CO2 and O2 compete for the reaction sites on the photosynthetic carbon-fixing enzyme, Rubisco. Increasing the concentration of CO2 in the atmosphere has two effects on the Rubisco reactions: increasing the rate of reaction with CO2 (carboxylation) and decreasing the rate of oxygenation. Both effects increase the rate of photosynthesis, since oxygenation is followed by photorespiration which releases CO2 (Farquhar et al., 1980). With increased photsynthesis, plants can develop faster, attaining the same final size in less time, or can increase their final mass. In the first case, the overall rate of litter production increases and so the soil carbon stock increases; in the second case, both the below-ground and above-ground carbon stocks increase. Both types of growth response to elevated CO2 have been observed (Masle, 2000).
The strength of the response of photosynthesis to an increase in CO2 concentration depends on the photosynthetic pathway used by the plant. Plants with a photosynthetic pathway known as C3 (all trees, nearly all plants of cold climates, and most agricultural crops including wheat and rice) generally show an increased rate of photosynthesis in response to increases in CO2 concentration above the present level (Koch and Mooney, 1996; Curtis, 1996; Mooney et al., 1999). Plants with the C4 photosynthetic pathway (tropical and many temperate grasses, some desert shrubs, and some crops including maize and sugar cane) already have a mechanism to concentrate CO2 and therefore show either no direct photo-synthetic response, or less response than C3 plants (Wand et al., 1999). Increased CO2 has also been reported to reduce plant respiration under some conditions (Drake et al., 1999), although this effect has been questioned.
Increased CO2 concentration allows the partial closure of stomata, restricting
water loss during transpiration and producing an increase in the ratio of carbon
gain to water loss ("water-use efficiency", WUE) (Field et al., 1995a;
Drake et al., 1997; Farquhar, 1997; Körner, 2000). This effect can lengthen
the duration of the growing season in seasonally dry ecosystems and can increase
NPP in both C3 and C4 plants.
Nitrogen-use efficiency also generally improves as carbon input increases, because plants can vary the ratio between carbon and nitrogen in tissues and require lower concentrations of photosynthetic enzymes in order to carry out photosynthesis at a given rate; for this reason, low nitrogen availability does not consistently limit plant responses to increased atmospheric CO2 (McGuire et al., 1995; Lloyd and Farquhar, 1996; Curtis and Wang, 1998; Norby et al., 1999; Körner, 2000). Increased CO2 concentration may also stimulate nitrogen fixation (Hungate et al., 1999; Vitousek and Field, 1999). Changes in tissue nutrient concentration may affect herbivory and decomposition, although long-term decomposition studies have shown that the effect of elevated CO2 in this respect is likely to be small (Norby and Cortufo, 1998) because changes in the C:N ratio of leaves are not consistently reflected in the C:N ratio of leaf litter due to nitrogen retranslocation (Norby et al., 1999).
The process of CO2 "fertilisation" thus involves direct effects on carbon assimilation and indirect effects such as those via water saving and interactions between the carbon and nitrogen cycles. Increasing CO2 can therefore lead to structural and physiological changes in plants (Pritchard et al., 1999) and can further affect plant competition and distribution patterns due to responses of different species. Field studies show that the relative stimulation of NPP tends to be greater in low-productivity years, suggesting that improvements in water- and nutrient-use efficiency can be more important than direct NPP stimulation (Luo et al., 1999).
Although NPP stimulation is not automatically reflected in increased plant biomass, additional carbon is expected to enter the soil, via accelerated ontogeny, which reduces lifespan and results in more rapid shoot death, or by enhanced root turnover or exudation (Koch and Mooney, 1996; Allen et al., 2000). Because the soil microbial community is generally limited by the availability of organic substrates, enhanced addition of labile carbon to the soil tends to increase heterotrophic respiration unless inhibited by other factors such as low temperature (Hungate et al., 1997; Schlesinger and Andrews, 2000). Field studies have indicated increases in soil organic matter, and increases in soil respiration of about 30%, under elevated CO2 (Schlesinger and Andrews, 2000). The potential role of the soil as a carbon sink under elevated CO2 is crucial to understanding NEP and long-term carbon dynamics, but remains insufficiently well understood (Trumbore, 2000).
C3 crops show an average increase in NPP of around 33% for a doubling of atmospheric CO2 (Koch and Mooney, 1996). Grassland and crop studies combined show an average biomass increase of 14%, with a wide range of responses among individual studies (Mooney et al., 1999). In cold climates, low temperatures restrict the photosynthetic response to elevated CO2. In tropical grasslands and savannas, C4 grasses are dominant, so it has been assumed that trees and C3 grasses would gain a competitive advantage at high CO2 (Gifford, 1992; Collatz et al., 1998). This is supported by carbon isotope evidence from the last glacial maximum, which suggests that low CO2 favours C4 plants (Street-Perrott et al., 1998). However, field experiments suggest a more complex picture with C4 plants sometimes doing better than C3 under elevated CO2 due to improved WUE at the ecosystem level (Owensby et al., 1993; Polley et al., 1996). Highly productive forest ecosystems have the greatest potential for absolute increases in productivity due to CO2 effects. Long-term field studies on young trees have typically shown a stimulation of photosynthesis of about 60% for a doubling of CO2 (Saxe et al., 1998; Norby et al., 1999). A FACE experiment in a fast growing young pine forest showed an increase of 25% in NPP for an increase in atmospheric CO2 to 560 ppm (DeLucia et al., 1999). Some of this additional NPP is allocated to root metabolism and associated microbes; soil CO2 efflux increases, returning a part (but not all) of the extra NPP to the atmosphere (Allen et al., 2000). The response of mature forests to increases in atmospheric CO2 concentration has not been shown experimentally; it may be different from that of young forests for various reasons, including changes in leaf C:N ratios and stomatal responses to water vapour deficits as trees mature (Curtis and Wang, 1998; Norby et al., 1999).
At high CO2 concentrations there can be no further increase in photosynthesis with increasing CO2 (Farquhar et al., 1980), except through further stomatal closure, which may produce continued increases in WUE in water-limited environments. The shape of the response curve of global NPP at higher CO2 concentrations than present is uncertain because the response at the level of gas exchange is modified by incompletely understood plant- and ecosystem-level processes (Luo et al., 1999). Based on photosynthetic physiology, it is likely that the additional carbon that could be taken up globally by enhanced photosynthsis as a direct consequence of rising atmospheric CO2 concentration is small at atmospheric concentrations above 800 to 1,000 ppm. Experimental studies indicate that some ecosystems show greatly reduced CO2 fertilisation at lower concentrations than this (Körner, 2000).
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