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).