Establishing a Diverse Assemblage of Native Grasses and Forbs on a Knapweed-Infested Site in the Bass River Recreation Area, Ottawa County, Michigan

Transcription

1 Grand Valley State University Student Summer Scholars Undergraduate Research and Creative Practice Establishing a Diverse Assemblage of Native Grasses and Forbs on a Knapweed-Infested Site in the Bass River Recreation Area, Ottawa County, Michigan Corey K. Kapolka Grand Valley State University Neil W. MacDonald Grand Valley State University, macdonan@gvsu.edu Follow this and additional works at: Recommended Citation Kapolka, Corey K. and MacDonald, Neil W., "Establishing a Diverse Assemblage of Native Grasses and Forbs on a Knapweed-Infested Site in the Bass River Recreation Area, Ottawa County, Michigan" (2009). Student Summer Scholars This Open Access is brought to you for free and open access by the Undergraduate Research and Creative Practice at ScholarWorks@GVSU. It has been accepted for inclusion in Student Summer Scholars by an authorized administrator of ScholarWorks@GVSU. For more information, please contact scholarworks@gvsu.edu.

2 Establishing a Diverse Assemblage of Native Grasses and Forbs on a Knapweed-Infested Site in the Bass River Recreation Area, Ottawa County, Michigan Abstract Corey K. Kapolka and Neil W. MacDonald Department of Biology, Grand Valley State University Report Date: September 11, 2009 Extensive areas of degraded lands and remnant natural areas in the upper Midwest have been invaded by the non-native perennial, spotted knapweed (Centaurea maculosa Lam.). Reestablishment of native plant communities requires the application of effective control measures. The objective of our study was to examine the interactive effects of mowing and chemical site preparation treatments (herbicides) combined with hand pulling on spotted knapweed control and native plant establishment on a knapweed-infested site in western Michigan. Initial mowing and herbicide treatments were applied to forty-eight plots in the summer of 2008, and we seeded these plots with a mixture of native grasses and forbs in the spring of We hand pulled seed-producing knapweed from selected plots in mid-summer, 2009, and determined residual knapweed densities and native plant occurrence on all plots in late July, All site preparation treatments began to reduce the knapweed soil seedbank, while both glyphosate and clopyralid herbicides substantially reduced mature spotted knapweed densities. Hand pulling effectively reduced seed-producing knapweed densities to less than 0.5 plants m -2 on mowed and glyphosate-treated plots; hand pulling was unnecessary on clopyralid plots because mature knapweed were totally absent in Only clopyralid, however, reduced juvenile and seedling knapweed densities significantly. Planted native warm-season grasses were present on all treatment combinations, but full development of a diverse native plant community is expected to take several years. Some combination of herbicide treatment and hand pulling of knapweed is recommended to facilitate reestablishment of a native plant community on knapweed-infested sites. Keywords Spotted knapweed, mowing, hand pulling, herbicides, glyphosate, clopyralid, native grasses Introduction Invasive exotic species present a significant threat to native ecosystems throughout the Midwest, outcompeting or otherwise displacing native forbs and grasses and disrupting ecosystem establishment and stability. One particularly aggressive exotic plant now found commonly throughout North America is spotted knapweed (Centaurea maculosa Lam.). Spotted knapweed is very successful at invading disturbed ecosystems, quickly establishing dominance through prolific seeding and competitive inhibition (Sheley et al., 1998). Heavily infested sites 1

3 are typically treated with broadleaf-specific herbicides in order to clear out existing knapweed populations and encourage native reestablishment (Rice et al., 1997; Sheley et al., 2000), but herbicide treatments may not be appropriate for use in all circumstances. Native plant establishment could be inhibited along with knapweed, and where increased native plant diversity is an objective, alternate methods of site preparation may be warranted. Previous studies indicate that physical damage and inhibition of spotted knapweed by mowing may require repeated applications in order to have appreciable effectiveness (Rinella et al., 2001), but it is a relatively simple option for controlling the weed on a large scale. Another non-herbicidal option is to physically remove entire plants by hand (Sheley et al., 1998; Abella, 2001), however there are very few published studies that have included hand pulling as a treatment (e.g., Lutgen and Rillig, 2004) and anecdotal evidence is inconclusive as to its longterm effectiveness as a means of knapweed control. In addition to removing mature knapweed, juveniles and the soil seedbank must also be addressed when restoring an infested site. Knapweed seeds can persist in a seedbank for many years (Davis et al., 1993), and may easily retake a site even after all adult knapweed are eliminated. Therefore, any methodologies which aim to eliminate knapweed from a site in favor of a native plant population should also include measures to eliminate multiple generation classes, not only the conspicuous adults (Carpinelli et al., 2004). Recent work suggests that reestablishing a diverse native plant community will provide additional resistance to reinvasion (Pokorny et al., 2005; Sheley and Half, 2006; Maron and Marler, 2007), but developing restoration strategies that include diverse native plant mixtures remains a major research priority (Maron and Marler, 2008). Our study site, the Bass River Recreation Area (located in Section 12, T7N R15W, Ottawa County, Michigan; 43 o 00'49" N, 86 o 01'47" W), has been used previously for a field experiment evaluating the viability of using native warm-season grasses as a first step to restoring a native plant community on a knapweed-infested site (MacDonald et al., 2003) and, more recently, for documenting the effectiveness of mid-spring burning in further transitioning the site from knapweed infestation toward native grassland (MacDonald et al., 2007). This project is thus a continuation of previous related work focused on the restoration of native Michigan plant communities in areas heavily invaded by spotted knapweed. Methods and Materials Initial Site Preparation Treatments: In July, 2008, we established a series of plots at the Bass River Recreation Area to initiate a new long-term research project at the study site. The site preparation methods completed in 2008 included mowing, alone and in combination with application of either clopyralid (Transline, 6 ml concentrate per plot on 8/19/2008) or glyphosate (Round-Up, 178 ml concentrate per plot on 9/7/2008) herbicides to control weed competition. Mowing alone represents a low-disturbance treatment that could be used in remnant natural areas prior to interseeding with desired native species (Packard and Mutel, 1997). Clopyralid is a selective herbicide that provides control of spotted knapweed and other broadleaf weeds in areas where desirable native grasses already exist, but it has residual soil effects that may inhibit establishment of certain native forbs, especially legumes. In comparison, glyphosate is a broad-spectrum herbicide that kills all pre-existing vegetation, has no residual soil activity and thus should not inhibit establishment of native forbs, but may allow more rapid resurgence of spotted knapweed (Sheley et al., 2001; MacDonald et al., 2003). 2

4 Experimental Design: The field experiment was configured in a randomized complete block design with a total of 12 treatment combinations (site preparation, hand pulling, and a future burning treatment) and four replications for a total of 48, 5 m 5 m plots. We incorporated hand pulling of adult knapweed as a full-plot factor in 2009, but burning will not be introduced as a treatment factor until after the initial establishment of native plants has occurred. Seed Bank Study: In late March, 2009, before spring germination of knapweed seed commenced, we collected seed bank samples from the upper 5 cm of soil of each plot using a 4.5-cm diameter metal corer (5 samples per plot, 398 cm 3 soil total). We also collected representative samples from 5 m 5 m plots in adjacent knapweed-infested areas (16 composite samples total, from four plots near each replicate block) to provide an untreated control comparison for the initial site-preparation treatments. Composite soil samples from each plot were placed on top of sterile potting soil in plastic pots. The pots were arranged in a greenhouse in a randomized block design similar to that of the field experiment, but also incorporating four of the untreated control samples in each block. The greenhouse was maintained at its normal temperature and lighting, and the samples were watered daily to encourage germination of all residual knapweed seeds. Germinated seedlings were counted and removed once a week from April through June, 2009, similar to the procedures employed by MacDonald et al. (2007). Soil Sampling, Seeding, and Site Maintenance: In early May, we took soil samples from all plots for baseline analyses of gravel content, ph, organic carbon, and soil texture. Samples were taken from the upper 15 cm of soil at four locations on each plot with a bucket auger. Samples were composited by plot, air-dried, and passed through a 2-mm sieve prior to analyses. Bulk soil samples were weighed prior to sieving, and the gravel fraction was weighed after sieving to determine percent gravel (% by weight). We analyzed all samples (n = 48) for ph (1:1 soil:water), organic C (H 2 SO 4 -K 2 Cr 2 O 7 oxidation), and texture (hydrometer). Laboratory methods followed standard soil analytical procedures (Klute, 1986; Page et al., 1982). Repeated measurement errors, based upon 17% replication, were 0.7% for ph, 5.1% for organic C, 1.5% for sand, 11.0% for silt, and 10.2% for clay. We obtained a diverse seed mix containing both native grasses and forbs from the Michigan Wildflower Farm (Portland, Michigan). Species in the seed mix included Indian grass, switchgrass, big bluestem, and little bluestem (~60% grasses) along with butterfly weed, sand tickseed, prairie coreopsis, tall coreopsis, catsfoot, western sunflower, rough blazing star, wild lupine, round-headed bush clover, wild bergamot, horsemint, yellow coneflower, black-eyed Susan, early goldenrod, gray goldenrod, showy goldenrod, goat s rue, common spiderwort, and hoary vervain (~40% forbs). This seed mix approximates the native species composition of southern Michigan dry sand prairies, dry-mesic prairies, and oak barrens, and is appropriate to the excessively drained sandy soils at the study site. We broadcast the seeds by hand at a rate of 23 kg ha -1 (20 lb acre -1 ) on all plots in mid-may, 2009, and lightly raked the seed in to improve soil:seed contact. We mowed the areas surrounding the plots in early July, 2009 to control spotted knapweed in the buffers between the plots and to facilitate plot access for subsequent treatments and measurements. Hand Pulling Treatment: We hand pulled and removed all adult (bolted) knapweed from the designated plots in early July, 2009 prior to knapweed seed set and dispersal. Plants were individually removed by hand using a common garden weed puller, and separated into tops 3

5 and roots. We determined the number of separate plants removed from each plot by counting the individual taproots, and dried and weighed the removed knapweed tops to estimate the annual reduction in mature aboveground knapweed biomass from these plots. We also pulled, counted, and weighed any additional mature knapweed observed on these plots in mid-july when followup vegetation evaluations were performed. Knapweed removal data (numbers and biomass) reported in this paper represent the totals removed by hand pulling in both early and mid July. Vegetation Evaluation: We sampled the study plot vegetation in mid-july, 2009 to determine the effects of site preparation and hand pulling treatments on the density of juvenile (seedlings plus unbolted rosettes) and mature knapweed, and to survey the presence of native species of grasses and forbs. Presence of native species and densities of juvenile and mature knapweed were determined from five randomly located 0.25-m 2 (50 cm 50 cm) subplots on each plot. Mature knapweed plants on non-hand-pulled subplots were clipped off at ground level, dried, and weighed to estimate spotted knapweed aboveground biomass on these plots. Statistical Analyses: Plant density and biomass data were converted to a m -2 basis to facilitate comparisons using a standard areal unit, and these data were statistically analyzed in that form. All data were analyzed using a balanced factorial design, including three initial site preparation treatments, two hand pulling treatments, and four replications, with a total of 48 independent sampling units in the experiment (the future burning treatment will have two levels). Data were tested for equality of variance with Bartlett s test (Steel and Torrie, 1980) and normality with Lilliefor s test using SYSTAT (version 4; Wilkinson 1989). Equal variances and normal distributions were not present for most of the plant data, so we used nonparametric, permutational multivariate analyses of variance (PERMANOVA, McArdle and Anderson, 2001, Anderson, 2005) to test for treatment effects in these situations as we have done previously (MacDonald et al., 2007). We used pairwise comparisons within PERMANOVA to identify significant differences among means. Analyses of variance were based on Bray-Curtis dissimilarities for knapweed density data and on Euclidean distances for biomass data (McArdle and Anderson, 2001). Probabilities are based on unrestricted permutation of raw data using 4,999 permutations for each analysis. Significance for all analyses was accepted at p < Results Soil Analyses: We determined the soils on all plots to be sands to loamy sands that were slightly alkaline, low in organic carbon, and contained fair amounts of gravel (Fig. 1). No statistically significant differences among treatment combinations with regards to soil composition were observed, which removes underlying variation in soil properties as a possible confounding factor when interpreting treatment effects. Seedbank Study: On average, 98% of all the knapweed seedlings counted germinated during the first month of the seedbank study (by the end of April, 2009). All three site preparation treatments tended (p = 0.10) to reduce the knapweed seedbank to below levels recorded from untreated control plots, with clopyralid having the greatest observed effect (Fig. 2) as compared to the untreated control plots. 4

6 Figure 1. Soil properties (mean + SD) of 48 plots at the Bass River Recreation Area, Ottawa County, Michigan. Data were collected via sieving and weighing (gravel, % of bulk soil weight), hydrometer method (texture, % of < 2mm soil weight), and chemical analyses (ph and organic carbon). Figure 2. Site preparation effects on spotted knapweed seedbank after one year at the Bass River Recreation Area, Ottawa County, Michigan. Error bars represent one standard error (n = 16) for each mean. Probability related to site preparation treatment main effect comparison p =

7 Treatment Effects on Knapweed and Native Plant Populations: Both clopyralid and glyphosate treatments dramatically reduced mature knapweed presence and biomass (Fig. 3 and 4), but only clopyralid also had a significant effect in reducing spotted knapweed juvenile numbers compared to mowing alone (Fig. 5). An average of 44 mature plants per square meter were removed from mowed plots designated for pulling (Fig. 3), which compared well to the 47 m -2 counted on unpulled mowed plots (Fig. 6). Only 3 plants m -2 needed to be removed from plots treated with glyphosate (Fig. 3), which also was consistent with the 2.5 plants m -2 counted on unpulled glyphosate-treated plots (Fig. 6). No mature spotted knapweed plants were removed from clopyralid-treated plots in 2009 as none were observed in either mid (Fig. 3) or late (Fig. 6) July. Pulling was as effective as glyphosate in reducing adult knapweed densities on mowed plots (Fig. 6), and potentially more effective than glyphosate at reducing juvenile numbers on mowed plots (Fig. 5). Planted native warm-season grass species successfully established to some extent on all treatment combinations, with an average of 21% of sampled subplots possessing germinated native warm-season grass seedlings of the species we had previously seeded. There were no significant differences among treatments in presence of the planted grasses. No planted native forbs were observed on any plots during the first year of this study. Discussion All initial site preparation treatments began to reduce the spotted knapweed soil seedbank, although the effects were not significant (p = 0.10). In our study, average seedbank densities on untreated control plots (417 m -2 ) were very similar to approximate seedbank densities on moderately infested, unburned remnant prairies in Michigan (estimated as half of annual seedfall to be 300 to 700 m -2, Emery and Gross, 2005). Effective prevention of knapweed resurgence on infested sites is very dependent on a reduction in seedbank densities through time (Carpinelli et al., 2004; Story et al., 2008), and some means of preventing reseeding from mature plants is needed to permanently reduce the soil seedbank. For example, MacDonald et al. (2007) found that knapweed seedbank densities after three years of annual burning were reduced to 52 m -2, similar to knapweed seed bank densities (32-42 m -2 ) reported by Davis et al. (1993) after seven years of experimental suppression of seed production using annual herbicide (2,4-D) treatments. In comparison, knapweed seedbank densities were reduced to 19 m years after introduction of seed head-attacking insects as a biological control measure in western Montana (Story et al., 2008). Without some type of continued suppression of reseeding from mature knapweed, soil seedbank densities are likely to return very quickly to the greater than 400 m -2 found on untreated plots in this study. As expected, hand pulling greatly reduced mature knapweed densities, but when we returned two weeks later to the site to sample for residual knapweed and seeded native presence, we still found bolted or flowering adult knapweed present in low densities on previously pulled plots. This was due to late flowering of some mature individuals, along with some expected error in attempting to pull every bolted knapweed on a large plot. Because of the presence of multiple age classes, we anticipate effectively controlling knapweed on the site will require additional years of hand pulling effort until both the existing knapweed population and the knapweed seedbank are depleted. In order to expedite the process, a more aggressive approach to remove the multiple age classes of knapweed including seedlings and unbolted rosettes rather than just the reproductive adults alone could be incorporated into a treatment or management plan using hand pulling as a primary control method. Previous reports suggest that persistent and careful 6

8 Figure 3. Total mature knapweed removals from hand-pulled plots one year following initial treatments at the Bass River Recreation Area. Error bars indicate one standard error (n = 8) for each mean. Different letters above bars indicate significant differences among the means (p < 0.05). Figure 4. Total knapweed biomass removals from hand-pulled plots one year following herbicide treatments at the Bass River Recreation Area. Error bars represent one standard error (n = 8) for each mean. Different letters above bars indicate significant differences among the means (p < 0.05). Figure 5. Residual spotted knapweed juveniles per square meter across all treatment combinations in mid July. Error bars indicate one standard error (n = 8) for each mean. Different letters above bars indicate significant differences among main effect site preparation means (p < 0.05). Figure 6. Residual spotted knapweed adults per square meter across all treatment combinations in mid July. Error bars represent one standard error (n = 8) for each mean. Different letters above Not Pulled bars indicate significant differences among those site preparation means (p < 0.05). 7

9 hand pulling can control knapweed (Sheley et al., 1998), but in the absence of parallel efforts to restore a diverse, invasion-resistant native plant community, even several years of hand pulling may not effectively reduce spotted knapweed cover on infested sites (Lutgen and Rillig, 2004). We found that initial treatments with either clopyralid or glyphosate were effective in reducing mature spotted knapweed densities, similar to effects reported by Sheley et al. (2000; 2001). We also found clopyralid most effective in reducing juvenile presence across all treatment combinations, while mowing and glyphosate had much lesser impacts. These results also are consistent with previous reports that the effect of glyphosate on suppressing spotted knapweed was less long-lived than that of clopyralid (Sheley et al., 2000; 2001). Even though we had only targeted mature knapweed plants for hand removal, hand pulling also appeared to reduce juvenile densities on mowed plots, but had no significant effect on juvenile densities on glyphosatetreated plots. Non-herbicided plots tended to have very high densities of mature individuals, many with large root systems which could produce juvenile clones from mature root stock. We identified each individual root system as one individual, so on pulled plots the clonal juveniles were removed along with the root stock from which they originated. However, on unpulled nonherbicided plots, it was difficult at times to differentiate a true individual juvenile from a clonal juvenile, and it is likely that some clonal juveniles were counted as individuals on the non-pulled plots. On glyphosate-treated plots, most of the mature knapweed removed by hand pulling were younger plants that had bolted for the first time, and thus tended to be single-stemmed plants as opposed to older plants with attached clonal juveniles. As noted before, many of the mature knapweed removed from mowed plots were well-established plants with multiple clonal juveniles attached to the primary rootstock, so hand-pulling would remove these juveniles along with the main bolted mature plant, and thus could result in an apparent reduction in juvenile as well as mature plant densities. Based on our results, hand pulling was as effective as the initial herbicide treatments in reducing mature knapweed numbers across all treatment combinations. But this does not tell us anything about the logistics of the strategies employed, which can be just as important as their effectiveness. We found hand pulling required a great deal of time and effort to clear heavily infested non-herbicided plots (6 to 8 person hours per 5 m 5 m plot), longer than we had first anticipated. The time and amount of labor required to treat an area should also be factored into the expense of hand pulling as a potential method for spotted knapweed removal, as either could be a prohibitive factor for certain operations. Lutgen and Rillig (2004) found that up to four years of hand pulling alone did not reduce spotted knapweed cover below that of an untreated control, while treatment with herbicides (picloram or clopyralid+2,4-d) combined with mowing or hand pulling reduced spotted knapweed cover significantly, often to very low levels. Our initial results are consistent with these observations, and taken together, suggest the need for a combination of chemical and mechanical treatments to provide the most effective control of spotted knapweed in efforts to restore or reestablish native plant communities on infested sites. Initial establishment of seeded native grasses was somewhat sparse, but ecosystem restoration is a long-term process, and full development of a native plant community is expected to take several years or more (Packard and Mutel, 1997). Initiation of burning may be required before competition from non-native species, including spotted knapweed, can be effectively controlled (MacDonald et al., 2007). Burning also may be needed to break the dormancy of and prepare an appropriate seedbed for some or all of the seeded native forbs (Packard and Mutel, 1997). Effective control and long-term suppression of spotted knapweed on previously infested sites will depend on the development of and competition from a diverse native plant community 8

10 (Pokorny et al., 2005, Sheley and Half, 2006, MacDonald et al., 2007; Maron and Marler, 2007; 2008). The long-term interactive effects of the initial site preparation treatments, hand pulling, and burning will provide additional insight into the most effective strategy for establishing such a diverse native plant community on degraded and knapweed-infested sites similar to the one at our study site. Management Implications Hand pulling can be a viable method of reducing mature knapweed densities, and we found it to be as effective as the non-selective herbicide glyphosate. Clopyralid, however, was even more effective at reducing mature plant densities, and was also effective at reducing densities of the juvenile age classes of spotted knapweed, probably as a result of its residual soil activity. Hand pulling could therefore be useful in removing mature knapweed from sensitive sites which have residual native plant diversity present that could be harmed by herbicide use. Pulling does, however, require a large investment of time and effort in order to achieve effects similar to herbicide treatment, and this limitation must be taken into account by any entities planning a strategy for restoring a knapweed-infested site. Groups with large volunteer corps or individuals with small areas they wish to restore would likely find pulling a viable method. To be effective, hand pulling needs to be applied consistently and persistently over several years until the multiple age classes of spotted knapweed, including the seedbank, are depleted (N.W. MacDonald, personal observation). Glyphosate appears to be most appropriate for initial site preparation treatments in areas that are heavily infested with spotted knapweed and lack a residual native plant population, and where a diverse native plant community will be reestablished from seed. Clopyralid may be most suitable for treatment of areas where remnant populations of native grasses and herbicide-resistant native forbs are already present and can respond rapidly to selective removal of spotted knapweed. Hand pulling offers promise as a follow-up to herbicide applications once a native plant community is reestablished through seeding or is released from knapweed competition, but where additional herbicide applications may need to be restricted to prevent damage to the redeveloping native plant community. The interaction of burning with these knapweed control treatments is not well understood, but is a planned component of this study and will be implemented as an additional treatment factor in the future. Acknowledgments We thank the Grand Valley State University Student Summer Scholars Program for providing funding and other support for the project, Glenn Palmgren and Charles Ehrlich of the Michigan Department of Natural Resources for permission to use the study site at the Bass River Recreation Area, David Hillger of Dow Agrosciences for providing the Transline herbicide used in this study, and Diane Laughlin, Star Santiago, and Jennifer Burmeister of the Grand Valley State University Biology Department for logistical support. 9

11 Literature Cited Abella, S.R Effectiveness of different management strategies for controlling spotted knapweed in remnant and restored prairies. Ecological Restoration 19: Anderson, M. J PERMANOVA: a FORTRAN computer program for permutational multivariate analysis of variance. Department of Statistics, University of Auckland, Auckland, New Zealand. Carpinelli, M.F., R.L. Sheley and B.D. Maxwell Revegetating weed-infested rangeland with niche-differentiated desirable species. Journal of Range Management 57: Davis, E.S., P.K. Fay, T.K. Chicoine and C.A. Lacey Persistence of spotted knapweed (Centaurea maculosa) seed in soil. Weed Science 41: Emery, S.M., and K.L. Gross Effects of timing of prescribed fire on the demography of an invasive plant, spotted knapweed Centaurea maculosa. Journal of Applied Ecology 42: Klute, A. (ed.) Methods of soil analysis. Part 1: Physical and mineralogical properties, 2 nd edition. American Society of Agronomy and Soil Science Society of America, Madison, Wisconsin. Lutgen, E.R., and M.C. Rillig Influence of spotted knapweed (Centaurea maculosa) management treatments on arbuscular mycorrhizae and soil aggregation. Weed Science 52(1): MacDonald, N.W., M.T. Koetje and B.J. Perry Native warm-season grass establishment on spotted knapweed-infested gravel mine spoils. Journal of Soil and Water Conservation 58: MacDonald, N.W., B.T. Scull and S.R. Abella Mid-spring burning reduces spotted knapweed and increases native grasses during a Michigan experimental grassland experiment. Restoration Ecology 15: Maron, J., and M. Marler Native plant diversity resists invasion at both low and high resource levels. Ecology 88: Maron, J.L., and M. Marler Field-based competitive impacts between invaders and natives at varying resource supply. Journal of Ecology 96: McArdle, B.H. and M.J. Anderson Fitting multivariate models to community data: a comment on distance-based redundancy analysis. Ecology 82: Packard, S., and C.F. Mutel (ed.) The tallgrass restoration handbook. Island Press, Covelo, California. 10

12 Page, A.L., R.H. Miller, and D.R. Keeney (ed.) Methods of soil analysis. Part 2: Chemical and microbiological properties, 2 nd edition. American Society of Agronomy and Soil Science Society of America, Madison, Wisconsin. Pokorny, M.L., R.L. Sheley, C.A. Zabinski, R.E. Engel, T.J. Svejcar, and J.J. Borkowski Plant functional group diversity as a mechanism for invasion resistance. Restoration Ecology 13: Rice, P.M., J.C. Toney, D.J. Bedunah and C.E. Carlson Plant community diversity and growth form responses to herbicide applications for control of Centaurea maculosa. Journal of Applied Ecology 34: Rinella, M.J., J.S. Jacobs, R.L. Sheley and J.J. Borkowski Spotted knapweed response to season and frequency of mowing. Journal of Range Management 54: Sheley, R.S., C.A. Duncan, M.B. Halstvedt and J.S. Jacobs Spotted knapweed and grass response to herbicide treatments. Journal of Range Management 53: Sheley, R.L., and M.L. Half Enhancing native forb establishment and persistence using a rich seed mixture. Restoration Ecology 14: Sheley, R.L., J.S. Jacobs, and M.F. Carpinelli Distribution, biology, and management of diffuse knapweed (Centaurea diffusa) and spotted knapweed (Centaurea maculosa). Weed Technology 12: Sheley, R.L., J.S. Jacobs, and D.E. Lucas Revegetating spotted knapweed infested rangeland in a single entry. Journal of Range Management 54: Steel, R.G.D. and J.H. Torrie Principles and procedures of statistics. 2nd edition. McGraw-Hill Book Company, New York. Story, J.M., L. Smith, J.G. Corn, and L.J. White Influence of seed head-attacking biological control agents on spotted knapweed reproductive potential in western Montana over a 30-year period. Environmental Entomology 37(2): Wilkinson, L SYSTAT: the system for statistics, version 4. SYSTAT Inc., Evanston, Illinois. 11