Seed germination traits reveal naturalization potential: global insights from temperate European herbaceous species

1. Seed germination is a key stage in a plant's life cycle, influencing regeneration from seed by determining the post‐germination environment, plant fitness and evolutionary potential. Therefore, seed germination traits are expected to play a fundamental role in the naturalization of alien seed plants; yet broad‐scale empirical evidence of this remains limited.
2. Using seed germination data for 1146 native temperate European herbaceous species, we tested whether species that have become naturalized outside their native range differ from non‐naturalized species in overall germinability (final germination proportion) and in their germination responses to six environmental cues across temperate, tropical dry and tropical humid macroclimatic zones of naturalization. We also assessed whether germinability and responses to these cues are associated with the geographic extent of naturalization, using a phylogenetically informed meta‐analysis that integrates 18,596 standardized laboratory germination records with global naturalization data.
3. Naturalization was a common phenomenon, with 60% of species having naturalized in temperate regions and over 30% having naturalized in tropical regions. Naturalized species showed consistently higher overall germinability, germination at lower temperatures and higher requirements for seed scarification compared to non‐naturalized species, while other germination traits varied with the macroclimatic zone of naturalization. The extent of naturalization was also positively, though weakly, related to higher germinability and to the same germination traits that distinguished naturalized from non‐naturalized species.
4. Synthesis . This study provides global‐scale evidence that the naturalization of European herbaceous species is related to specific germination traits acquired in the native range. Our findings indicate that traits such as high germinability, low stratification requirements and responsiveness to scarification act as preadaptations that facilitate naturalization by increasing opportunities for establishment. They also show that standardized laboratory germination tests using seeds sourced from native populations represent a cost‐effective tool for improving global risk assessments and for predicting naturalization potential under climate change through alterations in regeneration from seed.