Geographic patterns and environmental drivers of seed traits of a relict tree species

Seed traits related to recruitment can directly affect plant fitness and persistence. Phenotypic variability in seed traits among populations could increase species resilience and reduce the risk of extinction under climate change. However, seed trait variations along geographic gradients of relict mountain tree species remain poorly explored despite their vulnerability to environmental changes. Here, we collected seeds of Euptelea pleiospermum from 18 populations across its natural distribution in China, measured seed morphology and seed nutrients, and performed a germination test. We investigated geographic patterns of seed traits and analyzed the relationships between environmental factors and seed traits. We also analyzed the relationships between seed intrinsic attributes and seed germination percentage. In addition, we explored the direct and indirect effects of climatic and edaphic variables on seed germination percentage. We found substantial variation in seed traits of this species among populations. Seed mass decreased from low to high latitude, and seed size (length and width) decreased with longitude from west to east. Seed germination timing increased from low to high altitude. Temperature and soil phosphorus determined the geographic variation of seed traits. Seed mass and seed nitrogen had positive effects on seed germination percentage. Seed intrinsic attributes, rather than maternal environmental factors, were the dominant drivers of the variability in seed germination percentage. However, maternal environmental variables could indirectly affect the seed germination percentage through their effects on seed morphology and nutrients. These results demonstrate that among-population seed trait variations are mainly driven by climatic variables and soil nutrients, and indicate that climate warming is likely to alter seed germination patterns by shifting seed intrinsic attributes. Our study provides insight into how mountain tree species regulate seed traits and germination time to adapt to heterogeneous environments and improves our power to predict how relict plants may respond to climate change.