Should Forest Restoration with Natural Hybrids Be Allowed

One of the greatest challenges of forest restoration is the choice of appropriate plant material that is adapted to local environments, provides the desired ecological services, and does not compromise the local biodiversity. Plant restoration programs use native seeds, a mixture of local and introduced seeds of the same species or allopatric species. However, restoration programs rarely use local hybrid individuals if a local species exhibits natural hybridization. Hybridization is an important evolutionary process that can result in new genotypic combinations that might be suited better to the target habitat than the parental species and allow adaptation to rapid environmental change. European forests are generally under strict legal regulations so that only certified seeds can be used for their restoration. The European certification procedure for seeds from forest trees has been devised to commercialize clearly identified species with known adaptation to a given area. Natural hybrids, for which taxonomic status is uncertain and fitness is generally unknown, are excluded, which is not the case of artificial hybrids because they have been previously tested in the field. Here we discuss the pressing need to include natural hybrid populations as a source of certified seeds for restoration purposes. Using the example of two ash species and their well-adapted hybrids along the Loire River in France, we show why European forest guidelines must be made more flexible to allow the provision of stable ecosystem services.