Manon C M Hess
email: hess@tourduvalat.org

François Mesléard

Elise Buisson

Priority effects: Emerging principles for invasive plant species management

Keywords: Invasion resistance, Restoration, Community assembly history, Historical contingency

Many anthropic activities generate soil disturbances, favoring competitive, fast growing invasive plant species at the expense of natives. Active restoration of invasionresistant plant communities is increasingly recognized as a relevant strategy to combat invasive plant colonization in disturbed areas, but results are often unsatisfying. Historical competitive advantage through early emergence and quick growth, but also self-induced soil modifications that can persist after their removal or death, commonly termed "soil legacies".

In fertile and stable conditions, the reduction of invasive species priority effects must be coupled with the restoration of an invasion-resistant native plant cover to avoid reinvasion and secondary invasions. The second strategy is to bring about situations in which the restored native species are more likely to exert strong priority effects, decreasing invasion success. For this purpose, we sketch possible options open to restorationists based on resource or nonresource mechanisms. First, we discuss ways to maximize resource preemption by extending the time advance given to restored native species and manipulating restored species characteristics. Second, we consider the potential effect of increasing niche overlap between native and invasive species. Third, we introduce the potential manipulations of non-resource mechanisms, such as allelopathy, herbivory, disease, or the presence of mycorrhizae, to increase priority effects. This review incorporates recent research on priority effects to draw the outlines of priority effects-based restoration strategies and define future research questions that need to be addressed to test and improve these strategies.

effects, referred as "priority effects" (i.e. the effects generated by the order in which species arrive at a local site), can have a major role in community assembly and invasion success because they involve early colonists altering the performance of later colonists. Taking these priority effects into account in restoration projects is emerging as a relevant way to improve native species restoration success and prevent invasion. The present review discusses two strategies considering priority effects that would help to achieve the classic restoration goal of "more natives, less invasives". The first strategy relies on tackling priority effects of invasive plants using different management options adapted to local environmental conditions, including removal, reduction of propagule sources, or mitigation of soil legacies. Indeed, invasive plants often generate strong priority effects providing themselves a substantial

Introduction

The vast literature on biological invasions since Elton's seminal work (1958) testifies to the complexity of understanding the processes underlying invasion success [START_REF] Levine | Mechanisms underlying the impacts of exotic plant invasions[END_REF][START_REF] Hayes | Are there any consistent predictors of invasion success?[END_REF][START_REF] Simberloff | Biological invasions: much progress plus several controversies[END_REF]. According to deterministic theories, the outcome of an invasion depends on interactions between the invader and the physical and biological characteristics of the recipient environment [START_REF] Lonsdale | Global patterns of plant invasions and the concept of invasibility[END_REF]Williamson, 1999).

The intrinsic competitive superiority of invasive plant species (i.e. species introduced outside of their distribution areas which are able to grow and proliferate to become an autonomous viable population, and whose expansion can negatively impact local species and ecosystems; [START_REF] Richardson | Naturalization and invasion of alien plants: concepts and definitions[END_REF][START_REF] Mooney | Invasive alien species: the nature of the problem[END_REF] in acquiring resources has long been considered the critical mechanism determining invasion success [START_REF] Sax | The paradox of invasion[END_REF][START_REF] Vila | Are invasive plant species better competitors than native plant species?evidence from pair-wise experiments[END_REF][START_REF] Pyšek | Traits associated with invasiveness in alien plants: where do we stand?[END_REF]. However, invasive species performance also appears to depend on the physical and biological conditions encountered in the introduced range: resource availability and fluctuation [START_REF] Davis | Fluctuating resources in plant communities: a general theory of invasibility[END_REF][START_REF] Shea | Community ecology theory as a framework for biological invasions[END_REF][START_REF] Firn | Early emergence and resource availability can competitively favour natives over a functionally similar invader[END_REF][START_REF] D'antonio | Interactions among invasive plants: lessons from Hawaii[END_REF], multi-trophic interactions involving enemies [START_REF] Mitchell | Biotic interactions and plant invasions[END_REF], and resident plant community composition [START_REF] Levine | Elton revisited: a review of evidence linking diversity and invasibility[END_REF][START_REF] Fridley | The invasion paradox: reconciling pattern and process in species invasions[END_REF]. In addition to the deterministic explanations, stochastic dispersal and historical processes also play a crucial role in determining invasion success [START_REF] Hubbell | The unified neutral theory of biodiversity and biogeography[END_REF][START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF][START_REF] Chase | Community assembly: when should history matter?[END_REF][START_REF] Lockwood | The role of propagule pressure in explaining species invasions[END_REF][START_REF] Dickson | Do priority effects benefit invasive plants more than native plants? An experiment with six grassland species[END_REF][START_REF] Wilsey | Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes[END_REF][START_REF] Young | Initial success of native grasses is contingent on multiple interactions among exotic grass competition, temporal priority, rainfall and site effects[END_REF].

Stochastic colonization combined with deterministic interactions between early and later colonizing species lead to priority effects [START_REF] Case | Invasion resistance arises in strongly interacting species-rich model competition communities[END_REF][START_REF] Chase | Disentangling the importance of ecological niches from stochastic processes across scales[END_REF], where early-arriving species affect the establishment, survival, growth or reproductive success of later-arriving species [START_REF] Helsen | A test of priority effect persistence in seminatural grasslands through the removal of plant functional groups during community assembly[END_REF]. There is growing evidence that priority effects play a crucial role in community assembly, especially in productive environments [START_REF] Chase | Community assembly: when should history matter?[END_REF][START_REF] Aronson | Long-term vegetation development of restored prairie pothole wetlands[END_REF][START_REF] Körner | Small differences in arrival time influence composition and productivity of plant communities[END_REF][START_REF] Young | Using priority effects to manipulate competitive relationships in restoration[END_REF] and can be implicated in invasion success [START_REF] Seabloom | Invasion, competitive dominance, and resource use by exotic and native California grassland species[END_REF]Corbin and D'Antonio, 2004;[START_REF] Abraham | California native and exotic perennial grasses differ in their response to soil nitrogen, exotic annual grass density, and order of emergence[END_REF][START_REF] Grman | Within-year soil legacies contribute to strong priority effects of exotics on native California grassland communities[END_REF]. One mechanism prevalent in priority effects is resource preemption [START_REF] Young | Community succession and assembly comparing, contrasting and combining paradigms in the context of ecological restoration[END_REF][START_REF] Fukami | Historical contingency in community assembly: integrating niches, species pools, and priority effects[END_REF]: the reduction of available resources (e.g. space, light, nutrients) by the early colonizers [START_REF] Vance | Interference competition and the coexistence of two competitors on a single limiting resource[END_REF]. This mechanism may allow even weak competitors to persist and maintain long-term dominance [START_REF] Ross | Occupation of biological space during seedling establishment[END_REF][START_REF] Chase | Stochastic community assembly causes higher biodiversity in more productive environments[END_REF]. Priority effects also arise from alterations of biotic (e.g. soil microorganisms) and abiotic (e.g. nutrient dynamics, allelochemicals) components of the environment, which can, in some contexts, limit colonization by subsequent colonizers [START_REF] Kourtev | Exotic plant species alter the microbial community structure and function in the soil[END_REF][START_REF] Mangla | Exotic invasive plant accumulates native soil pathogens which inhibit native plants[END_REF][START_REF] Corbin | Gone but not forgotten? Invasive plants' legacies on community and ecosystem properties[END_REF]. Disturbances leading to the removal of most or all plant individuals in a habitat patch initiate a new round of community assembly [START_REF] Fukami | Historical contingency in community assembly: integrating niches, species pools, and priority effects[END_REF], often favoring competitive, fast growing invasive species at the expense of natives [START_REF] Hobbs | Disturbance, diversity, and invasion: implications for conservation[END_REF][START_REF] Davis | Fluctuating resources in plant communities: a general theory of invasibility[END_REF]. Because of the wellrecognized issues raised by invasive species (i.e. human health, crop production, native biodiversity, economic; [START_REF] Vitousek | Introduced species: a significant component of human-caused global change[END_REF][START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF][START_REF] Simberloff | Biological invasions: much progress plus several controversies[END_REF] and because of the evolution of legal framework on invasive species (at least in Europe; Regulation (EU) No 1143/2014), there is clearly an urgent need to develop effective strategies to limit invasions, particularly in newly disturbed areas. However, although active restoration of plant communities is increasingly recognized as a relevant tool to combat invasions [START_REF] Middleton | The effect of restoration methods on the quality of the restoration and resistance to invasion by exotics[END_REF][START_REF] Hazelton | Phragmites australis management in the United States: 40 years of methods and outcomes[END_REF][START_REF] Byun | Ecological application of biotic resistance to control the invasion of an invasive plant, Ageratina altissima[END_REF], results are often far from satisfactory [START_REF] Kellogg | Colonization during early succession of restored freshwater marshes[END_REF]. This has notably been attributed to a failure to account for priority effects, which may play a decisive role in restoration success [START_REF] Young | Community succession and assembly comparing, contrasting and combining paradigms in the context of ecological restoration[END_REF][START_REF] Temperton | Assembly Rules and Restoration Ecology: Bridging the Gap Between Theory and Practice[END_REF][START_REF] Grman | Within-year soil legacies contribute to strong priority effects of exotics on native California grassland communities[END_REF][START_REF] Wilsey | Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes[END_REF]. Priority effects appear to offer a cost-effective approach to combatting invasive plant species [START_REF] Chadwell | Effects of pre-existing submersed vegetation and propagule pressure on the invasion success of Hydrilla verticillata[END_REF], but have only been recently considered for invasive species management.

Here, we discuss two non-exclusive restoration strategies to achieve the end goal "more natives, less invasives". The first strategy consists in tackling priority effects generated by invasive species, while the second is to bring about situations in which the native species are more likely to exert strong priority effects.

Dealing with invasive species priority effects

Priority effects are particularly advantageous to invasive species

Phenological differences between invasive and native species can substantially contribute to invasion success [START_REF] Wolkovich | The phenology of plant invasions: a community ecology perspective[END_REF]. Distinct phenology allows certain invasive species to fill vacant phenological niches and profit from temporally available space and resources (e.g. light, nutrients, pollinators), sometimes creating seasonal priority effects (i.e. priority effects operating seasonally on a within-year scale; [START_REF] Wolkovich | The phenology of plant invasions: a community ecology perspective[END_REF]. Numerous invasive species shares the strategy of being active early in the season as to get an early access to resources and acquire a competitive dominance [START_REF] Dyer | Intraspecific and diffuse competition: the response of Nassella pulchra in a California grassland[END_REF][START_REF] Seabloom | Invasion, competitive dominance, and resource use by exotic and native California grassland species[END_REF][START_REF] Munter | Seasonal Prescribed fire Effects on Cheatgrass and Native Mixed Grass prairie Vegetation[END_REF][START_REF] Wolkovich | The phenology of plant invasions: a community ecology perspective[END_REF], but others also profit of being active late in the season (e.g. in California, the invasiveness of Centaurea silsitialis arises from extending its growing season into the summer when competition from winter annual vegetation for soil water is minimal; [START_REF] Gerlach | Testing life history correlates of invasiveness using congeneric plant species[END_REF].

Several studies reported that invasive species generate stronger priority effects than natives [START_REF] Dickson | Do priority effects benefit invasive plants more than native plants? An experiment with six grassland species[END_REF][START_REF] Wilsey | Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes[END_REF]Stuble andSouza, 2016, but see Cleland et al., 2015). The generally higher growth rate of invasive species [START_REF] Rejmánek | What attributes make some plant species more invasive?[END_REF][START_REF] Reynolds | The effects of litter and temperature on the germination of native and exotic grasses in a coastal California grassland[END_REF][START_REF] Grotkopp | Assessing potential invasiveness of woody horticultural plant species using seedling growth rate traits[END_REF][START_REF] Marushia | Phenology as a basis for management of exotic annual plants in desert invasions[END_REF][START_REF] Dawson | The maximum relative growth rate of common UK plant species is positively associated with their global invasiveness[END_REF] was suggested to underlie this advantage [START_REF] Stevens | Competitive effect of two nonnative grasses on a native grass in southern Arizona[END_REF][START_REF] Dickson | Do priority effects benefit invasive plants more than native plants? An experiment with six grassland species[END_REF]. A higher growth rate creates a greater asymmetry in plant size [START_REF] Weiner | Asymmetric competition in plant populations[END_REF], resulting in a stronger competitive suppression of the later arriving species [START_REF] Dyer | Effects of competition on resource availability and growth of a California bunchgrass[END_REF][START_REF] Perry | Founder control and coexistence in a simple model of asymmetric competition for light[END_REF][START_REF] Ejrnaes | Community assembly in experimental Grasslands: Suitable environment or timely arrival?[END_REF]. [START_REF] Cleland | Priority effects vary with species identity and origin in an experiment varying the timing of seed arrival[END_REF] found that the stronger priority effects of invasive species were correlated to regeneration trait values, such as higher germination rate and higher light capture during seedling stage, reflecting higher biomass.

In addition to these seasonal advantages, invasive species can also profit from priority effects through self-induced soil condition modifications, whether biological, chemical or physical [START_REF] Corbin | Gone but not forgotten? Invasive plants' legacies on community and ecosystem properties[END_REF]. Modifications such as: (1) shifts in nutrient cycling [START_REF] Ehrenfeld | Effects of exotic plant invasions on soil nutrient cycling processes[END_REF][START_REF] Marchante | Short-and long-term impacts of Acacia longifolia invasion on the belowground processes of a Mediterranean coastal dune ecosystem[END_REF][START_REF] Flinn | Invasion and succession change the functional traits of serpentine plant communities[END_REF] and soil salinity [START_REF] Novoa | Constraints to native plant species establishment in coastal dune communities invaded by Carpobrotus edulis: implications for restoration[END_REF], (2) changes in soil microbial communities including pathogens and mycorrhizal fungi [START_REF] Kourtev | Exotic plant species alter the microbial community structure and function in the soil[END_REF][START_REF] Hawkes | Arbuscular mycorrhizal assemblages in native plant roots change in the presence of invasive exotic grasses[END_REF][START_REF] Mangla | Exotic invasive plant accumulates native soil pathogens which inhibit native plants[END_REF][START_REF] Stinson | Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms[END_REF][START_REF] Kardol | Microbe-mediated plant-soil feedback causes historical contingency effects in plant community assembly[END_REF][START_REF] Lorenzo | Effect of invasive Acacia dealbata link on soil microorganisms as determined by PCR-DGGE[END_REF], and (3) the release of allelochemicals [START_REF] Bais | Allelopathy and exotic plant invasion: from molecules and genes to species interactions[END_REF][START_REF] Stinson | Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms[END_REF][START_REF] Milchunas | Allelopathic cover crop prior to seeding is more important than subsequent grazing/mowing in grassland establishment[END_REF][START_REF] Grove | Direct and indirect effects of allelopathy in the soil legacy of an exotic plant invasion[END_REF] can all generate priority effects enhancing invasive species performance and inhibiting native plant species (Fig. 1A; [START_REF] Reinhart | Soil biota and invasive plants[END_REF][START_REF] Stinson | Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms[END_REF][START_REF] Van Der Putten | Plant-soil feedbacks: the past, the present and future challenges[END_REF][START_REF] Rodriguez-Echeverria | The effect of soil legacy on competition and invasion by Acacia dealbata Link[END_REF][START_REF] Meisner | Plant-soil feedbacks of exotic plant species across life forms: a meta-analysis[END_REF]. Such invader-mediated soil modifications can persist after the causal invasive species is removed or dies [START_REF] Corbin | Gone but not forgotten? Invasive plants' legacies on community and ecosystem properties[END_REF][START_REF] Hacker | Differing consequences of removing ecosystem-modifying invaders: significance of impact and community context to restoration potential[END_REF][START_REF] Hamman | Biogeochemical and microbial legacies of non-native grasses can affect restoration success[END_REF], and are commonly termed "soil legacies" (Fig. 1B). performance, and by modifying soil conditions. Priority effects are asymmetric between native and invasive species, so that invasive species are less impacted by arriving late compared with natives. (B) Plants can also generate priority effects through soil legacies after they were removed or died, that can impact subsequent colonization. While, for many invasive species, persistent soil legacies have been reported to hinder invasive recolonization, little is known about how native species soil legacies could limit invasive species establishment. Note that [START_REF] Grman | Within-year soil legacies contribute to strong priority effects of exotics on native California grassland communities[END_REF] found no impact of native species legacies on invasive species success.

Countering invasive species priority effects

When it comes to decrease the competitive dominance of an invasive plant species, it is essential to look for abiotic conditions to determine what actions need to be undertaken. In environments with high nutrient resource and water availability, the presence of invasive species could particularly hinder restoration of native communities because of their high competitive abilities [START_REF] Cox | Stability of exotic annual grasses following restoration efforts in southern California coastal sage scrub[END_REF][START_REF] Abraham | California native and exotic perennial grasses differ in their response to soil nitrogen, exotic annual grass density, and order of emergence[END_REF][START_REF] Grman | Within-year soil legacies contribute to strong priority effects of exotics on native California grassland communities[END_REF].

Countering invasive species competitive advantage can be achieved by applying intensive management techniques to reduce invasive species cover (i.e. herbicide applications, mechanical removal; Fig. 2). [START_REF] Marushia | Phenology as a basis for management of exotic annual plants in desert invasions[END_REF], by applying control methods (herbicide application) early in the season, tackled rapid and early emerging exotic annuals while minimizing impacts on native plants. In favorable conditions, many invasive species are likely to invade in response to the removal of one or more invaders [START_REF] D'antonio | Interactions among invasive plants: lessons from Hawaii[END_REF]. Thus, it is particularly relevant to reduce propagule sources in order to limit invasive species recolonization (Fig. 2; D' Antonio et al., 2017). Common methods to decrease nondesirable species seed bank include topsoil removal [START_REF] Hölzel | Assessing soil seed bank persistence in flood-meadows: the search for reliable traits[END_REF], effective mowing management (i.e. adjusted to phenological development; [START_REF] Milakovic | Management of roadside populations of invasive Ambrosia artemisiifolia by mowing[END_REF] and artificial flushing of invasive species to induce germination, followed by lethal interventions such as tillage or herbicide application [START_REF] Wolf | Summer water in a restored native grassland flushes annual grass seed bank but fails to increase native perennial cover[END_REF]. Prescribed burns for fire-prone species or supplying water are two techniques promoting germination [START_REF] Ooi | Dormancy classification and potential dormancy-breaking cues for shrub species from fire-prone south-eastern Australia[END_REF][START_REF] Wolf | Summer water in a restored native grassland flushes annual grass seed bank but fails to increase native perennial cover[END_REF] that could be used to flush invasive plant species and tackle seasonal priority advantage early in the season [START_REF] Wolkovich | The phenology of plant invasions: a community ecology perspective[END_REF][START_REF] Wainwright | Seasonal priority effects: implications for invasion and restoration in a semi-arid system[END_REF][START_REF] Wilsey | Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes[END_REF]. Establishing early-emerging and competitive native species (e.g. cover crops) is another option that can help reducing competition from early-germinating invasive species. Indeed, restoring early-emerging species can directly decrease invasive species performance [START_REF] Blackshaw | Reduced herbicide doses in field crops: a review[END_REF] and indirectly favor desired native species [START_REF] Perry | Native cover crops suppress exotic annuals and favor native perennials in a greenhouse competition experiment[END_REF]. However, so far, these strategies are little explored. To successfully counter seasonal priority effects generated by invasive species, it is crucial to better understand invasive species phenology, requirements and possible interactions with native species, so as to improve existing management techniques (i.e. artificial invasive species flushing, the use of cover crops) and develop new ones.

To limit reinvasion and secondary invasions, invasive species reduction must be coupled with revegetation strategies (Fig. 2; [START_REF] Pearson | Secondary invasion: the bane of weed management[END_REF] directed towards the limitation of multiple co-occurring invasive plant species. However, because of soil legacies, invasive species removal and propagule pressure reduction are sometimes unlikely to lead to recovery of native communities [START_REF] Suding | Alternative states and positive feedbacks in restoration ecology[END_REF][START_REF] Ehrenfeld | Feedback in the plant-soil system[END_REF][START_REF] Corbin | Gone but not forgotten? Invasive plants' legacies on community and ecosystem properties[END_REF][START_REF] Jordan | Soiloccupancy effects of invasive and native grassland plant species on composition and diversity of mycorrhizal associations[END_REF][START_REF] Van Der Putten | Plant-soil feedbacks: the past, the present and future challenges[END_REF], and often favor secondary invasions [START_REF] Dickie | Belowground legacies of Pinus contorta invasion and removal result in multiple mechanisms of invasional meltdown[END_REF][START_REF] Grove | Persistence of a soil legacy following removal of a nitrogen-fixing invader[END_REF][START_REF] Yelenik | Self-reinforcing impacts of plant invasions change over time[END_REF]. When the site was invaded prior to disturbance, it may then be necessary to include measures to deal with soil legacies, rather than simply eliminate invasive species populations (Fig. 2). Soil legacies can be mitigated by adjusting soil properties, typically via topsoil removal or soil amendments [START_REF] Kulmatiski | Activated carbon as a restoration tool: potential for control of invasive plants in abandoned agricultural fields[END_REF][START_REF] Buisson | Reintroduction of Nassella pulchra to California coastal grasslands: effects of topsoil removal, plant neighbour removal and grazing[END_REF][START_REF] Perry | Immobilizing nitrogen to control plant invasion[END_REF]. Carbon addition can help lowering nitrogen availability by stimulating nitrogen immobilization [START_REF] Baer | Soil resources regulate productivity and diversity in newly established tallgrass prairie[END_REF], thereby reducing invasive species performance and concurrently increasing desired species growth [START_REF] Alpert | Carbon addition as a countermeasure against biological invasion by plants[END_REF][START_REF] Blumenthal | Soil carbon addition controls weeds and facilitates prairie restoration[END_REF][START_REF] Eschen | Carbon addition alters vegetation composition on ex-arable fields[END_REF] see Perry et al., 2010 for review). However, in the case of restored communities reassembling from seeds, nitrogen management may have no direct positive effect unless a headstart is given to natives (i.e. invasive species are controlled the first growing season; [START_REF] James | Managing soil nitrogen to restore annual grass-infested plant communities: effective strategy or incomplete framework?[END_REF]. Furthermore, success of carbon addition to decrease invasive species dominance also mainly depends on the condition that invasive species is nitrophilic relative to native species [START_REF] Blumenthal | Soil carbon addition controls weeds and facilitates prairie restoration[END_REF].

To face soil legacies, another restoration approach is to establish species that are tolerant to invasive species legacies [START_REF] Perry | Screening of grassland plants for restoration after spotted knapweed invasion[END_REF], or that could mitigate legacies before establishing the target community [START_REF] Jordan | Soil modification by invasive plants: effects on native and invasive species of mixed-grass prairies[END_REF][START_REF] Eviner | The effects of plant-soil feedbacks on invasive plants: mechanisms and potential management options[END_REF][START_REF] Leger | What seeds to plant in the Great Basin? Comparing traits prioritized in native plant cultivars and releases with those that promote survival in the field[END_REF][START_REF] Vink | Soil conditioning effects of native and exotic grassland perennials on the establishment of native and exotic plants[END_REF]. In this sense, restoration of non-susceptible species to Centaurea maculosa's allelopathic compounds prevented reinvasion and possibly facilitate native species recovery [START_REF] Callaway | Natural selection for resistance to the allelopathic effects of invasive plants[END_REF][START_REF] Thorpe | Root exudate is allelopathic in invaded community but not in native community: field evidence for the novel weapons hypothesis[END_REF]. [START_REF] Herron | Influence of nutrient availability on the interaction between spotted knapweed and bluebunch wheatgrass[END_REF] also showed that establishing native species decreasing nitrogen availability through high nitrogen uptake decreases the prevalence of invasive species favored by soil nitrogen enrichment.

Taking soil legacies into account in restoration projects is however challenging. Soil legacies are difficult to predict and to assess (involving chemical analyses, determination of microbial communities' abundance and composition), and are species-specific [START_REF] Bezemer | Plant species and functional group effects on abiotic and microbial soil properties and plant-soil feedback responses in two grasslands[END_REF][START_REF] Yelenik | Functional group identity does not predict invader impacts: differential effects of nitrogen-fixing exotic plants on ecosystem function[END_REF][START_REF] Bardgett | Aboveground-Belowground Linkages: Biotic Interactions, Ecosystem Processes, and Global Change[END_REF]. Furthermore, their persistence depends on characteristics of the invaded ecosystem (e.g. soil mineralization rates; [START_REF] Stock | Impacts of invading N 2-fixing Acacia species on patterns of nutrient cycling in two Cape ecosystems: evidence from soil incubation studies and 15 N natural abundance values[END_REF], on their nature [START_REF] Levine | Mechanisms underlying the impacts of exotic plant invasions[END_REF][START_REF] Corbin | Gone but not forgotten? Invasive plants' legacies on community and ecosystem properties[END_REF], and on the duration of invasion [START_REF] Marchante | Short-and long-term impacts of Acacia longifolia invasion on the belowground processes of a Mediterranean coastal dune ecosystem[END_REF][START_REF] Kulmatiski | Long-term plant growth legacies overwhelm short-term plant growth effects on soil microbial community structure[END_REF]. For example, while allelopathic compounds are generally short-lived in the soil (i.e. hours to days; [START_REF] Blair | New techniques and findings in the study of a candidate allelochemical implicated in invasion success[END_REF]Reigosa et al., 2006), increased nitrogen levels generated by a nitrogen-fixing invasive species can persist for decades (e.g. 35 years; Maron and Jeffries, 2001). Long-term studies suggest, however, that invasion impacts on ecosystems, such as increased nitrogen levels, can shift over time [START_REF] Yelenik | Self-reinforcing impacts of plant invasions change over time[END_REF]. All this makes it hard to predict the amplitude and persistence of soil legacies for a particular invasive species in a given environment, and calls for high levels of costly technical expertise. Thus, soil legacies are not systematically given the weight they deserve in restoration projects. Yet restoration would clearly benefit from accounting for soil legacies, especially when an invasive species is implicated in strong and persistent legacies and has dominated the target environment for several growing seasons (Fig. 2; [START_REF] Marchante | Short-and long-term impacts of Acacia longifolia invasion on the belowground processes of a Mediterranean coastal dune ecosystem[END_REF][START_REF] Kulmatiski | Long-term plant growth legacies overwhelm short-term plant growth effects on soil microbial community structure[END_REF]. A better understanding of how invasive species induce strong and persistent soil legacies in the habitat they commonly invade would reduce the need for complex and expensive analyses, facilitating development of effective restoration strategies. Cost-effective methods, such as native species germination or survival tests on soil with potential legacies should be developed to rapidly assess their extent. Fig. 2. Decision support to counter invasive plant species priority effects. When invasive species facilitate the establishment of natives (e.g. in some harsh environments), they should not be removed and can be used as nurse species for restored species. Conversely, when invasive species hinder native species establishment (e.g. in some fertile and stable environments), it is essential to decrease their abundance and prevent secondary invasions, notably via removal and seed bank reduction. After removal, soil legacies generated by invasive species can lower restoration success. These legacies should be particularly considered when the removed invasive species is known to produce strong and/or persistent legacies (e.g. nitrogen levels), and/or or was present in abundance and/or for long duration.

Legacies mitigation methods such as topsoil removal, amendments (e.g. carbon addition), or intermediate planting should be adjusted to the nature and intensity of legacies. Finally, the restoration of native species adapted to local conditions and invasion pressure should be undertaken to limit reinvasion and secondary invasions, except when revegetation facilitates invasions (e.g. in some harsh environments).

In harsh environments with extremely limited resources and/or stressful conditions (e.g. extreme temperatures, excessive solar radiation, unstable substrates), invasive species removal often lead to a lower success of a native cover restoration [START_REF] D'antonio | Exotic plant species as problems and solutions in ecological restoration: a synthesis[END_REF]. In some cases, invasive species are used as nurse plants to facilitate the establishment of native species (Fig. 2; [START_REF] Becerra | The widely invasive tree Pinus radiata facilitates regeneration of native woody species in a semi-arid ecosystem[END_REF][START_REF] Hanslin | Positive responses of coastal dune plants to soil conditioning by the invasive Lupinus nootkatensis[END_REF]. The removal of an invasive species may not result in additional invasions [START_REF] D'antonio | Interactions among invasive plants: lessons from Hawaii[END_REF]: the likelihood of other stress-adapted species being present and able to respond quickly is low [START_REF] Harms | Vegetation response following invasive tamarisk (Tamarix spp.) removal and implications for riparian restoration[END_REF], and these systems constrain species to low productivity or capacity to accumulate biomass [START_REF] D'antonio | Interactions among invasive plants: lessons from Hawaii[END_REF]. Managers may therefore have ample time to control a secondary invasive species because they commonly have low population growth rates [START_REF] Funk | Resource-use efficiency and plant invasion in low-resource systems[END_REF].

Strengthening native species priority effects

Restoring native communities after a disturbance can have opposite consequences on invasion success depending on abiotic conditions. In harsh environments, native species can create microclimatic conditions that are more favorable to invasive species establishment than the surrounding [START_REF] Lenz | Shade facilitates an invasive stem succulent in a chenopod shrubland in South Australia[END_REF][START_REF] Cavieres | Nurse effect of the native cushion plant Azorella monantha on the invasive non-native Taraxacum officinale in the high-Andes of central Chile[END_REF][START_REF] Mason | Arrival order among native plant functional groups does not affect invasibility of constructed dune communities[END_REF]. In such cases, the restoration of a vegetation cover may not be the best option. Removal of invasive species followed by the control of secondary invasions without active revegetation may be a more suitable strategy. Resource availability, especially nitrogen, also strongly influences invasion success through modifications in competition intensity between species [START_REF] Davis | Fluctuating resources in plant communities: a general theory of invasibility[END_REF][START_REF] Davis | Experimental support for a resource-based mechanistic model of invasibility[END_REF]. Environmental harshness has also been assumed to decrease the importance of stochastic factors because of strong niche selection [START_REF] Chase | Drought mediates the importance of stochastic community assembly[END_REF][START_REF] Kardol | Resource availability mediates the importance of priority effects in plant community assembly and ecosystem function[END_REF]. In this sense, [START_REF] Kardol | Resource availability mediates the importance of priority effects in plant community assembly and ecosystem function[END_REF] found weaker priority effects under low nutrient supply. In nitrogen-limited systems, restoration of nitrogen-fixing species can also favor the establishment of fast-growing invasive species that overgrow and shade slower-growing native species [START_REF] Huenneke | Effects of soil resources on plant invasion and community structure in Californian serpentine grassland[END_REF][START_REF] Maron | A native nitrogen-fixing shrub facilitates weed invasion[END_REF]Corbin and D'Antonio, 2004b). In such conditions, it may be advisable to restore a native plant cover adapted to low levels of nitrogen and to consider avoiding nitrogen-fixing species and soil nitrogen amendments.

In fertile and relatively stable conditions, restoration of invasionresistant native plant species is increasingly considered to protect disturbed sites from re-invasion or secondary invasions [START_REF] Perry | Light competition for invasive species control: a model of cover crop-weed competition and implications for Phalaris arundinacea control in sedge meadow wetlands[END_REF][START_REF] Buckley | The role of research for integrated management of invasive species, invaded landscapes and communities[END_REF][START_REF] Middleton | The effect of restoration methods on the quality of the restoration and resistance to invasion by exotics[END_REF][START_REF] Byun | Plant functional group identity and diversity determine biotic resistance to invasion by an exotic grass[END_REF][START_REF] Pearson | Secondary invasion: the bane of weed management[END_REF]. Environmental conditions influence the magnitude of priority effects [START_REF] Collinge | Transient patterns in the assembly of vernal pool plant communities[END_REF][START_REF] Kardol | Resource availability mediates the importance of priority effects in plant community assembly and ecosystem function[END_REF][START_REF] Symons | Timing is everything: priority effects alter community invasibility after disturbance[END_REF], with stronger impact in productive environments [START_REF] Kardol | Resource availability mediates the importance of priority effects in plant community assembly and ecosystem function[END_REF][START_REF] Young | Using priority effects to manipulate competitive relationships in restoration[END_REF]. The strength of priority effects also varies with the identity of the earlier-and the laterarriving species [START_REF] Dickson | Do priority effects benefit invasive plants more than native plants? An experiment with six grassland species[END_REF][START_REF] Von Gillhaussen | Priority effects of time of arrival of plant functional groups override sowing interval or density effects: a grassland experiment[END_REF][START_REF] Cleland | Priority effects vary with species identity and origin in an experiment varying the timing of seed arrival[END_REF][START_REF] Wilsey | Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes[END_REF][START_REF] Stuble | Priority effects: natives, but not exotics, pay to arrive late[END_REF], invasive species being less negatively impacted by arriving late than native species (Fig. 1A; [START_REF] Wilsey | Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes[END_REF][START_REF] Stuble | Priority effects: natives, but not exotics, pay to arrive late[END_REF], raising the need to restore native species producing strong priority effects.

Recent work on priority effects also states that the strength of priority effects is notably driven by (1) the impact a species has on resource levels [START_REF] Fargione | Community assembly and invasion: an experimental test of neutral versus niche processes[END_REF][START_REF] Fukami | Historical contingency in community assembly: integrating niches, species pools, and priority effects[END_REF], (2) the overlap between competitive species in resource needs [START_REF] Funk | Restoration through reassembly: plant traits and invasion resistance[END_REF][START_REF] Vannette | Historical contingency in species interactions: towards niche-based predictions[END_REF], and (3) the impact a species has on nonresource components of the environment [START_REF] Bever | Soil community feedback and the coexistence of competitors: conceptual frameworks and empirical tests[END_REF][START_REF] Levine | A meta-analysis of biotic resistance to exotic plant invasions[END_REF][START_REF] Goldstein | Applying competition theory to invasion: resource impacts indicate invasion mechanisms in California shrublands[END_REF]. The following sections will discuss how these emerging properties could be used to reinforce the priority effects of restored native species in invaded habitats (see Figure A1 in Appendix for summary).

Increasing resource preemption

Does the duration of time advance matters?

Numerous studies reported that giving native species a short time advance (one to few weeks) suffices to substantially decrease invasion success in grassland systems [START_REF] Firn | Early emergence and resource availability can competitively favour natives over a functionally similar invader[END_REF][START_REF] Grman | Within-year soil legacies contribute to strong priority effects of exotics on native California grassland communities[END_REF][START_REF] Vaughn | Short-term priority over exotic annuals increases the initial density and longer-term cover of native perennial grasses[END_REF][START_REF] Young | Using priority effects to manipulate competitive relationships in restoration[END_REF]. [START_REF] Grman | Within-year soil legacies contribute to strong priority effects of exotics on native California grassland communities[END_REF] found that native species establishment only five weeks before invasive species introduction reduced invasive biomass by 85%, against an 8% decrease when natives and invasives were planted simultaneously. [START_REF] Firn | Early emergence and resource availability can competitively favour natives over a functionally similar invader[END_REF] also found a strong effect of giving a threeweek head-start to native grasses on an invasive grass performance. However, few studies investigated the importance of the duration of the time interval between native establishment and invasive species colonization. Asymmetry in plant size has been advocated as one of the most important aspect of priority effect [START_REF] Wilsey | Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes[END_REF], suggesting that extending duration interval between native species establishment and the later invasion event may give a size advantage strengthening native priority effects. In this sense, von Gillhaussen et al. ( 2014) found that a six-week head-start resulted in stronger priority effects than a threeweek head-start. [START_REF] Young | Using priority effects to manipulate competitive relationships in restoration[END_REF] tested the effect of giving the native perennials a twoweek or a one-year seeding advantage over exotic annuals in a four-year experiment. It respectively resulted in a native cover increase of 68% or 128% compared to when natives and exotics were sown at the same time. The positive effect of increasing time advance appeared however inconstant between years and sites, with sometimes an absence of benefit.

Better understand how the duration of time advance given to the restored native species influence invasion success would be crucial to develop cost-effective priority effects-based revegetation strategies. To give natives a time advantage over invasives, native species should be actively restored as soon as possible after the disturbance on an invasive species-free soil [START_REF] Stevens | Competitive effect of two nonnative grasses on a native grass in southern Arizona[END_REF], and a particular attention must be payed to invasive species control in the initial weeks. Providing a short-term priority (several days) could also be achieved by "pre-germinating" native species seeds. Pre-treatments including seed priming and cold stratification can help ensuring a rapid and complete germination and overcome seed dormancy [START_REF] Halmer | Methods to improve seed performance in the field[END_REF]. These treatments therefore appear as opportunities for improving native emergence speed and create priority effects over invasives, but remain yet untested.

Although the eventual success of extending time advance can be judged only against the persistence of priority effects over long periods (i.e. more than one growing season), longterm studies are rare. [START_REF] Vaughn | Short-term priority over exotic annuals increases the initial density and longer-term cover of native perennial grasses[END_REF] showed that the effect of a two-week advance in planting can remain visible after three years, favoring native perennials over exotic annuals. [START_REF] Werner | Persistent asymmetrical priority effects in a California grassland restoration experiment[END_REF] highlighted differences in persistence of a one-year priority between functional groups: the grass priority over forbs was still visible after six to eight years, but the forb priority over grasses did not persist. Designing efficient, cost-effective restoration strategies that allow native species to maintain their dominance over invasive species in the long term calls for more studies on mechanisms (i.e. duration of time advance, disturbance regime, resource availability, dynamic of sown communities) influencing the persistence of priority effects over long periods.

Manipulate species composition and density

The resource competition model [START_REF] Tilman | Perspectives on Plant Competition[END_REF] predicts that the more a species reduces the availability of limited resources, the less these resources are available for later colonizers. Because resource preemption has been identified as one of the main driver of priority effects [START_REF] Fukami | Historical contingency in community assembly: integrating niches, species pools, and priority effects[END_REF], high resource preemption would lead to strong priority effects [START_REF] Vannette | Historical contingency in species interactions: towards niche-based predictions[END_REF]. [START_REF] Fargione | Community assembly and invasion: an experimental test of neutral versus niche processes[END_REF] found that C4-grasses inhibited the most the later arriving invasive species, most likely because this guild reduced soil nitrate to the lowest levels compared with other tested functional guilds. This result suggests that restoring native species leading to a strong and rapid reduction of limited resources could help enhancing priority effects. Accordingly, research efforts should be directed towards the identification of species having the ability to (1) rapidly occupy of above-and/or belowground space, thereby limiting light and space availability (often considered as two primary determinants of invasive species germination and establishment; D' Antonio et al., 2001;Corbin and D'Antonio, 2004a;[START_REF] Iponga | Superiority in competition for light: a crucial attribute defining the impact of the invasive alien tree Schinus molle (Anacardiaceae) in South African savanna[END_REF], and (2) rapidly and effectively preempt soil nutrients, especially in low productivity environments where there is likely to be less above-ground competition for light [START_REF] Dietz | Recognition that causal processes change during plant invasion helps explain conflicts in evidence[END_REF][START_REF] Gioria | Resource competition in plant invasions: emerging patterns and research needs[END_REF].

At small scale (10m 2 or less), many studies support the widespread assumption that species diversity confers invasion resistance [START_REF] Tilman | Community invasibility, recruitment limitation, and grassland biodiversity[END_REF][START_REF] Levine | Elton revisited: a review of evidence linking diversity and invasibility[END_REF][START_REF] Levine | A meta-analysis of biotic resistance to exotic plant invasions[END_REF][START_REF] Carter | High richness and dense seeding enhance grassland restoration establishment but have little effect on drought response[END_REF], due to fuller use of resources by resident species ("complementarity effect"; [START_REF] Robinson | Invasibility of experimental habitat islands in a California winter annual grassland[END_REF][START_REF] Lavorel | Invasibility and diversity of plant communities: from patterns to processes[END_REF][START_REF] Levine | Elton revisited: a review of evidence linking diversity and invasibility[END_REF][START_REF] Larson | Using prairie restoration to curtail invasion of Canada thistle: the importance of limiting similarity and seed mix richness[END_REF], or due to the increased probability of a species being present to be a strong competitor for the invasive species when increasing the number of species in a community ("sampling effect": [START_REF] Kennedy | Biodiversity as a barrier to ecological invasion[END_REF][START_REF] Wardle | Experimental demonstration that plant diversity reduces invasibility-evidence of a biological mechanism or a consequence of sampling effect?[END_REF][START_REF] Lavorel | Invasibility and diversity of plant communities: from patterns to processes[END_REF][START_REF] Goslee | Optimizing ecosystem function by manipulating pasture community composition[END_REF]. Increasing diversity has been reported to increase primary productivity in grassland systems [START_REF] Hector | BUGS in the analysis of biodiversity experiments: species richness and composition are of similar importance for grassland productivity[END_REF], suggesting that diverse communities produce higher rates of biomass and could therefore exert a stronger asymmetric competition with later colonists. Two studies supported the fact that diversity strengthens priority effects in protist and aquatic plant communities [START_REF] Jiang | Species diversity, invasion, and alternative community states in sequentially assembled communities[END_REF][START_REF] Viana | Disentangling the roles of diversity resistance and priority effects in community assembly[END_REF], but more studies investigating this relationship in plant communities are needed.

The density of individuals also modulates priority effects [START_REF] Weiner | Asymmetric competition in plant populations[END_REF] in the sense that establishing more individuals should lead to increased resource acquisition and competition intensity [START_REF] Goldberg | Components of resource competition in plant communities[END_REF][START_REF] Lockwood | The role of propagule pressure in explaining species invasions[END_REF]. The benefit of increasing sowing density may stabilize over time, since the law of constant yields predicts that evenaged populations grown at different densities show the same overall productivity after a certain period of time, with higher number of individuals in high densities but lower standing biomass per individual [START_REF] Drew | Stand density management: an alternative approach and its application to Douglas-fir plantations[END_REF]. Consistently, von Gillhaussen et al.

(2014) found sowing density (1.5, 2.5 and 5 g/m 2 ) only had a weak influence on aboveground productivity. Increased density is however often associated with improved invasion resistance in short time scales [START_REF] Gerhardt | Abiotic constraints eclipse biotic resistance in determining invasibility along experimental vernal pool gradients[END_REF][START_REF] Carter | High richness and dense seeding enhance grassland restoration establishment but have little effect on drought response[END_REF][START_REF] Vaughn | Short-term priority over exotic annuals increases the initial density and longer-term cover of native perennial grasses[END_REF][START_REF] Yannelli | Preventing plant invasions at early stages of revegetation: the role of limiting similarity in seed size and seed density[END_REF]. Accordingly, [START_REF] Yannelli | Preventing plant invasions at early stages of revegetation: the role of limiting similarity in seed size and seed density[END_REF] found that sowing communities at high density (10 g.m 2 ) is more effective in suppressing invasive species than low density (1 g.m 2 ). The improved invasion-resistance of high density community may be related to the inability of low density community to fully exploit available resources. Since we are looking for solutions to design restored communities rapidly exerting strong priority effects, increasing sowing densities is an attractive option. However, it remains to determine effective sowing thresholds depending on species used and environmental conditions.

Increasing niche overlap: Applying limiting similarity

Niche overlap, referring to resource use similarity between co-occuring species independent of their rate of resource consumption [START_REF] Pianka | Niche overlap and diffuse competition[END_REF][START_REF] Petraitis | The representation of niche breadth and overlap on Tilman's consumer-resource graphs[END_REF], has been hypothesized to influence invasion-resistance [START_REF] Abrams | The theory of limiting similarity[END_REF][START_REF] Funk | Restoration through reassembly: plant traits and invasion resistance[END_REF] and more recently priority effects [START_REF] Vannette | Historical contingency in species interactions: towards niche-based predictions[END_REF]. Niche overlap is derived from the limiting similarity concept, predicting that species most similar to the invasive species should provide greater invasion resistance because of greater overlap in resource use [START_REF] Abrams | The theory of limiting similarity[END_REF].

Accordingly, a high degree of similarity in resource use between first and later colonizers should strengthen priority effects of the recipient species. Attempts to use limiting similarity to limit plant invasions often resulted in failures [START_REF] Symstad | A test of the effects of functional group richness and composition on grassland invasibility[END_REF][START_REF] Emery | Limiting similarity between invaders and dominant species in herbaceous plant communities?[END_REF][START_REF] Turnbull | Experimental invasion by legumes reveals non-random assembly rules in grassland communities[END_REF][START_REF] Price | Can limiting similarity increase invasion resistance? A metaanalysis of experimental studies[END_REF], highlighting the complexity of selecting plant species having a sufficient degree of niche overlap. With current knowledge, using the limiting similarity concept to limit invasions appears premature. An emerging, more promising strategy consists in focusing on the identification of key functional traits playing a substantial role in invasion resistance and priority effects [START_REF] Drenovsky | Designing invasion-resistant plant communities: the role of plant functional traits[END_REF][START_REF] Cleland | Strengthening invasion filters to reassemble native plant communities: soil resources and phenological overlap[END_REF]. For example, [START_REF] Cleland | Strengthening invasion filters to reassemble native plant communities: soil resources and phenological overlap[END_REF] identified phenology as an important determinant of invasion success: high phenological overlap between exotic annual grasses and restored forb species successfully resulted in a decreased abundance of invasive species. These results suggest that restoring early active perennial species may be particularly relevant to decrease the competitive dominance of early active annual invasive species in the long term. Further investigations are needed to determine how and in which situations such trait-based strategies are efficient.

Manipulating non-resource components

Few studies have explored ways to enhance invasion-resistance of restored communities by exploiting non-resource priority effects [START_REF] Bever | Soil community feedback and the coexistence of competitors: conceptual frameworks and empirical tests[END_REF][START_REF] Levine | A meta-analysis of biotic resistance to exotic plant invasions[END_REF] induced by the release of allelopathic compounds, the manipulation of mycorrhizae, or the promotion of pathogens or herbivory [START_REF] Goldstein | Applying competition theory to invasion: resource impacts indicate invasion mechanisms in California shrublands[END_REF]. Non-resource priority effects could act through a direct negative impact on the target invasive species, or by an indirect improvement of native species success. The use of allelopathy (i.e. the exudation of chemical compounds influencing the growth of other plants or microorganisms) for invasive species control has received special attention, especially in suppressing weeds in agricultural systems [START_REF] Bhowmik | Challenges and opportunities in implementing allelopathy for natural weed management[END_REF][START_REF] Milchunas | Allelopathic cover crop prior to seeding is more important than subsequent grazing/mowing in grassland establishment[END_REF][START_REF] Jabran | Allelopathy for weed control in agricultural systems[END_REF][START_REF] Jabran | Allelopathy for weed control in agricultural systems[END_REF]. The establishment of native allelopathic species can directly reduce the biomass of the target invasive species [START_REF] Callaway | Novel weapons: invasive success and the evolution of increased competitive ability[END_REF], and indirectly facilitate the desired later-arriving native species [START_REF] Perry | Native cover crops suppress exotic annuals and favor native perennials in a greenhouse competition experiment[END_REF]. Indeed, allelopathy is relatively ineffective in interactions between species that frequently co-occur [START_REF] Fitter | Making allelopathy respectable[END_REF] and is more intense in novel interactions, such as between native and exotic species [START_REF] Callaway | Novel weapons: invasive success and the evolution of increased competitive ability[END_REF][START_REF] Thorpe | Root exudate is allelopathic in invaded community but not in native community: field evidence for the novel weapons hypothesis[END_REF]. By being established first, native allelopathic species could induce stronger priority effects reducing invasion success, but such assumption needs to be tested. Since the allelopathic effect vary depending on species [START_REF] Prati | Allelopathic inhibition of germination by Alliaria petiolata (Brassicaceae)[END_REF], community density [START_REF] Weidenhamer | Density-dependent phytotoxicity: distinguishing resource competition and allelopathic interference in plants[END_REF], climate conditions [START_REF] May | An assessment of the allelopathic potential of Eucalyptus[END_REF][START_REF] Blair | A lack of evidence for an ecological role of the putative allelochemical (±)-catechin in spotted knapweed invasion success[END_REF], and substrate characteristics [START_REF] Parepa | Testing for allelopathy in invasive plants: it all depends on the substrate![END_REF], using allelopathic native species to limit invasion appears complex and may be limited to a set of invasive species. The effectiveness of invasive control strategies based on allelopathic species needs further investigations, in particular the potential use of native allelopathic species to suppress several invasive species.

In addition to plant-plant interactions, biotic resistance can also arise from consumption by herbivores and disease [START_REF] Levine | A meta-analysis of biotic resistance to exotic plant invasions[END_REF]. Introducing coevolved natural predators or parasites from the native region of the invasive species has been implemented for controlling well-established invasive populations, with mixed results [START_REF] Clewley | The effectiveness of classical biological control of invasive plants[END_REF].

Native herbivores can have various impacts on invasives [START_REF] Maron | When do herbivores affect plant invasion? Evidence for the natural enemies and biotic resistance hypotheses[END_REF][START_REF] Levine | A meta-analysis of biotic resistance to exotic plant invasions[END_REF], sometimes contributing (i.e. invasive plants are maladapted to deter consumption by native herbivores; [START_REF] Parker | Biotic resistance to plant invasions? Native herbivores prefer nonnative plants[END_REF][START_REF] Morrison | Herbivore preference for native vs. exotic plants: generalist herbivores from multiple continents prefer exotic plants that are evolutionarily naïve[END_REF][START_REF] Petruzzella | Potential for biotic resistance from herbivores to tropical and subtropical plant invasions in aquatic ecosystems[END_REF][START_REF] Zhang | Herbivory enhances the resistance of mangrove forest to cordgrass invasion[END_REF] or not (i.e. native herbivores are maladapted to consume invasive plants; [START_REF] Keane | Exotic plant invasions and the enemy release hypothesis[END_REF][START_REF] Liu | Testing the enemy release hypothesis: a review and meta-analysis[END_REF][START_REF] Xiong | A snail prefers native over exotic freshwater plants: implications for the enemy release hypotheses[END_REF] to biotic resistance. However, because herbivores have been reported to create disturbances facilitating the establishment of invasive species [START_REF] Mack | Temperate grasslands vulnerable to plant invasion: characteristics and consequences[END_REF][START_REF] Hobbs | Disturbance, diversity, and invasion: implications for conservation[END_REF], and because young restored native species may be negatively impacted by trampling (Clear-Hill and Silvertown, 1997), using herbivores in early stages of restoration may not be an advisable option.

Mycorrhizal fungi, forming symbiotic relationships with 80-90% of terrestrial plants [START_REF] Smith | Mycorrhizal symbiosis, 3rd ed[END_REF], often strongly influence plant growth and reproduction [START_REF] Koide | Effects of mycorrhizal fungi on plant populations[END_REF], plant community structure [START_REF] Van Der Heijden | Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity[END_REF]Hartnett anrd Wilson, 1999[START_REF] Klironomos | Feedback with soil biota contributes to plant rarity and invasiveness in communities[END_REF], and invasion success [START_REF] Klironomos | Feedback with soil biota contributes to plant rarity and invasiveness in communities[END_REF]Callaway et al., 2004). Soil inoculation of arbuscular mycorrhizal fungi can reduce the performance of agricultural non-mycorrhizal weeds [START_REF] Jordan | Arbuscular-mycorrhizal fungi: potential roles in weed management[END_REF][START_REF] Vatovec | Responsiveness of certain agronomic weed species to arbuscular mycorrhizal fungi[END_REF][START_REF] Rinaudo | Mycorrhizal fungi suppress aggressive agricultural weeds[END_REF][START_REF] Veiga | Can arbuscular mycorrhizal fungi reduce the growth of agricultural weeds?[END_REF], raising a potential application in managing non-hosts invasive species (e.g. from Chenopodiaceae and Cruciferae families; [START_REF] Wang | Phylogenetic distribution and evolution of mycorrhizas in land plants[END_REF]. In the case of non-hosts invasive species, establishing species having the ability to increase mycorrhizal inoculum potential would facilitate arbuscular mycorrhizal fungi-dependent native species [START_REF] Eviner | The effects of plant-soil feedbacks on invasive plants: mechanisms and potential management options[END_REF], and may enhance their competitive abilities over later arriving invasive species [START_REF] Smith | Arbuscular mycorrhizae promote establishment of prairie species in a tallgrass prairie restoration[END_REF]. In the cases where the presence of mycorrhizae increases invasion success of host invasive species [START_REF] Marler | Mycorrhizae indirectly enhance competitive effects of an invasive forb on a native bunchgrass[END_REF][START_REF] Smith | Mycorrhizal symbiosis, 3rd ed[END_REF], mycorrhizae suppression through fungicide application combined with restoration of nonmycorrhizal species may help limiting invasive species. The feasibility and effectiveness of this approach needs however to be investigated, since mycorrhizae are sometimes essential in some species assemblages [START_REF] Dostálek | The effect of AMF suppression on plant species composition in a nutrient-poor dry grassland[END_REF].

Overall, whether native herbivores, parasites and symbionts could create priority effects reducing invasive species success remains untested, so that an application in restoration is premature. Because interactions between invasive species and native enemies or symbionts are species or trait-specific [START_REF] Veiga | Can arbuscular mycorrhizal fungi reduce the growth of agricultural weeds?[END_REF][START_REF] Grutters | Plant traits and plant biogeography control the biotic resistance provided by generalist herbivores[END_REF][START_REF] Zhang | Herbivory enhances the resistance of mangrove forest to cordgrass invasion[END_REF], it may be relevant to develop non-resource-based restoration strategies for the most noxious invasive species.

Conclusion

Recent research suggests that better considering priority effects of both invasive and native species in restoration strategies could significantly help reducing invasive species colonization on disturbed areas. When invasive plants arrive or emerge earlier than natives, a size-related advantage can hamper native community restoration success, often impelling to reduce or remove invasive propagule sources. Yet, after removal, invasive plants can still threaten restoration success through persisting soil legacies, especially when the invasive species have long been present or when they were very abundant. The processes underlying the magnitude and persistence of soil legacies are however still poorly understood. Research efforts should be directed towards this topic, as well as towards developing cost-effective and rapid methods of assessing invasives-induced soil modifications. In order to avoid reinvasion and secondary invasions, invasive species removal must often be coupled with the restoration of native species. However, before undertaking revegetation, it is advisable to ensure that it will not lead to invasive species facilitation, such as in some harsh environments.

Invasion-resistance of restored native species could be increased by manipulating resource-and non-resource-based priority effects, especially in productive environments.

Resource preemption, driving priority effects, may be enhanced by extending native species time advance over invasives and by manipulating the characteristics of the restored native species. Several studies reported a high benefit of giving only few weeks of advance, and the amplitude of the benefit was often correlated to variations in environmental conditions (e.g. climate, rainfall, soil fertility). Extending time advance showed mixed results and has been yet poorly studied, raising the need to multiply studies in order to define durations of time advance which are the most effective and how this effectiveness varies depending on environmental conditions. Resource preemption could also be enhanced by manipulating the characteristics of the restored species (selecting species having traits associated to strong and rapid resource preemption, increasing species diversity or sowing density), but such strategies remain largely untested in the field. Priority effects have also been suggested to be influenced by niche overlap between species, but attempts to use functional similarities to control invasive species often showed unsatisfying results. Focusing on key functional traits playing a substantial role in invasion success (e.g. phenology) appeared more promising, but studies are lacking to evaluate the relevance of this method. The manipulation of non-resource priority effects to limit invasions has been yet poorly studied. Restoring allelopathic native species may decrease invasion success by directly reducing the target invasive species biomass and indirectly facilitating native species. To investigate the potential of this method, research is needed on the interactions between allelopathy and priority effects as well as the use on the ability of native allelopathic species to suppress several invasive species. The manipulation of other non-resource mechanisms to increase priority effects of natives, such as natural enemies of invasive species or mycorrhizae, appears today premature.

Combinations between different priority effect-based strategies have not been explored yet, but may potentially enhance invasive species control. When invasive species are present, it may be relevant to simultaneously tackle their priority effects and increase those of desired native species. Different strategies could also be successively used over time. For example, establishing a community dominated by one competitive species, producing a high rate of biomass, may be an effective way to rapidly increase native cover and counter immediate invasion risk, while subsequently adding seeds from diverse species may help stabilize the community in the long term.

Fig. 1 .

 1 Fig. 1. Mechanisms driving the consequences of priority effects when the plant individual is present (A), or has been removed or died (B). (A) Early arriving species can limit colonization success of subsequent colonizing species by gaining a size-related competitive advantage, by generating positive plant-soil feedbacks improving its own

Acknowledgements

We gratefully thank D. Simberloff for his helpful comments, as well as M. Sweetko for reviewing the English. No conflicts of interest have been declared.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ecoleng.2018.11.011.