Mixing source populations increases genetic diversity of restored rare plant populations

The genetic diversity of germplasm used in reintroduction and restoration efforts can influence how resulting populations establish, reproduce, and evolve over time, particularly in disturbed and changing conditions. Regional admixture provenancing, mixing seeds derived from multiple populations within the same region as the target site, has been suggested to produce genetically diverse germplasm. Yet little empirical evidence shows how genetic diversity in germplasm resulting from this approach compares to source populations, or how it varies in restored populations. Here, we use neutral molecular markers to follow genetic diversity through production and use of germplasm when mixing multiple source populations in nursery production beds. Castilleja levisecta is a rare species experiencing inbreeding depression in remaining populations, with a federal recovery plan requiring the re-establishment of populations in areas where it has been extirpated. Specifically, we track diversity from wild-collected source populations through different production approaches and reintroductions using two propagule types. We show that measures of genetic diversity, inbreeding, and relatedness change during the production and use of material produced with a regional admixture provenancing approach, with the step at which source populations are mixed and germplasm type used influencing whether all source populations are equally represented. While genetic diversity increased throughout the process, inbreeding and relatedness increased in nursery production beds but decreased in reintroductions, with the lowest inbreeding and relatedness in populations restored using seeds rather than plugs. The results highlight the importance of taking an integrated approach informed by research when planning and implementing reintroductions with mixed-source germplasm.