Vasiliki Balogianni1,2, Margherita Gioria1,2,3, Amanuel Gebremichael1,2,4, Mauricio Mantoani1,2 and Bruce Osborne1,2
1UCD School of Biology and Environmental Science, University College Dublin, Belfield, Dublin, Ireland
2UCD Earth Institute, University College Dublin, Belfield, Dublin 4, Ireland
3Institute of Botany, Czech Academy of Sciences, CZ-252 43 Průhonice, Czech Republic
4Teagasc, Environmental Research Centre, Johnstown Castle, Wexford, Ireland
Invasive alien plants can promote long-term changes in invaded plant communities, as well as, their revegetation potential by altering not only the diversity and composition of the standing vegetation but also the soil seed bank. Here, we assessed changes in the vegetation (above- and below-ground) associated with two major invasive herbaceous species in Ireland, Gunnera tinctoria and Impatiens glandulifera. Changes in the vegetation were monitored throughout the growing season at three coastal grassland communities for G. tinctoria and three riparian communities for I. glandulifera, as these represent the typical habitats invaded by these species in Ireland and where both species are dominant in the standing vegetation. Soil seed banks were collected in March 2018 and November 2018, to capture both the more persistent and transient components of the seed bank, respectively.
Preliminary analyses indicate that G. tinctoria dominated the more persistent component of the soil seed bank and caused the displacement of several native species from species-rich coastal grassland communities. Impatiens glandulifera had instead become invasive in already species-poor and degraded communities, where giant hogweed (Heracleum mantegazzianum) was also present. Both invaders promoted the homogenization of the seed flora towards an over-representation of seeds of native problematic species and/or those of other alien species. However, seed banks invaded by G. tinctoria were highly distinct from uninvaded seed banks, while those invaded by I. glandulifera were often similar to uninvaded ones. This indicates differences in the reversibility of the impacts of these invaders on the above- and below-ground flora and in the efforts required to restore pre-invasion conditions. Moreover, I. glandulifera suppressed the growth of giant hogweed, which can be regarded as a positive effect. The large, persistent seed bank formed by G. tinctoria in the invaded communities confirmed the need to develop control measures targeting the seed bank and not only the standing vegetation for the long-term management of this species.