Intraspecific variability in seed mass, germination, and seedling growth in the narrow endemic Iberodes littoralis...

The study of seed trait variation is essential for developing effective conservation strategies, understanding plant recruitment, and predicting the long-term persistence of endemic/rare plant species under environmental change. This study examines the seed ecology of Iberodes littoralis subsp. gallaecica, a narrow endemic to coastal dune systems in north-western Spain. Seeds collected from five populations over 7 years were grown in a glasshouse to investigate variation in seed mass, germination rate, time to germination, and seedling relative growth rate (RGR). To evaluate the role of resource availability, seeds were sown in two substrate types: dune sand alone and mixed with compost. Seed traits varied significantly among years and populations. Overall, older seeds germinated less and took longer to germinate than younger seeds. Heavier seeds had greater germination percentages, but the effect of seed mass on time to germination differed among years. Seeds produced under higher maternal rainfall were lighter, which in turn influenced germination timing and seedling RGR, indicating an indirect effect of maternal environment on seed performance. Germination rates were lower in the substrate amended with compost, but those seedlings had a higher RGR. These results suggest that using fresh seeds maximizes restoration success, and when fresh material is unavailable, the largest seeds should be selected. While fertilisation can enhance early seedling growth, it may not improve germination success. Our findings highlight the complex interplay among seed age, seed mass, population differences and substrate conditions in shaping germination and early growth of this narrow endemic.