Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO2 concentrations: Has its importance been underestimated?

Abstract. Climate change will include correlated increases in temperature and atmospheric CO 2 concentration (C a ). Rising temperatures will increase the ratio of photorespiratory loss of carbon to photosynthetic gain, whilst rising C a will have an opposing effect. The mechanism of these effects at the level of carboxylation in C 3 photosynthesis are quantitatively well understood and provide a basis for models of the response of leaf and canopy carbon exchange to climate change. The principles of such a model are referred to here and used to quantitatively examine the implications of concurrent increase in temperature and C a . Simulations of leaf photosynthesis show the increase, with elevation of C a from 350 to 650 μmol mol ‐1 , in light saturated rates of CO 2 uptake (A sat ) and maximum quantum yields (φ) to rise with temperature. An increase in C a from 350 to 650 μmol mol ‐1 can increase A sat by 20% at 10°C and by 105% at 35°C, and can raise the temperature optimum of A sat by 5°C. This pattern of change agrees closely with experimental data. At the canopy level, simulations also suggest a strong interaction of increased temperature and CO 2 concentration. Predictions are compared with the findings of long‐term field studies. The principles used here suggest that elevated C a will alter both the magnitude of the response of leaf and canopy carbon gain to rising temperature, and sometimes, the direction of response. Findings question the value of models for predicting plant production in response to climate change which ignore the direct effects of rising C a and the modifications that rising C a imposes on the temperature response of net CO 2 exchange.