Differentiation of seed germination traits along a small-scale hydroperiod zonation in Mediterranean temporary ponds

Understanding the regeneration strategies of ephemeral vegetation in Mediterranean temporary ponds (MTPs) is crucial to achieve a deeper view on the possible effects of shifting water regimes on this endangered habitat, whose conservation is considered a priority in Europe (Council Directive 92/43/CEE; code 3170*). In this study, we tested the hypothesis that germination traits vary along the small-scale plant zonation that characterizes MTPs. Despite being a highly variable habitat, three different zones related to the hydroperiod can indeed be distinguished in MTPs: a long-lasting flooded area, followed by a short-lasting flooded area, and finally the pond’s outer belt (Fig. 1). Mature fruits or seeds (hereafter, ‘seeds’) of 13 species were collected along the zonation of MTPs in the Hyblean plateau (south-east Sicily, Italy). The germination response to different levels of temperature, thermoperiod (constant and alternating), light, and after-ripening was evaluated for each species. Using a thermal-time approach, base (Tb), optimal (To), and ceiling (Tc) cardinal temperatures for germination were calculated. The effect of each factor on final germination was analyzed using MCMCglmm models, accounting for the role of phylogeny, seed mass, and pond zonation. The shape of the germination niche across zones was evaluated with ordination analysis (PCA), performed on the species×seed traits matrix. Temperature preference varies significantly along the zonation, with species from the outer belt exhibiting higher To and Tc values than those from flooded areas. Furthermore, species from the long-lasting flooded area showed the highest germination percentages in darkness. Alternating temperatures and after-ripening, on the other hand, had a generally positive effect across all zones. At the species level, the ordination analysis pointed out a high variability in light and thermoperiod preferences among species from the short-lasting flooded area. Our findings support the hypothesis that temperature and light preferences for germination vary across the small-scale plant zonation of MTPs. Under current climatic conditions, this allows species from the outer belt to be among the first to germinate, while species from long-lasting flooded areas can germinate even under turbid water. The different thermoperiod and light preferences observed in species from the short-lasting flooded areas may reflect adaptations to cope with more pronounced fluctuations in water regimes characteristic of this intermediate zone. With projections of increased out-of-season rainfall and altered precipitation seasonality in the Mediterranean, our results highlight the importance of using germination traits to better understand the ecological dynamics of wetland plant communities under climate change.