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Effects of Elevated CO2 and Temperature on Grass-Legume Ecosystems
Reference
Lilley, J.M., Bolger, T.P. and Gifford, R.M. 2001. Productivity of Trifolium subterraneum and Phalaris aquatica under warmer, higher CO2 conditions. New Phytologist 150: 371-383.

What was done
The authors grew monocultures and mixed swards of Trifolium subterraneum and Phalaris aquatica in field tunnels fumigated with air containing 380 and 690 ppm CO2. In addition, swards were exposed to ambient or elevated (ambient + 3.4C) air temperatures. Thus, the interactive effects of elevated CO2 and air temperature on biomass production in these grassland communities were investigated.

What was learned
After one year of differential treatment exposure, monocultures of Trifolium subterraneum grown at an atmospheric CO2 concentration of 690 ppm had produced 19% more foliage than plants grown at ambient CO2, while monocultures of Phalaris aquatica displayed no significant growth response to atmospheric CO2 enrichment. Exposure to elevated air temperature reduced foliage growth in monocultures of Trifolium subterraneum by 28% at ambient CO2, while plants grown in elevated CO2 still displayed a positive growth enhancement of 8%. Foliage growth of Phalaris aquatica monocultures, however, was not significantly affected by air temperature at either atmospheric CO2 concentration.

When grown in mixed swards at ambient air temperature, elevated CO2 significantly stimulated foliage production of Trifolium subterraneum and Phalaris aquatica by 31 and 40%, respectively, while warming alone had no significant effect on mixed foliage production. Interestingly, the leguminous Trifolium species did not increase its abundance when grown in competition with the Phalaris species, so that atmospheric CO2 enrichment did not alter the proportion of the nitrogen-fixing Trifolium species within mixed grassland swards. Finally, at elevated air temperature, atmospheric CO2 enrichment stimulated foliage production in mixed swards by 23%, again without affecting the proportional presence of either grassland species.

What it means
As the atmospheric CO2 concentration rises, it is likely that temperate pasture ecosystems composed of Trifolium subterraneum and Phalaris aquatica mixtures will respond favorably by increasing their foliage production. In addition, if air temperatures rise in the future, regardless of the cause, such ecosystems should continue to exhibit increases in foliage production. Furthermore, increases in the air's CO2 concentration should not alter the competitiveness between these two species, thus helping to maintain the relative contributions of each species to the biodiversity that presently exists in temperate grasslands.