Recovery of photosynthesis of boreal conifers during spring:: a comparison of two models

Two models of the effects of air temperature on the recovery of photosynthetic capacity of boreal conifers during spring were compared with respect to three aspects: (i) An examination of the ecophysiological assumptions of the models revealed several differences between the models, the most important ones being related to photosynthetic dormancy during winter, reversibility of the recovery, and effects of short term frosts. (ii) Model simulations were applied to deduce the predictions of the models for the springtime recovery of photosynthetic capacity. Both models predicted an increase in the photosynthetic capacity as a result of rising air temperatures. However, the model developed and parameterised for Scots pine predicted earlier recovery than the model developed and parameterised for Norway spruce, and there was also more variation in the predicted recovery with the former than with the latter. Furthermore, in the case of the Scots pine model the photosynthetic capacity fluctuated with fluctuations of air temperature, i.e. the recovery was drastically reversed in several cases, whereas the reverse played generally only a minor role in the quite straightforwardly increasing photosynthetic capacity predicted by the Norway spruce model. A review of the empirical findings available in the literature suggests that a new model unifying some of the ecophysiological assumptions of the current models should be developed. (iii) Additional simulations were carried out to examine the implications of the ecophysiological differences between the models for the photosynthetic production of Scots pine in central Finnish conditions. For the photosynthetic production calculated for the first half of the year, the Norway spruce model as parameterised for Scots pine predicted on an average 23.3% higher values than the Scots pine model. Thus, also with respect to estimation of the carbon sequestration of forests, further unifying model development is called for. (C) 2002 Elsevier Science B.V. All rights reserved.