Choosing Native Species for Restoring Crested Wheatgrass Fields on the Great Plains of Northeast Montana.
Hundreds of thousands of hectares of public land in eastern Montana, U.S.A. are retired agricultural fields planted with non-native crested wheatgrass (Agropyron cristatum) to prevent soil erosion in the late 1930's during or shortly after the great drought of the 20th Century. These fields have low plant species diversity and lack the shrubs and forbs lo support strong populations of pronghorn antelope (Antilocapra americana), greater sage grouse (Centrocercus urophasianus) and Baird's sparrow (Ammodramus bairdii) as well as many other species compared to native grasslands or sagebrush steppe (Reynolds and Trost, 1980; Urness, 1986; Rotder et al., 2015). For example Lloyd and Martin (2005) found nestlings of chestnut-collared longspurs grew more slowly and had a 17% lower chance of surviving in crested wheatgrass fields compared lo native grasslands of northeast Montana. Restoring native plant diversity to these fields lo increase wildlife and pollinator diversity is a goal of land managers (Bakker and Wilson, 2004; Dumroese et al., 2016).
Most crested wheatgrass fields on the Great Plains have resisted significant invasion by native species and are still virtual monocultures, even after eight decades (McHenry and Newell, 1947; Smoliak et al., 1967; Looman and Heinrichs, 1973; Wilson, 1989), indicating that it is a formidable competitor (Christian and Wilson, 1999; Jordan et al, 2008). Nonetheless, some native species are better able to coexist with crested wheatgrass than others (Gunnell et al, 2010). Most attempts to return native species diversity to crested wheatgrass pastures either on the Great Plains or in the Great Basin have had little success because, even after cultivation or herbicide treatments, crested wheatgrass returns from a strong seedbank (Ambrose and Wilson, 2003; Wilson et al., 2004; Fansler and Mangold, 2011; Pehrson and Sowell, 2011; McAdoo et at., 2017). Therefore, native species that can persist with crested wheatgrass over (lie long term would be desirable for restoration, but have not previously been identified.
Crested wheatgrass has been successfully planted on a wide variety of soils (Knowles and Buglass, 1980). Soil type has a strong influence on vegetation (Trudgill, 1988) and is an important factor determining restoration outcomes (Baer et al., 2010; Abella et al., 2015). The ability of native species lo reestablish in crested wheatgrass fields may depend on soil texture, and edaphic differences are one possible explanatory variable for differential rates of secondary succession in crested wheatgrass fields.
Our study look advantage of an inadvertent experimental design begun ca. 80 v ago when fields were broken and planted with crested wheatgrass in a mosaic of native grassland. Our objective was to document which native species were most successful at reestablishing and persisting in crested wheatgrass fields and whether their ability to establish and persist depended on soil properties. Our results could help managers identify which native species should be planted in northeast Montana and possibly other parts of the Great Plains in order to promote a more diverse native composition.
MATERIALS AND METHODS
The semi-arid grasslands of eastern Montana were homesteaded in the early 1900s, during which time hundreds of thousands of hectares of native grassland were plowed and planted with wheat. During the great drought of 1928-1937 there was a mass exodus from the region and much of the farm fields were abandoned. The Agricultural Adjustment Act of 1933 and the Bankhead/Jones Act of 1937 authorized the U.S. Department of Agriculture to purchase these abandoned homesteads (Land Utilization or LU lands), and the Soil Conservation Service was given the responsibility of stabilizing the soil and returning the land to productivity (Lorenz, 1986). This stabilization was accomplished with crested wheatgrass, first introduced into the U.S. in 1898 (Roglerand Lorenz, 1983; Lorenz, 1986). The crested wheatgrass cultivar, Fairway, was developed in Canada at the same time as a similar selection in Montana. It is a diploid and can be referred to Agropyron cristatum var. pectinatum (M. Bieb.) Roshev. ex B. Fedtsch. (Dewey, 1986). Crested wheatgrass has been planted on six to 10 million ha in the Northern Great Plains (Lesica and DeLuca, 1996). Fairway was the only cultivar in mass production at the time and was used almost exclusively in reclaiming Montana I.U lands in the late 1930's. Our study sites were all located on these LU lands.
The Bureau of Land Management (BLM) provided us a list of potential study sites located on the Great Plains in northeast Montana, east of the Rocky Mountains, between the towns of Fort Benton on the northwest and Roundup on the southeast. Each site encompassed a crested wheatgrass field and adjacent native grassland on similar soil and topography and in the same grazing pasture. We randomly selected 21 BLM sites for sampling, stratified by soil texture classes presented in existing soil surveys, in addition to three sites on private land representing sandier soils (Table 1). Elevations range from 885 ill to 1190 m. Climate across the area is continental with cold winters, hot summers and most rainfall between April and September. Mean January minimum and July maximum temperatures at Fort Benton are -13 C and 30 C respectively, while mean January minimum and July maximum temperatures at Roundup are -11 C and 32 C respectively. Fort Benton receives an annual average of 353 mm precipitation, and Roundup receives 319 cm. January through June precipitation was 19 mm and 102 mm above average for Fort Benton and 6 mm and 114 mm above average for Roundup for 2010 and 2011 respectively.
Native grasslands adjacent to crested wheatgrass fields were mixed-grass prairie dominated by Agropyron smithii, Agropyron dasystachyum, Stipa comata, Poo secunda and Bouteloua gracilis. Wyoming big sagebrush (Artemisia tridentata ssp. uyomingensis) was common at several of the sites. Common forbs included Achillea millefolium, Allium textile, Selaginella densa, Taraxacum spp. and Vicia americana. Vascular plant nomenclature follows Lesica (2012). To the best of our knowledge none of the study sites had any recent wild or prescribed fire or human-caused disturbance other than livestock grazing since being planted with crested wheatgrass.
Our study was conducted in June of 2010 and 2011, during the height of the growing season when species had emerged above ground and were identifiable. We located two representative circular, 0.8 ha macroplots (50 m radius) at each site. The "native" macroplot was dominated by native vegetation in ground that was never tilled within 50 m of the crested wheatgrass field. The "crested" macroplot was in the crested wheatgrass field within 50 m of the plow line marking the edge of the native vegetation. In all but three cases, the centers of these two macroplots were within 150 m of each other (mean = 114 m). The close proximity of the two macroplots indicates that a native plant's abundance in the crested macroplot will depend mainly oil its ability to coexist with crested wheatgrass, not on its dispersal ability.
We located four 0.02 ha sample plots (8 m radius) within each macroplot, one on each principal compass bearing at randomly-chosen distances from the macroplot center (Fig. 1). We estimated canopy cover of all vascular plants observed in each sample plot using the following cover classes: trace- (T-) = <0.05%, T = 0.5%, T+= 0.6-0.9%, 1%,2%,5%, 10%, 15%, etc. We located three 1-[m.sup.2] microplots in each sample plot 4 m from the center, one on each of three evenly-spaced, randomly-chosen radii. This provides 12 microplots per macroplot. The number of individuals of all native perennial species was counted in each microplot. Individual stems or clumps of stems were counted for rhizomatous species. We did not attempt to quantify Selaginella densa in density microplots given individual ramets and/or genets cannot be distinguished. We attempted to distinguish western wheatgrass (Agropyron smithii) from thick-spike wheatgrass (Agropyron dasystachyum), but the two species can be very similar morphologically (Lavin, 2012), and we may have erred occasionally.
We collected three soil samples from haphazardly-chosen locations near the plot center in the native macroplot and near the center of the crested wheatgrass field. We collected soil from the top 12 cm after discarding the surface organic layer. The three samples from each location were combined and analyzed for particle-size distribution using a modified Bouvoucos hydrometer method (Day, 1965). Soil pH and conductivity (a measure of salinity) were measured on each combined sample using hand-held Oakton Acorn pH 5 and ECTestr 11 meters.
We measured the abundance of native perennial plants in crested macroplots in order to determine which species are best able to coexist with crested wheatgrass. There are three non-destructive measures of species abundance commonly used in describing vegetation: frequency, density and canopy cover (i.e., dominance) (Curtis, 1959; Mueller-Dombois and Ellenberg, 1974; Greig-Smith, 1983). For each species in our study, frequency is the number of crested macroplots in which the species occurred in at least one microplot, providing a measure of the plants ecological amplitude across the entire study area. Species density is the mean across crested macroplots of the number of individuals per 1-[m.sup.2] averaged over the 12 microplots in each of these macroplots. Canopy cover is the mean of the midpoints of each of the four sample-plot cover-class estimates for each crested macroplot. Only sites at which a species occurred in native and/or crested microplots were included to calculate mean density and canopy cover for that species.
In order to help determine the best species to use for restoration we require measures of how well each species coexisted with crested wheatgrass relative to others. To this end we calculated relative measures of frequency, density and canopy cover as follows:
relative frequency of species A = frequency of species A x 100/sum of frequencies of all species 
relative density of species A = mean density of species A x 100/sum of mean densities of all species 
relative dominance of species A = mean canopy cover of species A x 100/sum of mean canopy cover of all species 
We added these three measures to obtain an Importance Value (IV) as proposed by Curtis (1959) and widely used in vegetation ecology (Mueller-Dombois and Ellenberg, 1974; Greig-Smith, 1983). Only crested macroplots in which a species occurred at least once in the adjacent native macroplot were used in calculations of relative density and dominance. Many of the native species encountered in our study were rare across and within macroplots and did not provide enough data to be evaluated for appropriateness as restoration species. Only a subset of native species occurring in density microplots in at least two crested macroplots with a mean density [greater than or equal to] 0.04/[m.sup.2] were judged to be common enough to provide adequate data to be included in analyses.
We used principal components analysis (PCA) to explore the relationship among soil variables. Paired-sample t-tests were used to compare soil variables between native vegetation and adjacent crested wheatgrass fields across all sites. Statistical significance was assigned at P [less than or equal to] 0.05; P-values were not adjusted for multiple comparisons (Stewart-Oaten, 1995). We use linear regression analysis to assess the association between canopy cover of native species and the soil. Analyses were performed using SYSTAT 13 (Systat Software Inc.. Chicago, Illinois, U.S.A).
SOILS AND BIG SAGEBRUSH CANOPY COVER
There were strong correlations among all soil variables except salinity. The first principal component in the crested wheatgrass soil PCA explained 65% of the variation in the five soil variables and had strong positive loadings by % sand (0.97) and pH (0.75) and strong negative loadings by % silt (--0.93) and % clay (-0.85). Percent sand in soils of crested wheatgrass fields was bimodally distributed with 50% the low point between the two modes. We used these results to classify' soils into two groups. Soils with <50% sand were classified as "heavy;" these include soils classified as clay, clay loam and loam. Soils with >50% sand were classified as "light" and included sandy loam and loamy sand. There were no consistent differences between soils supporting native vegetation and crested wheatgrass across all site pairs (n = 24) for pH (P = 0.64), electrical conductivity (P = 0.21),% sand (P = 0.78), % silt (P = 0.12), or % clay (P = 0.34).
Canopy cover of big sagebrush in native macroplots varied from 0 to 29% across the study sites (mean = 13%) and was greater than 1% at 18 of 24 sites. Half of the six sites lacking big sagebrush were on heavy soil and half on light soil. There was no difference in mean big sagebrush canopy cover between the two types of soil where it occurred in native macroplots (!' = 0.71). Correlations between big sagebrush cover and soil variables in native vegetation were all weak (r < 0.15). There were no strong correlations between big sagebrush canopy cover in crested macroplots and any crested wheatgrass field soil variables (r [less than or equal to] 0.22).
NATIVE SPECIES AND CRESTED WHEATGRASS
We recorded 191 species of vascular plants (not including Selaginella densa) in the macroploLs across all sites. Of these 152 were native perennials. Forty-one species of native perennials occurred in microplots of both native and crested macroplots at one or more sites. However, only 29 of these native species occurred in density microplots in two or more crested macroplots with a mean density [greater than or equal to]0.04/[m.sup.2] and were, therefore, included in subsequent analyses. Importance value, frequency, mean canopy cover and mean density for these 29 species are provided in Table 2.
Abundance of big sagebrush as well as the common grasses Pun secunda, Agropyron smithii, Koelaria macrantha, Stipa comata and Bouteloua gracilis were approximately equal across the soil types. The forbs Vicia arrmicana, Sphaeralcea coccinea and Gaura coccinea were also common across all soils. However, Lomatium foeniculaceum, Iva axillaris, Psoralen argophylla and Antennaria parvifolia were more common on heavy soils, while Astragalus missouriensis and Allium textile were better represented on light soils (Table 2).
Choosing which species to plant is an important decision when planning the restoration of fields after crested wheatgrass has been reduced with cultivation or herbicide. Our study identified 29 species of native plants that persisted with densities above 0.04/[m.sup.2] in crested macroplots from at least two of the crested wheatgrass fields studied (Table 2). Because crested macroploLs were in close proximity to a source of native seed, we suggest that the differences in abundance among these 29 species were due to their ability to coexist with crested wheatgrass and not to differential dispersal.
Big sagebrush and several grass species generally were more successful reinvading and persisting (higher importance values) than most forbs, primarily because forbs overall had lower canopy cover at our study sites (Table 2). However, forbs are needed for functional diversity and to support insects such as pollinators (Dumroese el al,, 2016) as well as sage grouse (Klebenow and Gray, 1967; Peterson, 1970).
We found some evidence that ability of native species to persist in crested wheatgrass fields differed with soil type. These differences may be important when selecting forbs for a seed mix. We expect that we would have found more differences in IVs had we sampled sites with soils having >80% sand. Importance Values are meant to integrate three measures useful for choosing restoration species: frequency, density and cover. In most cases we predict that species with higher IV will persist better with crested wheatgrass in restoration efforts; however, there may be exceptions. Green needlegrass (Stipa viridula) had a relatively low IV, but was successful persisting with crested wheatgrass on sandy loams 200 km southeast of our study area (McWilliams and Van Cleave, 1960). It may also be useful to examine the three measures separately when choosing species. More common species are those able to establish in a wider range of ecological settings. Large species with higher canopy cover often have greater biomass but lower densities, while small plants can occur at higher densities but have lower biomass. On light soils, Cerastium arvense and Symphothrichum falcalum had high IVs because of high densities al the only site where they occurred. Their low frequency in light-soil macroplots should be taken into consideration when planning seed mixes.
Our study used systematic observations of native plant persistence in crested wheatgrass fields to inform restoration tactics for these fields. Based on our data, good candidates for seed mixes across all soil types were ihe shrubs in the genus Artemisia, the grasses Agropyron smithii, Stipa comata, Poa secunda and Bouteloua gracilis, and forbs Vicia americana, Sphaeralcea coccinea and Gaura coccinea. Eomatium foeniculaceum, Iva axillaris, Psoralen argophylla are expected to do well on heavy soils, while Astragalus missouriensis and Allium textile are more appropriate for fields on lighter soils. Our results provide information for restoration in northeastern Montana but may be less informative elsewhere. Our approach can be applied more generally throughout ihe Great Plains wherever crested wheatgrass has been abundantly planted.
Acknowledgments.--Our study was funded by the Montana Bureau of Land Management (BLM). Adam Carr, Dan Brunkhorst, Mike Barrick and Dustin Crowe of BLM identified potential study sites. We are grateful to the many ranch families that allowed us to conduct our study on their grazing allotments. Bill and Dana Milton allowed us to collect data on their ranch. Winsor Lowe and anonymous reviewers provided helpful comments on the manuscript.
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PETER LESICA and STEPHEN V. COOPER, Conservation Biology Research, 929 Locust Street, Missoula, Montana 59802. Submitted 28 August 2018; accepted 20 December 2018.
Caption: FIG. 1.--Sample design for crested macroplot. Sample plots and microplots were located in a stratified-random manner
TABLE 1.--Soil texture class, % sand, pH and electrical conductivity of soil from the center of cresled wheatgrass fields at 24 study sites Site name Latitude/longitude Soil texture HEAVYSOIL Boxelder 47.218[degrees]N 108.466[degrees]W Loam Duck Creek 47.192[degrees]N 108.641[degrees]W Silty-clay East Christina 47.375[degrees]N 109.258[degrees]W Clay-loam Erie 47.481[degrees]N 109.311[degrees]W Loam Hansom Dam 47.292[degrees]N 108.343[degrees]W Clav-loam Kosir 47.334[degrees]N 108.695[degrees]W Clay Maidenhead 47.112[degrees]N 109.042[degrees]W Clay-loam Moulton 47.045[degrees]N 108.762[degrees]W Clay Wolf 47.332[degrees]N 108.663[degrees]W Clay Chippewa 47.078[degrees]N 108.901[degrees]W Clay-loam Grass Range 47.038[degrees]N 108.787[degrees]W Loam Grass Range East 47.074[degrees]N 108.710[degrees]W Clay Manuel 47.092[degrees]N 108.153[degrees]W Loam Maruska 47.373[degrees]N 108.801[degrees]W Clay Milwaukee 47.518[degrees]N 109.336[degrees]W Clay-loam West Bohemian 47.331[degrees]N 108.851[degrees]W Sandy- clay-loam LIGHT SOIL Carl Spring 46.982[degrees]N 108.849[degrees]W Loamy-sand Gallatin 47.727[degrees]N 109.473[degrees]W Sandy-loam Loma 47.924[degrees]N 110.500[degrees]W Sandy-loam Milton 1 46.573[degrees]N 108.434[degrees]W Loamy-sand Milton 2 46.535[degrees]N 108.410[degrees]W Sandy-loam Milton 3 46.557[degrees]N 108.404[degrees]W Sandy-loam Rifle Range 46.022[degrees]N 108.522[degrees]W Loamy-sand Windmill 47.088[degrees]N 108.186[degrees]W Loam Site name % sand pH EC ([micro]S) HEAVYSOIL Boxelder 46 7.06 85 Duck Creek 48 7.46 130 East Christina 32 6.81 46 Erie 30 6.64 77 Hansom Dam 30 7.72 184 Kosir 26 8.05 120 Maidenhead 28 7.12 170 Moulton 28 6.91 158 Wolf 30 6.62 152 Chippewa 28 6.62 132 Grass Range 38 6.41 132 Grass Range East 22 7.01 70 Manuel 46 7.94 160 Maruska 24 7.56 330 Milwaukee 22 6.95 340 West Bohemian 49 7.32 445 LIGHT SOIL Carl Spring 77 7.56 227 Gallatin 55 8.12 185 Loma 64 7.85 224 Milton 1 75 7.83 296 Milton 2 67 7.45 152 Milton 3 72 8.2 199 Rifle Range 79 8.13 185 Windmill 52 7.55 290 TABLE 2.--Importance Value (IV), frequency (number of sites), mean density and mean canopy cover, for perennial native plants in crested macroplots. Only native species occurring in density microplots in at least two crested macroplots, with a mean density [greater than or equal to]0.04/[m.sup.2] in one are included Mean density Mean Frequency (/1[m. % IV (8 sites) sup.2]) cover LIGHT SOIL Shrubs Artemisia frigida 10.20 6 0.09 0.4 Artemisia tridentata 25.66 3 0.05 7.1 Opuntia polvacantha 8.16 5 0.04 0.5 Graminoids Agropyron dasvstachyum 0.07 0 0.00 0.0 Agropyron smithii 55.93 6 1.06 8.9 Aristida purpurea 4.16 3 0.01 0.1 Bouteloua gracilis 14.16 7 0.22 0.4 Carex eleocharis 6.28 4 0.06 0.1 Koeleria macrantha 6.81 5 0.03 0.1 Poa secunda 20.53 6 0.33 2.2 Schedonnardus paniculaius -- Stipa comata 20.21 6 0.35 1.9 Stipa viridula 5.50 4 0.03 0.0 Forbs Achillea millefolium 6.57 1 0.13 0.9 Allium textile 5.26 4 0.02 0.0 Symphyotrichum falcalum 14.51 1 0.30 2.4 Astragalus agrestis 3.57 2 0.05 0.1 Astragalus missouriensis 5.37 3 0.03 0.4 Picradeniopsis oppositifolia 2.71 1 0.07 0.0 Cerastium arvense 38.50 1 1.14 4.5 Comandra umbellata -- Gaura coccinea 6.94 0.01 1.5 Iva axillaris -- Liatris punctata 1.79 1 0.03 0.0 Lomatium foeniculaceum -- Phlox hoodii 1.53 1 0.01 0.0 Pediomelum argophyllum 1.23 1 <0.01 0.0 Sphaeralcea coccinea 12.09 7 0.13 0.4 Vicia americana 22.27 4 0.56 1.9 Mean density Mean Frequency (/1[m. % IV (16 sites) sup.2]) cover HEAVY SOIL Shrubs Artemisia frigida 11.85 10 0.09 0.3 Artemisia tridentata 27.21 6 0.02 2.9 Opuntia polvacantha 5.63 8 0.01 0.0 Graminoids Agropyron dasvstachyum 9.08 3 0.03 0.8 Agropyron smithii 26.53 8 0.32 1.3 Aristida purpurea 6.45 1 0.12 0.2 Bouteloua gracilis 12.15 8 0.03 0.8 Carex eleocharis 10.06 5 0.19 0.0 Koeleria macrantha 18.43 12 0.16 0.7 Poa secunda 26.57 15 0.34 0.7 Schedonnardus paniculaius 6.90 2 0.06 0.5 Stipa comata 12.45 8 0.09 0.5 Stipa viridula 8.88 7 0.03 0.5 Forbs Achillea millefolium 8.30 6 0.09 0.2 Allium textile 2.18 3 0.01 0.0 Symphyotrichum falcalum 9.31 6 0.11 0.2 Astragalus agrestis -- Astragalus missouriensis 0.71 1 <0.01 0.0 Picradeniopsis oppositifolia 2.95 2 0.05 0.0 Cerastium arvense 2.71 1 0.05 0.0 Comandra umbellata 2.41 2 0.03 0.0 Gaura coccinea 5.37 6 0.04 0.0 Iva axillaris 6.15 2 0.10 0.2 Liatris punctata 1.86 2 0.02 0.0 Lomatium foeniculaceum 19.65 6 0.20 1.1 Phlox hoodii 0.70 1 <0.01 0.0 Pediomelum argophyllum 5.45 4 0.06 0.1 Sphaeralcea coccinea 10.06 11 0.07 0.1 Vicia americana 35.18 14 0.43 1.4
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|Title Annotation:||Notes and Discussion Piece|
|Author:||Lesica, Peter; Cooper, Stephen V.|
|Publication:||The American Midland Naturalist|
|Date:||Apr 1, 2019|
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