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DOT Project Number:  90-00-LRTF-707

Fiscal Year:  2007

Award:  $11,693

Principal Investigator:  Kathryn Yurkonis, Department of Ecology, Evolution and Organismal Biology, Iowa State University, yurkonis@iastate.edu

Other Project Participants:  Brian Wilsey and Kirk Moloney, Department of Ecology, Evolution and Organismal Biology, Iowa State University

Summary Report: 

PLANT DISTRIBUTIONS IN PRAIRIE RESTORATIONS AND THEIR CONSEQUENCES FOR DIVERSITY AND EXOTIC SPECIES INVASION

Project Summary

Many studies have addressed how disturbance such as fire and mowing can be used to increase species diversity in prairie reconstructions.  However, few studies have asked if another important aspect of prairie restoration, initial seeding methods, affect restoration success.  Of particular interest is whether or not drilled and broadcast seeded tallgrass prairie reconstructions differ from one another.  With this study we measured several aspects of plant community structure to test our hypothesis that drill-seeded prairie reconstructions would be less diverse and more invaded than broadcast seeded restorations due to differences in initial seed spacing between plantings.  We sampled drilled and broadcast seeded prairie reconstructions across Iowa to determine if drilled and broadcast seeded prairie reconstructions differed in (1) species diversity, (2) warm season grass and exotic species abundance, and (3) plant distributions.  Generally, species diversity was similar between these planting types.  However, the effects of planting type on native warm season grass and exotic species abundance and plant distribution were location specific.  Ongoing research is investigating the potential mechanisms behind these differences in abundance and plant distribution and their future impacts on these types of plantings. 

Management Implications

  • Our findings suggest that drill and broadcast seeding are interchangeable when assessing restoration success through the lens of plant species diversity.

  • Drill seeding may result in greater native warm season grass and exotic species abundance. 

  • Drill and broadcast seeding produce communities with different plant distributions.  These differences may affect future diversity within the plantings and need to be further investigated. 

  • The effects of drilled vs. broadcast seeding were location specific and may be related to location preparation and management, including seed mix composition, time of planting, local propagule pool, or use of fire.  The ways these factors interact to affect drilled and broadcast plantings need to be further studied.

Background

Prairie restorations are planted and managed using various techniques in attempts to construct communities that are comparable to remnants (Packard and Mutel 1997).  As most site managers are well aware, decisions regarding which techniques to incorporate at various stages through the planting and management process may have lasting effects on restoration success.  Incorporating post-planting disturbance such as fire and mowing can reduce exotic species and increase species diversity  (Copeland et al. 2002, Williams et al. 2007).  However, little is known about how initial seeding methods may affect restoration success.  Two common methods are seed drilling and broadcast seeding, which differ in the depth and placement of seeds during planting.  Although anecdotal results suggest that seed drilling and broadcast seeding are interchangeable (Kirt 1990), their ability to produce comparably structured plantings has not been empirically investigated using diverse seed mixes.  With this study, we compared drilled and broadcast seeded plantings in five sites across Iowa to determine if they differ in species diversity and composition.  We also measured a third aspect of the restoration, how plants are distributed in space, which may also be important in determining future diversity and resource use (Tilman and Kareiva 1997, Stoll and Prati 2001, De Boeck et al. 2006).  Plant distribution describes the ways in which individuals or stems of the same species are grouped in space.  Plant distribution may be important in determining restoration success and future stability (Stoll and Prati 2001, Bolker et al. 2003, Bartha et al. 2004, Lortie et al. 2005, De Boeck et al. 2006).  However, we know very little about how the spatial relationships among seeds at planting might affect final plant distributions and the future impact of those distributions on a restoration. 

In what ways could drilled and broadcast plantings differ?

Drill and broadcast seeding differ in two ways which may irreversibly affect restoration success.  First, drill seeding plants seeds deeper than broadcast seeding.  Drill seeding typically places seeds 1 cm below the soil surface, while broadcast seeding places seeds on the soil surface.  Deeper seeding in drilled plantings may promote germination in some species and inhibit germination in others (Redmann and Qi 1992, Ambrose and Wilson 2003).  Second, drilled and broadcast plantings presumably differ in the intensity of seedling competition due to their arrangement at planting.  Seedlings in drilled rows have closer neighbors and would experience more competition (Lortie et al. 2005, Milbau et al. 2007) than seedlings spread further apart in a broadcast planting (Figure 1).  In a study of establishing grasslands, survival of neighboring seedlings was most influenced by neighbors within a 3 cm distance (Milbau et al. 2007).  Among close neighbors, strong competitors (Stoll and Prati 2001, Lortie et al. 2005) or early emerging seedlings (Ross and Harper 1972, Korner et al. 2008) are most likely to persist.  If this effect of neighbor distance is important in grassland establishment then drilled plantings with close neighbors should be less diverse than otherwise similar broadcast plantings.

Changing seeding method may also affect how plants establish in space resulting in large or small patches of the same species.  Large patches of the same species may form when competition among close neighbors excludes weaker competitors, as in a drilled planting.  In a broadcasting planting greater distances among neighboring seedlings may result in less competition and more numerous and smaller patches (Stoll and Prati 2001).  The way plants are distributed in space may also affect local resource uptake (De Boeck et al. 2006), exotic species invasion (Bergelson et al. 1993, Melbourne et al. 2007), and future species interactions (Silvertown et al. 1992, Racz and Karsai 2006).  If seeding method dictates plant distributions in this way, then drilled plantings with shorter distances among neighboring seedlings should have larger and more numerous patches. 

Finally, the distribution and proportion of open spaces (bare ground) in drilled vs. broadcast plantings may affect exotic species invasion.  Invasion is typically greater in larger more contiguous open spaces, as in drilled plantings, over smaller more dispersed spaces, as in broadcast plantings (Goldberg and Werner 1983, Aguilera and Lauenroth 1993, Kennedy et al. 2002, Milbau et al. 2007) (Figure 1).  Invasion may also be related to patch size, where larger patches may not use resources as completely and are thus more susceptible to invasion (Tilman et al. 1996, Naeem et al. 2000, Wilsey and Polley 2002, Melbourne et al. 2007, He and Lamont 2008.  If this effect of neighbor distance and patch size on invasion is important, then drilled plantings with large uniform spaces between rows should have greater exotic species abundance than otherwise similar broadcast plantings.

Project Objectives

Otherwise similar drilled and broadcast grasslands plantings may differ due to differences in the depth and placement of seeds at planting.  This study tests for differences between paired drilled and broadcast seeded tallgrass prairie plantings to determine if there are long-term effects of planting method after several growing seasons.  We measured diversity, species composition and plant distributions in three locations throughout Iowa.  Within each location, the same seed mix was either drill or broadcast seeded into equal sized plantings.  We test the hypotheses that:

(1) drilled plantings would be less diverse due to more competition among seedlings,

(2) warm season grasses and exotic species would be more abundant in drilled plantings due to competitive exclusion of close neighbors and more contiguous places for exotic species invasion, and

(3) warm season grasses and exotic species would occur in larger patches in drilled plantings due to competitive exclusion and more contiguous places for establishment. 

These results will be useful for improving restoration plans and expand our understanding of how initial planting methods might affect future restoration success (Bartha et al. 2004).

Methods

Study Sites

Three locations were sampled in the summer of 2007, a seven year old reconstruction at Peterson Park (Story County, a four year old reconstruction at Lakeside Lab (Dickinson County), and three sites at Neal Smith National Wildlife Refuge (Jasper County).  All of the sites were in annual crop production before planting.  Although many other sites in Iowa have been planted in similar ways, these are the most closely matched drilled and broadcast plantings that have been located in the state. 

The Peterson Park and Lakeside Lab sites are in the Des Moines Lobe landform region of Iowa and each contains a drill and a broadcast seeded planting.  The drilled and broadcast areas were planted with the same seed mix and then managed in the same way within each site.  The Peterson Park site (lat 42º 05´ N, long 93º 35´ W) was planted in the fall of 1999 by the Story County Conservation Board.  The site is located in the Skunk River floodplain and contains moderately to well drained fine loam mixed Cumulic Hapludolls (DeWitt 1984).  Mean annual temperature is 8.8ºC and mean annual precipitation is 837 mm.  The site was divided into two sections, each planted with a seed mix containing 20 native species collected in bulk from three locations in Story County, Iowa.  The northern 3.5 ha was planted at 15.6 kg pure live seed/ha with a broadcast seeder and cultipacked after seeding.  The southern 1.9 ha was drilled with the same seed mix, mixed from the three sites in a similar ratio, at 16.8 kg pure live seed/ha.  The most abundant species within the bulk mix were Solidago rigidia (Stiff goldenrod), Ratibida pinnata (Yellow coneflower), Elymus canadensis (Canada wildrye), Andropogon gerardi (Big bluestem), Sorghastrum nutans (Indian grass), and Elymus virginicus (Virginia wildrye).  The entire site was burned in the springs of 2004, 2005, and 2006.  The western half of the site was burned (½ of the drilled and ½ of the broadcast planting) in the fall of 2006 and broadcast interseeded to increase species diversity (B. Gleason, Story County Conservation Board, Ames, IA, personal communication).  As a result, sampling for this study was restricted to the eastern portion of the site.

The Lakeside Lab site (lat 95º 10´ N, long 43º 23´ W) is a 9.3 ha planting located on a south facing slope at the Iowa Lakeside Laboratory (P.I.’s: Arnold van Der Valk, Daryl Smith, and Tom Rosburg).  Mean annual temperature is 7.2ºC and mean annual rainfall is 725 mm.  Soils are predominantly fine loam mixed Typic Hapludolls on 2-9% slope with some Cumulic Hapludolls (Dankert 1983).  Soil series run East-West across the site and plantings were established with an equal proportion within each soil type (North-South).  Sections (1.0 ha) drilled or broadcast with pure live seed during the spring of 2003 were sampled for this study.  The site was disked twice and leveled with a cultipacker before planting.  Both drilled and broadcast areas were drilled with the annual Avena sativa (oat) as a cover crop (17.4 kg/ha) in the spring of 2002.  A seed mix consisting of 37 forbs and 9 grasses was added at 12.0 kg pure live seed/ha within both plantings.  The most abundant forbs (>10 seeds/m2) in the mixture were Ratibida pinnata (Yellow cone flower), Rudbeckia hirta (Black eyed susan), Solidago rigida (Stiff goldenrod), and Petalostomum purpurea (Purple prairie clover).  The most abundant grasses were Schizachyrium scoparium (Little bluestem), Koelaria macrantha (Junegrass) and Sorghastrum nutans (Indian grass).  Both plantings have been mowed twice yearly (spring and late summer) to control thistles, primarily Cirsium arvense (Canada thistle) and Carduus nutans (Musk thistle).

At Neal Smith National Wildlife Refuge (NSNWR, U.S. Fish and Wildlife Service, Prairie City, IA) we sampled three separate sites containing replicated drilled and broadcast plantings as part of the ongoing Thistle Suppression Research Plot study (P.I. Diane Larsen and Pauline Drobney).  NSNWR is located on the southern Iowa drift plain where the mean annual temperature is 9.4ºC and the mean annual precipitation is 836 mm.  Three fields within the refuge were each planted with 12 replicate (12.2 m x 12.2 m) plots of 12 seeding and diversity treatments in a full factorial design.  All three fields, Production, Orbweaver and Harmeson were treated with herbicide prior to planting.  Soils are relatively consistent within fields, but do vary among fields.  Production is predominantly Aquertic Argiudolls, Orbweaver Oxyaquic Argiudolls, both formed in loess, and Harmeson Mollic Hapludalfs formed in glacial till (Nestrud and Woster 1979).  In July 2007, we sampled spring planted (2005), medium-diversity, drilled and broadcast plots (2 seeding treatments x 12 replicates x 3 fields = 72 plots) within the Thistle Suppression Research Plots.  These plots were either drilled or broadcast seeded with a 20 species seed mix where 70 percent of the seeds (by numbers) were grasses and 30 percent forbs.  The mix contained 13 forbs and seven grasses with Elymus canadensis L. (Canada wild rye) seeded in a higher proportion as a cover crop.  Plots were seeded at approximately 60 seeds/m2.  All plots were mowed once in the first year and had not been burned. 

Vegetation sampling

In each of the plantings at Peterson Park and Lakeside Lab we sampled ten 1 m2 quadrats.  Quadrats were located randomly along and away from a transect through the longest portion of the planting. We used a 70 m transect at Peterson Pits, and a 100 m transect at Lakeside Laboratory.  At NSNWR we sampled a 1 m2 quadrat randomly located to the north of the central marker in each plot.  All species were recorded and species relative abundance measured through point intercept sampling within a 1 m2 sampling frame placed over each quadrat (Jonasson 1988, Frank and McNaughton 1990).  We recorded the identity of every leaf and stem touching each of 40 pins dropped uniformly across the quadrat.  A small value (0.5 touch) was added for each species with no hits when calculating diversity measures to account for species that were not captured (Bowman et al. 2006).  Species relative abundance was determined by dividing the total touches for species i in a quadrat by the total touches in the quadrat.  These data were used to determine species richness (S), Simpson’s diversity (1/D), where D = ∑pi2 and pi = relative abundance of species i, and evenness ([1/D]/S) at the quadrat scale (Smith and Wilson 1996, Wilsey et al. 2005).    

To quantify plant distributions, each 1 m2 quadrat was divided into 64– 12.5 x 12.5 cm square cells using metal rods passed through the vegetation.  The species occupying 50% or more of the aboveground space in each cell was recorded onto an 8 x 8 grid.  This method generates a map of the locally abundant species throughout the quadrat.  Each map was imported into the program (Gardner 1999, Gardner and Urban 2007) which summarized the number and size of patches within each quadrat.  A patch was defined as a group of neighboring cells containing the same species.  Neighboring cells included the four adjacent and four diagonal cells from a focal cell.  With these data we computed several simple metrics: number of species per map, mean patch area, and patch mean-squared radius.  Patch mean-squared radius is a measure of patch dispersion in meters where larger mean-squared radius values indicate that a larger area is needed to encompass the patch than one with smaller values (Gardner 1999). 

We quantified plant distributions in two ways within each quadrat.  First, we calculated mean patch size (m2) and dispersion (m) across all patches within each quadrat, irrespective of their species occupancy.  Second, we determined the proportion of the quadrat covered by native warm season (C4) grasses and the mean size and dispersion of C4 grass patches.  C4 grasses were of particular interest because they can dominate restorations despite efforts to promote realistic native species composition (Sluis 2002, Derner et al. 2004, Martin et al. 2005).  For the C4 grass analysis, each quadrat map was simplified into two classes, native warm season grass and ‘other’, and then summarized via QRULE.  While the first analysis tests for differences in general patch structure, the second analysis tests if dominant species occupy space, and potentially utilize resources, within each planting in different ways.  We repeated this analysis for exotic species.  However, because exotic species were not abundant in space exotic species distribution data are not presented for NSNWR.

Data analysis

We used analysis of variance (ANOVA; PROC GLM; SAS version 9.1) to test for quadrat scale differences in species diversity, exotic species composition, and plant distributions between drilled and broadcast plantings.  Peterson Park and Lakeside Lab data were analyzed separately from NSNWR data due to differences in within site replication.  Native warm season grass relative abundance at NSNWR and exotic species relative abundance data at all three locations were arcsine squareroot transformed to meet normality assumptions.  At NSNWR, patch size and mean-squared radius data were log transformed to meet normality assumptions.  However, exotic species at Peterson Park and native warm season grasses at Lakeside Lab were not recorded as occupying cells in several quadrats and normality assumptions could not be met with data transformations for their distribution metrics.  A non-parametric Kruskal-Wallis test was performed to test for differences in plant distributions at these locations. 

Results

Do drilled and broadcast seeded prairie reconstructions differ in species diversity?

At Peterson Park and Lakeside Lab there was no main effect of planting type on quadrat Simpson’s diversity, evenness or species richness (Table 1; Figure 2).  Because there was a site x planting type interaction for species richness (Table 1; Figure 2), we also separately considered the effect of planting type on species richness within each site.  At Peterson Park, there were no differences in quadrat species richness (F1, 18 = 0.07; p > 0.05) between plantings, while quadrat species richness was higher in the broadcast planting at Lakeside Lab (F 1, 18 = 5.27; p = 0.03). 

At NSNWR, there was also no effect of planting type on Simpson’s diversity, species richness, or evenness, although there were differences among sites in Simpson’s diversity and species richness (Table 2; Figure 3).  Approximately one-quarter of the species in the seed mix established within the quadrats and native planted species comprised half of the species within each quadrat.

Do drilled and broadcast seeded prairie reconstructions differ in warm season grass and exotic species abundance?

Of the species that established from the seed mix, the native warm season grasses, Andropogon gerardi, Sorghastrum nutans, and Schizachyrium scoparium were abundant (by number of point intercept touches) at both Peterson Park and Lakeside Lab in addition to Elymus canadensis at Lakeside Lab.  The most abundant exotic species at Peterson Park were Poa pratensis and Bromus inermis.  The most abundant exotic species at Lakeside Lab were Bromus inermis, Poa pratensis, Elymus repens, and Dactylis glomerata.  Despite similarities in the species that occurred between sites, there were significant differences in the relative abundance of native warm season grasses, planted forbs, and exotic species between sites (Table 1; Fig. 4).  The Peterson Park plantings were dominated by native warm season grasses with few exotic species and Lakeside Lab plantings were dominated by exotic species with fewer native warm season grasses (Fig. 4).  There were also differences in the effect of planting type within each site (Table 1; Fig. 4).  At Peterson Park relative abundance of native warm season grasses (F1,18 = 0.82; p > 0.05; Fig. 4) and exotic species (F1,18 = 0.25, p > 0.05; Fig. 4) were similar between plantings.  However, the drilled planting at Lakeside Lab had higher exotic species relative abundance (F1,18 = 18.32; p < 0.001) and lower relative abundance of native warm season grasses (F1,18 = 8.22; p < 0.05) than the broadcast planting.

At NSNWR, despite low recruitment from the seed mix, over seventy-five percent of the leaf hits were from native planted species (Fig. 5).  The plantings were primarily comprised of Elymus canadensis that was seeded as a cover crop with the seed mix.  Setaria viridis was the most common exotic weed alongside the native Conyza canadensis.  Exotic species abundance was similar between planting types (Table 2, Fig. 5).  However, native warm season grasses (Andropogon gerardii, Panicum virgatum, Schizachyrium scoparium, Bouteloua curtipendula, and Sorghastrum nutans) were more abundant in the drilled plantings (F2,66 = 4.05; p < 0.05; Fig. 5).

Do drilled and broadcast seeded prairie reconstructions differ in their plant distributions?

At Peterson Park and Lakeside Lab mean patch size and dispersion per quadrat were not significantly different between sites or planting types (Table 3; Fig. 6).  However, there were differences in how groups of similar species established and were distributed between planting types.  At Peterson Park, native warm season (C4) grasses were recorded in over half of the cells in both plantings.  C4 grasses occupied more cells per quadrat (F1,18 = 5.18; p = 0.0352; Fig. 7), and occurred in larger patches per quadrat (F1,18 = 6.00; p = 0.0247; Fig. 7) in the drilled than in the broadcast planting.  However, there was no effect of planting type on patch dispersion (Χ2 = 0.5714; df = 1; p > 0.05; Fig. 7).  Exotic species occupied few cells within quadrats at Peterson Park.  Exotic species were recorded in four broadcast and one drilled quadrat at Peterson Park and there was no effect of planting type on the number of cells occupied by exotic species per quadrat (Χ2 = 2.2208; df = 1; p > 0.05, Fig. 7), mean patch size (Χ2 = 1.9371; df = 1; p > 0.05, Fig. 7), or dispersion (Χ2 = 1.8054; df = 1; p > 0.05, Fig. 7).

At Lakeside Lab, these suites of species were distributed differently.  Exotic species occupied many cells, in some cases comprising the entire quadrat.  In the drilled planting, exotic species occupied more cells (F1,18 = 19.82, p = 0.0003, Fig. 7) and collectively occurred in larger (Χ2 = 8.6914; df = 1; p = 0.0032, Fig. 7) similarly dispersed (Χ2 = 3.0400; df = 1; p = 0.0812, Fig. 7) patches than in the broadcast planting.  C4 grasses were recorded in at least one cell in each broadcast quadrat and in four drilled quadrats.  C4 grasses occupied a larger proportion of space (Χ2 = 6.2325; df = 1; p = 0.0125, Fig. 7) and were more dispersed (Χ2 = 6.0142; df = 1; p = 0.0142, Fig. 7) in broadcast quadrats, but did not differ in patch size (Χ2 = 3.6708; df = 1; p > 0.05, Fig. 7) between planting types.

At NSNWR, plants generally established in space in similar ways between planting types (Table 4; Fig. 8).  There was no effect of planting type on the number of species recorded within quadrat maps, mean patch size, or mean patch mean-squared radius (Table 4).  However, there was a planting effect on native warm season grass distributions.  Drilled plantings contained a greater proportion of C4 grass attributed cells (F1,60 = 4.39; p < 0.05).  C4 grasses were distributed in the same number (F1,60 = 2.55; p > 0.05) of similarly sized (F1,60 = 3.15; p < 0.10), but more dispersed (F1,60 = 4.27; p < 0.05) patches in drilled over broadcast plantings (Table 5; Fig. 9).

Discussion

Few studies have considered whether seeding methods have lasting effects on diverse tallgrass prairie reconstructions.  To address this question, we tested whether drill and broadcast seeding diverse tallgrass prairie seed mixes produces similar or different communities.  We sampled five sites in three locations throughout Iowa which differed in overall management and site conditions.  Within each location seeds where either drilled or broadcast seeded into otherwise similar plantings. 

In all three locations we found that drilled and broadcast plantings were similar in species diversity, evenness, and with one exception (Lakeside Lab) species richness.  However, the effect of planting on species composition and plant distribution varied among locations.  At the more invaded Lakeside Lab site, exotic species were more abundant and native warm season grasses less abundant in the drilled versus broadcast plantings.  In contrast, native warm season grasses were more abundant in drilled plantings at NSNWR.  We also found that the most abundant species within a site were distributed differently between drilled and broadcast plantings at each site.  Although drill and broadcast seeding produced similarly diverse plantings at these locations, present differences in species composition and the ways plants are distributed may affect how each planting changes in the future. 

Do drilled and broadcast seeded prairie reconstructions differ in species diversity?

Our hypothesis that diversity would be lower in drilled plantings due to greater competitive exclusion at establishment was not supported.  There was no effect of planting type on quadrat Simpson’s diversity, evenness, and, in one site, species richness.  These results are consistent with studies of drilled and broadcast plantings in other systems (3-6 planted species, primarily perennial grasses) which also found no differences in species richness between planting types (Montalvo et al. 2002, Sheley et al. 2006).  Drill and broadcast seeding appear to be interchangeable when measuring restoration success based on species diversity.  However, diversity metrics are only one aspect to consider when judging restoration success and others may be more important for understanding how restored communities might develop in the future (SER 2004).

Do drilled and broadcast seeded prairie reconstructions differ in warm season grass and exotic species abundance?

We found location specific effects of planting type on species abundance, likely due to overriding effects of seed mix composition, timing of planting, and the ways each location was restored including use of fire (Howe 1994) vs. mowing (Williams et al. 2007).  There was no effect of planting type on C4 or exotic species abundance at Peterson Park (fall seeded and burned), but there was at Lakeside Lab (spring seeded and mowed) and NSNWR (spring seeded and mowed).  Native grass abundance was lower and exotic species abundance higher in the Lakeside Lab drilled planting.  In the younger NSNWR sites, there was no effect of planting type on exotic abundance, but warm season grasses were more abundant in drilled plantings across the three sites.  Greater depth of seeding in drilled plantings may have favored native warm season grasses (Redmann and Qi 1992, Bakker et al. 2003) at NSNWR.  This outcome supports other studies of drilled and broadcast plantings which have found higher survivorship (Bakker et al. 2003) and biomass (Jackson 1999) and increased density (Sheley et al. 2006) of native grasses in drilled plantings.  Future studies are needed to determine if this difference disappears with age of the planting, as may have occurred at Peterson Park and Lakeside Lab, or if this outcome was determined by other location characteristics.    

Do drilled and broadcast seeded prairie reconstructions differ in their plant distributions?

Our hypothesis that plants of the same species would occur in larger patches in drilled plantings due to competition and more available open space after seeding was not supported.  At all three locations mean patch structure was similar between plantings.  However, native warm season (C4) grasses and exotic species were distributed differently between plantings, when they were abundant.  Because species richness was similar between plantings this was not likely the result of competitive exclusion leading to larger patch sizes as predicted.  This distribution may be a function of the spatial arrangement of seeds and bareground at planting, or other aspects of the drilling process including how seeds are sorted onto the landscape and seed movement along the ground once planted (Packard and Mutel 1997).

Results from Lakeside Lab suggest that planting method can influence invasion when there is greater invasion pressure.  Exotic species were more abundant and occupied more space in drilled quadrats at Lakeside Lab.  Exotic plants may have established more extensively in this drilled planting as a result of larger spaces for establishment (Bergelson et al. 1993) and feedbacks that promoted their persistence (Bergelson 1990).

Differences in plant distribution between drilled and broadcast plantings in each location may affect how the plantings will develop in the future.  Results from a concurrent experiment at the Iowa State University Horticulture Research Station suggest that present plant distributions can be very important for determining future vegetation composition.  In an experiment designed to test if changing plant distributions can maintain species diversity longer when like plants are grouped together, Monarda fistulosa spread less in plots with large patches of the same species than in plots with small patches.  This result is consistent with findings from less complex systems where clumping of individuals of the same species leads to more constant species diversity through time (Stoll and Prati 2001, Idjadi and Karlson 2007).  However, there are conflicting results in the literature on whether or not maintaining small (Stoll and Prati 2001) or large (Wilsey and Polley 2002) patches of the same species is a desirable management goal and future work is needed to resolve this question.  The effect of present differences in exotic and C4 grass distributions between these plantings on vegetative spread and species diversity needs to be further investigated.

Conclusions and implications

As we examine restorations to determine what aspects are and are not restorable (Hobbs 2007, Miller and Hobbs 2007), we need to consider how species use space and how spatial heterogeneity develops within plantings as a result of initial conditions and/or subsequent management (Bartha et al. 2004).  Considering initial species distributions in the restoration process has been important in wetland (Liu et al. 2004) and aquatic (Sleeman et al. 2005) systems and needs to be addressed in grassland restoration.  This is the first study that we are aware of that takes such a fine-scale spatial approach to assessing grassland restoration success.  We demonstrate that distributions of dominant species in space differ among variously restored grasslands despite having similar levels of diversity.  The mechanisms that generated these patterns and the implications of different plant distributions for future species diversity are being further investigated in ongoing experimental studies at Iowa State University.  With such an understanding of the effects of plant distribution on restoration success we may be able to more readily predict aspects of long term sustainability (SER2004) within restorations. 

Education and Outreach

This research has been included in several education/outreach activities and a portion of this work has already been accepted for publication in Restoration Ecology (Yurkonis et al. in press).  Various stages of the study have involved three undergraduate student workers (Kim Wahl, Thomas Moeller, and Adam Asche).  Findings from the experimental plots were presented at the 2007 Joint Meetings of the Ecological Society of America and Society for Ecological Restoration in San Jose, CA.  Data collected at NSNWR was presented at the Neal Smith National Wildlife Refuge Spring 2008 Reporting Symposium and will be presented at the 2008 Ecological Society of America Meeting in Milwaukee, WI.  Additional manuscripts will be submitted in the coming year.

Acknowledgements

We thank those that have planned, planted, and manage the reconstructions sampled for this study.  Joe Kooiker and Ben Gleason helped with the Peterson Park plantings, Arnold van der Valk, Daryl Smith, and Tom Rosburg planned the Lakeside Lab plantings, and Pauline Drobney and Diane Larsen planned and implemented the Neal Smith National Wildlife Refuge plantings.  Arnold van der Valk, Daryl Smith, Tom Rosburg, and Pauline Drobney provided valuable feedback on this project.  Undergraduates Kim Wahl, Thomas Moeller, and Adam Asche helped to collect data.  This project was also funded in part by the Iowa Prairie Network, the Iowa Native Plant Society, and the Iowa Lakeside Laboratory.

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