INTRODUCTION
At the global scale, the rate of species extinction due to environmental
changes (e.g. land-use alterations or climate warming) is accelerating
at such a pace that the sixth mass extinction has been invoked (Barnosky
et al., 2011; Bellard et al., 2012; Ceballos et al., 2015). At finer
scales, this process translates into local extirpations of species
failing to cope with rapid and abrupt changes (Hylander & Ehrlén,
2013). Plant species may avoid or delay local extinction by surviving
outside of their optimal conditions or through various strategies
promoting in-situ persistence (Marini et al., 2012; Saar et al.,
2012). Overall, persistence operates as a counteracting process to local
extinction (Auffret et al., 2017). Consequently, analyzing persistence
strategies should be critical in conservation biogeography and
ecological studies because it may help estimate local extinction risk
related to current or anticipated environmental changes.
To successfully persist in an area under changing environmental
conditions, plants must be effective in i) acquiring, using and
conserving resources (Ackerly & Cornwell, 2007; Saar et al., 2012), ii)
occupying new neighboring space through seed dispersal and/or clonal
ability (Cody & Overton, 1996; Jiménez-Alfaro et al., 2016), and iii)
recovering after disturbance (Martínková et al., 2020; Pausas et al.,
2018). How these functions play out under specific circumstances can be
assessed through trait-based approaches (Klimešová et al., 2019;
Ottaviani et al., 2020a; Pérez-Harguindeguy et al., 2013; Weiher et al.,
1999). A good example is offered by seasonal and disturbance-prone
temperate grasslands hosting many perennial herbaceous species with
different persistence strategies (Klimešová et al., 2016a, 2021;
Ottaviani et al., 2020b). A major group comprises long-lived clonal
species, capable of both sexual and vegetative reproduction and
investing considerable resources belowground into bud-bearing and
carbohydrate storage organs (Janovský & Herben, 2020; Klimešová et al.,
2016a). Clonal species coexist with many non-clonal species
distinguished by shorter lifespan, relying only on regeneration from
seeds, and investing fewer resources into bud bank and carbohydrate
storage (Martínková et al., 2020). All of these plants can cope
effectively with seasonally cold climates and recurrent disturbances
(e.g. mowing, grazing, fire; Klimešová et al., 2018, 2021; Ottaviani et
al., 2020b). Some of them are also able to deal with increasingly drier
seasons, nutrient deposition or altered management regimes (Fischer et
al., 2020; Qian et al., 2021). Yet, only those distinguished by high
investments into persistence strategies (e.g. bud and seed bank, clonal
multiplication) may avoid or delay local extinction caused by ongoing
and predicted environmental changes (Marini et al., 2012; Rosbakh &
Poschlod, 2021; Saar et al., 2012).
Another potentially critical example in the analyses of persistence
strategies is constituted by insular systems. These are “natural
laboratories” (Warren et al., 2015; Whittaker et al., 2017) and
suitable models to examine persistence strategies because plants with
limited dispersal on steadily isolated islands tend to exhibit adaptive
strategies to successfully survive (Cody & Overton, 1996; Conti et al.,
2021; Ottaviani et al., 2020a). At the same time, insular systems are
particularly vulnerable to species extinctions linked to environmental
changes (Courchamp et al., 2014; Macinnis-Ng et al., 2021; Veron et al.,
2019). The study of plant functional traits in insular systems has
boosted in recent years, providing important insights into the
eco-evolutionary dynamics of these systems (e.g. Biddick et al., 2019;
Biddick & Burns, 2021; García-Verdugo et al., 2020; Negoita et al.,
2016; Schrader et al., 2021; Spasojevic et al., 2014; Taylor et al.,
2019). Yet, most of this research targeted dispersal and resource
acquisition traits, neglecting other important, non-acquisitive
functions (but see Aikio et al., 2020), which are needed for better
understanding how plant traits promote persistence and help avoid local
extinction (Auffret et al., 2017). For example, plants occurring in
highly and steadily isolated places (e.g. distant from species sources
preventing or limiting gene flow) should be i) conservative in the way
they use resources, ii) grow slower but become older, and iii) allocate
more into vegetative reproduction, and when regenerating sexually,
producing fewer and heavier seeds with limited dispersal – all
strategies indicative of enhanced local persistence (refer to Ottaviani
et al., 2020a for an overview).
Trait-based studies are abundant, yet they are often carried out at the
interspecific level because differences among species are usually larger
than within species (Hulshof & Swenson, 2010; Klimešová et al., 2019;
Pérez-Harguindeguy et al., 2013). In the last decade, however, a large
body of evidence is showing that intraspecific trait variability can
considerably contribute to plant ability to cope with environmental and
biotic changes (e.g. Conti et al., 2018; Hulshof & Swenson, 2010;
Kichenin et al., 2013; Midolo et al., 2019; Siefert et al., 2015; Violle
et al., 2012). Exploring trait patterns at the inter- and intraspecific
level may provide a clearer picture of the persistence strategies
deployed by insular biota to cope with insularity-related extinction
risk – a valuable insight for functional ecology (Weiher et al., 1999)
and conservation biogeography (Richardson & Whittaker, 2010).
In this study, we explore links between persistence-related traits at
the inter- and intraspecific level with variation in environmental
(soil, climate) and biogeographical (insularity) conditions in edaphic
islands. These are terrestrial island-like systems defined by the patchy
distribution in a landscape of i) discrete bedrock or soil type (e.g.
serpentine soils; Harrison et al., 2006; Kazakou et al., 2008; Hulshof
& Spasojevic, 2020), or ii) topographic discontinuity (e.g. inselbergs;
de Paula et al., 2019; Ottaviani et al., 2016). We focus on edaphic
islands constituted by rocky outcrops in Central Europe, which host
temperate dry grasslands formed by perennial clonal and non-clonal plant
species confined to the outcrops. Due to the nature of any edaphic
island system (Kazakou et al., 2008; Spasojevic et al., 2014), we expect
soil properties and insularity to play a greater role in shaping
persistence-related trait patterns than climate. Because clonal and
non-clonal plant species represent distinct life-history strategies in
temperate grasslands (Klimešová et al., 2016a; Martínková et al., 2020),
we treat them separately. Specifically, we ask:
(Q1) Do clonal and non-clonal plant species consistently exhibit
enhanced persistence abilities and more resource-conservative strategies
under harsher environmental conditions – i.e. drier and less fertile
soils primarily, and, to a lesser extent, warmer and more seasonal
climate? If so, do they achieve this through distinct inter- and
intraspecific trait responses to the environment?
(Q2) Do clonal and non-clonal plant species experiencing stronger
insularity (i.e. growing on smaller and/or more isolated edaphic
islands) consistently exhibit enhanced persistence abilities and more
resource-conservative strategies? If so, do they achieve this through
distinct inter- and intraspecific trait responses to insularity?