Genetic identity and diversity can have significant effects on plant morphology and productivity, with cascading effects on associated species. However, whether and how the importance of genetic diversity changes across life history stages is seldom tested. Further, the effects of genetic diversity on population, community, and ecosystem processes may depend not only on the number of genotypes (i.e., richness), but also on the relatedness of genotypes. Seagrass systems provide some of the best evidence for the ecological effects of genetic diversity among adult plants, yet whether seeds, which are the ultimate source of seagrass genetic diversity, amplify or negate these effects is not known. We directly manipulated seagrass (Zostera marina) genetic relatedness by collecting seeds from reproductive shoots and controlling the number of maternal families present. We tested the effects of seed diversity on germination, seedling morphology, and seedling productivity by comparing experimental assemblages of seeds collected from single reproductive shoots ("monocultures") to assemblages of seeds collected from multiple reproductive shoots ("polycultures"). We also partitioned among diversity mechanisms to assess the relative importance of complementarity and selection effects at early life history stages.
Results/Conclusions
There was no difference in the proportion of seeds that germinated in monocultures and polycultures, but there were significant effects of seed diversity on above- and below-ground seedling traits. Above ground, seedlings from polycultures had larger shoots and more leaves on average than seedlings from monocultures at the end of the one-year experiment. Similarly, there were clear diversity effects below ground: polyculture seedlings had longer roots and greater below ground biomass than monoculture seedlings. The positive effects of diversity in our experiment were due primarily to trait-independent complementarity among seedlings, with negligible effects of selection (i.e., dominance and trait-dependent complementarity). Neither genetic relatedness of the reproductive shoots used to create each polyculture nor genetic relatedness of the seedlings in each polyculture were predictive of polyculture morphology or biomass. Our results suggest that effects of seagrass genetic diversity on morphology and productivity manifest long before shoots are fully mature. In addition, they emphasize the need to consider all life history stages, and particularly sexual stages, when evaluating the ecological importance of genetic diversity.