FURTHER RESPONSES TO CO2 ENRICHMENT IN BRITISH HERBACEOUS SPECIES

1. CO2-enrichment experiments have been performed on 15 British herbac eous species of widely differing ecology The conditions of growth were very similar to those used in a previous study and involved full-ligh t glasshouse conditions, non-Limiting supplies of water and mineral nu trients and a daytime mean temperature of 18 degrees C. Four CO2 treat ments were maintained (350, 500, 650 or 800 vpm) over periods of 49 or 52 days. 2. Hyperbolic functions were fitted to yield vs CO2 concentr ation. The functions were used to generate predictions of Q(540/350) ( the quotient of the 'present' yield which is predicted for the CO2 reg ime expected by the year 2050) and Q(700/350) (the quotient predicted for a doubling of the present ambient CO2 concentration). Values of Q( 540/350) for whole-plant dry weight ranged from below 1.00 to 1.19. Th e mean Value of whole-plant Q(700/350) for eight species of 'competiti ve' functional type was 1.13. Six species of 'stress-tolerant' or 'rud eral' type had a mean Q(700/350) Of only 1.07. 3. The new data support and amplify an earlier conclusion that high CO2 responsiveness is nor mal only within the competitive functional type (or 'strategy') and it s close relations. A simplified and more broadly based general predict ion now gives a fitted percentage increase after approximately 7 weeks ' growth of 27% for species of broadly competitive strategy In the cen tre of the range of functional types the fitted values now range from 13 to 20%, and at the far extremes, the value for species of either th e ruderal or the stress-tolerant type is now 6%. The gradient of this response is statistically significant, but less steep than that previo usly reported.