Introduction

Scope of problem

A major component of ecological restoration is the removal of exotic species and their replacement with desirable native assemblages. However, complexities exist which frustrate these efforts,leading to,in many instances,sub-adequate results.^2,6,7 Exotic species may leave a legacy of changes in the system after their removal, including a buried seed bank or chemical or physical alterations to the site, making long-term restoration difficult. For example, firetree (Myrica faya) and buffelgrass (Cenchrus ciliaris) have been found to alter soil-available nitrogen (N) compared to native vegetation^14,15 while others, like cheatgrass, cause a redistribution of N within the soil profile.¹⁶ Similarly, species such as saltcedar (Tamarix ramosissima) can alter soil salinity and pH in the rooting zone, restricting germination and establishment of desired species.¹⁷ Many invasive species (e.g., cheatgrass) establish feedback systems leading to their persistence on the landscape.¹⁸ Other alterations in plant-soil feedback mechanisms attributable to invasive species and their impacts on successful restoration are still coming to light.¹⁹

Figure 1.

Cheatgrass (Bromus tectorum) has become a significant ecological pest in the Intermountain Region, occupying millions of hectares of western rangelands.

Cheatgrass rapidly invades Great Basin landscapes through expression of key physiological and morphological traits (Fig. 1). First, cheatgrass uses available water resources more efficiently than many native species by initiating early season germination and root and shoot growth at lower temperatures.^20-23 Second, cheatgrass can grow in extremely high population concentrations, with documented densities as high as 10,000 plants/m².²⁴ Cheatgrass is also highly efficient at using available N, resulting in expressed vigor and seed production.²⁵ Cheatgrass further alters ecological functions such as soil N cycling and wildfire frequency, making conditions less suitable for native plants and more suited to its own perpetuation.^26,27 Once cheatgrass abundance has crossed an ecological threshold of dominance within a location, reconversion of the site to its original state becomes extremely difficult.

Despite the ecological advantages exhibited by cheatgrass and other exotic weeds in disturbed sites, some native plants have been shown to have the capacity to resist annual grass invasion and build local site resilience.^28-32 Mature native perennial plants entrenched in areas of operative ecological function, for example, can be effective competitors against cheatgrass proliferation; however, lack of competitiveness at the seedling establishment stage remains a crucial barrier to restoration.^30,33,34 The vast scale of disturbance and large size of typical restoration efforts in the Intermountain sagebrush steppe make seeding, often with little or no seedbed preparation, the most feasible restoration treatment. Therefore, any improvements in landscape scale restoration from seed in this region would lead to meaningful changes in land management. Further investigation is needed to determine which native species can effectively compete against cheatgrass and other weeds during the beginning stages of restoration. One group that shows potential are the ruderal, or early seral, native plants.

Figure 2.

Conceptualization of initial floristic theory for rangelands of the Intermountain West. Modified from Egler³⁷ to include plant functional groups common to successional stages in the Intermountain West.

Plant community succession

Plant community succession is defined as a directional change in species composition or structure of a community over time.³⁵ While Thoreau³⁶ may have first coined the term “succession” in an ecological context, it is typically Clements³⁷ who receives credit for first codifying the theory. Clements viewed succession as a progression through 6 stages: (1) nudation, (2) migration, (3) establishment and growth of plants, (4) competition, (5) reaction, and finally (6) stabilization.³⁸ He viewed each seral step as distinct communities, with each subsequent community outcompeting the previous in a predictable, linear fashion. Clements' early ideas have been modified and replaced with newer models as ecologists have gained a deeper understanding of the underlying processes.³⁷ It is now understood that succession can have multiple possible trajectories and that initial species present following disturbance play a prominent role in the path of succession and future species composition.^22,39,40

One of the significant advances in successional theory was the idea of initial floristics presented by Egler.³⁷ He proposed that propagules of species from several successional stages can often be present at the time of disturbance and the onset of succession, either through long-term persistence in the seed bank or intermittent introductions from off-site. Those species of different seral stages then germinate or mature at different rates, and modify or utilize available resources, leading to shifts in community dominance. We see a similar pattern in Intermountain Western rangelands (Fig. 2).

Grime⁴¹ defined life-history traits of species that make them adapted to various stages of succession, classifying species as ruderal, competitive, or stress-tolerant. Ruderal species are associated with short life spans and high seed production making them better adapted to and more competitive in severely disturbed environments. Competitive species are those expected to maximize vegetative growth in productive and relatively stable environmental conditions, monopolizing the capture of resources by the spatially dynamic placement of roots and shoots.⁴² Finally, stress-tolerant species are associated with low disturbance habitats and have reductions in vegetative and reproductive allocations, lending them towards adaptation in high-stress environments. In layman's terms, in the Intermountain West, these categories can generally be described as early (ruderal), mid (competitive), and late (stress-tolerant) seral species, respectively.

Succession as a management tool – the value of early seral species

Natural succession after disturbance in the Intermountain West begins with the dominance of early seral species (i.e., mostly annual forbs and some perennials resprouting after fire) followed by establishment of perennial grasses and forbs and further transitioning to a suite of long-lived climax species.⁴³ Intermountain early seral species have evolved to colonize sites immediately following disturbance and are adapted to local soil and climatic conditions. They further fill an essential niche in space and time, reducing post-disturbance erosion and offering food sources for pollinators until later seral species become fully established.⁴⁴

Early seral species have been shown to alter soil biology and nutrient cycling in ways that ultimately favor later-successional species.^45,46 In a study examining the interactions between native annual plants and arbuscular mycorrhizal fungi (AMF), Busby et al.⁴⁵ found certain early seral species produced a more rapid increase in AMF compared to late seral species. Kieffer Stube⁴⁶ further found a significant effect on AMF richness or abundance from native early seral species. Understanding these relationships may help practitioners to restore soil biology and facilitate full site recovery. These and other processes, associated with early succession, need more exploration.

Early seral colonizers have been shown to manipulate soil chemical properties.They may hold dominance over the plant community until available N is depleted to the point where they no longer have a competitive advantage, resulting in the successional transition toward mid- and late-seral species (Fig. 3).^26,27,47,48 In contrast, invasive exotic species typically alter soil N dynamics in ways foreign to the naturally evolved community. In Intermountain shrublands, high levels of N produced in disturbance regimes have been linked to supporting the persistence of invasive weeds over native species.²⁷

The differential capacity of each species to acquire resources and nutrients drives succession. Therefore, rebuilding soil biology and chemistry can significantly enhance progression from a disturbed condition to the desired plant community.⁴⁹ Knowing this, managers can modify and manipulate successional processes to direct plant community dynamics and create invasion-resistant soils via N management.⁴⁸

Figure 3.

Plant mortality following disturbance creates high levels of mineralized N. Natural plant succession lowers N levels back to those suited for the potential natural community (PNC). Based on Reitstetter and Rittenhouse.²⁷

Regional studies in the Intermountain cold-desert region confirm the concept that the starting point of succession strongly influences the long-term trajectory. Hoelzle et al.⁵⁰ looked at community composition of sagebrush steppe seedings after 25 years of development. They found different compositions of early vs. late seral seed mixes can end at significantly different communities after long periods of succession time. For example, seeding a predominantly early seral mixture resulted in increased exotics and mid seral shrubs, while perennial grass mixtures led to greater dominance by perennial grasses. However, their “early seral” mixture included over 70% exotic weed seed such as cheatgrass, kochia (Kochia scoparia) and Russian thistle (Salsola tragus) and was not designed to test the effects of early seral natives.⁵⁰ McLendon and Redente²⁶ showed the magnitude of disturbance in the upper soil profile affected the trajectory and duration of the succession of species contained in the pre-existing soil seed bank in the Colorado sagebrush steppe. Plots with greater levels of disturbance were dominated by exotic annuals for the first 2 to 3 years and ultimately transitioned to a community dominated by mid and late seral shrubs. In contrast, low-severity disturbance treatments responded with a greater presence of perennial grasses.²⁶ Stevenson et al.⁴⁹ followed succession of early vs. late seral plantings within fumigated and non-fumigated plots in the Piceance Basin of northwest Colorado. They found seed mixture composition (early vs. late seral) significantly impacted species richness and productivity. After seven years, the early seral mix resulted in greater species richness and aboveground biomass than the late seral mix. These studies support the concept that available successional components (plants from a residual seedbank or disturbance survivors) have a strong influence on later species composition.

Because succession in semiarid communities of the Intermountain West occurs slowly - decades to centuries if not interrupted⁵¹ - the current practice of seeding mid- to late-seral seed mixes in post-disturbance landscapes may create a temporal lag between the removal or control of exotics and the actualization of the desired native plant community.⁵² Tilley et al.⁵³ reported after four years, many native grasses, especially long-lived rhizomatous perennials, had not reached their full expression at a restored site in southeastern Idaho. Only after ten years did they reach high densities due to rhizomatous spread. Similarly, mean cover of the late seral rhizomatous grass species western wheatgrass (Pascopyrum smithii) seeded into post-fire locations in western Utah increased 14-fold between 3 years post-fire and 16 years post-fire.⁵² Likewise, in a long-term succession study in southeastern Idaho, the dominant shrub Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) did not regain a significant onsite presence until 15 years post-fire,⁵⁴ leading the authors to conclude some conditions (not all defined by current research) of a more mature community may be required for later seral species to become established.

Figure 4.

Above- and below-ground diversity of growth forms and root structures fills spatial niches and increases resilience against invasion. Image courtesy of Jeremy Maestas and Maja Smith, Sage Grouse Initiative. Used with permission.

A potential strategy that may “even the playing field” against undesirable exotic weedy species such as cheatgrass, is to include “home-grown” weedy species in restoration strategies.^9,13,55,56 The notion of increasing resilience through increasing species diversity is well understood. Several studies have shown, for example, greater functional group diversity can translate to increased resistance against invasion (Fig. 4).^57,58 Similar to the resilience expressed by a healthy, diverse mature plant community, establishment of early seral species, which exhibit greater overlap in survival strategies with the invasive annuals in the Intermountain West, offer greater competitive advantages than late seral species.

Numerous greenhouse and field studies have shown that, in comparison to late seral species, early seral species are more effective at competing against, and even suppressing, exotic species. This outcome could be especially valuable to restoration efforts during the critical first years of post-seeding establishment. In controlled studies, several native annual Intermountain forbs were found to be highly competitive against cheatgrass.¹² Bristly fiddleneck (Amsinckia tessellatta) significantly reduced cheatgrass biomass, while fiddleneck and western tansymustard (Descurainia pinnata) reduced cheatgrass seed production by >79%.¹¹ In a similar greenhouse study, Perry et al.⁵⁶ found the annual native forbs, annual ragweed (Ambrosia artemisiifolia), and common sunflower (Helianthus annuus), reduced biomass of several weeds, including cheatgrass, Japanese brome (Bromus arvensis), Canada thistle (Cirsium arvense) and whitetop (Cardaria draba). Herron et al.⁹ likewise found a significant reduction in weed cover in plots seeded with early seral native annuals compared to mid and late seral perennials. Kieffer Stube⁴⁶ found early seral native species reduced cheatgrass biomass and density during establishment. Finally, Uselman et al.¹¹ showed early seral natives generally had earlier germination and better survival than late seral natives.

Figure 5.

Generalized conceptualization of seral stage progression and site resilience. Following disturbance, early seral species quickly establish providing initial resilience. The early seral species then naturally decrease in abundance as mid-seral perennial species increase. The ultimate “climax” community in the Intermountain West is often a mixture of late-seral shrubs and long-lived perennial grasses. Each seral group provides cover and resilience for a period following disturbance. Seeding a climax-only seed mixture leaves a resiliency gap allowing for establishment and proliferation of weeds. Developing restoration strategies that include temporal and functional species diversity can increase resilience throughout the life of the sere.

Application in Intermountain rangelands

It has been demonstrated that early seral species, especially annuals and short-lived perennials, occupy an ecological niche similar to many of the region's exotic grass invaders and are more capable of competing against potential invaders than the more dissimilar late seral species.⁵⁹ Therefore, it may be beneficial to include site-adapted, early seral species that have evolved to colonize a site following disturbance in restoration seed mixes to promote the establishment of the target plant community and build site resiliency.^9,11,55 Developing restoration strategies including species diversity in space and time can increase resiliency throughout the entire sequence of succession, eliminating gaps of low resilience (Fig. 5). Succession management strategies involving multiple seral stages could be used in heavily infested sites to improve restoration establishment outcomes and drive succession in a desired direction. This strategy may also be used in sites of low to moderate weed invasion where prevention of further degradation is the stated goal.

Contemporary restorationists have largely excluded early seral species as components of restorative seed mixtures.^13,51,57 Seed mix designs are usually based on efforts to duplicate the pre-disturbance, “climax”, plant community. Historically, the NRCS' state and transition models and Ecological Site Descriptions (ESDs) have provided preferred information on species' relative abundance to be used as a baseline for designing seed mixtures for sagebrush steppe restoration.⁶⁰ However, while these resources provide valuable information regarding the historical natural community and prospective successional trajectories, they do not include comprehensive information concerning the constituent species of early and mid-succession.

Current seeding guides and recommendations for Intermountain rangelands overlook principles of plant community succession.^51,61 For example, NRCS Idaho seeding guide Technical Note 24³ mentions succession only once, in the description for fern bush (Chamaebatiaria millefolium). However, it does mention several species as being “short-lived” and likely to be replaced over time by longer-lived species. Some of these “temporary” species include squirreltail (Elymus elymoides and E. multisetus), slender wheatgrass (E. trachycaulus), and smoothstem blazingstar (Mentzelia laevicaulus). Several other species, including blue and Lewis' flax (Linum perenne and L. lewisii), and several beardtongue species (Penstemon spp.) are noted as being short-lived, but this information is provided largely as a seed production factor for seed producers and not as a guiding principle for restorationists. Information provided by Lambert⁶² follows a similar pattern of disregard. Similarly, Jensen et al.⁶³ exclude succession theory in their planting guide but do include short-lived perennial grasses like slender wheatgrass in many of their seed mixture recommendations. Finally, Monsen et al.⁶¹ mention succession and warn against planting species that might inhibit the natural successional pathways. They also suggest using a combination of species to “initiate natural successional processes” but do not go so far as making recommendations based on the successional status of the species.

Seeding recommendations developed by some researchers do implicitly follow successional theory by employing the practice of using nurse crops to assist in site capture and reducing soil erosion.⁵⁶ Recommended species, especially for post-fire erosion control, can include non-native grasses like common wheat (Triticum aestivum), sterile triticale (xTriticosecale), or annual ryegrass (Lolium multiflorum).⁶⁴ Krueger-Mangold et al.¹² recommend using cover crops (i.e., nurse crops) to push competition in favor of native species. The assumption is these species will provide quick cover, die before reproducing, and not present a nuisance or obstacle to restoration goals. However, experience suggests an inherent risk with using exotic species in a novel environment.⁶⁵ Additionally, these non-native species will be deficient in characteristics critical to augmenting the site such that it is optimal for the establishment and health of succeeding late-seral plant populations.

A few short-lived native perennial grasses are included in restoration protocols and are produced within the commercial market. Slender wheatgrass and bottlebrush squirreltail are currently recommended and used as nurse crops.^3,4,64 Mooney and Drake⁶⁶ argued that the potential of native species as nurse crops has not been fully explored and more research in the field is required. Thus, a more extensive palette, inclusive of early seral forb species, is needed to fully capitalize on natural successional habitat progressions.

Which species offer effective early characteristics for restoration purposes?

Determining which species are of potential worth in succession-based restoration is problematic. Species present following disturbance have been documented, and at least a partial understanding of common traits shared by early seral species exists. In some cases, information about early seral tendencies for a species can be found in regional floras⁶⁷ and other sources. However, no comprehensive lists have been published grouping Intermountain species by seral category. Also, few long-term analyses have been published tracking successional phases from which to draw conclusions. McLendon and Redente²⁶ followed the succession within each of 4 types of disturbance in the Piceance Basin of northwest Colorado for 12 years. Their results included identification of one native early seral species, scarlet globemallow (Sphaeralcea coccinea). They identified a mid-seral group consisting of several native perennial grasses including Indian ricegrass (Achnatherum hymenoides), needle and thread grass (Hesperostipa comata), thickspike wheatgrass (Elymus lanceolatus) and western wheatgrass (Pascopyrum smithii), as well as two shrubs; rubber rabbitbrush (Ericameria nauseosa), and yellow rabbitbrush (Chrysothamnus viscidiflorus). The late seral component consisted of sagebrush and junegrass (Koeleria macrantha). Hoelzle et al.⁵⁰ found similar assemblages when they followed plant community succession for 25 years in sagebrush steppe in northwest Colorado. Their findings indicated several short-lived colonizers, including bottlebrush squirreltail, multilobed groundsel (Senecio multilobatus), scarlet globemallow, yellow rabbitbrush, rubber rabbitbrush, and broom snakeweed (Gutierrezia sarothrea). Ott et al.⁶⁸ mentioned several native species as being present immediately following wildfire or chaining treatments in western Utah, including common sunflower, hoary tansyaster (Machaeranthera canescens), scarlet globemallow, coyote tobacco (Nicotiana attenuata), and multilobed groundsel.

While studies tracking temporal succession are scarce, opportunities for observing ongoing succession under disturbed conditions are abundant. Along any stretch of highway in the Intermountain Region, identification of early seral native species is easily accomplished by observing those persisting and thriving in cyclically disturbed conditions. Intermountain roadsides also serve as ideal collection sites for adapted plant materials.⁵⁵ Highway corridors often experience frequent traffic, soil compaction, repeated mowing treatments, and recurrent fire. Often a distinct plant community occupies the first few meters of the roadside (Fig. 6) and further away phases into more mature habitat. There is typically a high presence of invasive weeds like cheatgrass and tumble mustard (Sisymbrium altissimum), but the same space may also host native species. Rubber rabbitbrush often makes up the overstory, especially further from the road where mowing is less frequent. Grasses here can include sand dropseed (Sporobolus cryptandrus) and purple threeawn (Aristida purpurea). Upright native biennial and annual forbs consist of curlycup gumweed (Grindelia squarrosa), common sunflower, and bristly fiddleneck, and there may be a component of prostrate forbs like bigbract verbena (Verbena bracteata). Moving further away from the roadside, the plant community transitions, sometimes very distinctly, to a late seral, perennial-dominated system (Table 1).

Figure 6.

Intermountain roadways demonstrate plant community succession. Early seral members commonly include curlycup gumweed (Grindelia squarrosa), common sunflower (Helianthus annuus), rubber rabbitbrush (Ericameria nauseosa), and sand dropseed (Sporobolus cryptandrus).

Development of early seral plant materials

While wildland-collected seed of Intermountain West early seral species may be available in limited quantities, few released germplasm sources from this group exist. Several cultivars of slender wheatgrass have been developed and released. ‘Pryor' was released by the Bridger, Montana NRCS Plant Materials Center and the Montana and Wyoming Agricultural Experiment Stations in 1988. ‘San Luis’ was released in 1984 by the Colorado, and New Mexico Agricultural Experiment Stations, NRCS, and the Upper Colorado Environmental Plant Center.⁶⁹ More recently, ‘First Strike’ was released cooperatively in 2006 by the Agricultural Research Service (USDA-ARS) and the U.S. Army.⁴ Likewise, the Logan, Utah USDA-ARS Forage and Range Research Laboratory (FRRL) has made several germplasm source releases of squirreltail, including ‘Toe Jam Creek,’ ‘Pleasant Valley,’ ‘Antelope Creek,’ ‘Fish Creek,’ and ‘Rattlesnake’ bottlebrush squirreltail, as well as ‘Sand Hollow’ big squirreltail (Elymus multisetus).⁴

Regional early seral forb releases are poorly represented in the plant materials catalog. Most early seral research to date for the western U.S. has been conducted on native annuals, few of which are well-suited for commercial seed production. See de Queiroz et al.⁷⁰ for a discussion on Great Basin annual forb seed increase. Many species, such as foothill deervetch (Lotus humistratus), bigbract verbena, and sixweeks fescue (Vulpia octoflora)^9,71 are low-statured or prostrate in growth habit, making seed harvest difficult (Fig. 7). Still others such as blazingstar (Mentzelia spp.) and slender phlox (Microsteris gracilis)¹⁰ produce limited quantities of seed, making them economically unsustainable and prohibitively costly for most restoration projects. Development and release of early seral species that serve the same function, while having better seed production attributes, would be both beneficial for restoration and more likely sustainable amid market forces.

Table 1

Native Intermountain rangeland species presented as short-lived, early seral colonizers of semiarid lands within various publications. Species are grouped by family and then alphabetically within families.

A species showing promise is hoary tansyaster. Barak et al.⁷¹ found a close relative, tansyleaf tansyaster (M. tanacetifolia), to be a potentially valuable species for restoration of cheatgrass-invaded rangelands. Because it lacks seed dormancy, hoary tansyaster has been observed to germinate after late summer and fall rains, a beneficial trait as it allows the species to seasonally compete with cheatgrass.^71,72 In greenhouse competition studies,Parkinson et al.⁷² found biomass of hoary tansyaster was not reduced after 12 weeks of growth in the presence of the native grasses, bottlebrush squirreltail and Sandberg bluegrass (Poa secunda) when compared to plants growing alone. However, Tilley et al.⁷³ did not find hoary tansyaster to reduce cheatgrass biomass or volume when the two species were planted together. Cheatgrass at a density of 100 plants/m² significantly reduced total biomass and relative growth rate of hoary tansyaster shoots, but not rate of root growth.⁷³ ‘Amethyst’ is a selected class germplasm of hoary tansyaster released by the Aberdeen, Idaho PMC (IDPMC) in 2015, that is commercially available.⁷⁴

Figure 7.

Bigbract verbena (Verbena bracteata) is a common Intermountain West early seral forb. Despite its colonizing abilities, its prostrate growth habit inhibits efficient seed production. Selecting early seral species with suitable seed production attributes can keep costs down and facilitate their adoption for use in restoration practices.

Another native species currently under evaluation and selection at IDPMC is curlycup gumweed, a common native forb found along roadsides and disturbed sites throughout the west (Fig. 8).⁷⁵ This forb exhibits significant benefits for pollinators⁷⁵ and has been documented as a dietary component of greater sage-grouse.⁷⁶ Recent studies have also indicated curlycup gumweed possesses fire-retardant capabilities making it a valuable component of fuel break greenstrips in the west.⁷⁵ Despite these benefits, only minimal acceptance of gumweed is prevalent with regard to use in rangeland restoration. In fact, this species has been designated as a weed by several counties in Wyoming,⁷⁷ where it is actively sprayed by county weed crews despite its being a native species.

Figure 8.

Curlycup gumweed (Grindelia squarrosa), a native, early seral forb, is being investigated for plant germplasm release by the NRCS Aberdeen Plant Materials Center (Idaho). Its attractiveness to native pollinators, seed production attributes, and ability to establish in disturbed soils make it a worthy candidate for succession management in the Intermountain West.

Additional early seral forb and grass plant releases are needed to adequately supply early seral species for conservation and restoration projects. Developers of plant materials would benefit from a framework of filters through which potential species for research and evaluation could be identified. Questions that could be posed include 1) is the species competitive against invasives? 2) how effective is the species in a pioneer role within a disturbed site? 3) does the species possess suitable seed production attributes for large scale production and commercial success? and 4) what is the effective geographic range and area of adaptation? Ironically, a good search tool for identification of suitable early seral natives may be Weeds of the West.¹

A strategy for identifying early seral native species with high competition potential is to make seed collections from highly disturbed sites. Leger⁵⁵ suggested managing disturbed sites undergoing competitive pressure from introduced annuals could lead to the rapid evolutionary selection of competitive native phenotypes. These high competition areas would naturally create selection pressure for the most adaptive traits for disturbed site restoration. This idea is intriguing but has yet to be fully explored.

Risks and questions

Ultimately the acceptance of early seral native weeds for use in restoration necessitates answers to two questions. First, will early seral natives migrate outside of the restoration site and degrade communities where they are not wanted? Second, will the addition of early seral native species facilitate a transition to the desired plant community, or will they hinder successional processes and ultimate climax species composition?

Intuitively, the answer to the first question is “no.” Early seral species are, by definition, not likely to invade and persist in a healthy climax community⁴¹ and should only occupy a degraded site. For example, non-local germplasms of curlycup gumweed⁷⁸ and common sunflower, both North American native species, are becoming increasingly common along roadsides and in disturbed areas around the west leading some to conclude they are invasive. However, there is no evidence of these species becoming a problem in healthy, species-rich landscapes. Long-term monitoring is needed to fully support the non-invasive idea, but it seems likely apparent increases in sunflower and gumweed result from conditions of perpetual disturbance.

Answering the second question is more complicated. It is conceivable that overloading a restoration seed mixture with highly competitive early seral species could prevent or delay the longer-lived perennial species from becoming adequately established. In some instances, early seral species could simply be too competitive to be useful, in which case using those particular early seral native species for assisted succession may not lead to a full transition back to the climax community or may send succession on an unwanted path.^13,50 It is also possible the inclusion of early seral species might not provide enough benefit to warrant the added cost.

An additional concern with respect to the use of some early seral native species is their suitability for use in grazing lands. Many native forbs found in Weeds of the West are included due to low palatability or toxicity.¹ While these concerns are prevalent among ranchers, a better understanding of succession dynamics may help dispel these fears. First, early seral plant communities are ephemeral and will transition with time to later seral plant assemblages, and their long-term persistence is unlikely. Of greater significance, however, are the long-term benefits of successful restoration to western rangelands including increased resilience, decreased risk of catastrophic wildfire and improved range condition.

The next steps in developing succession management protocols for the Intermountain West should include field trials to evaluate the role, benefits, and risks of early seral native species. Field-scale trials of varying seed mixtures are also needed to compare ratios of seeds representing various successional and functional groups to determine the best mix compositions. Also, studies are needed to assess the efficacy of stepwise establishment of early, mid- and late-seral species. Long-term monitoring, as opposed to the standard 2 to 4-year studies suitable for career-dependent publications, will also be needed to observe successional change on a meaningful time scale. Such studies could examine the cost of multi-successional restoration seed mixes versus the potential for additional restoration measures when late seral mixes fail and will help guide future restoration decision strategies.

Conclusions

Early successional species, especially annuals and short-lived perennials, represent an integral component of the plant community. Evolution and selection over the lifetime of an ecosystem have produced species with traits pivotal in successful habitat restoration. Many of these species are similar in morphology and function to some of the region's most troublesome exotic invaders. Early seral species are well adapted to establish in disturbed sites under difficult conditions. While these traits are often associated with a perception of weediness, they should be recognized as providing advantages to enhancing competitiveness that will reduce the impact of invasion from exotics and conserve native site processes.⁶⁸ Unfortunately, due to ignorance of disturbed site dynamics, many of these species have been branded as unfit for restoration purposes.

Succession management is an underutilized tool in the western sagebrush steppe. While the benefits of morphological diversity have been widely accepted, the temporal diversity of successional groups has yet to be fully incorporated into practice. By integrating the ideas of plant community succession in restoration plans, restorationists can naturally and more successfully alter site conditions and guide succession towards the desired state. One obstacle to achieving this goal is the limited number of early seral plant materials currently available at the scale needed to support restoration efforts. The BLM, NRCS, US Fish and Wildlife Service (FWS), and other agencies have recently developed a Native Plant Strategy with the mantra to “use the right plant at the right time.”⁷⁹ Conservation practitioners should strive to use the right plant to meet resource objectives, whether for forage production, disturbed site reclamation, or native habitat restoration. The species of choice could be an introduced cultivar, a long-lived, local, perennial ecotype, or it might just be an unassuming, short-lived native with little forage value that has evolved over millennia to colonize disturbed sites and pave the way for climax species. If a species is on a weed list that does not mean it should be automatically excluded from consideration in reclamation and restoration seedings.

Weeds have evolved, for good and bad, to quickly invade and colonize sites opened by disturbance. This is a trait restorationists should be exploiting rather than resisting. Successional management and early seral natives are tools at the disposal of ecologists, which, if properly utilized, could generate meaningful strides toward improved restoration outcomes. Developers of plant materials should continue to evaluate new species for conservation and restoration value, including those designated as native weeds, because sometimes a weed is the right plant, in the right place, at the right time.

Declaration of competing interest

Co-author Jason Karl is the current Editor in Chief of Rangelands but was not involved in the review or decision process for this manuscript.