A first checklist of the alien-dominated vegetation in Italy

This study provides a first step toward the knowledge of the alien-dominated and co-dominated plant communities present in Italy. The first ever checklist of the alien phytocoenoses described or reported in literature for the Italian territory has been compiled, produced by data-mining in national and local thematic literature. The resulting vegetation-type draft-list has been checked in the light of the most recent syntaxonomic documentation and updated with regards to syntaxonomy and nomenclature, with special reference to the frame proposed in the Italian Vegetation Prodrome. The list includes 27 vascular and one bryophyte vegetation classes, hosting 194 low rank alien-dominated syntaxa. The different vegetation types detected for each syntaxonomic class and macro-vegetation group, defined by physiognomical and ecological attributes, are discussed.


Introduction
Biological invasions are an ever-increasing global process arising from the intentional or accidental human-mediated introduction of species to areas outside their native range, overcoming natural dispersal mechanisms and biogeographic barriers ( The consideration and debate on Invasive Alien Species (IAS) date back to long ago (Allan 1936;Egler 1942;Baker 1948;Elton 1958). In the last few decades, invasion science has emerged, becoming a relevant discipline of its own (Richardson et al. 2000; Richardson and Pyšek 2006; Richardson 2011), also as a consequence of the increasing rates of alien species' introductions at the global scale (Seebens et al. 2017(Seebens et al. , 2018 with no exception in Europe (DAISIE 2009)  An alien organism needs to overcome geographical, environmental, and reproductive barriers to colonize a new region and spread over wide areas. In this process, some factors and traits can be more significant than others in explaining its success and therefore its invasiveness (Van Kleunen et al. 2015). Particularly, the concept of invasiveness of plant species has been integrated with that of propagule pressure and of "invasibility" of habitats and plant communities, i.e. the susceptibility of an environment to invasions by alien species, as different habitats and phytocoenoses may be more invasible than others and show different degrees of resistance/resilience (Rejmánek 1989 Pyšek et al. 2010aPyšek et al. , 2010b. This seems to some extent more important than alien species number, suggesting the hypothesis that the more an alien taxon becomes dominant in a vegetation type, the stronger the impact on native species diversity might be. This approach gave a pulse to research on alien plant species assemblages, and recently investigations on IAS have been addressed also at the community scale and benefitted from the currently available large da- A study by Chytrý et al. (2008) demonstrated that the habitat type is a reliable predictor of the level of plant invasion. This paper took into account patterns of plant invasions across habitats at the European scale, suggesting precious insights for biodiversity conservation and habitat monitoring, especially when considering the acknowledged correspondence between syntaxonomic types and habitats sensu Dir. 92 Despite a long-dated Italian tradition of phytosociological studies, a national synthesis of the alien-dominated (and co-dominated) plant communities thriving in Italy has never been produced yet. Therefore, the present research aims at taking the first step toward the filling of this knowledge gap.
The European and national projects dedicated to IAS, which involved and currently still involve research academic centres and institutions, are numerous in Italy. Some of them have helped to gather a significant amount of information on invasive plant and animal species. Among the most recent, addressing non-native plant species, their impact and their management, as well as the awareness of the general public on these topics, we can mention: LIFE ASAP (http://www.lifeasap.eu), LIFE GESTIRE IP2020 (http://www.naturachevale.it/il-progetto/life-gestire-2020/), LIFE REDUNE (http://www.liferedune.it), MARITTIMO ALIEM (http://interreg-maritime.eu/web/aliem).
The present work is the result of a research agreement between SISV (the Italian Society for Vegetation Science) and ISPRA (the Italian Institute for Environmental Protection and Research), with financial support from the Italian Ministry of Environment, aimed at supporting the implementation of Regulation EU 2014/1143 (updated by EU Reg. 2017/1263) on the prevention and management of the introduction and spread of invasive alien species. In particular, the general agreement focused on: i) the update of the Database of Italian Alien Species (DIAS) with reference to their impacts on the ecosystems and the most threatened habitats and ii) the identification of the alien-dominated or co-dominated plant communities occurring in Italy. The outcomes of the latter are here presented. The research on alien plant communities and habitats was carried out through the collaboration of a wide working group of experts led by a Coordinating Committee composed by SISV members.

Methods
A dedicated SISV working team, formed by national and local experts, collected all the thematic literature related to terrestrial and freshwater alien-dominated and co-dominated vegetation. On the basis of this bibliographic dataset, a selection of all vegetation data was carried out, with special attention to nomenclature and syntaxonomic classification. All the existing national and regional vegetation databases (e.g. LiSy -http://www.scienzadellavegetazione.it/sisv/lisy/index.jsp; Poldini et al. 1985;Poldini 1991Poldini , 2002Poldini , 2009Gallizia Vuerich et al. 1999;Brullo et al. 2001;Gigante et al. 2012;Landucci et al. 2012;Evangelista et al. 2016), together with the regional bibliographic sources (e.g. Poldini 1989;Poldini and Vidali 1989;Poldini et al. 1991Poldini et al. , 1999, were consulted. Data concerning each phytocoenosis were selected if one or more alien species played a substantial role in the analysed vegetation unit. In particular, the SISV Coordinating Committee collected and checked the information and selected the data whenever: 1. the analysed low rank syntaxon (association, subassociation, phytocoenon) was dominated or co-dominated by one or more alien plant species; a cover value ≥ 3 according to the "Braun-Blanquet" scale (Braun-Blanquet 1979) for alien species cover in each relevé has been set as threshold; if, instead of the "Braun-Blanquet" scale, the "Pignatti" scale was used in the bibliographical reference (Pignatti and Mengarda 1962), the cover value threshold was set to ≥ 2, i.e. 20-40%; 2. the alien species name was included in the name of the syntaxon, by that implying that it is a characteristic/differential/diagnostic or somehow important taxon for the definition of the syntaxon, or even dominating and determining its vertical structure (as stated in Art. 10b of the International Code of Phytosociological Nomenclature: Theurillat et al. 2020).
Starting from this selected dataset, a draft syntaxonomic list was produced. The names of the plant communities and their syntaxonomic attributions at higher ranks have been acknowledged (and are here reported) in the same form as published by the original Authors. In case of inconsistencies or discrepancies, a specific comment has been provided.
The syntaxonomic scheme has then been updated in accordance with the Italian Vegetation Prodrome (Biondi et al. 2014; http://www.prodromo-vegetazione-italia. org/), which however does not take into considerations syntaxa below the alliance level. Relevant differences and discrepancies with other syntaxonomic frames, especial- php), to which we referred also to identify the species to be considered as aliens to Italy, including archaeophytes (e.g. Arundo donax, Acanthus mollis subsp. mollis) as well. New hybridogenic species due to xeno-speciation events (e.g. some species of Oenothera or Vitis) were taken into account, when considered as alien taxa by Galasso et al. (2018). In case of species considered alien to an administrative region and native to another one, only the communities reported for the region where the species is alien have been recorded (e.g.: a community dominated by Acanthus mollis subsp. mollis was considered alien to Liguria but not to Sicily, where this species is considered as native, see Minissale et al. 2019).
This process allowed to produce a first checklist of the Italian alien-dominated plant communities. On this ground, some statistics were calculated considering the number of alien vegetation units with reference to i) each resulting syntaxonomic class and ii) homogeneous groups of the resulting syntaxonomic classes, clustered according to their physiognomic and ecological characteristics.

Results and discussion
A comprehensive and annotated checklist of the alien-dominated and co-dominated plant communities occurring in Italy is provided in Appendix I. All phytocoenoses have been framed in an updated syntaxonomic scheme. A specific bibliographical list with references for all the syntaxa quoted in the checklist is available in Appendix II.
The checklist includes a total of 27 classes of vascular plant vegetation and one of bryophyte vegetation, including plant communities dominated or co-dominated by aliens to Italy. The total number of low rank syntaxa (associations/subassociations/communities) amounts to 194.
The number of communities for each class is reported in Fig. 1. As expected, the class with the highest number of alien-dominated plant communities resulted by far Stellarietea mediae, followed by Artemisietea vulgaris. The presence of some higher-rank syntaxa named after and mainly formed by IAS (e.g. the class Robinietea, the order Nicotiano glaucae-Ricinetalia communis) is worth to be noted, which highlights the coenological and physiognomic-structural autonomy of these communities. The 27 identified classes and some subordinate syntaxa have been grouped in clusters based on their physiognomy and ecology. The considered groups are as follows: Results of the community rates per class groups are shown in Fig. 2. The group including the classes of annual ruderal herbaceous vegetation (Stellarietea mediae and Polygono arenastri-Poetea annuae) hosts about 25% of the total number of the detected alien-dominated communities. Stellarietea mediae is by definition characterized by high rates of alien plant species, especially archaeophytes (http://www.prodromo-vegetazione-italia.org/). However, data analysis showed that this explains only part of the story. Actually, the involved alien species and genera are mainly represented by neophytes (e.g. Robinia pseudoacacia, Artemisia sp.pl., Amaranthus sp.pl., Erigeron sp.pl., Euphorbia sp.pl., Solidago sp.pl.). Indeed, it is known that neophytes mostly occur in strongly anthropogenic areas, whose habitats appear to be not only the most invaded, but also the most invasible (Pyšek et  . This is indirectly confirmed in this study, by the fact that many of the less represented groups of alien-dominated communities refer to scarcely invasible environments, such as screes (Thlaspietea rodundifolii) or Mediterranean grass-dominated vegetation (Lygeo-Stipetea). Unexpectedly, our data seem to suggest that psammophilous coastal vegetation experiences low rates of alien community occurrence (Figs. 1, 2). However, to correctly interpret this outcome, it should be considered that the collected data might outline a biased picture due to a lack of syntaxonomic investigation and classification for some communities. In support of this hypothesis, it should be noted that many alien coastal communities dominated by Yucca sp.pl., Agave sp.pl., Opuntia sp.pl. or Acacia sp.pl. have been noted although not syntaxonomically described yet ( Other habitats also linked to water (and to the related high levels of disturbance) show a high number of alien-dominated communities. It is the case of riparian non-woody habitats, or of annual and perennial herbaceous hygrophilous and hygro-nitrophilous vegetation types (Fig. 2), which reach high percentages (the latter more than 18%).
Also the alien communities attributed by many authors to Querco-Fagetea have a prevalent mesohygrophilous character (Appendix I). The most relevant woody invasive alien species, also found in the anthropogenic neophytic woody vegetation (e.g. Robinia pseudoacacia, Ailanthus altissima, Amorpha fruticosa, and many others) can be defined as "ecosystem engineers", i.e. species that shape habitats and/or cause changes to their state and resources availability ( In the checklist we also recorded a bryophyte community dominated by the alien Campylopus introflexus, which in Europe is considered a neophytic moss, introduced from the Southern hemisphere and rapidly ex- of which are considered invasive in at least one region. Our checklist shows that, based on the current knowledge, less than one hundred of these species (precisely 88) perform a dominant or co-dominant role in the considered plant communities. Only five of these (Ailanthus altissima, Elodea nuttallii, Lagarosiphon major, Myriophyllum aquaticum, Cenchrus setaceus) are considered IAS of Union Concern (see https://ec.europa.eu/environment/nature/ invasivealien/list/index_en.htm), however this number grows considerably when downscaling at the national level, with 76 of them (i.e. more than 86% of the total) to be considered as IAS in Italy (Galasso et al. 2018). Again, this can be traced back to the fact that most of these IAS are neophytes, many have been introduced in relatively recent times, and probably there was not enough time to give rise to such a degree of invasion to be relevant at European scale yet. Additionally, also at the national scale these invasions are often very localized or at very early stages, and their study from the phytosociological point of view is still a minor topic (although emergent).
At the same time, the continuous rate of introduction-naturalisation-invasion of new alien plants is an ongoing process that should be detected just in early stages, in order to prevent serious damage to native biodiversity. The numbers here reported raise the alarm for planning conservation biodiversity studies, monitoring protocols and management activities.

Conclusion
The here presented first checklist of the alien-dominated plant communities in Italy should not be considered exhaustive. It is the first step toward a better understanding of distribution, ecology and invasion processes of alien species at community level in this country. Our review represents a screenshot of the current knowledge and suggests that a serious lack of investigation for certain vegetation types (and probably for certain areas of the country) has to be highlighted.
The present checklist of the alien vegetation in Italy can represent a very useful tool, not only for stimulating further studies and investigations but also for prevention, management and monitoring purposes. As emphasized by Olaczek (1982), the phytosociological school, taking into account the whole floristic composition of plant communities, was one of the first approaches able to detect the effects of alien species on the diversity of natural phytocoenoses and to include these new communities in the syntaxonomical vegetation classification. In fact, the "floristic and phylogenetic homogenization" (e.g. Pino et al. 2009) and the "degeneration of phytocoenoses" by means of a progressive modification of structure and floristic composition due to the alien species invasion, concepts expressed by Olaczek (1982) and Faliński (1998a, 1998b), can transform a native plant community into an athropogenic one, or even in a "novel ecosystem" (Lugo 2015), susceptible to be classified in a new syntaxonomic frame.
This becomes particularly important when considering the close link between plant communities syntaxonomi-