Productivity of Trifolium subterraneum and Phalaris aquatica under warmer, high CO2 conditions

Despite the importance of grass-legume pastoral ecosystems worldwide, there is little known about the impacts of concurrent increase in temperature and atmospheric CO2 concentration on their productivity. Pure and mixed swards of subterranean clover (Trifolium subterraneum) and phalaris (Phalaris aquatica) were established under ambient and warmed (+3.4 degreesC) air temperatures, at ambient and 690 mu mol mol(-1) CO2 concentrations in field tunnels in temperate south-eastern Australia. Over one year, elevated CO2 increased clover foliage growth in the monoculture by 19%, and by 31% in the mixture. Warming reduced clover monoculture herbage production at ambient CO2 by 28% and reduced the growth enhancement by elevated CO2 to +8%. Forage growth of phalaris monoculture was not affected significantly by either factor. Forage growth of the mixture was increased by 34% in response to higher CO2, but unaffected by warming. Elevated CO2 combined with warming increased forage growth of the mixed sward by 23%. Concurrent rise in atmospheric CO2 concentration and temperature increased productivity of subterranean clover-phalaris swards. However, longer term effects on species competition and persistence may modify this conclusion.