Annual pattern of photosynthesis in Scots pine in the boreal zone

To detect seasonal changes in photosynthetic rate in the field, a set of 18,000 photosynthetic measurements made between April and October on three shoots of Scots pine growing near the northern timberline was studied. The measurements were analyzed in the framework of an optimal stomatal control model of photosynthesis, in which irradiance (photosynthetically active radiation, I), air humidity and ambient temperature are driving variables. All driving variables were monitored concomitantly with gas exchange measurements throughout the growing season. The model has nine parameters, of which six were assumed to be constant over the growing season and were fixed based on previous information. The three variable parameters were the initial slope (alpha) and saturation value (gamma) of the light-response curve of carboxylation efficiency in the intercellular cavity, and the cost of transpiration (lambda), in carbon units, regulating the degree of stomatal opening. These parameters could not be estimated independently, nor could their values be satisfactorily found by standard nonlinear regression techniques. A Monte Carlo based simulation procedure was devised to analyze the best-fit parameters and their mutual correlations near the minimum of the residual sum of squares. This was accomplished by replacing the saturation value of the light-response curve with a linearity parameter that determined the shape of the curve. In the best fit solutions, only alpha and lambda varied from day to day, whereas the shape of the curve was constant (i.e., gamma was proportional to alpha). Both alpha and lambda showed consistent patterns from spring to autumn, but the seasonal variation was considerably greater for alpha than for lambda. The optimal stomatal control model with the seven fixed and two daily parameter values gave a good overall fit for photosynthetic rate over the season (PEV > 95%).