Genetic diversity matters for restoration of a threatened saltmarsh plant in harsh environments

1. While genetic diversity is theorised to increase the restoration success of degraded plant populations, previous studies examining its impacts on restoration outcomes have yielded mixed results. 2. We hypothesise that the importance of genetic diversity for the performance of plants targeted for restoration increases with environmental stress and varies with specific plant traits related to different plant successive life stages. To test this, we conducted a fully factorial common garden experiment, crossing salinity and genetic diversity under low- and high-nitrogen conditions on the growth and reproduction of Scirpus mariqueter, a threatened endemic saltmarsh plant in the Yangtze Estuary. 3. The impacts of genetic diversity varied across stress gradients and metrics. Under high stress (high salinity, low nutrients), greater diversity enhanced reproductive traits such as clonal growth and seed production, but not under low stress. Plant growth traits showed no diversity effects regardless of stress. 4. Synthesis: Our results highlight the importance of considering intraspecific genetic diversity in ecological restoration and help explain the context-dependent effects of diversity.