Belowground competition drives invasive plant impact on native species regardless of nitrogen availability

Abstract

Plant invasions and eutrophication are pervasive drivers of global change that cause biodiversity loss. Yet, how invasive plant impacts on native species, and the mechanisms underpinning these impacts, vary in relation to increasing nitrogen (N) availability remains unclear. Competition is often invoked as a likely mechanism, but the relative importance of the above and belowground components of this is poorly understood, particularly under differing levels of N availability. To help resolve these issues, we quantified the impact of a globally invasive grass species, Agrostis capillaris, on two co-occurring native New Zealand grasses, and vice versa. We explicitly separated above- and belowground interactions amongst these species experimentally and incorporated an N addition treatment. We found that competition with the invader had large negative impacts on native species growth (biomass decreased by half), resource capture (total N content decreased by up to 75%) and even nutrient stoichiometry (native species tissue C:N ratios increased). Surprisingly, these impacts were driven directly and indirectly by belowground competition, regardless of N availability. Higher root biomass likely enhanced the invasive grass's competitive superiority belowground, indicating that root traits may be useful tools for understanding invasive plant impacts. Our study shows that belowground competition can be more important in driving invasive plant impacts than aboveground competition in both low and high fertility ecosystems, including those experiencing N enrichment due to global change. This can help to improve predictions of how two key drivers of global change, plant species invasions and eutrophication, impact native species diversity.

Elymus athericus encroachment in Wadden Sea salt marshes is driven by surface elevation change

“…Furthermore, we are unable to rule out effects of belowground competition, as this study only included the assessment of aboveground biomass. An experiment using the grass species Agrostis capillaris , for example, showed that the negative effect of this invasive species on the native vegetation was driven by belowground competition (Broadbent, Stevens, Peltzer, Ostle, & Orwin, ). Interestingly, the authors found this effect regardless of N availability.…”

Biogeographic differences in soil biota promote invasive grass response to nutrient addition relative to co-occurring species despite lack of belowground enemy release

“…Other factors, such as disturbance and priority effects, i.e. where the first species to arrive following a disturbance ultimately dominates the community (Seabloom et al 2003), or superior competitive abilities (Sun et al 2014;Broadbent et al 2017), likely underlie the invasions of other grass species, including A. capillaris and A. odoratum. In combination with findings from previous studies, our results suggest that predicting which invasive plant species are most likely to benefit from belowground enemy release will be difficult, due to large variation within functional groups and across different habitats in the introduced range.…”

Recent advances in understanding grasslands

“…A recent paper by Broadbent et al . 13 highlighted the potential importance of root competition in the interactions between invasive and native grasses in New Zealand. This small-scale study only investigated the relationship between three species but demonstrated the importance of belowground competition, a mechanism that has received very little attention, in driving their interactions and highlights this as an area in need of future research.…”