Elevated carbon dioxide and fire reduce biomass of native grass species when grown in competition with invasive exotic grasses in a savanna experimental system

Invasive exotic grasses are known to increase fire severity and frequency in a number of fire prone systems. If elevated atmospheric carbon dioxide (CO2) increases growth rates and biomass production of these grasses, or increases their competitive ability, fire regimes may be further affected with detrimental consequences for native vegetation composition and diversity. Mesocosms containing three invasive exotic C-4 grasses (Andropogon gayanus, Cenchrus polystachios, Cenchrus ciliaris), and three native C-4 grasses (Heteropogon triticeus, Heteropogon contortus, Eriachne triseta), all from savanna woodlands of northern Australia, were grown as mixtures in field soil in glasshouses under elevated (700 ppm) and ambient (385 ppm) CO2, burnt and assessed for resprouting response. We found no effect of elevated CO2 on community biomass, however biomass of the invasive exotic grasses was much greater than that of native grasses at both CO2 concentrations. Species' responses to elevated CO2 were varied. Two native grasses (H. triticeus and H. contortus) had significantly less biomass under elevated compared with ambient CO2 after burning, indicating an effect of elevated CO2 on resprouting response of these species when grown in competition. These results suggest that although overall productivity of this community may not change with increases in CO2 and fire frequency, community composition and species interactions in this fire-prone community may alter, shifting to a more exotic-dominated community and potentially resulting in an intensified fire frequency due to positive feedbacks.