Repeated landscape-scale treatments following fire ... - CiteSeerX

Biol Invasions DOI 10.1007/s10530-015-0847-x

ORIGINAL PAPER

Repeated landscape-scale treatments following fire suppress a non-native annual grass and promote recovery of native perennial vegetation Seth M. Munson • A. Lexine Long • Cheryl Decker • Katie A. Johnson • Kathleen Walsh • Mark E. Miller

Received: 30 July 2014 / Accepted: 24 January 2015 Ó Springer International Publishing Switzerland (outside the USA) 2015

Abstract Invasive non-native species pose a large threat to restoration efforts following large-scale disturbances. Bromus tectorum (cheatgrass) is a nonnative annual grass in the western U.S. that both spreads quickly following fire and accelerates the fire cycle. Herbicide and seeding applications are common restoration practices to break the positive fire-invasion feedback loop and recover native perennial species, but their interactive effects have infrequently been tested at the landscape-scale and repeated in time to encourage long-lasting effects. We determined the

Electronic supplementary material The online version of this article (doi:10.1007/s10530-015-0847-x) contains supplementary material, which is available to authorized users. S. M. Munson (&) A. L. Long U.S. Geological Survey – Southwest Biological Science Center, 2255 N. Gemini Dr., Flagstaff, AZ 86001, USA e-mail: [email protected] C. Decker M. E. Miller National Park Service – Southeast Utah Group, Moab, UT 84532, USA K. A. Johnson National Park Service – Lassen Volcanic National Park, Mineral, CA 96063, USA K. Walsh National Park Service – Zion National Park, Springdale, UT 84767, USA

efficacy of repeated post-fire application of the herbicide imazapic and seeding treatments to suppress Bromus abundance and promote perennial vegetation recovery. We found that the selective herbicide reduced Bromus cover by *30 % and density by [50 % across our study sites, but had a strong initial negative effect on seeded species. The most effective treatment to promote perennial seeded species cover was seeding them alone followed by herbicide application 3 years later when the seeded species had established. The efficacy of the treatments was strongly influenced by water availability, as precipitation positively affected the density and cover of Bromus; soil texture and aspect secondarily influenced Bromus abundance and seeded species cover by modifying water retention in this semi-arid region. Warmer temperatures positively affected the nonnative annual grass in the cool-season, but negatively affected seeded perennial species in the warm-season, suggesting an important role of seasonality in a region projected to experience large increases in warming in the future. Our results highlight the importance of environmental interactions and repeated treatments in influencing restoration outcomes at the landscapescale. Keywords Bromus tectorum Cheatgrass Colorado Plateau Herbicide and seeding restoration treatments Imazapic Invasive species control Pinyon–juniper woodland Semi-arid Zion National Park

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Introduction Two common goals of restoration efforts at degraded sites are the establishment of native perennial vegetation and the reduction of invasive non-native plant species. Perennial vegetation can protect soil surfaces from erosion, create wildlife habitat, and promote the recovery of ecosystem function, whereas invasive non-native annual plants can displace perennial vegetation, suppress species diversity, and limit ecosystem recovery (D’Antonio and Vitousek 1992). Land managers in dryland ecosystems face tremendous challenges to meet restoration goals because low water availability limits the establishment of perennial vegetation and variable water availability favors fast-growing invasive non-native plant species that can take advantage of soil moisture for short periods of time when it is available (Booth et al. 2003). These challenges to restoration are likely to become more pronounced as the threats of land-use and disturbance intensification, including fire, create larger tracts of disturbed land than what existed historically (Dale et al. 2001; Westerling et al. 2006). Long-lasting restoration treatments are critically needed by land managers at the landscape-scale to promote perennial vegetation cover and limit the spread of invasive nonnative plant species. Bromus tectorum (cheatgrass) is an invasive annual grass from Eurasia that has rapidly spread across 22.7 million hectares of the western U.S. (Duncan et al. 2004) and greatly limits perennial revegetation following disturbance. Bromus overwinters as a seedling and is able to take advantage of soil moisture and nutrients in the early spring before many native perennial species begin to grow (Mack and Pyke 1983). Due to its early phenology, Bromus senesces by late spring to early summer and leaves abundant litter and dead biomass on the ground, which are extremely flammable as fine fuels (Knapp 1996). As a result, Bromus can alter the fire cycle from an average of 400? years in pinyon–juniper woodlands to\5 years (Floyd et al. 2004). Furthermore, post-fire landscapes are prime environments for additional Bromus expansion, since it is a prolific seed producer and can establish quickly after a fire, whereas many native species are slow to recover (Stewart and Hull 1949). This early emergence and rapid spread can result in a positive feedback loop for fire-cycles and Bromus

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invasion, thereby perpetuating larger and more intense fires that may harm native perennial species that may not be fire-adapted (Brooks et al. 2004) and irreversibly altering ecosystem properties (D’Antonio and Vitousek 1992). Herbicide application coupled with the seeding of native species is a common restoration approach to interrupt the non-native grass fire-cycle and encourage perennial vegetation establishment (DiTomaso 2000). Typically applied at relatively small sites, herbicide is often used to control pre- and post-emergence of undesirable species following fires (Thode et al. 2010). However, its efficacy to control the invasion of Bromus has not been widely tested at the larger landscape-scale ([100 ha; Morris et al. 2009). To guide effective management practices following large disturbances, there is a tremendous need to assess the successes and failures of landscape-scale restoration treatments (Menz et al. 2013). Such assessments need to weigh how much physical factors, including climate, soil, and topographic characteristics, contribute to invasibility and intended restoration outcomes. In addition to suppressing the abundance and spread of Bromus, another measure of the efficacy of herbicide treatment is whether it has unintended consequences on the performance of non-target species, including those used in the post-fire seed mixes. There are conflicting reports about the optimal amount and timing of imazapic herbicide to control Bromus and minimize impacts to native vegetation, including those used in seed mixes (Morris et al. 2009; Owen et al. 2011; Shinn and Thill 2004). Fall application (pre-emergence of Bromus) at even low rates has been shown to negatively impact Bromus and minimize impacts to the emergence of native perennial vegetation, whereas spring (postemergence of Bromus) is not always as effective (Matchett et al. 2009; Vollmer and Vollmer 2008). Testing how herbicide and seed mix treatments interact is essential for promoting recovery with minimal sideeffects. Finally, there is little knowledge on the efficacy of repeated restoration treatments to promote perennial native and limit non-native plant species, and encourage a resilient, self-supporting plant community (Munson and Lauenroth 2014). The primary objective of our study was to assess the efficacy of landscape-scale post-fire management actions to reduce B. tectorum and enhance native

Treatments suppress non-native and promote native vegetation

Our study took place from 2010 to 2012 at two sites within the perimeter of the Kolob Fire, which occurred

in June 2006 at Zion National Park in southern Utah (37°9.380 N, 113°29.350 W; Fig. 1). The Kolob Terrace site occurs at 1,340–1,500 m elevation and is dominated by Juniperus osteosperma and Pinus monophylla with herbaceous and shrub species in the understory (Appendix S1 in ESM). The Kolob Terrace site is on shale and sandstone deposits over a midPleistocene basaltic lava flow (Mortensen et al. 1977). The Crater Hill site occurs 7 km south of the Kolob Terrace site at 1,290–1,420 m elevation and is also dominated by the same species on mixed aeolian and alluvial deposits within a younger basaltic lava flow at the base of the Crater Hill cinder cone. Long-term (1904–2013) mean annual precipitation at Zion National Park is 380 (±111 S.D.) mm, the majority

Fig. 1 Location of the study transects at the Kolob Terrace (N = 60) and Crater Hill (N = 60) sites at Zion National Park in southern Utah. The zoomed-in figure inset to the right shows the different combination of treatments (C control, H herbicide

only, S seeding only, SH seeding and herbicide) in 2006 (horizontally displayed) and 2009 (vertically displayed) with a unique symbol (e.g., open circle) for all the transects at the two sites

perennial species. Our specific objectives were to: (1) determine how the aerial application of herbicide and a native perennial seed mix interacted with environmental factors to influence the abundances of B. tectorum and seeded species; (2) assess how these post-fire management actions influenced plant community composition, including non-target species.

Methods Study area

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of which occurs in the cool-season (October–April). Long-term mean annual temperature is 16.3 (±0.8) °C. Fire and treatment history The Kolob Fire started in June 2006 in the southwest corner of Zion National Park and burned as a highintensity crown fire onto adjacent Bureau of Land Management, state, and private lands (Fig. 1). The fire was the largest in the history of the park, burning a total of 7,135 ha, causing high mortality of perennial vegetation and opening up large areas for B. tectorum invasion. A 3,422 ha area affected by the fire was treated in late October—early November 2006 by aerial application of 0.6 L ha-1 of the selective herbicide imazapic (PlateauÒ) combined with 0.4 L ha-1 surfactant (LiberateÒ) delivered in 30 L ha-1 of water by a helicopter. An aerial seed mix application of 195 ha occurred at the same time as the herbicide treatment over a portion of the area treated with herbicide. The locally-sourced seed mix over the entire seeded area was composed of Elymus elymoides (1,210 kg or approx. 510 M seeds), Sporobolus cryptandrus (438 kg or approx. 5.12 B seeds), Penstemon palmerii (110 kg or approx. 142 M seeds), and Sphaeralcea coccinea (55 kg or approx. 60 M seeds). Through post-fire monitoring efforts (Brisbin et al. 2013; Thode et al. 2010), it was determined that the first herbicide treatment had minimal effectiveness in certain areas, likely influenced by application after Bromus germination (pre-emergence imazapic application is recommended). Therefore, a second treatment occurred in mid-late September 2009 and consisted of aerially broadcasting imazapic herbicide (0.6 L ha-1) with surfactant (0.4 L ha-1) across 1,257 ha. A seed mix was concurrently applied from a helicopter on 357 ha over a portion of the area treated with herbicide. The seed mix in 2009 had some of the same species as the mix in 2006: Penstemon palmerii (697 kg or approx. 900 M seeds), Sphaeralcea grossulariifolia (200 kg or approx. 220 M seeds), Elymus elymoides (110 kg or approx. 47 M seeds), and Sporobolus cryptandrus (32 kg or approx. 374 M seeds). Herbicide at the Crater Hill site was delivered in 30 L ha-1 water, as it was in 2006, but was delivered in 60 L ha-1 water at the Kolob Terrace site to increase penetration through a high density of dead woody vegetation.

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Sampling design A total of sixty 30 m transects at the Kolob Terrace site and sixty 30 m transects at the Crater Hill site were established. At the Kolob Terrace site, transects were established in a full factorial design and randomly selected and assigned to represent the fullinteraction between 2006 treatments: (1) control (no herbicide or seeding), (2) herbicide only, (3) seeding only, (4) herbicide and seeding; and 2009 treatments: (1) control, (2) herbicide only (Fig. 1). The sixty transects measured at the Kolob Terrace site expanded upon an existing set of forty-eight transects from a previous study that were used to monitor the effects of 2006 treatments (Brisbin et al. 2013). In contrast, all randomly selected and assigned transects at the Crater Hill site in 2006 were treated with: (1) herbicide only; and in 2009: (1) control, (2) herbicide and seeding. The sample size of transects was uneven across treatments due to differences in the number and combination of treatments at each site. However, the expanded number of transects after 2009 was intended to allow for greater precision in cover and density estimates after it was determined in 2006 that these estimates had high spatial variability. All transects at each site occurred in areas with similar surficial geology, burn severity, and tree density; were buffered by 15 m on all sides by the treatment assigned to them; occurred[30 m from each other, and[15 m from the nearest road (Thode et al. 2010). Vegetation and environmental measurements At each of the 30 m transects, we used a line-point intercept method to record plant species intercepted by points spaced 50 cm apart, resulting in 60 total points per transect. We collected Bromus density (number of individual plants) in five 0.5 m2 quadrats spaced 5 m apart along the first 25 m of the same transect used for line-point intercept. All vegetation measurements were conducted annually in April–June from 2010 to 2012. At the center of each of the transects, we recorded slope, aspect, and the percent cover class of rock fragments by size classes: gravel (2–75 mm), cobbles (7.5–25 cm), stones (25–60 cm), boulders ([60 cm), and bedrock (continuous rock). We also collected fifteen 0–10 cm soil subsamples with a corer of 7.6 cm in diameter at 2 m intervals along the 30 m transect, composited them, and analyzed texture using


particle size laser diffraction (Malvern Instruments Ltd., Worcestershire, UK). To determine the amount of herbicide applied, three water-sensitive deposition cards (52 9 76 mm) were placed at 5, 15, and 25 m along each of the transect locations before application. Following herbicide application, the cards were retrieved and the areal coverage of the herbicide droplets was read using automatic image analyzing software. We extracted mean monthly temperature (minimum, mean, maximum) and precipitation for each of the transect locations from the 4-km gridded PRISM dataset (PRISM 2014). Temperature and precipitation variables were compiled into cool (October–April) and warm (July–September) seasons. Data analysis We calculated plant species cover by summing the points intercepted by a plant species divided by the total number of points along the transect. Seeded species cover was calculated by summing species that were used in the seed mix. We used the average Bromus density across all five sampling quadrats. We selected the best transformations for these response variables using a Box–Cox procedure so the residuals were normally distributed with homogenous variance, but present untransformed data. We fit a mixed-effects model to explain Bromus density and cover, in addition to seeded species cover, which allowed us to account for fixed effects with both categorical (treatment, site) and continuous (climate, soil, and topographic attributes, as well as herbicide area) variables, in addition to uneven sampling design (Crawley 2007). Transect was included as a random effect in our mixed-effects model to control for pseudoreplication, and we found that an autoregressive moving average correlation structure was best to account for repeated measures (Pinheiro et al. 2014; nlme package in R). A Bonferonni outlier analysis was performed to identify significant transformed response variable outliers. Only explanatory variables found to be significant by zero-order correlations were included in the model. Due to multi-collinearity among temperature variables, we only included the temperature variable (mean, min., max.) that explained the most variance when it was significant. Because we used a mixed-effects model, it was necessary to calculate the marginal R2 (proportion of variance explained by fixed factors alone) and conditional R2 (proportion of

variance explained by both fixed and random factors) according to the r.squared.lme procedure (Nakagawa and Schielzeth 2013). The conditional R2 explains additional variability not accounted for by fixed effects at the transect-scale. We subsequently used hierarchical partitioning (Walsh and Mac Nally 2013; hier.part package in R), a statistical procedure that accounts for multi-collinearity among explanatory variables, to determine the relative % of the marginal R2 explained, and ANOVA with a Tukey HSD to compare treatment differences. We used non-metric multi-dimensional scaling (NMDS) and permutationbased MANOVAs with a Bray–Curtis similarity distance measure to assess differences in plant community composition between restoration treatments (not including Bromus, which was assessed separately). Power transformed cover (see above) of all 119 plant species was used in the NMDS distance matrix, and 500 iterations were conducted to reach a final stress of 0.09 in 3 dimensions. We further quantified species cover differences among treatments using pairwise t tests.

Results Fixed effects explained about half the density and cover of B. tectorum (density: marginal R2 = 0.45, P \ 0.0001; cover: marginal R2 = 0.58, P \ 0.0001), whereas repeated measures of transects explained more (density: conditional R2 = 0.53, P \ 0.0001; cover: marginal R2 = 0.87, P \ 0.0001) (Fig. 2a). Of the fixed effects, annual and cool season precipitation were positively and most strongly ([20 % of the marginal R2) related to Bromus density (annual: 2.0 ± 0.2 individuals m-2 mm-1, cool-season: 2.2 ± 0.2 individuals m-2 mm-1) and Bromus cover (annual: 0.13 % mm-1, cool-season: 0.14 % mm-1). There was no significant effect of the 2006 herbicide and seeding treatments on either Bromus density (P = 0.46) or cover (P = 0.11). There was a significant 2009 treatment effect on Bromus density and cover, which interacted with site and year of measurement (all P \ 0.05). Because of these significant interactions, we examined treatment effects on cover and density separately by site and year and found that there were 61 and 52 % reductions of Bromus density in treatment relative to control areas in 2011 at the Crater Hill and Kolob Terrace sites, respectively, and

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an additional 55 % reduction at the Kolob Terrace site in 2012 (Fig. 3). Bromus cover was reduced by 25 and 13 % in 2010 and 2011, respectively, in treatment relative to control areas at the Crater Hill site, and 34 % in 2010 at the Kolob Terrace site. It was not entirely possible to differentiate the herbicide and seeding treatment from the herbicide only treatment because the former treatment only occurred at the Crater Hill site. There was higher Bromus density on north- and west-facing aspects than east- and south-

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facing aspects (F3,337 = 8.09, P \ 0.0001), and higher Bromus cover on east- than north- and southfacing aspects (F3,337 = 4.02, P \ 0.01). Bromus density was negatively associated with gravel (r = -0.26, P \ 0.0001) and positively associated with silt (r = 0.16, P \ 0.01), whereas Bromus cover was positively associated with gravel (r = 0.22, P \ 0.0001) and sand (r = 0.21, P \ 0.0001), and negatively associated with areal coverage of the herbicide (r = -0.25, P \ 0.0001). Fixed effects explained nearly one-third of the variability in seeded perennial vegetation cover (marginal R2 = 0.30, P \ 0.0001), and repeated measures of transects explained almost twice as much variability (conditional R2 = 0.55, P \ 0.0001; Fig. 2). The 2006 treatments explained 50 % of the marginal R2, whereas the 2009 treatments explained 19 %, and both treatments interacted with each other (F8,337 = 3.11, P \ 0.05). At the Crater Hill site, herbicide in 2006 and herbicide and seeding in 2009 resulted in 334 % more seeded perennial vegetation cover than herbicide in 2006 and no treatment in 2009 (Fig. 4). At the Kolob Terrace site, seeding in 2006 and herbicide in 2009 were most effective, producing 276 % more seeded perennial vegetation cover than the next most effective treatment, seeding in 2006 and no treatment in 2009. Seeded perennial vegetation cover was positively related to applied herbicide area in 2009 (r = 0.27, P \ 0.0001) and sand (r = 0.21, P \ 0.0001), and negatively related to Bromus density (r = -0.24, P \ 0.01) and summer maximum minimum temperatures (r = -0.19, P \ 0.01). There was no significant difference in plant community composition among 2006 treatments at the Kolob Terrace site (F3,56 = 1.22, P = 0.11), and 2006 treatment differences at the Crater Hill site could not be compared because there was only one treatment (herbicide only). However, there was a consistent difference between plant community composition between 2009 treated (herbicide and seeding at the Crater Hill site and herbicide only at the Kolob Terrace site) and control transects for all years of the study (Crater Hill: F1,58 = 10.98, P \ 0.0001; Kolob Terrace: F1,58 = 4.84, P \ 0.0001) (Fig. 5). The primary differences in plant community composition between treatments at the Crater Hill site were attributable to higher Erodium cicutarium (t = 20.04, P \ 0.0001) and Sisymbrium altissimum (t = 21.35, P \ 0.0001) cover, two early-growing


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Fig. 3 Density (solid) and cover (striped) of Bromus tectorum from 2010 to 2012 at the Crater Hill and Kolob Terrace sites in control (black) and treatment areas (gray) (Crater Hill treatment: herbicide and seeding; Kolob Terrace treatment: herbicide).

Error bars are standard errors. An asterisk represents a significant difference (P \ 0.0001) between control and treatment within a year

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bars are standard errors. An asterisk represents a significant difference (Tukey-adjusted P \ 0.0001) between the treatment it appears above and all other treatment types

non-native annual forbs, at the control relative to the treated transects; in contrast to higher cover of the perennial grass, Elymus elymoides (t = 4.03, P \ 0.05), and annuals, Vulpia octoflora (t = 8.42,

P \ 0.01), Machaeranthera tanacetifolia (t = 26.62, P \ 0.0001), and Lupinus kingii (t = 12.34, P \ 0.001) at the treated relative to control transects. Similar to the Crater Hill site, the differences between

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results show significant differences in plant community composition between control and herbicide and seeding (Crater Hill) or herbicide (Kolob Terrace) treatments

control and treatment transects at the Kolob Terrace site were attributable to higher Erodium (t = 23.99, P \ 0.0001), and lower Elymus (t = 6.75, P \ 0.01) and Vulpia (t = 9.10, P \ 0.01) at the control relative to the treatment transects. However in contrast to Crater Hill, Lupinus was higher (t = 5.47, P \ 0.05) at the control transects. An additional perennial grass, Sporobolus cryptandrus was higher (t = 4.27, P \ 0.05) at the treatment transects and the annual forb species, Lotus denticulatus, was higher (t = 5.47, P \ 0.05) at the control transects. The seeded Penstemon palmerii and Sphaeralcea spp. did not have high enough measured cover to detect treatment effects.

where herbicide was applied for 2 years following treatments (Thode et al. 2010), and the seedbank of Bromus was reduced 1 year following herbicide application (Brisbin et al. 2013). In our extended post-Kolob Fire monitoring, we were not able to detect a significant effect of the 2006 treatments on Bromus density and cover, nor an additive effect in areas that had been repeatedly treated. Collectively, these results demonstrate a B3 year efficacy of the herbicide with little benefit of multiple treatments that extend beyond the recovery time of Bromus. A first year treatment effect on cover and lagged second and third year effect on density suggests an initial suppression of the capacity of the annual invasive to grow and a delayed secondary effect on germination. The herbicide most likely had relatively strong delayed impacts on density through a direct effect on emergence from the seed bank and indirect reductions in seed production from less vigorous plants within the first year of treatment. Although previous results have shown a slight to no effect of seeding on reducing Bromus when it is used in combination with herbicide (Owen et al. 2011), we were not able to separate the effect of seeding from herbicide due to 2009 treatment differences between the Crater Hill and Kolob Terrace sites. Precipitation was equally to slightly more important than the 2009 treatments in explaining the density and cover of Bromus, demonstrating the effect water availability has on the spread of non-native species in semiarid regions. Cool-season precipitation, in particular, can drive the long-term abundance of Bromus on the Colorado Plateau (Munson et al. 2011) by influencing

Discussion Our results demonstrate that landscape-scale herbicide and seeding treatments had a transient effect on the density and cover of B. tectorum. Herbicide application reduced Bromus density by over half at the Kolob Terrace site in the second and third years and at the Crater Hill site in the second year after the treatments. Herbicide was more effective at reducing Bromus cover than density in the first year at both sites and into the second year at the Crater Hill site, although there was still above 20 % cover and over 200 individuals m-2 in the most effectively treated areas. Post-2006 monitoring of the Kolob Fire in another study that followed the first application of herbicide showed similar declines in Bromus density and cover in areas

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seed germination in the late fall, in addition to growth and seed production in the early spring. Fire-induced reductions in cover of native perennial species further release the limiting resource of water to be used by the non-native annual grass (Davis et al. 2000; Melgoza et al. 1990). Warmer annual and cool-season maximum temperatures were also related to high Bromus density and cover. Given the early phenology of this non-native grass and its growth limitation at cold temperatures (Evans and Young 1972), we attribute the influence of warmer temperatures in the cool-season as a means for Bromus to become physiologically active and access early spring soil moisture. A manipulative experiment in southern Utah demonstrated that Bromus advanced the start of its spring growth and had increased biomass and reproductive output in artificially warmed plots in a year that had adequate soil moisture (Zelikova et al. 2013), revealing the important interaction between temperature and precipitation on Bromus performance. Physical attributes had an important but secondary influence on the abundance of Bromus once accounting for restoration treatments and climate. North-facing slopes likely supported higher Bromus density because they receive less direct sunlight and resultant evaporative losses relative to south-facing slopes, although Kulpa et al. (2012) found higher Bromus density on south-facing slopes. West-facing slopes may have supported higher Bromus density due to a predominant rain shadow effect in the region (Bradley and Mustard 2006). In contrast, east-facing slopes may have supported higher cover because they are exposed to wind that moves Bromus seed in a predominant west to east direction in this region. Gravel likely impeded the density of Bromus by limiting soil space and water-holding capacity for germination. However, gravel may have increased cover, especially at the Crater Hill site that had high amounts of gravel-sized cinder, by intercepting rainfall, slowing sheetflow and decreasing evaporative losses (Poesen and Lavee 1994). The high water and nutrient-holding capacities of silt had a positive effect on Bromus density, which is a grain size that has previously shown to positively influence the establishment of B. tectorum (Miller et al. 2006). The higher Bromus density at the Kolob Terrace site and higher cover at the Crater Hill site highlights an important distinction in the capacity of an annual invasive to germinate and its capacity to grow, which may differ according to physical attributes of a site. From a management perspective, high cover of invasive annuals may be of greater concern due to its role in fueling fire in

ecosystems with historically sparse vegetation that did not burn frequently (Brooks et al. 2004). The 2006 seeding without herbicide treatment had a large effect on enhancing seeded perennial species cover 4–6 years after seeding, whereas herbicide on its own or in combination with seeding had a relatively small effect on seeded perennial species cover 1–3 years after treatment. Given the low germination and establishment rates in the Great Basin (Chambers et al. 2007), it was not surprising that seeding treatments on their own required more than 3 years to be effective on the adjoining Colorado Plateau. Furthermore, seeding may be necessary to overcome the low native perennial seed banks after fire on the Colorado Plateau, as is the case in the Great Basin (Humphrey and Schupp 2001). Higher cover of seeded species in the seeding only relative to the herbicide and seeding treatment in 2006 indicates that the herbicide may have suppressed the effectiveness of the seeding treatment. However, the most effective combination of treatments in creating perennial vegetation cover was seeding in 2006 and herbicide application in 2009 at the Kolob Terrace site and herbicide and seeding in 2009 at the Crater Hill site. The efficacy of herbicide is further supported because a high applied herbicide area on the ground, as indicated by the deposition cards, following the 2009 treatment and resultant low Bromus density was positively related to seeded species cover. Although inter-annual precipitation did not explain seeded species cover, the contrast in herbicide effects in promoting seeded species cover in the 2009 versus 2006 treatments may have been due to a wetter set of cool-season years following the 2009 (2010–2012: 317 mm) compared to the 2006 (2007–2009: 214 mm) treatments. These wetter conditions may have allowed seeded species to overcome any negative impacts of herbicide application. Our contrasting result of herbicide efficacy is similar to other studies because imazapic can have both a positive and negative effect on seeded species establishment, which likely depends on the interaction between optimal application rate and timing (Morris et al. 2009) with the environmental conditions of the site. Sand content may have been positively related to seeded species cover because it allows water to percolate outside the zone of high evaporative demand, controlled by high temperatures, which were negatively related to seeded species cover. Soils with high sand content may also have less imazapic persistence, which is positively associated

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with clay (Shaner and O’Connnor 1991). Differences between the conditional and marginal R2 indicate that 25 and 29 % of the variance for seeded species and Bromus covers, respectively, may be attributable to factors at the transect-level that we did not measure, such as inorganic nitrogen, which can be reduced in areas invaded by Bromus because it is rapidly taken up in the biomass of the annual grass (Evans et al. 2001). Alteration of plant community composition due to 2009 treatments was primarily due to lower cover of two non-native annual forbs in treated areas, a result consistent with a general reduction of annual species with imazapic treatment (Elseroad and Rudd 2011). However, we also found that the other annual forbs, such as Machaeranthera were higher in treated areas, and that the legume Lupinus was higher in control transects at the Crater Hill site and treatment transects at the Kolob Terrace site. Our results suggest variable tolerances of plant species to the herbicide due to differences they have in an important enzyme that the active ingredient of imazapic binds to and blocks (Shaner and O’Connnor 1991), and potential site characteristics that modify the uptake of the herbicide. We are uncertain why the early-growing native annual grass, Vulpia, was higher in treated areas given its similar life history and structure as B. tectorum. In contrast to many annual forbs, perennial grasses were relatively unaffected by herbicide, and increased if they were seeded (Sporobolus and Elymus). This differential effect of herbicide may be attributable to a greater abundance of mature tissue in perennial compared to annual plants. Mature tissue is less affected by the enzyme inhibition of imazapic relative to new tissue (Shaner and O’Connnor 1991; Shinn and Thill 2004). The herbicide only treatment in 2009 subsequent to the seeding only treatment in 2006 may have been the most effective combination in promoting seeded perennial grass cover due to the herbicide having a relatively low effect on perennial grasses that had 3 years to establish mature tissue following the 2006 treatment. However, herbicide can negatively affect the earlier germination and seedling stages of seeded perennial grasses (Shinn and Thill 2004).

Conclusions and management implications Post-fire imazapic herbicide and seeding treatments interacted with climate, soil, and topographic

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characteristics at the landscape-scale to affect the density and cover of the non-native annual grass B. tectorum and cover of seeded perennial plant species. Our results demonstrate a[50 % reduction of Bromus density and *30 % reduction in cover following herbicide application, but these effects were relatively short-lived, had no additive effect with prior herbicide treatments in the same area, and may have negatively affected the first seeding treatment to re-establish perennial species in 2006. Herbicide application 3 years after perennial grasses from the 2006 seeding treatment had time to establish was the most effective strategy in promoting perennial species cover. This increase in seeded perennial species, in combination with changes to non-target species resulted in altered plant community composition in the 2009 follow-up herbicide only and herbicide plus seeding treatments. We suggest that when planning herbicide application, managers carefully balance the optimal application rate between a minimum to control the non-native target species and a maximum to limit negative impacts to non-target native species. From a timeefficiency standpoint, the aerial application of herbicide at the landscape-scale (3,422 ha) in 2006 took \2 weeks with two helicopters. The same treatment using ground-based application would have taken [1 year with a 10 person crew (NPS 2006), prolonging the treatment into times of the year the herbicide is less likely to be effective. Landscape-scale treatments can therefore be effective at restoring large tracts of land, especially if they are first tested at smaller scales, as was the case prior to the treatments in this study (Matchett et al. 2009). The efficacy of restoration treatments is strongly regulated by water availability in this semi-arid region, as inter-annual precipitation strongly influenced the abundance of Bromus and likely influenced the establishment of perennial plant species over multiple years. Physical factors, including aspect and soil texture, secondarily contributed to Bromus density and cover, in addition to seeded perennial species cover at our dryland study site, most likely by influencing the retention and timing of water availability. We suggest that managers carefully consider short-term climate forecasts and the role physical attributes have in regulating water availability in restoration areas to maximize the probability of restoration success. Warming temperatures that are already occurring at Zion National Park and forecasted


to continue (Hansen et al. 2014) will likely create new opportunities for winter and spring non-native annual species to expand their growing season if water availability is sufficient for growth, as we found Bromus had a positive response to increasing temperatures, especially in the cool-season. The results of our study provide insight on strategies to address potential future increases of non-native annual plant species at the landscape-scale. Acknowledgments This project was supported by funding from the National Park and the U.S. Geological Survey. We thank Linda Kerr, Kristin Legg, and Nate Benson for their support of this project. We also thank seasonal field crews at Zion National Park for taking vegetation measurements, and Matt Van Scoyoc, Jered Hansen, Harland Goldstein, and Jane Zelikova for technical assistance. Author contributions: M.E.M., C.D., and K.A.J. acquired funding and designed the research, K.W. collected the data, S.M.M. and A.L.L. analyzed the data and led the writing, and all authors contributed to the writing. Any use of trade, product, or firm names in this article is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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