Invasion by Rhododendron ponticum depletes the native seed bank with long-term impacts after its removal

The soil seed bank plays an important role in determining what plant species emerge following the removal of monodominant invasive species. A depleted seed bank may provide a substantial barrier to site restoration, however, little is known about what changes occur in the seed bank during invasion and following clearance. This study used greenhouse germination trials to quantify and compare the seed bank present in 30 Scottish Atlantic oak woodland sites under three treatments: (1) sites featuring dense stands of the non-native invasive shrub Rhododendron ponticum; (2) sites that had been previously subject to dense R. ponticum stands but which had been cleared; and (3) uninvaded control sites. Seed banks of densely invaded and cleared sites had significantly lower species richness than those of uninvaded control sites with a lower abundance of graminoid, and to a lesser extent forb seeds than the uninvaded controls. The seed bank community composition differed significantly between the three site types. Uninvaded sites featured a wide array of species, densely invaded sites were dominated by R. ponticum seeds and cleared sites were dominated by birch (Betula sp.) seeds. Cleared sites contained very few R. ponticum seeds indicating that once effective clearance had been achieved, re-invasion would be unlikely to occur from the soil seed bank. Our findings revealed that the soil seed bank present in cleared sites was very different from the seed bank of uninvaded control sites, with clear implications for site restoration.