The effect of elevated CO2 concentration on growth of Phleum pratense L in different parts of the growth season

Seedlings of Phleum pratense L. (timothy) of the same age were grown in five sequential four-week periods during one growth season (May 5-September 23) at low (380 mu mol mol(-1)) and high (650 mu mol mol(-1)) CO2 concentration. The experiment was performed in 10 9 m(2) field plots surrounded by plastic foil walls (''field chambers'') in the relatively cool climate (10-13 degrees C mean temperature) of the west coast of Norway (59 degrees N latitude). Raising the CO2 concentration generally decreased the height of the grass (8-23%), especially at the beginning and end of the growth season. The number of shoots was significantly increased (13-42%) by CO2 enrichment in all growth periods except the last. Elevated CO2 did not influence the above-ground biomass (dry weight) in the first (May) and last (September) period, but increased it by 14-51% in the intervening periods (June-August). Positive effects of CO2 enrichment on plant biomass were correlated with positive effects on the number of shoots. Elevated CO2 concentrations resulted in 25-64% denser plant biomass (dry weight per unit air volume) in the different growth periods. In general, a positive ''chamber effect'' on plant height and dry weight was found in spite of the small air temperature differences between the insides and the outsides of the chambers. A greenhouse experiment showed that wind speeds above 3 m s(-1) strongly decreased height and dry weight of timothy seedlings. The reduced wind speeds inside the chambers could therefore explain the ''chamber effects'' found.