The ecology, distribution and invasiveness of Gunnera L. species in Connemara, Western Ireland

The huge herbaceous Gunnera tinctoria (Giant rhubarb), long naturalised in coastal Connemara, has recently spread rapidly and moved inland causing considerable concern. Its native habitat in South America comprises disturbed areas on volcanic soils. In Connemara it has colonised quarries and roadsides, spreading out from these, though rarely onto bog. Gravel spread from infested quarries has been an important dispersal factor. It is mostly associated with plant communities of damp and disturbed habitats, with other invasive aliens. In a growth experiment, it grew poorly in waterlogged peat or gravel and best in a drained mixture of both substrates. Gunnera manicata (Brazilian giant rhubarb) is here confirmed for Connemara and both species are mapped. G. manicata appears to occur through accidental transfer of rhizome parts, while G. tinctoria can also spread by its abundant seed. The exceptionally severe winter of 2009/2010 checked the growth of G. tinctoria away from the coast, but not G. manicata. It is likely that the previous long run of milder winters facilitated the recent rapid spread of G. tinctoria. Climate change hypotheses predict increased milder oceanic conditions for Western Europe, which could result in both species becoming more widely invasive.