Alien plant introductions and greenhouse gas emissions: Insights from Gunnera tinctoria invasions

Plant invasions represent a major global change in land/vegetation cover with the potential to significantly modify greenhouse gas (GHG) emissions. To get a better understanding of the impacts of terrestrial invasive plants on soil GHG emissions we report, firstly, on experiments conducted on invasive populations of the N-fixing herbaceous species Gunnera tinctoria in Ireland, and secondly, compare our results with published information based on a systematic review of the literature. For G. tinctoria populations, there was a >50% reduction in soil CO₂ emissions, mainly due to a reduction in autotrophic respiration, but with little impact on annual N₂O or CH₄ budgets. One year after the removal of G. tinctoria, soil GHG emissions returned to values comparable to uninvaded areas and this was associated with the reestablishment of the vegetation and an increased root biomass per unit area. If G. tinctoria covered 10% of abandoned agricultural land in Ireland, this could be associated with a reduction of approximately 8% (or 4.988 Mt CO₂eq y^-1) of the country's national CO₂ emissions. Comparisons of these results with literature values were 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. We found 46 studies that documented results for 16 species. From the studies that measured soil respiration, it was enhanced in only 45% of cases, questioning the assumption that invasive plants always increase soil CO₂ emissions. In 25 cases that analysed methane, CH₄ emissions increased in 76% of them, but all of these were conducted in wetlands. In only two cases were N-fixing species associated with enhanced N₂O emissions. Our results argue for more detailed and comprehensive assessments of the effect of plant invasions on GHG emissions and their global impact.