x 0.5-m plot was marked on alternating sides, and at a random distance of 1-3 m away from the transect (30 replicate plots). Plot frames, in place only during sampling, were viewed to assign a Daubenmire cover class (Daubenmire, 1959) for each species present. Species frequently overlapped so that values for some plots exceeded 100% cover. Only one seed bank species, Androsace occidentalis, completed its life cycle prior to the Jun. survey.

Data from Jun. and Aug. vegetation surveys were combined to account for the seasonal growth of all species. A combined species list for each plot was created and used for all measures of community structure and comparisons. Percent cover values in the combined list represent the peak cover value (i.e., Jun. or Aug. value) for each species present in a plot.

SEED BANK SURVEY

Soil samples were collected in Apr. 2006 by taking 12 soil cores (2 cm diameter, 6 cm depth) spaced approximately 0.5 m apart, within a 2 x 4 m plot adjacent to each vegetation plot. The 12 soil cores for each plot were homogenized into a composite sample. The 30 replicate samples comprised a total area of 0.11 [m.sup.2] (volume = 6786 [cm.sup.3]). Samples were collected in the spring, late enough to allow for overwintering (i.e., cold treatment) of the seed bank but prior to the flush of spring germination. Samples were transported and stored in coolers until processed (<1 wk). Each sample was passed through a sieve (4.75 [mm.sup.2] mesh) to remove root and bud material. Planting trays (20 x 20 x 3 cm depth), lined with fine mesh to reduce loss of soil, were filled with a sterile soil mix (1 : 3 ratio of sand : growing mix composed primarily of sphagnum peat moss, plus perlite, lime, gypsum and a wetting agent). Soil samples were spread <1 cm thick over the soil mix and placed in a temperature-controlled (range: 15-30 C) greenhouse at the University of Illinois at Urbana-Champaign. During daytime hours, natural light was supplemented with artificial light (1000 watt metal halide) during low light (below 900 [micro]mol x [m.sup.-2][s.sup.-1]). An automatic mist system supplemented hand watering to ensure no loss of seedlings through drying-rewetting periods. Sterile trays were placed on each bench to detect any seed contamination, which was zero. All trays were rotated on a regular basis to reduce any bench effect.

The seed bank (i.e., germinable seed bank) was determined by identifying species of individual seedlings as they emerged and counting number of individuals (i.e., density) between Apr. and Nov. 2006. Individuals were transplanted and grown to a larger size when necessary to verify species identification. Individuals that died before they could be identified were documented as 'un-identified' and were predominantly dicots. Trays were observed daily during the first three months, four times weekly for the next 2 mo and two times weekly during the last month. In Jul., when germination rates had declined precipitously, the upper 0.75 cm of soil was mixed to increase the germination of any remaining viable seeds.

DATA ANALYSIS

Data collected in field and germination trials were used to: summarize individual species information at the site level for both the vegetation and seed bank, summarize community structure and species composition at the site level for both the vegetation and seed bank, and analyze community structure separately for the vegetation and seed bank at the plot level.

Individual species information at the site level was used to generate importance values (IV). This measure was the only summary in which data were presented as relative values, calculations of which are detailed in Table 1. All other comparisons below use original (not relative) values. Species nomenclature and classification follow Gleason and Cronquist (1991).

At the site level, species richness, Shannon-Wiener diversity (H'), and evenness (E) were calculated for comparisons between vegetation and seed bank (Magurran, 1988). The similarity of species composition between the vegetation and seed bank was calculated using Jaccard's index (Mueller-Dombois and Ellenberg, 1974).

Also at the site level, two indices of floristic quality (Taft et al., 1997) were used to characterize the vegetation and seed bank. These indices have weighted values assigned to each species according to its level of affinity to intact native communities. Taft et al. (1997) assigned a value ('coefficient of conservatism'), a number between 0 and 10, to all plant species in the state of Illinois. A value of 10 corresponded to species associated with high quality natural areas, whereas a value of zero was assigned to native ruderal species. Nonnative, introduced species were also assigned a value of zero. Intermediate values were assigned to species found in the varying quality of habitats in between and to common species such as dominant grasses. The two indices used species composition at the site level and were calculated as follows:

* Mean Coefficient of Conservatism (Mean C) = [SIGMA]([CC.sub.s1] + [CC.sub.s2] + ...)/[SP.sub.site]; where CC = coefficient of conservatism for each species (S1, S2, etc.), and [SR.sub.site] = total (native and introduced) species richness.

Floristic Quality Index (FQI) = Mean C * [square root of][N.sub.site]; where [N.sub.site] = total native species richness.

At the plot level, statistical comparisons were conducted separately for the vegetation and seed bank, as direct comparisons were prohibited due to differences in sample methodology. Species were placed into the appropriate categories of three classifications: location of origin (native, introduced), life history (perennial, biennial, annual) and functional guild of perennial species (grass, forb, sedge, legume). Functional guild was restricted to perennial species to be able to assess the contribution of different guilds in the dominant life history (i.e., perennials) separate from the effects of life history. Legumes were dropped from statistical comparisons for the seed bank, as none germinated from the soil cores.

Statistical comparisons among categories within each classification were made using values at the plot level for species richness, % cover and density. For % cover and density, a value for each plot was obtained by summing the abundance of all individuals in each category (e.g., total % cover of all perennial individuals in a plot; Table 3). Plots that contained no individuals of a category were left as zero values and included in the statistical analysis. Their inclusion avoided inflation of species richness and abundance of rare categories (e.g., legumes) and maintained equal sample sizes. Values for frequency comparisons were calculated as follows: [SIGMA]([Freq.sub.s1] + [Freq.sub.s2] + ...)/[SR.sub.category]; where Freq = number of plots where species (S1, S2, etc.) is present, and [SR.sub.category] = number of species in category.

Some variables could not be transformed to meet normality assumptions of parametric tests. Therefore, nonparametric tests (Kruskal-Wallace) were used to test for differences among categories within classifications for species richness, % cover, density and frequency. Significant results within the life history and guild classifications were further examined between categories by using post hoc comparisons (Wilcoxon rank sum (WRS), a.k.a. Mann-Whitney) but only between categories that had overlapping standard errors (e.g., species richness of forbs vs. grasses in the vegetation). A Bonferroni correction (alpha/number of comparisons) was applied for multiple comparisons, for an experiment-wide statistical significance value of P < 0.005. All statistical analyses were run in SAS 9.1[R].

RESULTS

In the vegetation, a total of 46 species representing 17 families were identified (Table 1). Over half of the importance values (IV) belonged to two families, the Poaceae (IV = 75.6) and the Asteraceae (33.9). Individual species that dominated the site were the native perennial grass Schizachyrium scoparium (IV = 31.3) and three native perennial forbs Opuntia macrorhiza (19.0), Callirhoe triangulata (13.4) and Ambrosia psilostachya (12.8). In the Aug. vegetation survey, mean bare ground per plot was 8.8% cover (range: 0-37.5%), whereas mean % cover of dead vegetation (litter and standing dead vegetation) was 42% cover (range: 15-62.5%).

The seed bank had a total of 30 species representing 17 families (Table 1). Asteraceae (IV = 85.8) and Molluginaceae (30.6) combined to make up 58% of the importance values. The most important species in the seed bank were a native perennial forb Antennaria plantaginifolia (IV = 63.7), an introduced annual Mollugo verticillata (30.6), and two native annuals Conyza canadensis (19.1) and Linaria canadensis (15.4). No species of protected status were present. A total of 787 seedlings germinated from the samples, with a mean of 26 ([+ or -] 3 SE) (range: 4 to 64) seedlings per ~226 [cm.sup.3] soil sample.

[FIGURE 1 OMITTED]

COMPARISONS BETWEEN VEGETATION AND SEED BANK

At the site level, species richness was higher in the vegetation than the seed bank and fewer species dominated the seed bank than the vegetation (Fig. 1). Species diversity and evenness were greater for the vegetation (H' = 2.9, E = 0.8) than the seed bank (H' = 1.9, E = 0.6), reflecting the lower species richness and skewed densities caused by a few species in the seed bank (Fig. 1). The mean coefficient of conservatism was greater for the vegetation (mean C = 4.6) than the seed bank (3.1). The Floristic Quality Index (FQI) was two-fold greater for the vegetation (FQI = 29.9) than the seed bank (15.8).

In total, 43% (20 of 46 species) of species in the vegetation were represented in the seed bank (Table 1). Only two of four dominant species in the vegetation occurred in the seed bank, and these in low numbers (two seedlings of Schizachyrium scopanum and one seedling of Opuntia macrorhiza).

Reciprocally, 67% (20 of 30 species) of species in the seed bank occurred in the vegetation (Table 1). Of the four species with the highest importance values in the seed bank, Conyza canadensis had moderate importance in the vegetation (Vegetation IV = 6.15), whereas Linaria canadensis was uncommon there (3.1). Antennaria plantaginifolia and Mollugo verticillata, ranked first and second in importance value in the seed bank, respectively, were not encountered in the vegetation sampling, although rare individuals of A. plantaginifolia were observed outside of the plots.

The similarity of species composition between the vegetation and seed bank was 36 (Jaccard's index) for the entire flora (Table 2). Analyses of similarity by separating species into their classifications showed similar results for location of origin and life history (Table 2). However, in the guild classification, similarity was markedly lower in perennial forbs and markedly higher in perennial grasses and sedges compared to the flora as a whole.

CHARACTERIZATION OF VEGETATION AND SEED BANK

Location of Origin.--In the vegetation, at the site level, species richness was much greater for native than introduced species (Table 3). Likewise, mean species richness per plot and mean % cover per plot were significantly greater for native than introduced species. In the seed bank, the dominance by native species was also evident at the site and plot levels, with significantly greater values for native than introduced species at the plot level for mean species richness and mean density (Table 3).

Frequency of species did not differ in either the vegetation or the seed bank for location of origin, life history or guild classifications addressed below (Table 3). High variance in frequency of species in each category, as well as low number of species in some categories, contributed to this result.

Life history.--In the vegetation, at both the plot and site levels, species richness for perennials greatly exceeded biennials and annuals (Table 3). Also at the plot level, perennials had greater mean % cover per plot than biennials and annuals.

In the seed bank, life histories of species paralleled the vegetation only at the site level, with the greatest to least number of species represented by perennials, annuals and biennials (Table 3). Unlike the vegetation, mean species richness per plot was greater for annuals than perennials. Mean density per plot also differed between the low mean density of biennials and the dominant life histories (annual and perennial), as mean density of annual and perennials was similar (WRS, P > 0.005). Density of perennials was driven primarily by Antennaria plantaginifolia, whereas density of annuals was more evenly distributed, with four species dominating (Mollugo verticillata, Conyza canadensis, Linaria canadensis and Triodanis perfoliata) (Table 1).

Functional guild.--In the perennial vegetation, at the site and plot level, grasses and forbs were the dominant functional guilds, greatly surpassing the species richness of subdominant sedges and legumes (Table 3). Mean species richness per plot differed between the dominant and subdominant guilds, as post hoc tests showed no significant difference between grasses and forbs (WRS, P > 0.005). However, mean % cover per plot was significantly greater for grasses than forbs (WRS, P < 0.005).

In the seed bank, only three perennial guilds were present; legumes were absent (Table 3). At the site level, species richness decreased from grasses to forbs to sedges. At the plot level, mean species richness was low in all perennial guilds, but guilds differed significantly for both mean species richness and mean density per plot. The high density of forbs per plot was driven by Antennaria plantaginifolia.

DISCUSSION

Species composition and community structure differed between the vegetation and seed bank of this sand prairie. Species richness, diversity, evenness and floristic quality were higher in the vegetation than the seed bank. Species present in both the vegetation and seed bank had contrasting relative abundances, as dominant species in the vegetation were rare in the seed bank and vice versa. Fewer than half of the species in the vegetation were represented in the seed bank, whereas two-thirds of the species in the seed bank occurred in the vegetation. The relatively low similarity between the vegetation and seed bank could not be attributed to any one classification (location of origin, life history and functional guild), although perennial forbs were most underrepresented in the seed bank. Overall, this study indicates limited input from the vegetation into the seed bank and low recruitment from the seed bank.

The discord of species composition and relative abundances between the vegetation and seed bank is similar to previous grassland studies; species that dominate the seed bank are infrequent in the vegetation (Rabinowitz, 1981) and dominant vegetation does not retain a seed bank for regeneration (Bekker et al., 1997). The Jaccard's index value of 36 for similarity of species composition between the vegetation and seed bank was similar to earlier studies of grasslands (Bossuyt and Honnay, 2008).

In grasslands, competitive ability, opportunities for recruitment, seed production and longevity of the seed bank influence the discord between vegetation and seed banks. This study was not designed to determine which factor contributed most strongly to the low similarity (Jaccard's index) and disparities in species richness and evenness at this site (Fig. 1). Schizachyrium scoparium, the site's dominant grass that is largely absent from the seed bank, may have competitively excluded some species (Fargione and Tilman, 2005) and decreased recruitment from the seed bank. The bare ground that exists on site suggests that species present in the seed bank may be absent from the vegetation due to lack of appropriate environmental cues, such as disturbance (Rogers and Hartnett, 2001) or moisture (i.e., below average rainfall in 2005). Both competition and absence of environmental cues could cause some discord between the vegetation and seed bank. Additionally, the presence of species with abundant persistent seeds in the soil but absent from the vegetation may reflect historic opportunities of recruitment and establishment. For example, historic fire and grazing on the study site may have contributed to the two genera with the highest importance in the seed bank, as these genera have been documented to increase in the seed bank on other sites after fire (Antennaria in Abrams, 1988) and grazing (Mollugo in Renne and Tracy, 2007). Their absence from the vegetation at the study site may reflect the long intervals between fire and/or cessation of grazing in recent decades.

Native species dominated this site's vegetation, despite increased opportunity for introduced species due to its small size and location in an agricultural landscape (Lonsdale, 1999). Although it was common in North American grasslands for grazed sites to be 'improved' with non-native forage, an absence of deliberate introductions to the site may have kept propagule pressure of introduced species low (high propagule pressure has been proposed to explain probability of invasion) (Hierro et al., 2005). The dominance by native perennial grasses and forbs may also indicate the above-ground community's resilience to grazing and introduced species (Hobbs and Huenneke, 1992). However, it is unknown whether grazing-sensitive species were lost or altered in abundance before cessation of grazing.

The vegetation of this site was similar in species composition to other sand prairies in Illinois (Ebinger et al., 2006, and references therein). However, the 'stabilized' vegetation of this sand prairie, i.e., dominance of perennial grasses and forbs and low % cover of bare ground, contrasts with early successional sand habitat that may include lower abundance of perennial forb species and shifting sands (e.g., 'foredune' of Leicht-Young et al., 2009). In general, vegetation diversity is generally lower in sand prairies compared to loam prairies (Curtis, 1959), potentially driven by the low fertility (Tilman, 1987) and soil water-holding capacity in sand prairies.

In the seed bank, native species also greatly exceeded introduced species and may reflect the low importance of introduced species in the vegetation and low dispersal from adjacent land. Species richness and abundance of life history and functional guilds were skewed by the dominant species, Antennaria plantaginifolia, a native perennial forb, which was absent from the vegetation. Interpretation of the seed bank without A. plantaginifolia shows that the high density of annuals (Willms and Quinton, 1995; Fahnestock et al., 2003) and low representation of perennial forbs (Weaver and Mueller, 1942; Rabinowitz, 1981, but see Johnson and Anderson, 1986) were similar to other prairie seed bank studies. Only three of 15 perennial forb species in the vegetation were in the seed bank, and these three species were at low densities. The low richness and abundance of forb species may be influenced by low production of seed and/or high seed predation that reduce the number of seeds incorporated into the seed bank (Chambers and MacMahon, 1994). Finally, the methodology used, common to many seed bank studies and verified to capture much of their community structure (Gross, 1990), may have underestimated legumes in the seed bank because their hard seed coats inhibit rapid germination (Rice, 1989). Alternatively, they may have limiting mechanisms similar to non-leguminous forb species.

The seed bank of this site was similar to a sedge-dominated sand site that also had low occurrence of dominant vegetation species in the seed bank (Perez et al., 1998). In contrast, secondary sand dunes had higher densities of grasses from the vegetation in the seed bank (Leicht-Young et al., 2009). These similarities and differences could be due to multiple factors, including differences in species composition and habitat type.

At Thomson-Fulton, higher values of community structure (species richness, H' and E) (Fig. 1) in the vegetation than the seed bank indicate that most species have above-ground 'storage' (Warner and Chesson, 1985), in contrast to communities dependent on the seed bank for storage, such as desert annual communities (Facelli et al., 2005). Thus, the regeneration potential of most species lies in its existing vegetation through clonal growth and production of seeds not incorporated into the seed bank. Measurements of seed rain, vulnerability to seed predation, bud bank (i.e., underground vegetative structures, Benson et al., 2004) and dispersal would inform the potential for colonization by guilds underrepresented in the seed bank, especially perennial forbs. In addition, continued surveys of the existing vegetation, recruitment and establishment events would further the understanding of the long-term dynamics of the community.

In this sand prairie, knowledge of the vegetation and seed bank, and their dissimilarity, provides valuable information for its future management. Management for increased abundance of specific species or guilds, such as perennial forbs, may need to focus on overcoming potential seed limitation, as they are not likely to arise from the seed bank. The use of fire to stimulate interactions between the vegetation and seed bank may also be useful for future management of native and introduced species (Bowles et al., 2003). The low abundance of introduced species in the vegetation and seed bank indicates that introduced species are not likely to dominate a response to disturbance. However, disturbance should be approached with caution given its potential for altering community structure and composition. Overall, management to retain the existing community structure on site must take into consideration the low storage in the seed bank of most species in the vegetation.

This study indicates that the vegetation of this sand prairie is of relatively high quality, as shown by generally high values of community structure, the co-dominance of native grasses and forbs, and low invasion by introduced species. Ebinger et al. (2006) compared the floristic quality of Thomson-Fulton Sand Prairie to other sand prairies in the region. The vegetation component of this study supports their findings of relatively high vegetative floristic quality. However, this study adds knowledge that its seed bank does not reflect the vegetation, as underlined by its lower diversity, species richness and high importance of species not present in the vegetation. Furthermore, the difference in floristic quality values (mean C and FQI) between the vegetation and seed bank reflects both the generally high quality of the existing vegetation and prevalence of low 'coefficient of conservatism' species (Taft et al., 1997) in the seed bank.

Acknowledgments.--The authors thank Adrienne Edwards for assistance with initial research design, Brenda Molano-Flores and John Taft for valuable input to a draft of this manuscript, and Randy Nyboer and Ed Anderson (Illinois Dept. of Natural Resources (DNR)) for historical information on the study site. Funding for the study was provided to M. McNicoll by an Illinois DNR Wildlife Preservation Fund Small Project Grant, a grant from the Illinois Native Plant Society and a Philip W. Smith Award from the University of Illinois.

SUBMITTED 9 JUNE 2009

ACCEPTED 14 SEPTEMBER 2009

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MOLLY B. MCNICOLL (1) AND CAROL K. AUGSPURGER

Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana 61801

(1) Corresponding author: e-mail mcnicoll@life.illinois.edu
TABLE 1.--The species composition and community structure of the
vegetation and seed bank of Thomson-Fulton Sand Prairie. Species were
classified according to family, location of origin (N = native,
I = introduced), life history (P = perennial, A = annual, B = biennial)
and guild of perennial species (F = forb, G = grass, S = sedge,
L = legume). Species are ranked by their Importance Value (M in the
vegetation and followed by those species present in the seed bank only,
ranked by N. The four highest IVs are underlined for both the vegetation
and seed bank for ease of identification. See footnote for explanations
of calculations of relative values. * Androsace occidentalis was present
in early spring before the vegetation survey was conducted; Antennaria
plantoginifolia and Sporobolus cryptandrus were observed only as rare
individuals outside of vegetation plots. Empty cells represent species
not observed in seed bank or vegetation. Panicum depauperatum may
include individuals of P. linearifolium, as they were difficult to
distinguish in a vegetative state

                                                      Origin (N,I)
                                                     LifeI-list (P,
                                                      B, A) Guild
Family                Species                         (G, F, S, L)

Poaceae               Schizachyrium scoparium              N
Cactaceae             Opuntia macrorhiza                   N
Malvaceae             Callirhoe triangulata                N
Asteraceae            Ambrosia psilostachya                N
Poaceae               Panicum villosissimum                N
Cyperaceae            Carex pensylvanica                   N
Asteraceae            Solidago nemoralis                   N
Poaceae               Stipa spartea                        N
Asteraceae            Conyza canadensis                    N
Cyperaceae            Carex tonsa                          N
Poaceae               Koeleria macrantha                   N
Brassicaceae          Lepidium virginicum                  N
Polygalaceae          Polygala polygamy                    N
Poaceae               Panicum virgatum                     N
Commelinaceae         Tradescantia ohiensis                N
Fabaceae              Lespedeza capitata                   N
Poaceae               Poa pratensis                        I
Fabaceae              Tephrosia virginiana                 N
Scrophr lariaceae     Linaria canadensis                   N
Cyperaceae            Cyperus schmrinitzii                 N
Polygonaceae          Rumex acetosella                     I
Asteraceae            Krigia virginica                     N
Euphorbiaceae         Euphorbia corollata                  N
Poaceae               Paspalum setaceum                    N
Solanaceae            Physalis virginiana                  N
Poaceae               Eragrostis spectabilis               N
Cyperaceae            Carex muhlenbergii                   N
Poaceae               Calamovilfa longifolia               N
Poaceae               Leptoloma cognatum                   N
Poaceae               Panicum oligosanthes                 N
                      var. scribnerianum
Acteraceae            Brickellia eupatoriodes              N
Cyperaceae            Cyperus filiculmis                   N
Acteraceae            Corecpsis palmata                    N
Asclepiadaceae        Acclepias verticillata               N
Commelinaceae         Commelina erecta                     N
Acteraceae            Helianthus pauciforus                N
Asclepiadaceae        Acclepias cf., viridifora            N
Poaceae               Panicum depauperatum                 N
Plantaginaceae        Plantago patagonica                  I
Campanulaceae         Triodanis perfoliata                 N
Poaceae               Vulpia octofora                      N
Brassicaceae          Arabis lyrata                        N
Poaceae               Aristida tuberculosa                 N
Brassicaceae          Deccurainia pinnata                  N
Asteraceae            Erigeron strigmus                    N
Violaceae             Viola pedata                         N
Acteraceae            Antennaria plantaginifolia           N
Molluginaceae         Mollugo verticillata                 I
Poaceae               Sporobolus cryptandrus               N
Onagraceae            Oenothera rhombipetala               N
Caryophyllaceae       Silene antirrhina                    N
Primulaceae           Androsace occidentalis               N
Liliaceae             Allium sp.                           N?
Amaranthaceae         Amaranthus albus                     N
Caryophyllaceae       Arenaria serpyllifolia               I
Solanaceae            Solanum ptycanthum                   N
                      Un-identified
                      Grand Total

                                                    VEGETATION

                    Origin (N,I)
                    LifeHist (P,              Relative
                    B, A) Guild   Relative    Frequency    Importance
Family              (G, F, S,L)   % Cover        (%)          Value

Poaceae                P    G      23.5          7.8          31.3
Cactaceae              P    F      12.8          6.2          l9.0
Malvaceae              P    F       8.0          5.4          l3.4
Asteraceae             P    F       5.3          7.5          12.8
Poaceae                P    G       5.7          5.1          10.8
Cyperaceae             P    S       5.6          4.0           9.7
Asteraceae             P    F       4.8          3.5           8.3
Poaceae                P    G       4.6          3.5           8.1
Asteraceae             A            1.6          4.6           6.1
Cyperaceae             P    S       2.9          3.0           5.9
Poaceae                P    G       2.5          3.2           5.8
Brassicaceae           A            0.9          3.8           4.7
Polygalaceae           B            1.3          3.2           4.6
Poaceae                P    G       2.3          2.2           4.5
Commelinaceae          P    F       2.1          2.2           4.2
Fabaceae               P    L       1.2          2.7           3.9
Poaceae                P    G       1.4          2.4           3.8
Fabaceae               P    L       2.4          0.8           3.3
Scrophr lariaceae      A            0.7          2.4           3.1
Cyperaceae             P    S       0.9          2.2           3.0
Polygonaceae           P    F       1.1          1.9           3.0
Asteraceae             A            0.6          2.2           2.8
Euphorbiaceae          P    F       1.6          1.1           2.7
Poaceae                P    G       1.0          1.6           2.6
Solanaceae             P    F       0.3          1.9           2.2
Poaceae                P    G       0.3          1.6           2.0
Cyperaceae             P    S       0.3          1.6           1.9
Poaceae                P    G       0.5          1.3           1.8
Poaceae                P    G       0.7          1.1           1.8
Poaceae                P    G       0.4          1.1           1.5

Acteraceae             P    F       0.6          0.8           1.4
Cyperaceae             P    S       0.2          1.1           1.3
Acteraceae             P    F       0.4          0.8           1.2
Asclepiadaceae         P    F       0.1          0.8           1.0
Commelinaceae          P    F       0.1          0.8           1.0
Acteraceae             P    F       0.3          0.5           0.9
Asclepiadaceae         P    F       0.1          0.5           0.6
Poaceae                P    G       0.1          0.5           0.6
Plantaginaceae         A            0.1          0.5           0.6
Campanulaceae          A            0.1          0.5           0.6
Poaceae                A            0.1          0.5           0.6
Brassicaceae           B           <0.1          0.3           0.3
Poaceae                A           <0.1          0.3           0.3
Brassicaceae           A           <0.1          0.3           0.3
Asteraceae             B           <0.1          0.3           0.3
Violaceae              P    F      <0.1          0.3           0.3
Acteraceae             P    F                                  *
Molluginaceae          A
Poaceae                P    G                                  *
Onagraceae             B
Caryophyllaceae        A
Primulaceae            A                                       *
Liliaceae              P    F
Amaranthaceae          A
Caryophyllaceae        A
Solanaceae             A

                                  100          100           200

                                          SEED BANK

                                    Relative
                      Relative     Frequency     Importance
Family                 Density        (%)           Value

Poaceae                  0.3          1.2            1.4
Cactaceae                0.1          0.6            0.7
Malvaceae
Asteraceae
Poaceae                  0.1          0.6            0.7
Cyperaceae
Asteraceae
Poaceae
Asteraceae               8.4         10.7           19.1
Cyperaceae               0.1          0.6            0.7
Poaceae                  0.1          0.6            0.7
Brassicaceae             1.1          3.6            4.7
Polygalaceae             0.3          0.6            0.8
Poaceae
Commelinaceae            0.1          0.6            0.7
Fabaceae
Poaceae                  0.5          2.4            2.9
Fabaceae
Scrophr lariaceae        8.3          7.1           15.4
Cyperaceae
Polygonaceae             0.5          1.8            2.3
Asteraceae               0.3          1.2            1.4
Euphorbiaceae
Poaceae
Solanaceae
Poaceae                  1.5          3.0            4.5
Cyperaceae               0.5          1.8            2.3
Poaceae
Poaceae                  0.1          0.6            0.7
Poaceae                  0.5          1.2            1.7

Acteraceae
Cyperaceae               0.1          0.6            0.7
Acteraceae
Asclepiadaceae
Commelinaceae
Acteraceae
Asclepiadaceae
Poaceae
Plantaginaceae
Campanulaceae            6.9          6.5           13.4
Poaceae
Brassicaceae
Poaceae
Brassicaceae
Asteraceae               0.4          1.2            1.6
Violaceae
Acteraceae              45.9         17.9           63.7
Molluginaceae           15.8         14.9           30.6
Poaceae                  1.7          2.4            4.0
Onagraceae               0.8          3.0            3.7
Caryophyllaceae          1.7          1.8            3.4
Primulaceae              1.4          1.8            3.2
Liliaceae                0.1          0.6            0.7
Amaranthaceae            0.1          0.6            0.7
Caryophyllaceae          0.1          0.6            0.7
Solanaceae               0.1          0.6            0.7
                         2.2          9.5           11.7
                       100          100            200

Relative % cover = (([Cvr.sub.s]/[Cvr.sub.total]) X 100); where
[Cvr.sub.s] = total % cover of single species, Cvrtotal = total
% cover of all species; Relative density = ((DenS/Dentotal) X 100);
where DenS = total number of germinated seedlings of single species,
Dentotal = total number of all seedlings of all species; Relative
frequency = ((FregS/Fregtotal) X 100); where FregS = number of plots
(occurrences) where species is present, and Freqtotal = total of all
occurrences of all species; and Importance value (1V) = Relative %
cover (or relative density for seed bank) + Relative frequency. The
sum of IVs of 0 species in the vegetation (or seed bank) is 200

TABLE 2.--Similarity values (Jaccard's Index) comparing species
composition of vegetation and seed bank calculated for flora as
a whole and for each category of species classifications. No legumes
germinated from the seed bank (-)

ENTIRE FLORA            36

LOCATION OF ORIGIN
  Native                35
  Introduced            40

LIFE HISTORY
Perennial               35
Biennial                50
Annual                  33

FUNCTIONAL GUILD
Grass                   54
Forb                    18
Sedge                   60
Legume                  --

TABLE 3.--Parameters describing vegetation and seed bank community
structure according to location of origin, life history and functional
guild classifications in Thomson-Fulton Sand Prairie. Plot level and
frequency values are mean ([+ or -] SE). See Methods for explanation
of calculation of values. Statistical tests were conducted separately
for vegetation and seed bank data. All statistical tests among groups
within each classification were non parametric Kruskal-Wallace tests
(* = P < 0.005). See text for selected individual post hoc tests
within classifications

CLASSIFI-                   LOCATION                LIFE
CATION:                    OF ORIGIN                HISTORY

Category:           Native      Introduced    P     Perennial

SPECIES RICHNESS (SITE)
Vegetation            43            3                  34
Seed bank             26            4                  16

SPECIES RICHNESS PER PLOT
Vegetation        12.0 (0.6)    0.6 (0.1)     *    10.1 (0.5)
Seed bank          4.0 (0.3)    1.1 (0.1)     *     2.0 (0.2)

ABUNDANCE PER PLOT
Vegetation
  (% Cover)      167.0 (6.2)    4.3 (1.5)     *    160.9 (6.6)
Seed bank
  (Density)       21.1 (2.6)    5.1 (0.7)     *     13.7 (2.1)

FREQUENCY PER SPECIES (SITE)
Vegetation        8.2 (1.1)     6.0 (2.1)    ns     8.9 (1.3)
Seed bank         4.6 (0.4)     8.3 (5.6)    ns     3.9 (1.8)

                                                     FUNCTIONAL
CLASSIFI-                                            GUILD
CATION:                   LIFE HISTORY               (Perennials)

Category:         Biennial       Annual      P        Grass

SPECIES RICHNESS (SITE)
Vegetation            3            9                   12
Seed bank             3            11                   8

SPECIES RICHNESS PER PLOT
Vegetation        0.5 (0.1)    1.9 (0.2)     *      4.0 (0.3)
Seed bank         0.3 (0.1)    2.8 (0.2)     *      0.7 (0.2)

ABUNDANCE PER PLOT
Vegetation
  (% Cover)       2.6 (0.9)    7.8 (1.3)     *     73.7 (4.8)
Seed bank
  (Density)       0.4 (0.1)    11.6 (1.5)    *      1.3 (0.4)

FREQUENCY PER SPECIES (SITE)
Vegetation        4.7 (3.7)    6.2 (2.0)     ns     9.8 (2.2)
Seed bank         2.7 (1.2)    7.5 (2.4)     ns     2.5 (0.6)

CLASSIFI-
CATION:                  FUNCTIONAL GUILD (Perennials)

Category:           Forb         Sedge        Legume       P

SPECIES RICHNESS (SITE)
Vegetation           15            5             2
Seed bank            5             3             0

SPECIES RICHNESS PER PLOT
Vegetation       4.2 (0.3)     1.5 (0.2)     0.4 (0.1)     *
Seed bank        1.2 (0.1)     0.2 (0.1)         0         *

ABUNDANCE PER PLOT
Vegetation
  (% Cover)      64.4 (5.6)   17.0 (2.5)     6.2 (3.2)     *
Seed bank
  (Density)      12.2 (1.9)    0.2 (0.1)         0         *

FREQUENCY PER SPECIES (SITE)
Vegetation       8.5 (2.2)     8.8 (1.9)     6.5 (3.5)    ns
Seed bank        7.2 (5.7)     2.0 (1.0)         0        ns
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Author:McNicoll, Molly B.; Augspurger, Carol K.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2010
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