Mauricio C. Mantoani and Bruce A. Osborne

UCD School of Biology and Environmental Science and UCD Earth Institute, University College Dublin, Dublin, Belfield, Ireland

e-mail: Bruce.Osborne@ucd.ie

Plant invasions represent a major global change in land/vegetation cover and, because of documented alterations in C and N cycling and enhanced litter production, they may potentially result in significant increases in soil greenhouse gas (GHG) emissions. To assess the effects of invasive terrestrial plant species on GHG emissions, we performed a search of the literature and found 50 cases reporting 22 species. The results were, however, equivocal and only in 38% of the cases soil respiration increased, contradicting the common assumption that invasive plants increase CO₂ emissions. CH₄ emissions only increased in studies conducted in wetlands or areas prone to flooding and, of a total of four cases analysing N-fixing species, only two detected enhanced N₂O emissions. To get a better understanding of the impacts of invasive plants on GHG emissions, we conducted an experiment on invasive populations of the N-fixing species Gunnera tinctoria on Achill Island, Ireland. G. tinctoria showed > 50% lower soil CO₂ emissions, mainly due to a reduction in autotrophic respiration, with a strong seasonal dependency (Fig. 1), but with little change in N₂O or CH₄ emissions. Comparisons of these results with literature values is difficult because of the often low and limited sampling effort of previous investigations, a failure to assess all three major GHGs and because of marked seasonal variations. Consequently, there is considerable uncertainty about the likely impact of plant invasions on the land-atmosphere exchange of GHGs and their global significance. Based on the results for G. tinctoria there is, however, no reason to expect an enhancement of GHG emissions or that N-fixing species will increase N₂O emissions, as is often assumed. In part, the results may reflect species and site-specific results, but the limited regional coverage and the experimental deficiencies inherent in many previous investigations argue for more detailed and comprehensive assessments.