Paper Reviewed
Hager, H.A., Ryan, G.D. and Newman, J.A. 2020. Effects of elevated CO2 on competition between native and invasive grasses. Oecologia 192: 1099-1110.

One CO2-related hypothesis that has generated some concern in ecological circles, and which is the focus of the work of Hager et al. (2020), is that differential responses of atmospheric CO2 enrichment on the competitive abilities of invasive and non-invasive species may accelerate invasion and intensify its effects. But is this hypothesis correct?

Attempting to shed some light on this question the three Canadian researchers experimentally tested the effects of elevated CO2 on the competitive outcomes of four invasive and four native perennial grasses. The work was conducted over a 14-week period inside environmentally-controlled plexiglass growth chambers within the E.C. Bovey greenhouse at the University of Guelph, Canada. There, two C3 and two C4 native (noninvasive) and two C3 and two C4 non-native (invasive) grass species were grown under either ambient (390 ppm) or elevated (700 or 1000 ppm) CO2 levels, in monoculture or in combination. Adequate water and nutrients were supplied to the grasses throughout the duration of the experiment.

In summarizing their findings Hager et al. say "we found no indication that elevated CO2, at levels up to the highest projected to occur by 2100, alters the strength of aboveground competitive interactions for native C3 and C4 perennial grass seedlings competing with invasive C3 and C4 perennial grass seedlings under non-limiting water and soil nutrients." Thus, they conclude, "although invasive species are generally good competitors, they do not necessarily benefit from elevated CO2."

The above findings according to the authors, "corroborate those of previous competition-focused experiments that found little or no advantage of elevated CO2 on invasive grass success over native grasses," citing the works of Hely and Roxburgh (2005), Manea and Leishman (2011) and Larson et al. (2018). And so it is that the hypothesis of CO2 enrichment favoring the competitive abilities of invasive over non-invasive species is not supported by Hager et al.'s work or that of others before them.

References
Hely, S.E.L. and Roxburgh, S.H. 2005. The interactive effects of elevated CO2, temperature and initial size on growth and competition between a native C3 and an invasive C3 grass. Plant Ecology 177: 85-98.

Larson, C.D., Lehnhoff, E.A., Noffsinger, C. and Rew, L.J. 2018. Competition between cheatgrass and bluebunch wheatgrass is altered by temperature, resource availability, and atmospheric CO2 concentration. Oecologia 186: 855-868.

Manea, A. and Leishman, M.R. 2011. Competitive interactions between native and invasive exotic plant species are altered under elevated carbon dioxide. Oecologia 165: 735-744.