MODELING COMPARISON TO EVALUATE THE IMPORTANCE OF PHENOLOGY FOR THE EFFECTS OF CLIMATE-CHANGE ON GROWTH OF TEMPERATE-ZONE DECIDUOUS TREES

The importance of 3 phenological types of deciduous trees for the effects of climate change on growth was evaluated using the model FORGRO. The climate change scenarios used were a doubling of the CO2 concentration (700 mu mol mol(-1)) and an increase in temperature ranging from 0 to 7 degrees C. To elucidate the relative importance of photosynthesis and allocation for this evaluation, models with different levels of mechanistic detail of photosynthesis and allocation were used. The photosynthesis approach of FORGRO was compared to the Farquhar and von Caemmerer approach as formulated in PGEN (FORGRO-PGEN). Similarly, the allocation approach of FORGRO was compared to the transport-resistance approach, as formulated in the ITE-Edinburgh model (ITE-FORGRO). A sensitivity analysis was performed to ascertain whether the response of gross photosynthesis to a climate change scenario depends on the value assigned to parameters in these models, and to compare this sensitivity with the differences found between the phenological types. The differences in the response of gross photosynthesis (P-g) to the climate change scenarios between the phenological types were smaller according to ITE-FORGRO as compared to FORGRO. These differences are of a similar magnitude when comparing the 2 photosynthesis models. Furthermore, FORGRO-PGEN showed that the response of P-g to a 2x [CO2] increases with rising temperature, thus compensating for the increase in respiration. For both FORGRO and ITE-FORGRO this CO2 and temperature interaction was not found. Consequently, in these models the increase in respiration exceeded the increase in gross photosynthesis at the higher range of temperature rise. The sensitivity analysis showed that the models differ in the sensitivity of the response of P-g to a 2x [CO2] scenario combined with a temperature rise of 2 degrees C (C-700/T-2), when parameter values change by +/-25%. In FORGRO-PGEN, the magnitude of the response of P-g depended on the values of some of its parameters, especially those determining the Michaelis-Menten kinetics of Rubisco, which for these parameters exceeded the differences between the phenological types in this scenario. In both FORGRO and ITE-FORGRO this sensitivity is similar to or less than the difference between the phenological types in the C-700/T-2 scenario.