Gunnera tinctoria invasions increase, not decrease, earthworm abundance and diversity

Invasive plants often modify soil biotic communities through changes in soil physicochemical characteristics or the amount and/or quality of litter inputs. We assessed the impacts of Gunnera tinctoria invasions on soil and the earthworm community, on Achill Island, Co. Mayo, Ireland. We compared replicated (n = 5) areas invaded by G. tinctoria with uninvaded semi-natural grasslands, as well as with areas subjected to mechanical removal or herbicide treatment. Modifications in physiochemical properties included lower soil temperatures and higher soil pH during the summer in invaded areas, yet little effect on C and N stocks, or soil moisture. Marked differences in litter were observed, however, with invaded areas having c. 20-fold higher (above-ground) litter input than uninvaded ones, as well as lower C:N ratio (17 vs. 29). This was associated with a significantly higher overall abundance and biomass of earthworms in invaded plots (375 individuals m–2, 115 g biomass m–2), compared to the uninvaded control (130 individuals m–2, 45 g biomass m–2), with removal treatments having intermediate values. Earthworm communities comprised 10 species, typical for Irish grasslands, dominated by the common endogeic species Allolobophora chlorotica, Aporrectodea caliginosa and Aporrectodea rosea. Both earthworm species richness and Shannon diversity were significantly higher in invaded areas, but only in spring samples. Based on this new information, plant invaders may increase the abundance and diversity of earthworms, mainly due to much larger litter inputs, increased soil pH and possibly lower soil temperatures in the summer.typical of Irish grasslands

Post-Invasion Recovery of Plant Communities Colonised by Gunnera tinctoria after Mechanical Removal or Herbicide Application and Its Interaction with an Extreme Weather Event

The interventions that are required for both the control and post-invasion restoration of native plant communities depends on several factors, including the efficacy of the measures that are used and how these interact with environmental factors. Here, we report on the results of an experiment on the effects of mechanical removal and herbicide application on the invasive plant Gunnera tinctoria and how an extreme weather event impacted on the invader and on the recovery of native coastal grassland communities. Both removal protocols were largely effective in eradicating mature plants, but the mechanical removal treatment resulted in a major increase in the number of G. tinctoria seedlings, which was exacerbated by the extreme event. Nine months after removal, the number of native species had recovered to c. 80% of that in uninvaded grasslands. In contrast to seedlings, mature plants of G. tinctoria showed a significant reduction in above-ground production after the extreme weather event, although these had largely recovered after six months. Overall, our results indicate that post-control restoration of the plant community may be possible without further significant management interventions. Nevertheless, since some invasive plants survived, further monitoring is required to ensure that recolonisation does not occur.

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.