Leafy spurge (Euphorbia esula L.) is an invasive

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1 The Saga of Leafy Spurge (Euphorbia esula) in the Northern Great Plains MONTANA Leafy spurge (Euphorbia esula L.) is an invasive Eurasian perennial introduced into the United States as a contaminant of crop seed in the 1880s and 1890s. It typically forms monocultures in rangeland and natural areas of the northern Great Plains where, because of the latex that occurs in all parts of the plant, it is not consumed by naturally occurring herbivores. U.S. Geological Survey (USGS) scientists and their collaborators have been studying leafy spurge at Theodore Theodore Roosevelt National Park NORTH DAKOTA Tewaukon National Wildlife Refuge SOUTH DAKOTA Arrowwood National Wildlife Refuge MINNESOTA Roosevelt National Park (TRNP) and at Arrowwood and Tewaukon National Wildlife Refuges in North Dakota since Study findings have been published in Larson and Grace (2004), Larson and others (2006), Larson and others (2007), Jordan and others (2008), and Larson and others (2008). This fact sheet summarizes that body of research. Ecological Effects Because leafy spurge excludes other prairie plants (Belcher and Wilson, 1989), we were interested in understanding the mechanisms that allowed its dominance. We examined both above-ground (pollination) and below-ground (plant-soil feedback) processes. Pollination Although invasive plants commonly compete with native plants for light and nutrients, they also may compete for pollinator services by attracting pollinators away from native flowers. On the other hand, a dense stand of flowering invasive plants may attract more pollinators to the area and, in the process, enhance pollination of neighboring natives. If insects are carrying a mixed pollen load when they visit native flowers, however, they may deposit pollen of other species, rather than the pollen necessary for native flower reproduction. Leafy spurge provides large amounts of nectar and pollen in easily accessed flowers (cyathia) over a 4- to 6-week period in the spring, and sometimes for a shorter period in the fall at TRNP (Larson and others 2006). Its abundance is much greater than that of the native flowering plants in the vicinity of an infestation, which led us to ask if spurge had an effect on pollination of native plants. Specifically, we wondered (1) does visitation to native flowers by pollinator insects vary between infested and noninfested sites, (2) does the amount of conspecific pollen (pollen from the same species as the recipient) on stigmas vary between infested and noninfested sites, and (3) does infestation influence the number of pollen species and proportion of heterospecific pollen (pollen from different species than the recipient) on native plant stigmas? We found that although infestation by leafy spurge had an effect on pollinator visitation, the effect was neither consistent among species nor between years (Larson and others 2006; fig. 1). Nonetheless, there were fewer conspecific pollen grains on stigmas of native species within spurge infestations, and this did not vary between the 2 years of our study. We did not find a greater number of pollen species on native stigmas within infestations, likely because species diversity was depressed in these areas (Larson and others, 2006). Further studies of how the observed conspecific pollen deficit influences seed set for native plant populations are needed before firm conclusions can be drawn about how pollinator-mediated effects of leafy spurge influence native plant populations. Plant-Soil Feedback Accumulating evidence suggests that substantial interactions between invasive plants and soil biota may influence the outcome of invasion events. A common way of investigating these interactions is through plantsoil feedback experiments: sterilized and live soils are conditioned by repeatedly growing either invasive or native species in them over several growth cycles. The U.S. Department of the Interior U.S. Geological Survey Printed on recycled paper Fact Sheet August 2009

2 2.5 Campanula rotundifolia (bluebell) Linum lewisii (Lewis blue fax) Oxytropis lambertii (purple locoweed) Euphorbia esula (leafy spurge) EXPLANATION Bees Bumblebees NATURAL LOG [(VISITS PER 20 MINUTES) + 1] Flies Butterflies Wasps Infestation: No = not infested by leafy spurge; Yes = infested by leafy spurge Infestation YEAR No Yes No Yes No Yes No Yes No Yes No Yes Yes Yes Figure 1. Visits by pollinator taxa to native plant species and leafy spurge over 2 years. Native plant species are Campanula rotundifolia, Linum lewisii, and Oxytropis lambertii. performance of native species in the variously conditioned soils is. When we performed such an experiment with leafy spurge and a suite of native species common to the mixed-grass prairies of North Dakota (Jordan and others, 2008), we found that soils conditioned by leafy spurge had a negative effect on the growth of several native forb species, and further that this effect was mediated by soil biota living in unsterilized soil (fig. 2). This effect could help explain how leafy spurge becomes dominant, and may also help us develop strategic restoration plans that use species tolerant of changes in soil biota remaining even after an infestation has been controlled. Biological Control The biological control program for leafy spurge in the northern Great Plains is largely considered successful by land managers in the region (Hodur and others, 2006). Research by USGS scientists has demonstrated that the black flea beetles, Aphthona lacertosa/czwalinae (the two species cannot be distinguished in the field), have been effective in reducing populations of leafy spurge at TRNP (Larson and Grace, 2004) and at Arrowwood and Tewaukon National Wildlife Refuges (Larson and others, 2008). Integrated pest management, in which more than one management tool is applied in combination, also has been used in efforts to control leafy spurge. Our research indicates that if a successful biological control program has been established, addition of herbicides may produce quicker reductions in spurge populations but may also disrupt the interactions between flea beetles and leafy spurge, reducing the effectiveness of the biological control organisms (Larson and others, 2007). Herbicide effects were short lived; spurge populations 2 to 3 years after herbicide treatment were indistinguishable from those that had only biological control. Our results therefore suggest that managers consider the need for rapid control versus the potential disruption of a biological control program before deciding to use herbicides in situations where biological control is effective.

3 GRAMS PER INDIVIDUAL (LEAST SQUARE MEANS) Euphorbia esula (leafy spurge) Bouteloua gracilis (native grass) Aster ericoides (native forb) IU NU IS NS TREATMENT Agropyron cristatum (invasive grass) Koeleria pyramidata (native grass) Linum lewisii (native forb) IU NU IS NS TREATMENT Bromus inermis (invasive grass) Stipa viridula (native grass) Ratibida columnifera (native forb) IU NU IS NS TREATMENT Figure 2. Seedling biomass (mass of individual seedlings in a given area) of invasive and native species in response to soil conditioning and soil sterilization treatments. Soils were collected at Theodore Roosevelt National Park and conditioned by E. esula (leafy spurge) or native species (modified from Jordan and others, 2008). EXPLANATION IU invasive conditioning with unsterilized soil NU native conditioning with unsterilized soil IS invasive conditioning with sterilized soil NS native conditioning with sterilized soil Restoration of Previously Infested Mixed- Grass Prairie Native prairie species do not always return after leafy spurge declines. To investigate why this occurs and to try to overcome the problem, we have identified three types of sites in the south unit of TRNP: sites that were never infested by leafy spurge, sites that were previously infested but are now dominated by native species, and sites that were previously infested but are now dominated by exotic species. Ordination (fig. 3) clearly illustrates that plots previously infested by leafy spurge but now dominated by native species are very similar to those plots that were never infested. In contrast, plots that were previously infested by leafy spurge but are now dominated by other exotic species show little overlap with never infested plots. The presence of native species in the seedbank has ruled out propagule availability as the reason for lack of native recruitment (D. Haines, unpublished data, 2009), so the question becomes, what allows native recruitment in some areas but not in others? Our current work addresses this question in several ways: (1) we have done experimental plantings in the previously infested sites to see if differences in performance of native species we observed in the greenhouse are evident in the field; (2) collaborators in the

4 Agricultural Research Service (U.S. Department of Agriculture) are assessing pathogen loads on roots of our planted species; (3) collaborators at the University of Minnesota are assessing arbuscular mycorrhizal fungi communities associated with the planted species and leafy spurge; and (4) a graduate student at the University of Minnesota is examining effects of spurge root exudates (compounds released from roots into the soil) on native prairie species germination and growth. Our goal is to provide land managers with information that will assist them in developing successful restoration strategies for these degraded sites. AXIS 3 EXPLANATION Vegetation Never infested Previously infested, now native Previously infested, now exotic Partners and Collaborators Paulette Scherr, U.S. Fish and Wildlife Service, Arrowwood National Wildlife Refuge Kristine Askerooth, U.S. Fish and Wildlife Service, Tewaukon National Wildlife Refuge Nick Jordan, Sheri Huerd, Jennifer Larson, and Dustin Haines, University of Minnesota Tony Caesar and TheCan Caesar, U.S. Department of Agriculture, Agricultural Research Service Paula Andersen and Laurie Richardson, National Park Service, Theodore Roosevelt National Park Ron Royer and Margaret Royer, Minot State University References Cited Belcher, J.W., and Wilson, S.D., 1989, Leafy spurge and the species composition of a mixed-grass prairie: Journal of Range Management, v. 42, no. 2, p Hodur, N.M., Leistritz, F.L., and Bangsund, D.A., 2006, Biological control of leafy spurge utilization and implementation: Rangeland Ecology & Management, v. 59, issue 5, p Jordan, N.R., Larson, D.L., and Huerd, S.C., 2008, Soil modification by invasive plants effects on native and invasive species of mixed-grass prairies: Biological Invasions, v. 10, no. 2, p Larson, D.L., and Grace, J.B., 2004, Temporal dynamics of leafy spurge (Euphorbia esula) and two species of flea beetles (Aphthona spp.) used as biological control agents: Biological Control, v. 29, issue 2, p AXIS 2 Figure 3. Ordination (by nonmetric multidimensional scaling analysis) of plots with respect to plant species at Theodore Roosevelt National Park. Axis 2 accounts for 37 percent and axis 3 accounts for 28 percent of the variation in the data. Vegetation on the plots was the result of natural processes; no restoration was done. Larson, D.L., Grace, J.B., and Larson, J.L, 2008, Long-term dynamics of leafy spurge (Euphorbia esula) and its biocontrol agent, flea beetles in the genus Aphthona: Biological Control, v. 47, issue 2, p , doi: /j.biocontrol Larson, D.L., Grace, J.B., Rabie, P.A., and Andersen, P., 2007, Short-term disruption of a leafy spurge (Euphorbia esula) biocontrol program following herbicide application: Biological Control, v. 40, issue 1, p Larson, D.L., Royer, R.A., and Royer, M.R., 2006, Insect visitation and pollen deposition in an invaded prairie plant community: Biological Conservation, v. 130, issue 1, p By Diane L. Larson For more information, contact: Director U.S. Geological Survey Northern Prairie Wildlife Research Center th Street SE, Jamestown, ND Telephone: World Wide Web:

5 Tallgrass Prairie Restoration Seeding for Success Tallgrass prairie is one of the most imperiled ecosystems on Earth. A 2004 estimate indicated that only 2.4 percent of the original northern tallgrass prairie remained in the United States ( digitalcommons.unl.edu/usgssta_pub/45). If tallgrass prairie and the species dependent on it are to survive, management must include restoration of cropland and degraded prairies, in addition to preservation of the few remaining fragments. Despite the importance of restoration and its long history (the first tallgrass prairie restoration was started in 1935 at Curtis Prairie in Wisconsin), few studies have been undertaken with the goal of refining restoration practice (Rowe, 2010). This fact sheet contains the results of one such study, started in 2005, in which we compared three seeding methods (dormant-season broadcast, growing-season broadcast, and growing-season drill) fully crossed with low (10-), medium (20-), and high (34-species) seed mixes replicated 12 times on each of 9 former agricultural fields in Minnesota and Iowa (fig. 1). Plots were 12.2 x 12.2 meters (m) and occupied about 1.6 hectares (ha) (4 acres) of each field. A successful restoration is one in which and richness of planted species is maximized and of exotic and invasive species, especially the noxious weed Canada thistle (Cirsium arvense), is minimized. Details of the planting methods can be located in Larson and others (2011). Do Cover and Richness of Planted and Exotic Species Vary with Planting Method? Only one species in Minnesota and five species in Iowa failed to establish [see table 1 in Larson and others (2011) for complete species list and percent establishment]. As can be seen in figure 2, broadcasting seed during the growing season was never the best strategy. In Minnesota, broadcasting during the dormant season especially was favorable for forbs and was never detrimental to other planted functional s. In contrast, drilling seed during the growing season was the best strategy in Iowa, although planted legume suffered. Richness of planted species increased with the richness of the seed mix planted in all cases in Minnesota and Iowa. Bottom line if using a broadcast method, plant during the dormant season; if planting during the growing season, using a seed drill will produce better results. Can We Design a Seed Mix to Target a Particular Invasive Species? Many studies, including some in tallgrass prairies, have shown that plots with higher levels of species richness are more U.S. Department of the Interior U.S. Geological Survey Figure 1. MINNESOTA Fergus Falls Wetland Management District Meadows Heinola Diekmann Fahl Morris Wetland Management District Meeker Kandiyohi Litchfield Wetland Management District IOWA Neal Smith National Wildlife Refuge Harmison Orbweaver Production Study site location in Minnesota and Iowa. resistant to invasion than are plots with lower species richness (Middleton and others, 2010). The reason is that the presence of a greater number of functional s (for example, cool-season and warm-season grasses, annual and perennial forbs, legumes) will more effectively use resources, leaving less available for invaders. In addition, it has been shown experimentally that species more functionally similar to a particular invasive species will be more likely to exclude the invader (Fargione and Tilman, 2005). For example, a species that is actively growing and taking up nutrients in the spring may compete better with an invasive species that is trying to secure nutrients at the same time; however, neither of these mechanisms has been demonstrated in an operational prairie restoration setting. We used the restoration experiment described above to do just that (Larson and others, 2013). Only the six fields in Minnesota were used for this study due to lack of Canada thistle establishment in the Iowa fields. Having previously determined that planted species richness increased with seed mix richness, we asked if seed mix richness was negatively associated with of Canada thistle. There was no evidence that seed mix richness had any effect on of Canada thistle at our study sites. Not only was there no association with seed mix richness, Canada thistle was unrelated to planted species richness on plots in 2007 and 2010 (Larson and others, 2013). To evaluate the effects of functional similarity, species were separated into planted and nonplanted functional s as displayed in figure 3. Species in the same family as Canada thistle (Asteraceae) were expected to be more likely to have negative effects on Canada thistle ; however, this did not turn out to be the case, at least for the planted asters. Instead, locations that were suitable for planted asters also were suitable for Canada thistle, producing a positive association in the first year after planting. Perennial and annual/biennial asters that arose from the seedbank, however, did tend to have a negative effect on Canada thistle in later years (that is, a lag effect). Because of this time lag, we suspect the effects may be related to allelopathy rather than simple spaceoccupancy. Many of the nonplanted asters are weedy species that are invasive in their non-native range. For example, annual ragweed (Ambrosia artemisiifolia) has been shown to have allelopathic effects on Canada thistle in greenhouse trials (Perry and others, 2009). Planted cool-season grasses (primarily in the genus Elymus, wild rye) had consistent negative associations with Canada thistle early in the study, but a prescribed fire at all sites in spring 2009 may have weakened these short lived grasses and allowed Canada thistle Fact Sheet July 2013

6 Planted species richness Total planted Planted forb Planted legume Planted coolseason grass Planted warmseason grass Total exotic Dormant broadcast Growing-season broadcast Growing-season drill Minnesota Iowa Minnesota Iowa Minnesota Iowa Planted asters (same family as thistle) Planted nonasters forbs Planted coolseason grasses Planted warmseason grasses asters (same family as thistle) annual grasses annual/biennial forbs (some in aster family) legumes Canada thistle in 2006 in 2006 Canada thistle in 2007 in 2006 (lag) in 2007 in 2006 (lag) Canada thistle in 2010 in 2007 (lag) in 2010 Figure 2. Direction of effects of planting methods on planted species richness, total planted, of planted functional s, and total exotic five years after planting. Green arrows indicate that the method produced the most favorable response of the three, red arrows indicate that the method produced the least favorable response, and blue bars intermediate or no difference in the response variable. to encroach in Planted warm-season grasses, however, were beginning to show negative effects on Canada thistle in Yellow sweetclover (Melilotus officinalis) invaded some of the study sites and reduced Canada thistle in 2007 and Bottom line these results suggest that early, robust establishment of native species, whether they are planted or not, is important for reducing invasion by Canada thistle at the beginning of tallgrass prairie restorations. similarity, on the other hand, was not a good predictor of effect on Canada thistle. Seasonal death of annual grasses and weakening of cool-season grasses by a spring burn appeared to allow encroachment of Canada thistle. Therefore, management actions that reduce should be applied with caution when Canada thistle is a concern. Long-lived cool-season native grass species may have a more pronounced long-term suppressive effect on thistles than shorter lived species such as Canada wild rye (Elymus canadensis). References Cited Fargione, J. and Tilman, D., 2005, Niche differences in phenology and rooting depth promote coexistence with a dominant C-4 bunchgrass: Oecologia, v. 143, p Larson, D.L., Bright, J.B., Drobney, P., Larson, J.L., Palaia, N., Rabie, P.A., Vacek, and Wells, D., 2011, Effects of planting method and seed mix richness on the early stages of tallgrass prairie restoration: Biological Conservation, v. 144, p. 3,127 3,139. Figure 3. Relationship between functional s in 2006, 2007 and 2010 on of Canada thistle in 2006, 2007 and Note that plants growing in 2006 may have a direct effect on Canada thistle in 2006, but also a lag effect on thistle in 2007 and 2010 (see text). Pluses indicate a positive association, minuses a negative association; -/+ indicates that planting methods had different results. Empty cells indicate that there was no relationship between the functional and Canada thistle. The yellow rows distinguish functional s that are most similar to Canada thistle. Larson, D.L., Bright, J.B., Drobney, P., Larson, J.L., Palaia, N., Rabie, P.A., Vacek, S., and Wells, D., 2013, Using prairie restoration to curtail invasion of Canada thistle The importance of limiting similarity and seed mix richness: Biological Invasions, accessed March 2013, at Middleton, E.L., Bever, J.D., and Schultz, P.A., 2010, The effect of restoration methods on the quality of the restoration and resistance to invasion by exotics: Restoration Ecology, v. 18, p Perry, L.G., Cronin, S.A., and Paschke, M.W., 2009, Native crops suppress exotic annuals and favor native perennials in a greenhouse competition experiment: Plant Ecology, v. 204, p Rowe, H.I., 2010, Tricks of the trade Techniques and opinions from 38 experts in tallgrass prairie restoration: Restoration Ecology, v. 18, p Samson, F.B., Knopf, F.L., and Ostlie, Wayne, 2004, Great Plains ecosystems Past, present, and future: Wildlife Society Bulletin, v. 32, no. 1, p. 6 15, accessed May 2011, at edu/usgsstaffpub/45/. For further information, contact: Director, U.S. Geological Survey Northern Prairie Wildlife Research Center th Street Southeast Jamestown, North Dakota