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Effects of elevated CO2 on competition between native and invasive grasses

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Elevated atmospheric CO2 concentration increases the performance of invasive plants relative to natives when grown in monoculture, but it is unclear how that will affect the relative competitive abilities per se of invasive and native grasses grown together. We tested competitive outcomes for four native and four invasive perennial C3 and C4 grasses under ambient (390 ppm) and elevated (700 or 1000 ppm) CO2 concentrations in the greenhouse with non-limiting water and nutrients. We predicted that elevated CO2 would increase the competitive suppression of native grasses by invasive grasses. To test this, we determined the relative interaction intensity of biomass allocation for natives grown alone vs. those grown in native–invasive species pairs. We also measured photosynthetic traits that contribute to plant invasiveness and may be affected by elevated CO2 concentrations for species pairs in mixture to determine native–invasive relative performance. We found no effect of CO2 for the aboveground biomass and tiller production measures of interaction intensity or for relative performance for most of the measured photosynthetic traits. In competition, the invaders nearly always outperform natives in biomass and tiller production, regardless of CO2 level. The results suggest that increasing CO2 concentration alone has little effect on grass competitive outcomes under controlled conditions.

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We thank A. Patchett, K. Shukla, M. Melkic, K. Bolton, S. McGee, E. Staples, and J. Dale for help with the experiment; Y. Zheng, E. Lyons, H. Earl, and G. Otis for providing equipment; R. Dutton for managing the greenhouse, W. McCloskey for providing buffelgrass seed, C. Breton for data archiving assistance, and two anonymous reviewers for comments. This research was funded by Mitacs (HAH); Ontario Ministry of Agriculture, Food and Rural Affairs (all authors); Natural Sciences and Engineering Research Council of Canada (JAN); and J.D. Webster Postdoctoral Fellowship (GDR).

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HAH and GDR designed and executed the study. HAH analysed the data and wrote the manuscript. JAN advised on experimental design and statistics. HAH, GDR, and JAN edited the manuscript.

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Correspondence to Heather A. Hager.

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The authors declare that they have no conflicts of interest.

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Communicated by Yanjie Liu.

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Hager, H.A., Ryan, G.D. & Newman, J.A. Effects of elevated CO2 on competition between native and invasive grasses. Oecologia 192, 1099–1110 (2020).

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