MODELING COMPARISON TO EVALUATE THE IMPORTANCE OF PHENOLOGY AND SPRING FROST DAMAGE FOR THE EFFECTS OF CLIMATE-CHANGE ON GROWTH OF MIXED TEMPERATE-ZONE DECIDUOUS FORESTS

The importance of 3 phenological types of deciduous tree, and the effe cts of the occurrence of frost damage on growth of mixed-species fores ts, were evaluated using the models FORGRO and HYBRID, The climate cha nge scenarios used were a doubling of the CO2 concentration (700 mu mo l mol(-1)) and an increase in temperature ranging from 0 to 7 degrees C. Both FORGRO and HYBRID are mechanistic models treating eco-physiolo gical processes in detail. FORGRO highlights potential growth in manag ed forests where all individuals of one species are of the same age an d size, whereas HYBRID highlights growth in natural forests, including regeneration and mortality of individual trees that differ in age and size. Furthermore, the importance of inaccurate prediction of phenolo gical events and frost hardiness for growth in mixed-species stands wa s evaluated by comparing dynamic models to regression models. The dyna mic models predict the timing of phenological events annually and the progression of frost hardiness during dormancy, whereas the regression models represent empirical relationships between the change in the av erage date of phenological events with a rise in mean winter temperatu re and the level of frost hardiness at the moment of leaf unfolding. T he results of the climate change scenarios indicate for both FORGRO an d HYBRID that: (1) the differences in net primary production (NPP) of the 3 phenological types considered are enhanced when grown in a mixed -species stand compared to a monospecies stand; and (2) the effects of frost damage on growth are more prominent in mixed-species stands tha n in monospecies stands. Regarding the accuracy of the dynamic approac h compared to the regression approach for predicting the timing of lea f unfolding and spring frost damage, the dynamic approach for leaf unf olding results in a similar response of NPP to the regression approach , both for the monospecies and the mixed-species situation. The dynami c approach, however, yields larger differences in the NPP between the phenological types because the model predicts a greater advancement of leaf unfolding than does the regression approach. Comparing the regre ssion approach to the dynamic approach with regard to frost hardiness, the regression approach shows a greater frequency of frost damage; be cause, according to the dynamic approach the minimum level of frost ha rdiness is attained after the date of leaf unfolding, thus reducing th is frequency.