Effects of soil disturbance on success of a rare savanna forb, liatris scariosa var. nieuwlandii in habitats dominated by early and late successional grasses.
Historically, tallgrass prairie covered most of midwestern North America extending eastward in an area described as the "prairie peninsula" (Transeau, 1935). Within Illinois, less than one percent of the prairie presently remains (White, 1978) due to conversion to agriculture (Warner, 1994), urbanization and fire suppression (Baker, 1992; Warner, 1994). As a result, prairie restoration has become an important goal for conservationists.
Tallgrass prairie restoration usually focuses on accelerating succession by introducing seeds of dominant grass species, such as big bluestem (Andropogon gerardii), that rapidly develop a fuel base that increases with repeated buring (Schramm, 1992; Packard, 1994; Betz et al., 1999; Peet et al., 1975). Dominance by prairie grasses in restorations limits forb diversity (Dickson and Busby, 2010; McCain et al., 2010). When rare species are unable to compete for resources with aggressive species, conflicting goals can develop between maintaining diversity and rapidly establishing a tallgrass matrix.
Species coexistence is theorized to occur as a result of moderate levels of disturbance that reduce competition from dominant species (Fox, 1979; Huston, 1979; Pickett, 1980; Roxburgh et al., 2004). Patch disturbance is often required for interstitial species establishment (Petraitis et al., 1989) and can maintain or increase abundance of poorly competitive rare species (Pavlovic, 1994). Few field studies address the effect of dominant native grass abundance on rare prairie species or examine mechanisms, such as soil disturbance, that may mitigate such effects of competition (Collins and Barber, 1986). For example, small patch disturbance and mound building by burrowing animals redistributes soil and nutrients and causes shifts in species dominance (Inouye et al., 1987; Peart, 1987; Howe et al., 2006; Seifan et al., 2010). Larger scale disturbances by large mammals, including elk (Cervus canadensis) and bison (Bison bison), may remove vegetation and expose bare ground (Packer, 1963; Knapp et al., 1999), allowing early successional and less competitive species to regenerate (Oesterheld and Sala, 1990; Aguilera and Lauenroth, 1995; McMillan et al., 2011). These larger animals and such disturbances no longer occur naturally in eastern tall grass prairie (Hoffmeister, 2002). Non-native invasive species abundance is also expected to increase with soil disturbance (McIntyre and Lavorel, 1994; Kotanen, 1997; Daehler, 2003), complicating the notion that disturbance may benefit poortly competitive species.
Savanna blazing star (Liatris scariosa var. nieuwlandii, Asteraceae) is listed as Illinois threatened with 30 known populations in 18 Illinois counties (Herkert and Ebinger, 2002). It occurs in bur oak (Quercus macrocarpa) and white oak (Q. alba) savanna remnants or in disturbed grassland. This species is assumed to be a poor competitor. It occurs where partial tree canopy shade reduces grass competition and also colonizes open habitat where past cattle grazing has reduced grass cover (Bowles et al., 1988). It is an herbaceous perennial from a corm-like root system (Gleason and Cronquest, 1991). Flowering can occur in the second or third year with 15 to 45 flowering heads on a 3-10 dm scape. This species has the largest cauline leaves of the scariosa varieties, 2 to 5 cm wide, which differentiates it from the eastern L. s. var. novae-angliae (Gleason and Cronquest, 1991).
We used a field experiment to compare the performance of savanna blazingstar in disturbed and undisturbed patches in vegetation dominated by the early successional grass Danthonia spicata (poverty oats grass) and in restored grassland dominated by Andropogon gerardii. We also examine these effects on species richness and diversity as well as richness of dominant native and non-native species. We address three specific questions: (1) Are D. spicata and A. gerardii dominated communifies compositionally different? (2) How is L. s. nieuwlandii abundance affected by the dominant grass species? (3) How do L. s. nieuwlandii and associated vegetation respond to different scales of disturbance and to the dominant grasses? We expected that L. s. nieuwlandii abundance would decrease with increasing dominance by A. gerardii, but that this effect would be moderated by soil disturbance that would result in densities more similar to those found in D. spicata dominated vegetation. We also expected that species diversity would be reduced in vegetation dominated by A. gerardii compared to D. spicata and that non-native species richness would increase with disturbance.
Hickory Creek Barrens Nature Preserve is located in Will County, Illinois, USA (lat. 41[degrees]31.171'N, long. 87[degrees]54.565'W, 216 m elevation). The Preserve contains 232 ha of woodland, savanna, grassland, and floodplain vegetation (Bowles et al., 2002). The site's grassland soils are eroded silt loams (Soil Survey Staff, 2004) resulting from past overgrazing by cattle (Bowles et al., 2002). This grazing reduced cover of formerly dominant prairie vegetation. To restore this diversity, the Will County Forest Preserve District initiated tallgrass prairie restoration by broadcast sowing of Andropogon gerardii and other tall grass prairie species beginning in 1998, although A. gerardii was already present by the early 1990s (Bell & Bowles, pers. obs.).
Our study site was located within 8.5 ha of grassland containing one of the largest Illinois populations of Liatris scariosa nieuwlandii (Bowles et al., 2002). For this study, we used adjacent grassland vegetation dominated by Danthonia spicata, an early-successional grass (Scheiner, 1988), or by Andropogon gerardii, a late successional prairie grass (Collins and Barber, 1986). Both vegetation types occur on severely eroded Ozaukee silty clay loam (Soil Survey Staff, 2004). In the study area, vegetation dominated by D. spicata averaged 31.4% (0.05 SE) cover of bare ground with < 1/4 m high herbaceous species including Antennaria neglecta (field pussytoes) and Fragaria virginiana (barrens strawberry) (Fig. 1A) (Schroeder, 2007). Liatris scariosa nieuwlandii is most abundant in this area. Vegetation dominated by A. gerardii was formerly dominated by D. spicata but was converted to A. gerardii dominance by broadcast seeding and repeated burning (Bowles et al., 2002). Due to more dense cover by A. gerardii, this vegetation averaged 18.9% (0.04 SE) bare ground, and contained taller forbs including Solidago juncea (early goldenrod), Coreopsis palmata (stiff tickseed), and Monarda fistulosa (wild bergamot) (Fig. 1B) (Schroeder, 2007). Because of these changes, we refer to the D. spicata dominated vegetation as early successional. However, restored vegetation dominated by the late successional grass A. gerardii may not represent a true late successional stage.
During spring 2005, one 30 m transect was located in vegetation dominated by Andropogon gerardii. Two parallel 16 m transects were required to sample vegetation dominated by Danthonia spicata because of its narrow spatial area. Transect starting locations were randomly selected within each vegetation type. Orientation was north to south with a 2 m separation between the 16 m transects and a 100 m separation between transects in each vegetation type. Fifteen 1.5 x 1.5 m plots were aligned along transects at 2 m intervals. Each plot was randomly assigned one of three soil disturbance treatments: no disturbance (control), small disturbance, or large disturbance. Each plot contained nine nested 0.5 m x 0.5 m subplots. To mimic small animal patch disturbance, we drilled a 7.5 cm wide by 13 cm deep hole with a motorized auger using a 6.5 cm bit. One hole was drilled in the center of each subplot resulting in nine holes for each 1.5 x 1.5 m plot. We created larger disturbance patches by removing all vegetation and topsoil within the plot to a depth of 3 cm. Each vegetation type contained five no disturbance plots, five small disturbance plots, and five large disturbance plots, for a total of 15 plots with 135 subplots per vegetation type.
Seeds of Liatris scariosa nieuwlandii were collected on site and sown in plots during late fall 2005 to allow for natural stratification. These plots were unoccupied by L. s. nieuwlandii at the time of sowing. Based on a study of Liatris squarrosa (Baskin and Baskin, 1989) we assumed absence of a L. s. nieuwlandii seed bank. Seeds were not tested for viability. They were assumed to be viable if they had a thick, non-deteriorated seed coat that lacked signs of larval predation. Ten seeds were added to three randomly chosen subplots within each plot regardless of treatment. Within no disturbance and large scale disturbance plots, seeds were hand sown around the center of selected subplots and pressed lightly into the soil. For small scale disturbance plots, the same process was used, but seeds were distributed around the edge of each selected excavation.
[FIGURE 1 OMITTED]
DATA COLLECTION AND ANALYSIS
Data were collected during Aug. 2006 from each of the randomly selected subplots where Liatris scariosa nieuwlandii was sown. We recorded the presence of all vascular plants, density of L. s. nieuwlandii and cover of Andropogon gerardii, Danthonia spicata, Solidago juncea (early goldenrod) cover, Hieracium caespitosum (yellow hawkweed) cover and all woody species. All statistical analyses were calculated at the plot level by pooling compositional data and averaging density and cover data across the three subplots.
We used Sorensen's similarity coefficient to compare pooled species composition between the two communities. This coefficient was calculated as: QS = 2C/A + B where A and B equal the total number of species in sample 1 and 2 and C equals the species in common from the two samples. Intracommunity similarity was assessed by calculating ten indices per community using pair-wise plot combinations and calculating a mean value for each vegetation type. Intercommunity similarity was assessed by comparing undisturbed plot species by vegetation type and recalculating Sorensen's coefficient.
We used regression analysis to compare the effects of grass cover on Liatris scariosa nieuwlandii abundance. With Andropogon gerardii cover as an independent variable, we used linear regression to test whether it had a significant linear effect on L. s. nieuwlandii density and cover of Danthonia spicata. We also used linear regression to test for a similar effect of D. spicata on L. s. nieuwlandii. We then used an exponential model to determine whether these effects were stronger with a non-linear regression.
We used two-way analysis of variance (ANOVA) and Tukey-Kramer multiple comparisons to address how scales of disturbance and dominance by either Danthonia spicata or Andropogon gerardii effect Liatris scariosa nieuwlandii density, cover of dominant grasses, forbs, and woody vegetation, as well as native and alien species richness and species diversity. Subplots were averaged at the plot level for these tests. Simpson's inverse diversity was calculated for species frequency as: D = 1/[SIGMA][P.sub.i.sup.2] where [P.sub.i] is the proportional frequency of species i calculated from the subplot. Species cover data were Ln transformed to achieve normal distributions. In this analysis we use plots as the experimental units with inferences at the transect level within each vegetation type. Because the two vegetation types used in the study represent single landscape replicates, inferences cannot be made beyond the scale of this study.
Sorensen's similarity coefficient indicated 66% intercommunity similarity, an average of 79% (0.06 SD) similarity within the Danthonia spicata dominated vegetation and 89% (0.07 SD) similarity within the Andropogon gerardii vegetation. Both vegetation types had 22 species in common with 33 species in vegetation dominated by D. spicata and 25 species in vegetation dominated by A. gerardii.
EFFECTS OF GRASS DOMINANCE
Linear regression indicated a significant positive relationship between Liatris scariosa nieuwlandii density and Danthonia spicata cover ([R.sup.2] = 0.913, P = < 0.0001), and significant negative relationships between L. s. nieuwlandii density and Andropogon gerardii cover ([R.sup.2] = 0.438, P = 0.0371) and D. spicata and A. gerardii cover ([R.sup.2] = 0.549, P = 0.0143). An exponential non-linear model indicated stronger relationships between A. gerardii and both D. spicata and L. s. nieuwlandii (Fig. 2).
[FIGURE 2 OMITTED]
Within undisturbed plots, Liatris scariosa neiuwlandii averaged 1.73 (0.69 SE) juveniles per plot within Danthonia spicata dominated vegetation and were absent from the Andropogon gerardii dominated vegetation. Because of the short time period of this study, no L. s. nieuwlandii flowering plants developed from the seedlings established in either vegetation type. D. spicata averaged 20% cover as a dominant and was absent from A. gerardii dominated vegetation. Andropogon gerardii cover averaged 11% in D. spicata dominated vegetation, and 27% in vegetation in which it was the dominant grass.
DISTURBANCE EFFECTS VEGETATION TYPE
Scale of disturbance and interactions between these factors significantly affected abundance of Liatris scariosa nieuwlandii derived from planted seeds (Table 1, Fig. 3A). In association with Danthonia spicata, L. s. nieuwlandii had lower abundance in large disturbance patches (Fig. 3A). However, in association with Andropogon gerardii, L. s. nieuwlandii was absent from undisturbed plots but was present in disturbance plots (Fig. 3A). Cover of D. spicata also varied significantly in relation to disturbance and vegetation type and with the interactions between these factors (Table 1, Fig. 3B). It also had lower cover in large disturbance patches where it was dominant and was absent from undisturbed and small disturbance patches in association with A. gerardii (Fig. 3B).
Native species richness was significantly greater (10.7/[m.sup.2] + 0.6 SE) in vegetation dominated by Danthonia spicata compared with Andropogon gerardii (9.5/[m.sup.2] + 0.4 SE) and was lower in large scale disturbance patches (Fig. 3E). Non-native richness did not differ among vegetation types but was also reduced by large scale disturbance (Fig. 3F). Andropogon gerardii and Hieracium caespitosum varied significantly for both vegetation type and disturbance scale but did not have significant interactions (Table 1). They also had lower values in large scale disturbances patches, with greater cover of H. caespitosum in association with A. gerardii (Figs. 3D, E). Solidago juncea and Simpson's diversity differed significantly in relation to disturbance scale (Table 1). Solidago juncea cover reached 21% (4.2 SE) in undisturbed patches, 18% (4.7 SE) in small disturbance plots, and 7.0% (2.0 SE) in large disturbance plots (Fig. 3G). Simpson's diversity was 19.1 (0.9 SE) in undisturbed plots and 19.5 (1.0 SE) in small disturbance plots but only 14.9 (0.9 SE) in large disturbance patches (Fig. 3H). Total cover for woody species differed between vegetation type (Table 1) with greater cover (17% + 4.02 SE) associated with A. gerardii dominated vegetation than in D. spicata dominated vegetation (11% + 2.23 SE).
[FIGURE 3 OMITTED]
Our correlative and experimental data suggest that dominance by Andropogon gerardii has a strong negative influence on the abundance of Liatris scariosa nieuwlandii as well as Danthonia spicata. This effect apparently intensifies as succession proceeds and cover of A. gerardii increases and may be a direct result of competition for light and space for seed germination (Howe, 1995; Bullock et al., 1995; Morgan, 1997). These results are consistent with belowground competition theory (Casper and Jackson, 1997) whereby A. gerardii invades new habitat and reduces resources for existing D. spicata and L. s. nieuwlandii. Andropogon gerardii species is capable of developing a very dense root system within 3 y (Weaver, 1958), tolerating high soil temperature, and producing high levels of carbon gains during low water seasons (Knapp, 1985). It also produces multiple fillers by which it is capable of rapid vegetative spread.
Our results suggest that these competitive effects of Andropogon gerardii on Liatris scariosa nieuwlandii can be reduced by disturbance. The establishment of L. s. nieuwlandii in both small and large scale soil disturbances in A. gerardii dominated vegetation and its absence from controls indicates that disturbance may moderate the competitive effect of A. gerardii by allowing seedling establishment. This fits an expected model of species coexistence through disturbance (Huston, 1979; Fox, 1979; Pickett, 1980; Roxburgh et al., 2004), especially as succession proceeds (Collins and Barber, 1986). In contrast, there appears to be a much weaker competitive effect of Danthonia spicata on L. s. nieuwlandii, as abundance and cover of these species were positively correlated in undisturbed patches. Nevertheless, these species may occupy different stages of early succession as small scale disturbance appeared to favor establishment of both L. s. nieuwlandii and D. spicata in a less competitive environment, but only L. s. nieuwlandii responded to this disturbance scale in more competitive vegetation.
Disturbance caused microsite changes that alter nutrient and moisture availability also may have affected the response of Liatris scariosa nieuwlandii to disturbance. Changes in nutrient levels have been reported for burrow mounds (Inouye et al., 1987; Simkin et al., 2004; Seifan et al., 2010) while soil depressions could allow water to accumulate (Battaglia and Reid, 1993; Berlow et al., 2002). Large scale disturbances may decrease soil moisture and increase temperature in association with Danthonia spicata because of greater cover of bare ground (Bell and Ungar, 1981). This may have reduced abundance of L. s. nieuwlandii and cover of D. spicata. In contrast, soil disturbance in vegetation dominated by Andropogon gerardii may increase resource availability due to increased nutrient cycling in association with a dense grass matrix (Ojima et al., 1994) and less soil moisture loss from bare ground.
Lower native species richness in Andropogon gerardii dominated vegetation also fits an expected model for prairie restoration where dominance by grasses may limit forb diversity (Dickson and Busby, 2010; McCain et al., 2010; Lauenroth and Adler, 2008). The failure of large scale soil disturbance to reverse this process may have resulted from loss of early successional species from the immediate species pool or an inability to re-establish during the short time of this study.
Our study of soil disturbance differed from others (McIntyre and Lavorel, 1994; Kotanen, 1997) in that non-native species invasion was minimal. This was unusual considering the non-native species present at the site. For example, Hieracium caespitosum is an aggressive non-native species that may form dense colonies (Panebianco and Willemsen, 1976; Wilson et al., 1997), but its cover remained low in our large disturbance plots. The other three dominant non-natives, Chrysanthemum leucanthemum (oxeye daisy), Dacus carota (wild carrot), and Rumex acetosella (sheep sorrel) have the potential for extensive population growth (Carson and Pickett, 1990; Parendes and Jones, 2000) but remained at low levels. The short time period of this study may have reduced the potential for invasion by non-native species. Working in an established vegetation matrix also may have reduced the quantity of non-native seed sources in the local species pool (Foster, 2001).
These results suggest that reintroduction of small mammals that create soil disturbance may benefit management for Liatris scariosa nieuwlandii. More work is needed to investigate such processes with other early successional species during restoration.
Acknowledgments.--This research was funded in part through the Forest Preserve District of Will County. Nathan would like to thank Andrew Blackburn, Renee Euler, and Clayton Wooldridge for field assistance. Hira Walker, Glenn Harper, and two anonymous reviewers greatly improved this manuscript.
SUBMITTED 10 MAY 2011
ACCEPTED 12 MARCH 2012
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NATHAN C. SCHROEDER (1)
Department of Natural Resources and Environmental Sciences, 801 South Wright Street, University of Illinois Champaign
Urbana, Champaign 61820
TIMOTHY J. BELL
Department of Biological Sciences, Chicago State University, 9501 South King Drive, Chicago, Illinois 60628
MARLIN L. BOWLES
The Morton Arboretum, 4100 Illinois Route 53, Lisle, Illinois 60532
(1) Present address: Department of Natural Resources, Pueblo of Santa Ana, 02 Dove Road, Santa Ana Pueblo, New Mexico, 87004; e-mail: email@example.com
TABLE 1.--ANOVA F statistics and probabilities that selected species, woody vegetation, species richness, and diversity differ among disturbance treatments and vegetation types at Hickory Greek Barrens Nature Preserve, Illinois. Significant interactions indicate that disturbance treatments and vegetation types are not independent L. s. nieuwlandii A. gerardii density cover Source Df F P F P Vegetation Type 1 17.90 0.0003 12.26 0.0018 Disturbance 2 10.50 0.0005 14.84 <0.0001 Interaction 2 8.20 0.0019 2.34 0.1179 Residual 24 H. caespitosum D. spicata cover S. juncea cover cover Source F P F P F P Vegetation Type 28.44 <0.0001 0.18 0.6756 10.2200 0.0039 Disturbance 7.63 0.0027 11.98 0.0002 4.9700 0.0157 Interaction 7.84 0.0024 0.30 0.7460 1.1300 0.3390 Residual Woody species cover Native richness Source F P F P Vegetation Type 5.85 0.0235 6.14 0.0207 Disturbance 3.29 0.0545 20.84 <0.0001 Interaction 0.03 0.9730 1.92 0.1687 Residual Alien richness Simpson's diversity Source F P F P Vegetation Type 6.49 0.1770 0.23 0.1423 Disturbance 13.05 0.0001 19.15 <0.0001 Interaction 1.08 0.3550 2.61 0.0939 Residual