The Annual Cycle of Photosynthesis in Evergreen Conifers

The hypothetico-deductive modelling framework introduced in Chap. 2 is applied to modelling the annual cycle of photosynthesis in boreal and temperate evergreen conifers. The main emphasis is in the photosynthetic capacity of the needles, i.e., their inherent photosynthesis potential, which is an emergent manifestation of the physiological status of the photosynthetic machinery in the needles. Photosynthetic capacity is usually measured by means of the photosynthetic light curve. Unlike the models of the annual phenological cycle discussed in Chap. 3, models of photosynthetic capacity can be tested continuously on a daily basis. Since 1980, the annual cycle of photosynthetic capacity has been successfully simulated by means of different versions of a fully reversible fluctuating model, in which the changes in photosynthetic capacity follow the changes in air temperature. The effects of short-term frost are not taken into account in these models, but there is another model available for them. In further work, a synthesis of these models is called for. Moreover, the effects of solar radiation need to be taken into account in further work to attain better model realism, for the seasonality of photosynthetic capacity is largely caused by acclimation to different combinations of air temperature and solar radiation. The evidence for the effects of other environmental factors, such as soil temperature and ground frost, remains controversial, but these factors cannot be ruled out in further model development. Novel simulations using long-term air temperature records as input for the fluctuating model revealed large year-to-year variation in the springtime recovery of photosynthetic capacity.