Rhododendron ponticum depletes the native seed bank with long-term effects
Rhododendron ponticum (Ericaceae: EPPO Observation List of invasive alien plants) is one of the most damaging invasive plant species which threatens biodiversity in the United Kingdom. With outbreaks of the plant pathogen Phytophthora ramorum, increased efforts have been made to manage R. ponticum, as it acts as a host plant. However, following the clearing of dense monocultures of the species, it has been observed that native plant species in particular grasses and forbs fail to return even up to 30 years post-removal. The current study aimed to elucidate the impact of R. ponticum invasion, and its subsequent removal on the composition of the seed bank from Atlantic oak woodlands on the West coast of Scotland. Greenhouse germination experiments were conducted using soil collected from three site types and ten samples were taken from each site type: (1) uninvaded sites where R. ponticum had never been present (uninvaded); (2) sites with dense R. ponticum stands still present (invaded); (3) sites cleared between 10-30 years ago (cleared). Invaded sites and cleared sites had significantly lower abundance of grass species and to a lesser extent forb species compared to uninvaded controls. The seed bank community composition differed between the three soil types. Uninvaded sites had a high species richness of native species compared to invaded sites which were dominated by R. ponticum seeds and had half the species richness of uninvaded sites. Cleared sites had a significantly lower species richness compared to uninvaded sites and this was due to the cleared sites being dominated by birch (Betula pendula) seed. The results show that the soil seed bank in invaded sites and cleared sites is very different to uninvaded sites and this has implications for site restoration. Of particular importance is the lower species richness seen in cleared sites compared to uninvaded sites suggesting that re-seeding may be necessary post-removal.
Sources
Maaclean J, Mitchell RJ, Burslem DFRP, Genney D, Hall J, Pakeman RJ (2018) Invasion by Rhododendron ponticum depletes the native seed bank with long-term impacts after its removal. Biological Invasions 20, 375-384.