13.5.3  Temperature x CO2

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The tropics are warmer than temperate zones and as shown earlier (Figure 13.7) the impact of CO2 enrichment on CO2 assimilation is greater under warm conditions than under cool conditions. As temperature increases, gas solubilities decrease, but the ratio of solubilities of O2 to CO2 increases. Furthermore the specificity of Rubisco for CO2 decreases with increased temperature. Consequently photorespiration (relying on the oxygenase function of Rubisco) is favoured over CO2 assimilation as conditions become warmer, shown also as an increase in G* (Case study 1.1). As a result, the stimulation of CO2 assimilation due to increased atmospheric CO2 concentration also increases (Figure 13.7). Moreover, the temperature optimum for assimilation increases as atmospheric CO2 concentration increases. Finally, as Rubisco activation declines (a common outcome of CO2 enrichment) the temperature threshold for a positive impact of CO2 enrichment is increased. Overall, increased atmospheric CO2 concentration has a larger impact at high temperatures that at low temperatures. A priori, tropical locations should favour a large growth response due to CO2 enrichment.