Temperature

The temperature response of plant growth shows an optimum curve. At low temperature, elevated [CO2] has no or only small effects, as indicated by the lack of increase in photosynthesis. At elevated [CO2], the annual yield of ryegrass did not change in response to a 3°C increase in temperature, but the root fraction of total biomass decreased (Casella et al., 1996). However, the yield increased in spring and autumn, when temperatures were suboptimal, and decreased in summer. The decline in the CO2 effect in summer was due to increased canopy transpiration and, thus, to reduced water availability. Comparing the combined effects of doubled [CO2] and a temperature increase of 3°C with ambient [CO2] and ambient temperature, the annual grass yield increased (Casella et al., 1996). The higher yield was mainly produced in spring.

There was no seasonal variation in the relative yield increase of white clover at elevated compared with ambient [CO2] (Hebeisen et al., 2000), but the absolute increase was proportional to the yield. This indicates that the CO2 effect on growth was not directly affected by temperature. In this field experiment, temporal water deficits may have favoured carbohydrate allocation to the root. The higher photosynthesis expected at elevated temperature in summer increased root growth but not yield. The LAI decreased when the temperature increased by 2.5°C (Nijs et al., 1997) both at ambient and elevated [CO2]; however, the air temperature was above optimum during this experiment.

Guide to Alternative Fuels

Guide to Alternative Fuels

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