Evaluating the Effectiveness of Low Soil-Disturbance Treatments for Improving Native Plant Establishment in Stable Crested Wheatgrass Stands

Rangeland Ecology & Management 72 (2019) 237–248

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Evaluating the Effectiveness of Low Soil-Disturbance Treatments for Improving Native Plant Establishment in Stable Crested Wheatgrass Stands Christo Morris a, Lesley R. Morris b, Thomas A. Monaco c,⁎ a b c

Powder Basin Watershed Council, Baker City, OR 97814, USA Department of Animal and Rangeland Sciences, Oregon State University, Agriculture and Natural Resources Program, Eastern Oregon University, La Grande, OR 97850, USA US Department of Agriculture, Agricultural Research Service, Forage and Range Research Lab, Utah State University, Logan, UT 84322, USA

a r t i c l e

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Article history: Received 19 January 2018 Received in revised form 4 September 2018 Accepted 22 October 2018 Key Words: Agropyron cristatum and desertorum Great Basin land-use legacy propagule pressure sagebrush species diversity

a b s t r a c t Past seedings of crested wheatgrass (Agropyron cristatum [L.] Gaertn. and A. desertorum [Fisch. ex Link] Schult.) have the potential to persist as stable, near-monospecific stands, thereby necessitating active intervention to initiate greater species diversity and structural complexity of vegetation. However, the success of suppression treatments and native species seedings is limited by rapid recovery of crested wheatgrass and the influx of exotic annual weeds associated with herbicidal control and mechanical soil disturbances. We designed a long-term study to evaluate the efficacy of low-disturbance herbicide and seed-reduction treatments applied together or alone and either once or twice before seeding native species. Consecutive herbicide applications reduced crested wheatgrass density for up to 6−7 yr depending on study site, but seed removal did not reduce crested wheatgrass abundance; however, in some cases combining herbicide application with seed removal significantly increased densities of seeded species relative to herbicide alone, especially for the site with a more northern aspect. Although our low-disturbance treatments avoided the pitfalls of secondary exotic weed influx, we conclude that crested wheatgrass suppression must reduce established density to values much lower than 4 −7 plants/m2, a range that has not been obtained by ours or any previous study, in order to diminish its competitive influence on seed native species. In addition, our results indicated that site differences in environmental stress and land-use legacies exacerbate the well-recognized limitations of native species establishment and persistence in the Great Basin region. © 2018 The Society for Range Management. Published by Elsevier Inc. All rights reserved.

Introduction Seeding exotic forage grasses, such as crested wheatgrass (Agropyron cristatum [L.] Gaertn. and A. desertorum [Fisch. Ex Link] Schult.), 1 was one of the most widespread vegetation manipulations in North America during the past century (Scasta et al., 2015; Pilliod et al., 2017; Williams et al., 2017). Crested wheatgrass was used extensively first in the Great Plains (Holechek, 1981; Rogler and Lorenz, 1983) and then later in the Great Basin to increase forage for livestock grazing, suppress weeds, reduce wind and water erosion, and reduce fire risk (Young and Evans, 1983; Asay et al., 2001; Pellant et al., 2004; Davies et al., 2010). Although effective at meeting the desired objectives

⁎ Correspondence: Thomas A. Monaco, Forage and Range Research Laboratory, US Dept of Agriculture−Agricultural Research Service, Utah State University, Logan, UT 84322, USA. E-mail address: [email protected] (T.A. Monaco). 1 Plant taxonomy follows the US Department of Agriculture, Natural Resources Conservation Service, PLANTS Database (www.plants.usda.gov).

of its initial use, some stands remained stable for many decades with low floristic and structural diversity, particularly when seedings were large and postseeding efforts sought to eradicate competing native species (Christian and Wilson, 1999; Rayburn and Monaco, 2011; Nafus et al., 2016; Williams et al., 2017). This plant community dominance by crested wheatgrass is reflective of global patterns in grasslands where exotic species are favored by nutrient loading and vertebrate consumer pressures (Seabloom et al., 2015). The ecological implications of stable crested wheatgrass stands are now a major concern within both ecosystems (Pellant and Lysne, 2005; Rinella et al., 2016). In particular, crested wheatgrass dominance has been linked to low species diversity (i.e., Henderson and Naeth, 2005; MacDougall et al., 2008; Nafus et al., 2016) and stable stands fail to provide ideal forage quality for livestock grazing (e.g., Pendery and Provenza, 1987; Angell et al., 1990) or habitat quality for big sagebrush (Artemisia tridentata Nutt.)−obligate species in the Great Basin (Vale, 1974; Reynolds and Trost, 1980; McAdoo et al., 1989). Consequently, augmenting floristic and structural diversity by reducing crested wheatgrass dominance and interplanting shrubs and legumes are deemed necessary to increase herbage production of

https://doi.org/10.1016/j.rama.2018.10.009 1550-7424/© 2018 The Society for Range Management. Published by Elsevier Inc. All rights reserved.

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stable stands (Rumbaugh et al., 1982; Pendery and Provenza, 1987) and improve habitat quality for sagebrush-obligate birds and overall bird diversity (McAdoo et al., 1989; Doherty et al., 2010). Compared with historical crested wheatgrass seedings, which involve native species recolonizing over time (e.g., Gunnell, 2009; Nafus et al., 2016; Williams et al., 2017), seedings that remain in near-monospecific stands are considered highly resilient stable ecological states (Allen-Diaz and Bartolome, 1998; West, 2000; Stringham et al., 2003). High stand stability has been attributed to demographic factors of crested wheatgrass, including dominance of seed banks (Marlette and Anderson, 1986; Gunnell, 2009), higher seed production, seed longevity and seedling survival than native grasses (Pyke, 1990; Wilson and Pärtel, 2003), and the ability of stands to reseed themselves and spread to surrounding areas (Hull and Klomp, 1967; Pyke, 1990). Superior grazing and drought tolerance of crested wheatgrass relative to native grasses also increase stand persistence (Hull and Klomp, 1966; Looman and Heinrichs, 1973; McLean and van Ryswyk, 1973; Asay et al., 2001; Scasta et al., 2015). Furthermore, crested wheatgrass intensely competes with seedlings of both native and exotic annual weed species (Francis and Pyke, 1996; Davies et al., 2010; Gunnell et al., 2010; Rayburn and Monaco, 2011) and native species are suppressed even when seeded as a mixture (Schuman et al., 1982; Waldron et al., 2005; Knutson et al., 2014; Nafus et al., 2015). Land-use legacies from past disturbance, seeding method, and active prevention of native species recolonization also favored long-term dominance of crested wheatgrass and contributed to low floristic and structural diversity of stands (Morris et al., 2014; Nafus et al., 2016; Williams et al., 2017). Because numerous factors reinforce crested wheatgrass dominance and stand stability, opportunities to direct plant community transitions to alternative states may be particularly difficult (e.g., Briske et al., 2005) and may require addressing multiple ecological processes simultaneously with treatments to influence plant community change (e.g., KruegerMangold et al., 2006; Sheley and Smith, 2012). A limited number of studies have specifically sought to diversify stable crested wheatgrass stands in the Great Basin region by suppressing crested wheatgrass with either herbicide or soil disturbance (i.e., disking) and planting an array of native species to support greater floristic and structural diversity (Hulet et al., 2010; Fansler and Mangold, 2011; Newhall et al., 2011; Davies et al., 2013; McAdoo et al., 2013, 2017). Results from these studies emphasized three challenges associated with diversifying stable stands: 1) rapid recovery of crested wheatgrass to pretreatment levels following reductions in established-plant density; 2) failure of seeded species to persist; and 3) proliferation of exotic annual species following soil disturbance. Althugh previous studies did not evaluate repeated or combined treatments applications, the authors concluded that applying treatments over a longer time period are necessary to reduce crested wheatgrass recovery and increase the persistence of established native species (Hulet et al., 2010; Fansler and Mangold, 2011; McAdoo et al., 2017). These conclusions indicate that additional research is necessary to explore potential treatment options and address existing uncertainties (e.g., Pellant and Lysne, 2005; Frid and Wilmshurst, 2009; Grant-Hoffman et al., 2012). Interestingly, recommendations from previous studies in the Great Basin mirror those advocated for integrated weed management of perennial species because they can extend the time period of herbicide control and produce synergistic effects on target plants that exceed the effects of either treatment applied alone (Miller, 2016; Orloff et al., 2018). Accordingly, because herbicide treatments only temporarily suppress crested wheatgrass growth (e.g., Bakker et al., 1997; Davies et al., 2013; McAdoo et al., 2017), applications to stable crested wheatgrass stands in consecutive years may prevent rapid recovery of plants to pretreatment levels and create a longer timeframe for desirable seeded species to establish under reduced competition (Ambrose and Wilson, 2003; Hansen, 2007). Similarly, because crested wheatgrass stands can rapidly regenerate from seed banks (e.g., Marlette and Anderson, 1986; Pyke, 1990; Gunnell, 2009; Hulet et al., 2010), combining

herbicide with a second treatment capable of removing seed before it enters seed banks could decrease seedling density of crested wheatgrass and extend the period of suppression necessary to increase emergence and survival of seeded species (Schuman et al., 1982; Gunnell et al., 2010; Nafus et al., 2015). For example, seed production contributed most to survival and growth of crested wheatgrass populations, leading to the suggestion that this process should be targeted with control strategies (Hansen and Wilson, 2006; Hansen, 2007). Although combined treatments are potentially capable of simultaneously reducing crested wheatgrass seed input and providing herbicide control to seedlings as they emerge from seed bank, it is important to note that crested wheatgrass seed can remain viable in seed banks for at least 2− 4 yr (Pyke, 1990; Wilson and Pärtel, 2003). High seed longevity suggests that consecutive applications of combined treatments may yield better conditions for native species emergence when compared with treatments applied alone or only one time (Masters and Sheley, 2001; ReinhardtAdams and Galatowitsch, 2006). Unintended consequences associated with suppressing and/or eradicating dominant species are widely recognized in invasive plant management and ecological restoration (Buckley et al., 2007; Baer et al., 2009). These consequences typically include injury to nontarget desirable species (Rinella et al., 2009; Kettenring and Adams, 2011), increased soil resource availability that favors secondary invasion of exotic species (Davis et al., 2000; Courchamp et al., 2011; Pearson et al., 2016), and reinvasion of the target species from seed banks after eradicating established plants (Wilson et al., 2004a; Galatowitsch et al., 2016); all of which can be exacerbated when suppression and/or control treatments directly disturb soils (Cox and Anderson, 2004; Kotanen, 2004; Hierro et al., 2006). Similarly, the unintended consequences of mechanical treatments used to eradicate established crested wheatgrass by means of disturbing soils (disking) can increase both exotic weed cover and crested wheatgrass density (Hulet et al., 2010; Fansler and Mangold, 2011; McAdoo et al., 2017). These results indicate that soil disturbance should be avoided in order to minimize secondary weed invasions and reinvasion by crested wheatgrass and highlight the need to explore low-disturbance treatments capable of minimizing rapid reestablishment of crested wheatgrass from seed banks. Given the low success rates with applying only one treatment and high risk for unintended consequences from disturbance, our study objective was to evaluate the effectiveness of low soil-disturbance crested wheatgrass suppression treatments that included mowing inflorescences to reduce the addition of new seed into seed banks and herbicide applications to suppress crested wheatgrass seedlings and mature plants. Our experimental design included initiating studies in different years at each of two sites in order to address contingencies that influence restoration outcomes in semiarid ecosystems (Vaughn and Young, 2010; Wilson, 2015). In addition, because suppressing dominant species in grassdominated ecosystems alters coinciding temporal and spatial factors that influence species recruitment (e.g., Renne and Tracy, 2013), our treatments were also deployed either once (i.e., during the seeding year), in consecutive years (i.e., 1 yr before and again during the seeding year), alone, or in combination to explore treatment longevity and synergistic effects on crested wheatgrass abundance and seeded species establishment over a 6- to 7-yr period. We asked three questions: Do consecutive herbicide applications suppress crested wheatgrass density and cover for a longer timeframe than single applications? Do combined treatment applications of seed reduction and herbicide yield greater native species establishment than either treatment applied alone? and Do low-disturbance suppression methods lead to undesirable increases in exotic weeds or increases in crested wheatgrass? Methods Study Sites Two sites in southern Idaho (Cassia County) were selected for our study (i.e., Gunnell Pivot and Point Springs). Both sites were located

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on gentle slopes (2−5%), having very deep, well-drained silt-loam soils derived from alluvium; however, there is some loess at Point Springs (Noe and Kukachka, 1994). At Point Springs, potential vegetation would be dominated by bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] Á. Löve) and Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis Beetle & Young) with some Thurber’s needlegrass (Achnatherum thurberianum [Piper] Barkworth), arrowleaf balsamroot (Balsamorhiza sagittata [Pursh] Nutt.), Sandberg’s bluegrass (Poa secunda J. Presl.), and green rabbitbrush (Chrysothamnus viscidiflorus [Hook.] Nutt.) (Noe and Kukachka, 1994). At Gunnell Pivot, shadscale (Atriplex confertifolia [Torr. & Frém.] S. Watson), bottlebrush squirreltail (Elymus elymoides [Raf.] Swezey), needle-and-thread grass (Hesperostipa comata [Trin. & Rupr.] Barkworth), Indian ricegrass (Achnatherum hymenoides [Roem. & Schult.] Barkworth), and sand dropseed (Sporobolus cryptandrus [Torr.] A. Gray) would be the expected dominant plant species (Noe and Kukachka, 1994). The sites are about 19 km apart with Point Springs (1 585 m) on a north-facing slope and Gunnell Pivot (1 676 m) facing west. Consequently, our calculation of heat load indices for Gunnell Pivot and Point Springs varied (i.e., 2.58 vs. 2.49) due to greater potential annual incident radiation of the west-facing slope at Gunnell Pivot, creating higher maximum temperatures and drier conditions (McCune and Keon, 2002). Gunnell Pivot was located on a homestead with a history of cultivation and was continuously used for dry-land wheat production and irrigated alfalfa dating back to the early 1900s. In contrast, Point Springs did not have a history of land cultivation. Both sites were seeded with crested wheatgrass: Point Springs in the mid-1980s and Gunnell Pivot in the mid-1990s. After seeding with crested wheatgrass, both sites were grazed by cattle each spring before initiating our studies. The Point Springs site burned in 2007 due to a wildfire and was subsequently removed from grazing between 2007 and 2009 yet grazing resumed in 2012 for this site only. In the absence of recurring disturbances after seeding, absolute cover of crested wheatgrass across the eastern Great Basin is known to be highly variable (i.e., 4 − 32%) and inversely related to sagebrush cover, depending on soil texture (Williams et al., 2017). Relative to this gradient, our two sites did not contain mature sagebrush plants and crested wheatgrass dominated vegetation cover at both Point Springs and Gunnell Pivot in 2008 (i.e., 15.3% ± 1.1% and 19.4% ± 1.1% cover, respectively) and 2009 (i.e., 21.1% ± 1.2% and 9.1% ± 1% cover, respectively). This level of crested wheatgrass cover is suggested to fall within a threshold range (i.e., 15−20%), across which plant community diversity declines (Noe and Kukachka, 1994; Williams et al., 2017). The 30-yr average annual precipitation at the two sites is similar, with Point Springs averaging 341 mm and Gunnell Pivot averaging 348 mm. These averages are based on modeled data interpolated from surrounding weather stations (PRISM, 2018) because no weather data were collected on site and the nearest weather stations were 30 − 50 km away (NOAA, 2017). Both sites had lower than average precipitation in 2008, slightly above-average precipitation in 2009 and 2010, lower than average precipitation in 2012 and 2013, and higher than average in 2014 (Fig. 1). Treatments Treatments consisted of herbicide application and a seed removal treatment (via mowing of inflorescences), each combined with seeding of native species. Treatments were applied either once before seeding or in 2 consecutive yr before seeding. Treatments were repeated in time with seedings occurring in 2009 and 2010 at both sites (Table 1). The herbicide treatments were applied in the first week of June and consisted of glyphosate (Roundup Original Max, Monsanto Company, St. Louis, MO; 48.7% a.i.) applied at a rate of 3.5 L · ha−1 with a custom sprayer at 276 kPa and 4-m boom with eight flat fan nozzles (Teejet Technologies, Wheaton, IL). Treatments were in 130 L · ha −1 total

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spray solution, with a nonionic surfactant (IFA-S90, Intermountain Farmers Association, Salt Lake City, UT; 0.25% v/v). Glyphosate is a broad-spectrum herbicide, which at low doses has been used to stunt crested wheatgrass rather than kill it outright (Hulet et al., 2010; Fansler and Mangold, 2011; Davies et al., 2013). The seed removal treatment was achieved by using a flail mower (John Deere 390, Moline, IL) set to a height of 10 cm above the ground to minimize leaf damage. The treatment was applied when inflorescences contained developing seeds of crested wheatgrass in the soft dough stage. Harvesting seed at this developmental stage has been shown to yield unviable seed, even when inflorescences are allowed to cure in windrows (Horton et al., 1990; Anonymous, 1998). All plots were seeded in November using a Truax Flex II Series rangeland drill (New Hope, MN) with eight disk openers spaced 20 cm apart. The seed mix for all plots consisted of the following species (with application rates of pure live seed in kg · ha −1): bluebunch wheatgrass (3.36), bottlebrush squirreltail (3.36), Sandberg’s bluegrass (1.12), western yarrow (Achillea millifolium L. [0.34]), scarlet globemallow (Sphaeralcea coccinea [Nutt.] Rydb.; [0.56]), Wyoming big sagebrush (0.56), rubber rabbitbrush (Ericameria nauseosa [Pall. ex Pursh] G.L. Nesom & Baird; [0.56]) and shadscale (1.12). Treatment plots were placed in a randomized split-plot design with seed removal treatments as the whole plot and herbicide treatments as the split-plot factor. Treatments were applied to 7.6 × 7.6 m plots and replicated four times at each site per seeding year. Data Collection Vegetation measurements were taken each growing season (i.e., late June−mid July) from 2008 to 2014 for the 2009 seeding (except 2013) and from 2009 to 2014 for the 2010 seeding. Each census year, canopy cover measurements for all plant species were taken within a 1-m 2 sampling frame randomly placed twice within each plot at least 0.5 m from plot edges. Density counts for crested wheatgrass plants were also conducted using the same 1-m 2 frame, while density of seeded species was counted within a 0.66-m 2 frame that was randomly placed three times within each plot and straddled four drill-seeded rows. Statistical Analysis Response of crested wheatgrass (i.e., density and cover) was analyzed separately for the 2009 and 2010 seeding experiments as randomized complete block designs with mixed-effect models (α = 0.01); replication was a random effect and year, seed removal, and herbicide were fixed effects. Because the study of stand stability requires consideration of change over time (i.e., year), we focused only on interactive effects that included year as opposed to main effects that pooled data across years. Models were fit with standard least squares and the residual maximum likelihood (REML) estimation method in JMP ver. 13 (SAS Institute Inc., Cary, NC). Box-Cox transformations were applied to meet the assumptions of normality and homogeneity of variances. Post-hoc analyses of significant model effects (P b 0.05) were performed using Tukey’s honestly significant difference test to compare differences in the final census year. The influence of treatments on seeded species density was analyzed using nonparametric one-way Kruskal-Wallis rank sums tests due to the high number of zeros in the dataset. Separate models were run for 2009 and 2010 seeding experiments in each census year and study site (α = 0.01). When treatment differences were found, treatments with the highest density of seeded species were compared with the other treatments with pairwise Wilcoxon tests. Cover percentages for nine exotic weed species were also analyzed using nonparametric one-way Kruskal-Wallis rank sums tests due to the high number of zeros in the dataset. Weed species included cheatgrass (Bromus tectorum L.), saltlover (Halogeton glomeratus [M. Bieb.] C.A.), clasping pepperweed (Lepidium perfoliatum L. Mey.), western

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C. Morris et al. / Rangeland Ecology & Management 72 (2019) 237–248 140 Gunnell Pivot 120

Precipitation (mm)

Month 100

30-year mean

80 60 40 20 0 J F MAM J J A SO N D J FM AM J J A S ON D J F MAM J J A S ON D J F MAM J J A SO N D J F MAM J J A S ON D J FMA M J J A S ON D J F MAM J J A SO N D 2009

2008

2010

2011

2012

2013

2014

140 120

Point Springs

Precipitation (mm)

Month 100

30-year mean

80 60 40 20 0 J F MAM J J A S ON D J FMA M J J A S ON D J F MAM J J A SO N D J F MAM J J A SO N D J FM AM J J A S ON D J F MAM J J A S ON D J F MAM J J A SO N D

Figure 1. Total monthly precipitation for 2008−2014 and 30-yr monthly precipitation means modeled for Gunnell Pivot and Point Springs study sites using data from surrounding weather stations (PRISM, 2018).

tansymustard (Descurainia pinnata [Walter] Britton), cureveseed butterwort (Ceratocephala testiculata [Crantz] Roth), yellow salsify (Tragopogon dubius Scop.), prickly lettuce (Lactuca serriola L.), tall tumblemustard (Sisymbrium altissimum L.), and Russian thistle (Salsola kali L.). For each species, separate models were run for 2009 and 2010 seeding experiments in each census year and study site (α = 0.01). Dunnett’s tests were performed to determine if any of the treatments differed from the untreated control. All statistical analyses were performed using JMP ver. 13 (SAS Institute Inc., Cary, NC). Results Crested Wheatgrass Response Crested wheatgrass density and cover were not significantly influenced by seed removal treatments (Table 2). In contrast, the timeframe of reductions in crested wheatgrass density and cover relative to untreated plots differed among single- and consecutive-yr herbicide applications (see Table 2; Fig. 2). For the 2009 seeding, density and cover initially decreased in the first yr after treatment and values remained significantly lower in the consecutive-yr treatment relative to the noherbicide treatment. However, cover generally increased over time, Table 1 Timeline of treatment application, seeding, and census dates for experiments conducted at two crested wheatgrass–dominated plant communities in southern Idaho, United States. 2009 Seeding Yr 2008 2009 2010 2011 2012 2013 2014

Treatment X1 X

2010 Seeding Seeding X

Census X X X X

X

Treatment

Seeding

X1 X

X

Census X X X X X

Treatments were applied either once (1×; during seeding yr) or twice (2×; 1 yr before and during seeding yr). 1 Application of 2× treatment (1 yr before and during seeding yr).

even in the absence of treatment, whereas density remained lower than pretreatment values into the sixth yr. Similarly, crested wheatgrass cover remained significantly lower in the consecutive-yr compared with the no-herbicide treatment in the 2010 seeding experiment. In contrast, cover in the no-herbicide plots initially increased to approximately 30% and remained stable over time, while the single- and consecutive-yr treatments declined during the first 2 yr before increasing. By 2014, cover in the single-herbicide treatment was similar to the no-herbicide treatment. Although crested wheatgrass density in the 2010 seeding was generally lower in both herbicide treatments compared with the no-herbicide treatment for the first 3 yr post treatment, Table 2 Results (F-tests) of mixed-model analysis of variance of crested wheatgrass density and cover in response to seed removal and herbicide treatments. 2009 Seeding experiment

2010 Seeding experiment

Source

df

Density

Cover

df

Density

Cover

Yr Site Yr · Site Seed removal (SR) Yr · SR Site · SR Year · Site · SR Herbicide Year · Herb Site · Herb Year · Site · Herb Herb · SR Year · Herb · SR Site · Herb · SR Year · Site · Herb · SR

3 1 3 2 6 2 6 2 6 2 6 4 12 4 12

67.692 323.082 11.242 0.21 0.96 0.34 0.62 95.092 4.672 2.20 0.46 1.37 0.49 0.64 0.21

33.492 86.862 20.562 0.23 1.93 0.21 0.56 123.872 3.702 10.872 0.70 2.13 0.41 1.41 0.47

4 1 4 2 8 2 8 2 8 2 8 4 16 4 16

59.932 713.352 29.602 2.27 0.77 4.58 0.80 183.942 9.742 23.122 3.111 2.04 0.45 2.16 0.26

25.472 112.852 10.212 1.34 0.79 2.01 0.52 179.322 9.642 27.932 1.22 1.08 0.27 1.05 0.29

Treatments were applied once (during seeding yr), twice (yr before and during seeding yr), or not at all and were monitored over a 4-yr (2010 seeding) to 5-yr period (2009 seeding) at two crested wheatgrass–dominated plant communities in southern Idaho, United States. Bold font indicates significant effects that address crested wheatgrass resilience. 1 P b 0.01. 2 P b 0.0001.

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Figure 2. Mean ± SE density and cover of crested wheatgrass for the 2009 (a, c) and 2010 (b, d) seedings over a 7-yr period. Within a seeding year, lowercase letters indicate significant differences among no herbicide, herbicide applied during the year of seeding (1×), and herbicide applied 1 yr before and during seeding yr (2×) for final census date means (P b 0.05).

treatment differences were more pronounced at Gunnell Pivot than Point Springs during this time frame (see Table 2; Fig. 3). Crested wheatgrass density and cover were both generally lower at Gunnell Pivot, but differences were not consistent over time (see Table 2; Fig. 4). While density initially declined for the 2009 seeding at both sites, it increased more over time at Gunnell Pivot (see Fig. 4a). On the other hand, cover in the 2009 seeding experiment increased for both sites, attaining similar values by 2014, even though cover was initially higher, and experienced greater reductions in the first yr post treatment at Gunnell Pivot (see Fig. 4c). Cover was also similar between sites 5 yr post treatment in the 2010 seeding despite greater initial reductions at Gunnell Pivot.

between 2 and 9 seedlings/m 2 at the Point Springs site in the 2009 and 2010 seeding experiments, respectively (see Fig. 5c and d). Despite initially high forb/shrub establishment at Point Springs, fewer than 4 seedlings/m2 were found by 2014.

Exotic Weed Response Treatments did not affect any of the nine exotic weed species with the exception of Lepidium perfoliatum in 2012 at Gunnell Pivot (Table S1 [available online at https://doi.org/10.1016/j.rama.2018.10.009]). For this one case, cover of L. perfoliatum differed among treatments

Seeded Species Response Treatments designed to reduce crested wheatgrass abundance also influenced seeded-species density, which varied by census yr, seeding yr, and site (Table 3). At Gunnell Pivot 2 yr after the 2010 seeding, consecutive seed removal combined with herbicide (applied either once or twice) had higher density of native grass species than the control and five of the eight treatments (χ2 = 20.7; P = 0.008). At Point Springs 1 yr after the 2010 seeding, three of the four combined treatments had higher density of seeded forbs/shrubs than the control (χ 2 = 25.7; P = 0.0012). In contrast, none of the treatments resulted in higher density of seeded forbs/shrubs than the control by the second yr for the 2010 seeding at Point Springs, yet the combination of consecutive herbicide applications with either seed removal treatment resulted in higher density than a number of other treatments (χ 2 = 22.0; P = 0.0048). Lastly, 4 yr after the 2010 seeding at Point Springs, combining seed removal and herbicide application resulted in generally higher density of seeded forbs/shrubs than control plots or treatment types applied alone (χ 2 = 21.8; P = 0.0053). Density of seeded species was dynamic over time and varied greatly by site (Fig. 5). Overall establishment of native grasses and forbs/shrubs was greater at Point Springs than at Gunnell Pivot. Native grasses had greater initial establishment at Point Springs than at Gunnell Pivot in the 2009 and 2010 seedings, yet by 2014 fewer than 2 seedlings/m 2 survived at both sites (see Fig. 5a and b). Similarly, forb/shrub species generally did not establish at Gunnell Pivot, yet these species established

Figure 3. Mean ± SE density of crested wheatgrass for the 2010 seeding at Gunnell Pivot (a) and Point Springs (b) study sites over a 7-yr period.

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Figure 4. Mean ± SE density and cover of crested wheatgrass for the 2009 (a, c) and 2010 (b, d) seedings at Gunnell Pivot and Point Springs study sites over a 7-yr period. Within a seeding yr, lowercase letters indicate significant differences between study sites for final census date means (P b 0.05).

(χ 2 = 26.53; P = 0.0009) and, relative to the untreated control, was typically higher in treatments that included herbicide. Discussion Our results offer three points of clarification regarding the current perception of crested wheatgrass stand stability. First, although rapid recovery of crested wheatgrass following suppression on semiarid sites in the Great Basin is consistently reported (Hulet et al., 2010; Davies et al., 2013; McAdoo et al., 2017), we found that consecutive herbicide applications can suppress crested wheatgrass cover for up to 6−7 yr and over a longer timeframe than single-herbicide applications (see Fig. 2). However, crested wheatgrass density was not consistently suppressed for longer timeframes by consecutive herbicide applications, and density increased to levels matching untreated plots after 3 yr due to a few new seedlings per square meter recruiting from seed banks. Second, poor survival of established seeded species despite suppression of crested wheatgrass density and cover for up to 4 and 6 yr, respectively, reinforces the emerging recognition that low native plant establishment and seedling survival are primary challenges to restoration efforts (James et al., 2013; Kildisheva et al., 2016). Although the combined herbicide-seed removal treatments commonly resulted in the highest establishment of seeded species (i.e., for 17 of the 24 comparisons; see Table 3), it exceeded control plots in only three cases. These results suggest that any benefits to seedling establishment attributed to the combination of herbicide and seed reduction may have been overridden by environmental stresses experienced by native plant seedlings. Third, our results show that secondary invasion of exotic annual weeds that often accompanies the suppression of dominant perennial species (e.g., Symstad, 2004; Pearson et al., 2016) and soil disturbances in stable crested wheatgrass stands (e.g., Hulet et al., 2010; Davies et al., 2013) can be potentially avoided through the application of low-disturbance suppression treatments. Our low-disturbance suppression treatments also circumvented increases in crested wheatgrass density and cover from exceeding values in untreated plots (e.g., Hulet et al., 2010; Fansler and Mangold, 2011; McAdoo et al., 2017). However, lack of recruitment suggests that surviving crested wheatgrass plants likely acquired any excess soil resources stemming from reducing overall plant abundance.

Later, we discuss these points in an ecological context by raising three questions relevant to achieving greater floristic and structural diversity within the confines of stable crested wheatgrass stands while avoiding unintended consequences. What Is the Necessary Level of Crested Wheatgrass Suppression to Support Native Species Establishment? Although low plant species diversity is a hallmark of stable crested wheatgrass stands in western North America (Reynolds and Trost, 1980; Marlette and Anderson, 1986; MacDougall et al., 2008), recent work across a broad range of historical seedings indicates that, in the absence of fire or active efforts to stifle native species recolonization, species diversity does not decline until crested wheatgrass cover exceeds 15% (Williams et al., 2017). However, despite the capacity of all previous studies to suppress crested wheatgrass cover below this threshold value (Hulet et al., 2010; Fansler and Mangold, 2011; Davies et al., 2013; McAdoo et al., 2017) and our ability to lengthen the timeframe of suppression with consecutive herbicide applications, why was seedling recruitment and native species persistence so low? One possible answer is that the threshold density for a decline in established species is higher than the threshold for reestablishing native species, suggesting that competition between established crested wheatgrass plants and seedlings was still too intense even following suppression (e.g., Robertson, 1972; Bakker and Wilson, 2001; Leonard et al., 2008; Gunnell et al., 2010; Davies and Johnson, 2017). Although suppression of dominant grassland species often results in alleviating competition for limiting resources with subdominant species (e.g., Leyshon et al., 1981; McCain et al., 2010; Souza et al., 2011), competition from existing plants likely interfered with seedling establishment and persistence (Mueggler and Blaisdell, 1955; Brown and Archer, 1989; Waldron et al., 2005; Nafus et al., 2015). In support of this assertion, we also found crested wheatgrass cover increased following reductions in plant density (see Fig. 2), which is indicative of surviving plants being released from intraspecific competition and agrees with observations reported in mixed-grass prairie following crested wheatgrass suppression (Ambrose and Wilson, 2003). Reduced intraspecific competition has also been shown to increase seed production of remaining crested wheatgrass plants as the

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Table 3 Mean (±SE) seeded native grass and forb/shrub establishment density (seedlings • m−2) over a 4-yr (2010 seeding) to 5-yr period (2009 seeding) at two crested wheatgrass–dominated plant communities (Gunnell Pivot and Point Springs) in southern Idaho, United States. Yr

Control

Herbicide-1×

Herbicide-2×

Seed Rem.-1×

Seed Rem.-2×

Seed Rem.-1× Herbicide-1×

Seed Rem.-2× Herbicide-1×

Seed Rem.-1× Herbicide-2×

Seed Rem.-2× Herbicide-2×

Native Grass Density Gunnell Pivot 2009 Seeding 2010 6.62 (1.62) 7.19 (3.19) 4.92 (2.36) 2011 1.51 (0.87) 1.51 (1.35) 1.89 (1.14) 2014 0.67 (0.19) 0.58 (0.25) 1.58 (0.69)

9.84 (1.77) 0 0.44 (0.22)

7.00 (0.36) 1.77 (0.67) 0.25 (0.16)

7.76 (2.01) 0.50 (0.50 0.22 (0.22)

9.27 (3.00) 0.88 (0.56) 0.67 (0.38)

7.00 (1.21) 1.01 (0.21) 1.33 (0.72)

5.30 (2.87) 2.90 (1.26) 1.67 (0.87)

Native Grass Density Gunnell Pivot 2010 Seeding 2011 0 3.03 (1.35) 0 2012 0.50 (0.21) 3.41 (1.68) 1.39 (0.91) 2014 0.08 (0.08) 0.42 (0.08) 0.08 (0.08)

0.88 (0.43) 0.76 (0.76) 1.00 (0.58)

1.14 (0.52) 1.51 (0.50) 0.58 (0.21)

1.51 (0.36) 1.01 (0.54) 0.17 (0.17)

3.53 (0.92) 12.99 (2.55)1 0.42 (0.21)

2.02 (1.03) 6.43 (2.32) 0.58 (0.34)

4.54 (1.33) 10.47 (2.17)1 0.22 (0.22)

Native Grass Density Point Springs 2009 Seeding 2010 7.57 (2.18) 7.76 (1.67) 14.57 (2.84) 2011 1.64 (0.69) 1.77 (1.14) 1.18 (1.18) 2014 0.25 (0.08) 0 0.44 (0.44)

9.27 (2.59) 1.01 (0.68) 0

7.95 (2.50) 1.64 (1.00) 0.08 (0.08)

11.60 (1.10) 0.25 (0.25) 0

11.73 (1.81) 2.40 (1.39) 0

8.89 (2.13) 0.63 (0.24) 0.17 (0.17)

13.37 (1.65) 3.91 (1.72) 0.75 (0.25)

Native Grass Density Point Springs 2010 Seeding 2011 1.51 (0.71) 4.67 (1.36) 4.29 (0.78) 2012 1.01 (0.41) 8.20 (1.40) 6.56 (1.84) 2014 0 0.42 (0.21) 0

2.02 (1.38) 0.76 (0.44) 0

3.03 (0.41) 2.52 (1.71) 0

3.03 (1.27) 6.81 (4.04) 0.11 (0.11)

5.30 (1.04) 5.68 (3.19) 1.42 (0.64)

5.21 (0.73) 7.23 (3.38) 0.83 (0.44)

5.05 (1.46) 6.18 (1.04) 0.58 (0.28)

Native Forb/Shrub Gunnell Pivot 2009 Seeding 2010 0 0 0 2011 0 0 0 2014 0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0.38 (0.22) 0 0

0 0 0

Native Forb/Shrub Gunnell Pivot 2010 Seeding 2011 0.13 (0.13) 0.17 (0.17) 0.17 (0.17) 2012 0.13 (0.13) 0 0.50 (0.21) 2014 0 0.25 (0.16) 0.17 (0.17)

0.63 (0.24) 0.13 (0.13) 0

0.76 (0.48) 0.13 (0.13) 0

0.50 (0.21) 0.25 (0.25) 0.11 (0.11)

0.50 (0.00) 1.14 (0.43) 0.58 (0.28)

0.17 (0.17) 0 0.33 (0.19)

0.50 (0.29) 1.01 (0.50) 0.56 (0.29)

Native Forb/Shrub Point Springs 2009 Seeding 2010 0 0 0 2011 1.77 (0.60) 3.15 (1.17) 2.52 (1.67) 2014 0.22 (0.22) 0.08 (0.08) 0.50 (0.32)

0 2.19 (1.18) 0

0 0.67 (0.67) 0.44 (0.44)

0 0.76 (0.44) 0.17 (0.10)

0.95 (0.36) 4.79 (2.08) 0.17 (0.17)

0.76 (0.31) 0.76 (0.60) 0.17 (0.10)

0.50 (0.25) 1.51 (1.27) 1.00 (0.58)

Native Forb/Shrub Point Springs 2010 Seeding 2011 4.54 (0.80) 5.42 (0.48) 6.05 (0.50) 2012 1.51 (0.29) 8.58 (2.09) 8.58 (3.07) 2014 0.25 (0.16) 1.67 (0.38)1 1.50 (0.50)

2.02 (0.29) 1.39 (0.95) 0.42 (0.32)

6.56 (0.65) 1.77 (0.67) 0.33 (0.19)

16.90 (5.67)1 5.21 (3.06) 3.22 (1.18)1

17.91 (3.46)1 7.90 (0.94) 2.78 (0.80)1

14.51 (3.17)1 18.04 (3.85) 5.56 (1.72)1

14.13 (3.92) 15.98 (2.98) 6.83 (2.52)1

Treatments included herbicide applications and seed removal (Seed Rem.) that were applied either once (1×; during seeding yr) or over 2 consecutive yr (2×; 1 yr before and during seeding yr). Within a census yr, differences among treatments were determined with Kruskal-Wallis rank sums tests (χ2 statistic; df = 8; n = 4; P b 0.01). Bold font indicates treatments with the highest establishment. 1 Treatments that are significantly different than the control.

bare space between plants is increased (Buglass, 1964; McGinnies, 1971). Consequently, removing this obstacle to seedling establishment and persistence of native species may require suppressing crested wheatgrass to lower levels than have previously been achieved. Previous efforts, including those reported here, indicate that 4 − 7 established crested wheatgrass plants/m 2 was the lowest range in density that could be achieved through suppression treatments (Hulet et al., 2010; Fansler and Mangold, 2011; Davies et al., 2013; McAdoo et al., 2017). Thus, in order to alleviate intraspecific competition between crested wheatgrass and native species seedlings, suppression must reduce density below this range. At one extreme, complete eradication of exotic forage grasses is considered necessary for native warm season grasses to establish and persist in mesic prairie ecosystems (Wilson and Gerry, 1995; Barnes, 2004; MacDougall et al., 2008; Monroe et al., 2017). Even leaving 1 plant/m2 is roughly an order of magnitude lower than the density of a stable crested wheatgrass stand (Fansler and Mangold, 2011; Davies et al., 2013; McAdoo et al., 2017) and greatly narrows the density ratio between crested wheatgrass and established native seedlings. Although we do not know whether the highest establishment densities we achieved (i.e., 8 − 10 native seedlings/m 2; see Fig. 5) could have persisted longer had crested wheatgrass density been reduced to a tenth of native species, previous research suggests that the competitive ability of native seedlings is equal to crested wheatgrass at these ratios (Gunnell et al., 2010).

As density is reduced by suppression efforts, it is also important to consider how spatial aspects of crested wheatgrass stands influence seedling establishment (Rayburn and Monaco, 2011; Vaness et al., 2014). Suppression inevitably creates gaps in vegetation and sites with reduced competition for colonization (Cahill and Casper, 2002; Peters, 2002; Derner and Wu, 2004). For example, colonization of native forbs in old fields dominated by quackgrass (Elymus repens [L.] Gould) required creating gaps in resident vegetation at least 10-cm in diameter (Goldberg, 1987) and forb seedling growth and survival were also much greater in 18-cm gaps than in 6-cm gaps in Australian grasslands dominated by kangaroo grass (Themeda triandra Forssk.) (Morgan, 1998). Seedling establishment, survival, and biomass production of two native grasses also increased with gap sizes in the range of 10- to 40-cm diameter in semiarid grasslands where crested wheatgrass is a native component of the vegetation (Liu et al., 2014). Similarly, establishment of a forb species increased as the width of dead strips of crested wheatgrass increased up to 75 cm, but exotic weed growth was favored in strips wider than 50 cm (Shellenberg et al., 1994). Although these reports emphasize that creating gaps that are free of vegetation and competitive pressures from surviving crested wheatgrass plants are essential for native species seedling establishment and survival (i.e., Hook et al., 1994; Aguilera and Laurenroth, 1995; Wilson and Gerry, 1995), prudence must be exercised because as gap size increases, so do soil resource availability and susceptibility to exotic plant colonization (McConnaughay and Bazzaz, 1990; Bradford and Lauenroth, 2006; James et al., 2008). For example, even the creation of small gaps (i.e., 10 cm 2) or small

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Native grass density (plants . m 2 )

12

12

*

10

10

8

8 Gunnell Pivot

6

6

Point Springs

4

* 2010

2011

12

2012

2013

2014

(c) 2009 Seeding

2 0

10

8

8

6

6

4

4

*

2

2011

2011

2012

*

*

2013

2014

(d) 2010 Seeding

*

2

* 2010

2010

12

10

0

*

4

2 0

Native forb/shrub density (plants . m 2)

(b) 2010 Seeding

(a) 2009 Seeding

2012

2013

2014

0

2010

2011

2012

2013

2014

Year Figure 5. Mean ± SE density of seeded native grasses (a and b) and native forb/shrubs (c and d) for the 2009 and 2010 seeding, respectively, over a 5-yr period at Gunnell Pivot and Point Springs study sites. Within a seeding year, asterisks indicate significant differences between study sites (P b 0.05).

areas of soil surface disturbance (i.e., 100 cm 2) can facilitate invasive grass colonization, growth, and reproduction (Tozer et al., 2008; Leffler et al., 2016), and as gap size and connectivity of gaps between perennial bunchgrasses expand due to disturbance, susceptibility to annual grass invasion is known to increase for semiarid shrub-steppe communities across the Great Basin region (Reisner et al., 2013). What Environmental Factors Contributed to Low Native Species Survival? In addition to the competitive ability of crested wheatgrass interfering with native species (e.g., Harrington, 1991; Bakker and Wilson, 2001; Gunnell et al., 2010), our study reiterates that environmental factors presented an obstacle to seedling survival (Svejcar et al., 2017). Because seedling mortality limits the persistence of native species seeded in this region, even when control efforts are deployed to suppress competitive exotic species (Kulpa et al., 2012; Svejcar et al., 2014; Brabec et al., 2015), we partially attribute low survival of established seedlings to environmental stresses encountered for both seeding years and research sites (see Fig. 5). First, greater overall establishment of native species for the 2010 seeding but more rapid losses due to mortality for the 2009 seeding may be ascribed to total precipitation received in the years following seeding (November−October), which was above average for the former, and below average for the latter (see Fig. 1). We suspect that seedling establishment during the more favorable precipitation conditions of 2010 provided an advantage to seedling growth including greater seedling size and deeper roots that enabled seedlings to persist over the subsequent dry-summer growing seasons (2011 − 2014). In support of this interpretation, some studies have shown that larger seedlings at the initiation of drought have a higher probability of survival compared with smaller seedlings (e.g., Donovan et al., 1993; Gilbert et al., 2001; Cuesta et al., 2010), and higher seedling survival is likely achieved by seedlings rapidly accessing moist soil horizons before drought by growing deeper roots and allocating more overall growth to roots compared with alternative growth patterns (Padilla and Pugnaire, 2007; Ferguson et al., 2015). In contrast, seedlings that established during the drier 2009 conditions experienced both rapid and severe mortality when exposed to the low-precipitation years that followed seeding. The contrasting outcomes we observed between

the favorable and less-favorable seeding years supports the call for strategies to predict and exploit these suitable years in restoration planning (Hardegree et al., 2018) and build greater contingencies into diversification efforts through repeated seeding (Wilson et al., 2004b; Vaughn and Young, 2010; Wilson, 2015). Because sites with similar temperature and moisture regimes can portray vast differences in resilience to disturbance and environmental stress, including site recovery aided by restoration seedings (Chambers et al., 2014; Miller et al., 2015), we attribute some of the variation in native species establishment and persistence to differences in environmental stress among sites. Although sites experienced nearly identical precipitation patterns, they likely experienced distinct differences in moisture stress when considering slope aspects. For example, the westerly aspect at Gunnell Pivot and the northerly aspect at Point Springs dictate that the former experienced more direct incident radiation and higher cumulative heat load than the latter (e.g., McCune and Keon, 2002; Warren, 2008). Thus, we posit that Gunnell Pivot represents a drier and hotter site overall compared with Point Springs, due to perceivably higher rates of evapotranspiration. This difference in environmental stress likely contributed to initially lower crested wheatgrass density compared with Point Springs that persisted over the 5 − 6 yr of our study (i.e., Fig. 4a and b) and the differences we observed in native-seedling establishment and persistence between sites. Similarly, Davies and Bates (2017) reported sagebrush cover on northerly slopes was 19 times higher than drier southerly slopes 4 yr after seeding plots in the northern Great Basin. Variation in seedling establishment (i.e., colonization capacity) among slope aspect classes has been attributed to differences in water availability (Cantón et al., 2004; Bochet et al., 2009), a key environmental variable controlling seedling germination, growth, and survival in semiarid ecosystems (e.g., Call and Roundy, 1991; Cespedes et al., 2012; Liu et al., 2016). Accordingly, we suspect there was lower moisture stress associated with the northerly aspect of Point Springs, which contributed to greater overall initial native species establishment at Point Springs in both seeding yr and greater forb/ shrub establishment and survival in the 2010 seeding compared with Gunnell Pivot. Interestingly, despite resumption of grazing in 2012 at only Point Springs, this additional biotic stress did not detrimentally impact native species establishment and treatments generally had greater

C. Morris et al. / Rangeland Ecology & Management 72 (2019) 237–248

influence on establishment at this site compared with Gunnell Pivot. These results are consistent with others who found that slope aspect and heat load are important factors associated with resilience of native plant communities (Chambers et al., 2014; Bernards and Morris, 2017; Kimball et al., 2017) and restoration success in the Great Basin (Knutson et al., 2014; Davies and Bates, 2017). Variation in native seedling establishment and survival might also be a consequence of soil conditioning associated with long-term occupation by crested wheatgrass and land-use history. First, previous research has illustrated that crested wheatgrass creates a plant-soil feedback wherein it gains an advantage when growing in its own soil relative to other seeded species (Perkins and Hatfield, 2014). Crested wheatgrass conditions soils by modifying both nutrient availability and soil microbiota, which increases its own competitive ability and impedes establishment of other species, including native forbs (Jordan et al., 2008; Perkins and Nowak, 2013; Blank et al., 2015). Thus, overall low survival of native seedlings in our study may be mediated partially through legacy effects associated with modified soil nutrients, fungal associations, bacterial interactions, and accumulations of viruses that favor the long-term resident species (e.g., Klironomos, 2002; Wolfe and Klironomos, 2005; Blank et al., 2015). Second, we suspect that differences in land-use history between our study sites (i.e., soil cultivation and crop production at Gunnell Pivot, but not Point Springs) predisposed Gunnell Pivot to having fundamentally different crested wheatgrass abundance and a distinct cultivation legacy effect on seeded species performance compared with the Point Springs. For example, previous studies have shown that cultivation legacies result in lower crested wheatgrass density, modification of numerous soil properties, and reduced native species cover and density (Morris et al., 2014, 2016). Furthermore, the establishment of seeded native species was shown to be lower on sites with a history of cultivation and crested wheatgrass occupation compared with adjacent sites that had not experienced this land-use legacy (Monaco et al., 2018). These interpretations offer a plausible mechanism for overall lower crested wheatgrass density and consistently lower native species establishment and survival at Gunnell Pivot than Point Springs. Consequently, we suggest that future research and suppression efforts should not overlook the potential influences of soil conditioning and cultivation legacy effects on the stability of crested wheatgrass stands and restoration of native species. Why Was Exotic Species Invasion So Low? Soil disturbance is known to promote invasion of exotic species by increasing resource availability (Hobbs and Atkins, 1988; Beckstead and Augspurger, 2004; Leffler et al., 2016), especially in the Great Basin region that has experienced degradation and loss of native species (Bradford and Lauenroth, 2006; Chambers et al., 2007; Reisner et al., 2013). However, the unintended consequences of soil disturbance associated with soil cultivation promoting exotic species invasion in stable crested wheatgrass stands (e.g., Hulet et al., 2010; McAdoo et al., 2017) did not occur, even though nine potential exotic species were encountered in our study. The absence of exotic weed invasion is particularly important given the regional concern of cheatgrass expansion and evidence that soil disturbances increases its establishment (Beckstead and Augspurger, 2004; Leffler et al., 2016). Interestingly, even though we drill-seeded native species, which can disturb soils and increase exotic weed invasion in the Great Basin (Pierson et al., 2007; Miller et al., 2012; Ott et al., 2017), exotic weeds did not increase at our sites. The obvious conclusion is that our low-disturbance suppression treatments did not create suitable seedbed conditions to favor exotic annual establishment (e.g., Evans and Young, 1972; Mitchell et al., 2017). Alternatively, the abundance of cheatgrass in seed banks is known to decrease with crested wheatgrass canopy cover (Gunnell, 2009), suggesting that our sites may have had innately low seed bank densities for cheatgrass compared with those reported in other studies (Hulet et al., 2010; Davies et al., 2013).

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Management Implications Our results revealed that consecutive herbicide treatments suppress crested wheatgrass for up to 6 yr, combined treatments can improve native species establishment in certain cases, and secondary invasion by exotic annual weed species can be entirely avoided through the application low-disturbance suppression methods. Although reinvasion of crested wheatgrass and exotic weeds from seedbanks was avoided, native species establishment was not consistently improved with suppression efforts, suggesting that the low-disturbance treatments we employed failed to create favorable conditions for seedling recruitment. Consequently, we presume that the level of suppression required to alter stand stability and reduce interference from crested wheatgrass on native species is likely more extreme than has been achieved in ours, as well as previous studies. We conclude that restoring floristic and structural diversity to stable stands using low-disturbance suppression techniques will require striking a cautious balance between suppressing crested wheatgrass via the creation of suitably-sized gaps in vegetation to minimize competition with seeded species while avoiding soil disturbances that are known to cause secondary invasions. Because seeding crested wheatgrass is still a management practice following wildfire due to its ability to suppress exotic annual grasses (Pellant et al., 2004; Monaco et al., 2012; Williams and Monaco, 2012), our study adds to the growing evidence that crested wheatgrass may not serve as a “bridge species” to native species recovery as expected (Cox and Anderson, 2004). We also conclude that a greater understanding is needed of how sites that vary in environmental factors, including slope aspect and land-use legacies, influence the outcome of diversification efforts of stable created wheatgrass stands as has been done for the native communities in this region (Chambers et al., 2014). As others have noted, novel approaches are greatly needed to overcome the limitations to restoration success in the Great Basin (Svejcar et al., 2017), possibly with the use of native species specifically developed for such novel ecological conditions (Jones et al., 2010; Jones et al., 2015). Supplementary data to this article can be found online at https://doi. org/10.1016/j.rama.2018.10.009.

Acknowledgments Justin Williams oversaw data collection and contributed to the design of these studies. Lane Schumann of Arimo Corporation, Black Pine Ranches, was instrumental in site selection and animal management throughout the study. We also thank Kevin Gunnell and Tom Jones for reviewing previous drafts of the manuscript and offering insightful suggestions. Lastly, we thank many undergraduate students from Utah State University for their assistance with data collection and sample processing.

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