A carbon- and nitrogen-balance model, applying pipe model theory and a modi
fication of functional balance as growth-guiding rules, is presented for th
e grass stage pine seedlings. Three populations of Pinus merkusii Jungh. et
de Vriese, originating from northern and northeastern Thailand, were grown
under controlled environment for 47 weeks to obtain parameter information,
to evaluate the model performance and to investigate genotypic variation i
n various characteristics among the populations. Monte Carlo simulations we
re used to evaluate the sensitivity of the model behaviour to varying param
eter values and to calibrate the model for each population.
With given sets of parameter values, the simulated biomass development fitt
ed rather well the observed one during the experiment. The two most importa
nt parameters determining model performance were within-shoot shading and s
pecific nitrogen uptake rate of fine roots. The fit of simulated versus mea
sured fine roots had a major effect on acceptable model performance in Mont
e Carlo simulations. Significant variation in biomass growth, nitrogen use
efficiency, height, stem diameter, total carbon concentrations of stem and
fine roots, and total nitrogen concentrations of needles, transport roots a
nd fine roots was found among the populations. The observed genotypic varia
tion in seedling biomass and stem diameter was consistent with the geograph
ical distribution of the populations while the variation in the rest of the
measured characteristics was not. It seems that P. merkusii populations in
Thailand are adapted to more site specific conditions rather than climatic
conditions alone, and that the variation in biomass growth may result from
variation in internal carbon and nitrogen dynamics among the populations.