Impact of CO2 on crop growth

Crop production is affected by the atmospheric carbon dioxide (CO2) level. Photosynthetically active radiation (PAR) is used by the plant as energy in the photosynthesis process to convert CO2 into biomass. It is important to make a distinction in this process between C3 and C4 plants. The difference between C3 and C4 plants is the way the carbon fixation takes place. C4 plants are more efficient and the loss of carbon during the photorespiration process is negligible for C4 plants. C3 plant may lose up to 50% of their recently fixed carbon through photorespiration. This difference has suggested that C4 plants will not respond positively to rising levels of atmospheric CO2. However, it has been shown that atmospheric CO2 enrichment can, and does, elicit substantial photosynthetic enhancements in C4 species (Wand et al., 1999). Examples of C3 plants are potatoes, sugarbeet, wheat, barley, rice and most trees except mangrove. C4 plants are mainly found in the tropical regions and some examples are millet, maize and sugarcane. A third category is the so-called CAM (crassula-cean acid metabolism) plants, which have an optional C3 or C4 pathway of photosynthesis, depending on conditions: examples are cassava, pineapple and onions.

Modelling studies based on detailed descriptions of crop growth processes also indicate that biomass production and yield will increase under elevated CO2 levels. For example, Rotter and van Diepen (1994) showed that potential crop yields for several C3 plants in the Rhine Basin will increase by 15-30% in the next 50 years as a result of increased CO2 levels. According to their model, the expected increase in yield for maize, a C4 plant, will be only 3%, indicating that their model was indeed based on the assumption that C4 species do not benefit from higher CO2 levels.

Table 3.1. Increase of potential crop growth as a result of enhanced CO2 levels in percentages. A2 and B2 are the IPCC climate scenarios. (Source: CSCDGC, 2002.)

A2

B2

Crop

Period

(%)

(%)

Rice

2010-2030

20

10

2070-2100

40

20

Cotton

2010-2030

25

13

2070-2100

50

25

Wheat

2010-2030

20

10

2070-2100

40

20

Maize

2010-2030

10

5

2070-2100

20

10

Beet

2010-2030

10

5

2070-2100

20

10

Tomato

2010-2030

15

8

2070-2100

30

15

In addition to these theoretical approaches, experimental data have been collected to assess the impact of a CO2-enriched atmosphere on crop growth. A vast number of experiments have been carried out over the last decades to quantify the impact of increased CO2 levels on crop growth. The Center for the Study of Carbon Dioxide and Global Change in Tempe, Arizona (CSCDGC, 2002) has collected and combined results from this kind of experiments (Table 3.1). The example of rice shows that average biomass increases are 31% for increases in atmospheric CO2 concentrations of 300 ppm, but the variation is substantial (Fig. 3.5).

Fig. 3.5. Example of the impact of elevated CO2 levels on crop production according to experiments. Data are from 26 greenhouse experiments with CO2 concentrations of 650 ppm for rice. (Source: CSCDGC, 2002.)
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