EFFECTS OF ELEVATED CO2 AND SIMULATED SEASONAL-CHANGES IN TEMPERATURE ON THE SPECIES COMPOSITION AND GROWTH-RATES OF PASTURE TURVES

Large turves from a ryegrass/white clover based pasture were exposed to 350 or 700 µ11-1CO2 for a period of 217d in controlled environment rooms. The temperature was increased during the experiment from 10/4 °C day/night to 16/10 °C and finally to 22/16 °C. The turves were cut to a height of 2 cm at intervals and growth rates calculated from the regrowth. Growth rates over the duration of the experiment were 8% higher at elevated CO2; the difference between CO2., treatments being statistically significant only at the highest temperature. Species composition of the turves at 350 µ1l_l CO2 showed seasonal changes similar to those measured in the field. The effect of CO2 was to exaggerate the normal decline of ryegrass at wanner temperatures and increase the proportion of white clover. About 30% of the total growth rate was from other species (notably Bromus hordeaceus L. and Poa trivialis L.) and this fraction was similar between CO., levels. Root mass was measured at the end of the experiment and was 50% higher at elevated CO2. The modest above-ground response to CO2 was a result of CO2 stimulation occurring only at the higher temperature. Because of the CO2, x temperature interaction, the effect of CO2 in temperate regions will be seasonal. When this is matched with seasonal growth patterns of herbage species, a complex response of pasture communities to CO2 is possible. In our case, white clover was growing most strongly during the period of greatest CO2 stimulation and consequently its growth was enhanced more than that of ryegrass; however, the cooler season growth of ryegrass gives it a temporal niche which is little affected by CO2 and this may be important for ryegrass stability if it is an inherently poor responder to CO2. The results indicate that for temperate species the effects of competition at elevated CO2 cannot be easily determined from experiments conducted at a single temperature. © 1994 Annals of Botany Company.