How increasing atmospheric Carbon Dioxide concentration changes plant growth and water use by a tropical savanna.

In a 5 year experiment that was a world first, we quantified the responses of a natural savanna to the CO₂ concentrations expected in 2030 and 2070. We found that exposure to elevated concentrations of CO₂ altered the savanna ecosystem. Grasses grown at high CO₂ had 36% more above-ground biomass because the plants grown at high CO₂ pumped less water out of the soil through their stomata (the pores in the leaves); the grasses stayed greener longer and had prolonged growth into the dry season.

However the forage quality of the grasses was reduced, leaf protein dropped by 5%, because without fertiliser the savanna soil did not have enough nitrogen to support the larger plants. Hence we could expect that animals would have to eat more grass in order to fulfil their dietary requirements for nitrogen. The magnitude of the effect on soil water storage and plant growth depended on the condition of the landscape and the season. Less water was stored in the soil when grasses were heavily grazed and the CO₂- associated stimulation of plant growth disappeared during drought and was reduced during periods of high rainfall. Thus management and stocking rates will be critical.

Contact name: joseph.holtum@jcu.edu.au