The naturalization of an introduced species is a key stage during the invasion process. Therefore, identifying the traits that favor the naturalization of non-native species can help understand why some species are more successful when introduced to new regions. The ability and the requirement of a plant species to form a mutualism with mycorrhizal fungi, together with the types of associations formed may play a central role in the naturalization success of different plant species. To test the relationship between plant naturalization success and their mycorrhizal associations we analysed a database composed of mycorrhizal status and type for 1981 species, covering 155 families and 822 genera of plants from Europe and Asia, and matched it with the most comprehensive database of naturalized alien species across the world (GloNAF). In mainland regions, we found that the number of naturalized regions was highest for facultative mycorrhizal, followed by obligate mycorrhizal and lowest for non-mycorrhizal plants, suggesting that the ability of forming mycorrhizas is an advantage for introduced plants. We considered the following mycorrhizal types: arbuscular, ectomycorrhizal, ericoid and orchid mycorrhizal plants. Further, dual mycorrhizal species were those that included observations of arbuscular mycorrhizas as well as observations of ectomycorrhizas. Naturalization success (based on the number of naturalized regions) was highest for arbuscular mycorrhizal and dual mycorrhizal plants, which may be related to the low host specificity of arbuscular mycorrhizal fungi and the consequent high availability of arbuscular mycorrhizal fungal partners. However, these patterns of naturalization success were erased in islands, suggesting that the ability to form mycorrhizas may not be an advantage for establishing self-sustaining populations in isolated regions. Taken together our results show that mycorrhizal status and type play a central role in the naturalization process of introduced plants in many regions, but that their effect is modulated by other factors.

We compiled information on mycorrhizal status and mycorrhizal type from different sources: a database on mycorrhizal occurrence on vascular plant species across eastern Europe and north Asia (Akhmetzhanova et al. 2012), a database on mycorrhizal associations for plant species in central Europe (Hempel et al. 2013), a database on mycorrhizal associations for plants across Europe (Bueno et al. 2017) and a database on mycorrhizal occurrence on vascular plants from Europe and north Asia (Correia et al. 2018). We checked entries in several databases and their cited references to minimize errors. We included three mycorrhizal statuses: non-mycorrhizal, obligate mycorrhizal and facultative mycorrhizal. We classified each plant species into one mycorrhizal status according to observations of mycorrhizal associations (or the absence of mycorrhizal associations) directly on plant roots, at the individual plant level, following an approach repeatedly used in previous studies (Hempel et al. 2013, Bueno et al. 2017, Menzel et al. 2017, Correia et al. 2018). Non-mycorrhizal species were those that included only observations (on individual plants) of absence of mycorrhizal associations, and obligate mycorrhizal species were those that included only observations of mycorrhizal associations. Facultative species were those that included observations of mycorrhizal associations (either with arbuscular mycorrhizal or ectomycorrhizal fungi) as well as observations of absence of mycorrhizal associations. We considered a minimum of two independent observations for each mycorrhizal status (mycorrhizal associations and absence of mycorrhizal associations) to classify a species as facultative mycorrhizal in order to reduce the influence of observer errors. The number of species with observations of facultative ericoid mycorrhizal associations or facultative orchid mycorrhizal associations was too low to consider them in our analyses. It is important to note that these three mycorrhizal categories only indicate observed root colonization status and do not include information about functionality, such as mycorrhizal dependence. There were some cases of conflicting databases, which were caused by Correia et al. (2018) not including facultative ectomycorrhizal and dual mycorrhizal categories. In this regard, Correia et al. (2018) considered as obligate ectomycorrhizal plants those with observations of this mycorrhizal type, without any regard on facultative ectomycorrhizal species (ectomycorrhizal - non-mycorrhizal) or dual mycorrhizal species (arbuscular - ectomycorrhizal). Therefore, we checked every plant species classified as obligate ectomycorrhizal according to Correia et al. (2018) in their original data source and in other mycorrhizal databases to evaluate if any plant species was originally classified as facultative ectomycorrhizal or as dual mycorrhizal.

We considered five mycorrhizal types within the obligate mycorrhizal status: arbuscular mycorrhizal, ectomycorrhizal, ericoid mycorrhizal, orchid mycorrhizal and dual mycorrhizal (Smith and Read 2008, Moora 2014, Correia et al. 2018). Dual mycorrhizal species were those that included observations of arbuscular mycorrhizas as well as observations of ectomycorrhizas (at least two independent observations of each mycorrhizal type). The number of plant species with observations of dual mycorrhizal associations involving either ericoid or orchid mycorrhizal types was too low to consider them in our analyses. We standardized species names according to The Plant List (www.theplantlist.org) using the TPL function from the “taxonstand” package (Cayuela et al. 2012) 

                We use data on naturalization of vascular plant species from the GloNAF database version 1.2 (van Kleunen et al. 2019). The GloNAF database is the most comprehensive data source on naturalized vascular alien plant species. This database includes information of 13,939 taxa and 1029 regions, based on 210 data sources (van Kleunen et al. 2019). A region is defined here as the smallest geographic area for which a list of alien plant species is available. From the total number of regions a subset of 648 are considered as mainland regions and a subset of 381 as island regions. Both mainland and island regions vary widely in their surface area. Species names in the GloNAF database are already standardized according to The Plant List. We merged our database on mycorrhizal status and type with the GloNAF database. From the 3719 species in our mycorrhizal database 1981 were recorded as naturalized in at least one region according to the GloNAF database. For these 1981 species we calculated naturalization success as the number of regions where each species is reported to be naturalized. This would be an indicator of the tendency to spread, or invasiveness, of a plant species belonging to a particular group. We also calculated naturalization success as the area where each species is naturalized (adding up the areas of all the regions where the species is reported to be naturalized) to evaluate if we obtained the same patterns of naturalization success.

References

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