Patterns of biodiversity may vary across spatial scales and between taxonomic groups; therefore, specific studies are needed to provide insights into factors driving community structure. Semi-natural grasslands are among the most biodiverse ecosystems, providing a suitable model for examining key ecological mechanisms. We analysed dry grasslands in Transylvania (Romania), which harbor extraordinarily species-rich plant communities, including the

Competition from native species is a key mechanism for biotic resistance to invasion. Accelerated germination to pre-empt resources or delayed germination and induced dormancy until the next growing season are two alternative strategies for annual invasive plants to avoid the drawbacks of competition at the seed stage. In Ambrosia artemisiifolia, both of these tactics could theoretically increase its long-term fitness. However, their relative importance