Plant trait-insularity links: edaphic islands can operate as true islands, especially for clonal species
Strong insularity promoted belowground resource-conservation strategies across most edaphic island species while tending to reduce plant stature of non-clonal species (i.e. dwarfism; Biddick et al., 2019; Biddick & Burns, 2021; Carlquist, 1974) – both indicative of enhanced persistence abilities (Ottaviani et al., 2020a). Smaller plants with the ability to store resources belowground in dedicated organs (i.e. thick roots, rhizomes) may indicate adaptive strategies towards less costly economics, which may also prolong lifespan (Jiménez-Alfaro et al., 2016; Klimešová et al., 2016a; Saar et al., 2012). These results confirm that insularity can constrain plant growth and promote resource conservation. Additionally, greater insularity affects immigration rates and gene flow (MacArthur & Wilson, 1967; Warren et al., 2015), which may cascade on altered biotic interactions, such as competition, pollination and herbivory (Burns, 2019).
The widespread and consistent evidence that clonal species were more resource-conservative with increasing insularity may provide some insights relevant to their ecology and biogeography. Clonal species are well-known to be poor seed producers (Herben et al., 2015), which may limit their ability to reach distantly-located and/or tiny edaphic islands offering conditions they are specialized for. Yet, once they arrive and establish there, their persistence can be attained via two possible pathways. The first would imply species spreading laterally and multiplying vegetatively and foraging for resources over new areas. The second would involve long-term connection through clonal growth organs, such as rhizomes (Jónsdóttir & Watson, 1997). Both strategies result in an interconnected network of ramets able to share resources among them (Klimešová et al., 2019). The first pathway relates more to the spatial dimension of persistence strategies, while the second refers more to its temporal dimension (Klimešová et al., 2018). The latter pathway seems to be relevant for our edaphic islands, as suggested by high BDMC values (likely linked to a “grow slow-live long” strategy; Jónsdóttir & Watson, 1997; Klimešová et al., 2016a,b) and no response of lateral spread. Therefore, we may suggest that, especially for clonal species, the studied edaphic islands may be governed by similar ecological and biogeographical forces as true islands (Conti et al., 2021; Méndez-Castro et al., 2021; Ottaviani et al., 2020a).