The Effect of Elevated CO₂ Concentration on Growth of Phleum pratense L. in Different Parts of the Growth Season

Abstract

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 μmol mol⁻¹) and high (650 μmol mol⁻¹) CO₂ concentration. The experiment was performed in 10 9 m²field plots surrounded by plastic foil walls (“field chambers”) in the relatively cool climate (10-13°C mean temperature) of the west coast of Norway (59°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 CO₂enrichment 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.

Key Words:

• climate
• morphogenesis