Strong effect of soil and weak climate signal on edaphic island plant persistence
The studied edaphic islands associated with rocky outcrops are distinguished by shallow soils with high sand content, low water retention capacity and low nutrient availability. These harsh edaphic conditions contributed greatly to shaping the non-acquisitive trait patterns influencing the local persistence of perennial plant species. The evidence that soil properties operate as one of the key drivers of persistence strategies edaphic of island plants aligns with other studies (Kazakou et al., 2008; Spasojevic et al., 2014; Hulshof & Spasojevic, 2020). Increasing variability in soil depth was consistently related to higher BDMC values across all clonal species, suggesting that fine-scale soil heterogeneity may push these plants to be more conservative belowground, i.e. a “stay-where-you-are” adaptive strategy (Graae et al., 2018).
Non-clonal plants grew better in deeper and sandier soils, while plants on more fertile soils tended to grow older but at a slower pace, as for individuals of Carlina and Helianthemum . These findings challenge the notion that plants inhabiting harsher environments are slower in their growth and longer-lived (Nobis & Schweingruber, 2013). This may suggest that Carlina and Helianthemum have developed at the intraspecific level unique adaptive strategies to successfully persist in the distinct conditions provided by the edaphic islands, which do not necessarily constrain their growth (Doležal et al., 2021). Non-clonal plants also exhibited a greater ability to transport water through larger vessels in deeper, more variable and sandier soils. Yet, this may be explained by an adaptive hydraulic safety-efficiency functional trade-off (Drake et al., 2015). Larger vessels may imply quicker growth (as found for Helianthemum,probably facilitated by its deep rooting ability; Doležal et al., 2021) and higher evapotranspiration, but may also increase the risk of embolism associated with cold and arid spells typical of the highly-seasonal temperate dry grasslands.
Climate affected only a few traits, often showing inconsistent patterns. This flags a minor role of climate in shaping the persistence strategies of edaphic island plants compared to soil properties and insularity. The macroclimate can be considered the same across all the edaphic islands. Yet, microclimate (captured by the data-loggers) can differ within single islands because of variable slope aspect, inclination and solar radiation generated by the rugged terrain and dome-shaped topography of the outcrops (de Paula et al., 2019; Ottaviani et al., 2016). Still, for clonal species, we identified that warmer climates and reduced temperature fluctuations affected plant height negatively and BDMC positively. This result points towards more conservative resource economics under warmer conditions and may indicate that with the anticipated and exacerbating warming of the region, clonal species may be able to cope with these changes (Saar et al., 2012). For non-clonal species, results were even scantier and not pointing towards any generalizable trend.