RESPONSE TO CO2 ENRICHMENT IN 27 HERBACEOUS SPECIES

CO2-enrichment experiments were performed on 25 British native species of widely differing ecology. Two crops, one C3 (sunflower) and one C4 (maize), were also included. The background regime involved full-light, glasshouse conditions, non-limiting supplies of water and mineral nutrients and a daytime mean temperature of 18-degrees-C. Four CO2 treatments were maintained at nominal concentrations of 350, 500, 650 or 800 v.p.m. over a 56-day period. Hyperbolic functions were fitted to yield vs CO2 concentration. The functions were then used to generate predictions of Q540/350 (the quotient of present yield under the CO2 regime predicted for the year 2050) and Q700/350 (the quotient of present yield predicted for a doubling of ambient CO2 concentration). Values of Q540/350 for whole-plant dry weight ranged from below 1.01 to 1.49, the upper values being at least similar in magnitude to those already observed in C3 crops. The mean value of whole-plant Q700/350 for 11 species of near-competitive strategy was 1.43. Four species of stress-tolerant or ruderal strategy had a mean Q700/350 of only 1.05. High CO2 responsiveness was common only within the competitive strategy and its close relations. The fitted Q540/350 for species of the pure strategy was 1.38. In the centre of the strategic range the fitted value was 1.12, and at the far extreme, the value for species of ruderal or stress-tolerant strategy was only 1.03.