© 2003 Heron Publishing—Victoria, Canada
Annual pattern of photosynthesis in Scots pine in the boreal zone
Pertti Hari (1) and Annikki Mäkelä (1, 2)
1. Department of Forest Ecology, P.O. Box 27, FIN-00014, University of Helsinki, Finland / 2. Author to whom correspondence should be addressed (annikki.makela@helsinki.fi) / Received May 15, 2002; accepted July 20, 2002; published online January 31, 2003
Summary
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 (α) and saturation value (γ) of the light-response curve
of carboxylation efficiency in the intercellular cavity, and the cost of transpiration (λ), 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 α and λ varied from day to day, whereas the shape of the curve was constant (i.e., γ was proportional
to α). Both α and λ showed consistent patterns from spring to autumn, but the seasonal variation was considerably greater
for α than for λ. 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%).
Keywords:
Monte Carlo analysis, nonlinear parameter estimation, optimal control, seasonal production, stomatal conductance.
