Clonality in black locust (Robinia pseudoacacia L.) and implications for seed production

Abstract Key message The strong clonal growth of black locust ( Robinia pseudoacacia L.) not only influences the stand structure of natural or artificially established stands, but also the genetic composition of seed harvested from such clonal stands. For the commercial production of genetically diverse seeds, the stand structure should be taken into account or, at best, seeds harvested from seed orchards should be used. Context Black locust is characterised by intensive asexual reproduction through the formation of root suckers. By this means clonal structures can develop within black locust stands, in which ramets of a single clone can dominate extensive areas. Aims We want to analyse to what extent clonal structures within black locust stands negatively influence the genetic composition and diversity in seed harvested in such stands. We discuss how a potential reduction in genetic diversity can be reduced by measures taken during harvesting and whether the harvesting of seed orchards may be a better alternative. Methods We compare the genetic composition and diversity of parent trees and seed harvested from a clonal black locust stand with a seed orchard in which multiple ramets of selected clones were arranged in a randomised design. Results Within the clonal stand, parent contributions to the seed lot analysed proved to be strongly uneven. Selfing rates were high and large full-sib families dominated within the seed lot. Although the relatively strong pollination from unknown pollen donors, probably located outside of the stand, prevented a massive loss of alleles, high selfing rates and the formation of large full-sib families led to an unequal distribution of alleles within the progeny. Within the seed orchard—even though it had a lower number of clones than expected—the randomised design promoted a more diverse pollination pattern. Conclusion We conclude that for black locust, seed orchards have the greater potential to ensure a balanced genetic composition of harvested seed lots. If economic considerations make it necessary to harvest seed stands, this should only be done in a considered manner and, if possible, with knowledge of the clonal structures of the stand.