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?