A Process-Based Growth Model for the Grass Stage Pine Seedlings

A carbonand nitrogen-balance model, applying pipe model theory and a modification of functional balance as growth-guiding rules, is presented for the 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 in various characteristics among the populations. Monte Carlo simulations were used to evaluate the sensitivity of the model behaviour to varying parameter values and to calibrate the model for each population. With given sets of parameter values, the simulated biomass development fitted rather well the observed one during the experiment. The two most important parameters determining model performance were within-shoot shading and specific nitrogen uptake rate of fine roots. The fit of simulated versus measured fine roots had a major effect on acceptable model performance in Monte 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 and fine roots was found among the populations. The observed genotypic variation in seedling biomass and stem diameter was consistent with the geographical 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.


Introduction
Grass stage is an exceptional juvenile growth pattern occurring in several pine species, e.g.Pinus montezumae Lamb.and P. michoacana Mart. in Mexican highlands (Perry 1991), P. palustris Mill. in southeastern United States (Brown 1964), and P. merkusii Jungh.et de Vriese in mainland Southeast Asia (Cooling 1968).Following germination subsequently, the grass stage is an initial period of slow shoot growth with inhibited internodal elongation, and it ends when a period of rapid shoot growth with normal internodal elongation begins (Sirikul 1990).In field conditions, the grass stage may last three to five years (P.merkusii) (Cooling 1968) or even up to 15 years (P.palustris) (Brown 1964) before the rapid shoot growth initiates.
The slowly-developing terminal bud is well protected by long secondary needles and the seedlings also develop a thick secondary cortex in their short, branchless stem.Hence the grass stage has been considered as an adaptation to withstand fire in a seasonal climate (e.g.Cooling 1968).The grass stage increases the probability of the seedlings to survive after a ground fire but it also prolongs the time period the seedlings are susceptible to ground fires.In addition, the grass stage pine seedlings have to compete with ground layer vegetation for several years.Thus this juvenile growth period is a critical phase while regenerating forests of the given pine species.
In fire-dominated environment, nitrogen cycling is strongly affected by fire in several ways of which one is nitrogen loss through volatilisation (Raison 1979, Rundel 1981).Among other functions, nitrogen has an important role in the structural growth of plants (e.g.Kramer and Kozlowski 1979), and nitrogen deficiency decreases the demand for carbohydrates, causing starch accumulation (Birk and Matson 1986).Cooling (1968) reported numerous starch grains in the cells of a secondary cortex of P. merkusii seedlings.Adequate carbohydrate storage in the thick secondary cortex may be required before the seedlings emerge from the grass stage since Koskela et al. (1995) observed that shoot length increased as soon as the stem and taproot reached a certain volume.It is unknown to what extent the carbohydrate storing in the grass stage seed-lings is under genetic control and what is the role of possible imbalance between carbon intake and nitrogen uptake in the fire-dominated sites.Sirikul (1990) reported genotypic variation in shoot morphology and in the duration of the grass stage among mainland Southeast Asian populations of P. merkusii whereas the insular populations had no grass stage.He found that highaltitude mainland populations exhibit a pronounced and low-altitude ones less pronounced grass stage.It is not known, however, whether possible genotypic variation in biomass growth or allocation during the grass stage among the populations is also related to geographical distribution of the populations.
An analysis of growth during the grass stage should combine i) photosynthesis, ii) nitrogen uptake, and iii) dynamics of non-structural carbon and nitrogen.Process-based modelling approach provides means to combine various physiological processes which result in structural growth.A process-based growth model can be used as a research tool allowing the investigation of the underlying mechanisms of growth (Bossel 1991), and to answer management questions while considering the dynamic nature of biological processes (Battaglia and Sands 1998).In this approach, a plant is seen as a system consisting of state variables which represent different parts of the plant, and which interact with the environment and each other through material flows (e.g.Nikinmaa 1992).Hence, growth results from differences of material flows in and out of different parts of the plant.Resource acquisition from the surrounding environment and the need to transport water from roots to leaves have major effects on these material flows (e.g.Cannell and Dewar 1994).
The aims of this study were (1) to formulate a process-based growth model for the grass stage pine seedlings, (2) to obtain parameter information for the model with a controlled experiment, (3) to analyse the sensitivity of the model behaviour to varying parameter values with Monte Carlo simulations, and (4) to investigate whether possible genotypic variation in biomass growth, structural properties and total carbon and nitrogen concentrations among three populations of P. merkusii is consistent with the geographical distribution of the populations.

The Growth Model
A grass stage seedling is considered to be a dynamic system consisting of state variables such as biomass compartments (needles, branchless stem including wood and bark, transport roots and fine roots), and soluble carbon and nitrogen pools at plant-level.Photosynthesis, respiration, nitrogen uptake, and utilisation of soluble carbon and nitrogen in structural growth were included as physiological processes affecting the rate at which the seedling grows.Carbon and nitrogen flows for structural growth of the different biomass compartments is assumed to take place so that an adequate amount of nitrogen is taken up, and that transpiring biomass and woody structure are in balance.Changes in the state variables were calculated on daily basis.
The model is based on a mass balance approach (e.g.Thornley 1972, de Wit 1978) where the soluble carbon pool, C p (g C), is determined by daily photosynthesis, P (g C d -1 ), respiration, R (g C d -1 ), and carbon utilisation in structural growth, C g (g C d -1 ) (Fig. 1): The soluble nitrogen pool, N p (g N), is determined by daily nitrogen uptake, N d (g N d -1 ), and nitrogen utilisation in structural growth, N g (g N d -1 ): Initial sizes of C p and N p depend on soluble carbon and nitrogen concentrations, Cs i and Ns i (g C or N g -1 dry matter (DM)), and sizes of the biomass compartments, W i (g DM) (i = needles (n), stem (s), transport roots (tr) and fine roots (fr)) : Daily photosynthesis is assumed to be proportional to the daily photosynthesis of unshaded needles, P d (g C g -1 DM d -1 ), within-shoot shading, s (unitless), and needle biomass, Respiration is divided into maintenance respiration, R m (g C d -1 ), and growth respiration, R g (g C d -1 ), (e.g.Mäkelä 1986): The amount of daily maintenance respiration is proportional to respiring biomass in each compartment: R rW where the r i are compartment specific parameters (g C g -1 DM d -1 ).Growth respiration is proportional to the amount of carbon used for growth: where r g is a parameter (g C g -1 C).The daily amount of carbon used for growth is assumed to be proportional to C p : where b is a parameter (unitless).Nitrogen uptake is proportional to fine root biomass W fr and specific nitrogen uptake rate σ N (g N g -1 DM d -1 ): Size of a biomass compartment at a certain moment is: in which ∆W i is a compartment specific change in dry matter: where Ctot i are the total carbon concentrations of biomass compartments and η i are the proportions of C g which are allocated to each biomass compartment.Considering the time scale of the present study, it was assumed that no senescence took place during the experiment.Stem biomass is derived using the geometric dimensions (e.g.Mäkelä 1986): where φ (unitless) is an empirical stem form coefficient, ρ (g DM mm -3 DM) the wood density, h is the seedling height, and A s (mm 2 ) is the sapwood cross sectional area below the needles.
According to the pipe model theory (Shinozaki et al. 1964) which implies functional interconnections between different parts of a tree (e.g.Kaipiainen and Hari 1985, Hari et al. 1986, Mäkelä 1986), the sapwood area at a certain height and needle mass above this point can be related using a constant ratio ε s (mm 2 g -1 DM): Applying the same principle, transport root mass and needle mass can also be related using a constant ratio ε tr (g DM g -1 DM): In addition, it is assumed that height of a seedling during the grass stage is proportional to needle mass: where h 0 (mm) is initial height after germination is completed and β (mm g -1 DM) is a height growth parameter.The total amount of carbon used for structural growth, C g can be written as a sum of carbon allocated to the different biomass compartments, G i (g C d -1 ): and the total amount of nitrogen used in growth, N g depends on the amounts of carbon allocated to different biomass compartments: where n i (g N g -1 C) are organ specific nitrogen requirements in structural growth.The concept of functional balance (Brouwer 1962, Davidson 1969), which implies that carbohydrates are distributed between shoot and root growth so that the internal nutrient concentration remains stable, has been used as a growth-guiding rule in tree growth models (e.g.Mäkelä 1986, Nikinmaa 1992).It can be assumed that the metabolically active organs of a tree, needles and fine roots, take first priority in the utilisation of carbohydrates (e.g.Cannell and Dewar 1994), and that carbohydrates are distributed for fine root and needle growth so that the ratio of soluble nitrogen and carbon pools (N p :C p ) remains stable (cf.Thornley 1972, Reynolds and Thornley 1982, Mäkelä and Sievänen 1987).Hence the assimilation of carbon and the acquisition of nutrients should be in balance with the utilisation of these elements in growth (i.e. a tree maintains the amount of fine roots which is required for sufficient nitrogen uptake).The portion of C g which is first distributed between the metabolically active organs is denoted as C a (g C d -1 ) This is an auxiliary term and it is reduced from the equations when solving allocation coefficients.Thus the amounts of carbon allocated to fine root and needle growth are: where the multiplier z varies according to changes in N p :C p ratio.The amount of carbon allocated to stem is As sapwood and height growth are proportional to growth of needle mass, ∆A s and ∆h are and the amount of carbon allocated to transport roots is Allocation coefficients, η i , can be solved including equations 19, 20, 21 and 24 into the carbon balance (equation 17): It is known that the root proportion of total plant biomass increases almost linearly when nitrogen availability decreases to sub-optimal levels (e.g.Ingestad 1979, Ingestad andLund 1979), and that nitrogen deficiency affects decreasingly the ratio between non-structural nitrogen and carbon substrates (e.g. Green et al. 1994).In this study, it is assumed that the multiplier z depends on the N p :C p ratio as follows (Fig. 2): The more detailed assumptions and calculations to determine z and the initial values of N p and C p at the beginning of the simulations are given in Appendix 1.The presented process-based growth model, applying pipe model theory and a modification of functional balance as growth-guiding rules, includes totally 34 parameters and one multiplier.

Plant Material
Seeds of three Thai populations of P. merkusii (Table 1) were germinated in containers filled with a sand-peat mixture (1:1) in a walk-in growth chamber.The Huey Bong (HB) population from northern Thailand represented the high-altitude populations with pronounced grass stage pattern whereas the Khong Chiam (KC) and Sangkha (S) populations from northeastern Thailand represented the low-altitude populations with intermediate pattern (cf.Sirikul 1990).Germinated seedlings were transplanted into 40 cm-long PVC tubes (5.5 litres) filled with homogenized sand (grain size 0.1-0.6 mm) and thereafter fertilized once a week by applying 50 ml of a nutrient solution with a 0.1 % concentration of a nitrogen poor fertilizer (9-25-20, N-P 2 O 5 -K 2 O).Irrigation was arranged through small holes at the bottom of the pots which were kept in continuous contact with water.Soil water potential was measured with standard jet-fill tensiometers (model 2725, Eijkelkamp, Giesbeek, The Netherlands) at depths of 15 and 30 cm in four pots.Soil water potential remained fairly constant throughout the growing period at both depths, approximately at 600 and 400 Pa, respectively.The daily photoperiod consisted of 11 hours with a constant irradiance level of approximately 200 µmol m -2 s -1 and one-hour transition periods at the beginning and the end of the constant period.Air temperature during the photoperiod was kept at 26 °C ± 1 °C and during the dark period, at 18 °C ± 1 °C.Relative air humidity was set to 60 % ± 10 % and 70 % ± 10 %, during the light and dark periods, respectively.
At the age of 23, 29, 35, 41, and 47 weeks, four seedlings per population were harvested and separated into needles, stems, transport roots, and fine roots (defined as nonwoody, unsuberized or suberized roots with a diameter of 1 mm or less, cf.Kramer and Kozlowski 1979).While harvesting the seedlings, height, stem and sapwood diameters and length of the taproot were measured.Biomass compartments were dried to a constant mass at 105 °C (24 h) and weighed.
Total carbon and nitrogen concentrations were measured from samples of homogenised needles, stem, and transport roots per seedling with an elemental analyser (CHN-900, Leco CO., St Joseph, MI, USA) whereas all fine roots per seedling, due to the small dry mass, were composited and analysed using an another elemental analyser (CNS-1000) which requires less sample material.

Sensitivity Analysis of the Growth Model
List of symbols used in the growth model is presented in Table 2.The Monte Carlo simulation method (e.g.Spear and Hornberger 1980, Hornberger and Spear 1981, Hornberger and Cosby 1985) was applied for calibration and sensitivity analysis of the growth model.One Monte Carlo simulation consisted of 500 runs for a population.The behaviour of the model is determined by 14 parameters (Table 3) which were varied stochastically in each run, while other parameters of lesser importance (Table 4) were assigned constant values.For each run, values of these 14 parameters were randomly assigned from a priori uniform distributions which were selected based on the results of the present experiment and the literature.The model was run using the selected parameter values, and the resulting model output and the parameter values were stored.
The initial state of the model was fixed to correspond to the amount of biomass at the age of 23 weeks in each run.
After 500 runs, the results were classified according to a performance criterion into accepted or rejected subsets.As a performance criterion, it was required that model output simultaneously explain 85 % or more of the variances in needle, stem and transport root biomasses, and 65 % or more of the variance in fine root biomass during the experiment.In case of fine roots, the lower criterion was selected due to the identifying problems, which introduced additional variation in the measured fine root biomass.The fine root biomass values of five seedlings were considered to be outliers and excluded from the data before the calibration of the model, as recommended by Janssen and Heuberger (1995).
The two subsets were used to examine the importance of the parameters to model performance.Accepted and rejected subsets of the parameters do not differ from each other for nonimportant parameters.The more important a parameter is to model performance, the more the accepted and rejected parameter distributions differ from each other.The sensitivity analysis was based on the comparison of the cumulative distributions of the two subsets with Kolmogorov-Smirnov two-sample test.If the accepted and rejected parameter sets differed (P ≤ 0.05), the a priori distribution was narrowed according to the results, and a new Monte Carlo simulation was conducted.This was repeated until no differences were found between the accepted and rejected parameter sets.

Statistical Analysis
Variation in the size of the biomass compartments and structural properties (i.e.root:shoot ratio, nitrogen use efficiency (NUE) of the whole seedling, height, stem diameter, taproot length), and total carbon and nitrogen concentrations among the populations were analysed using twoway analysis of variance.The arcsin transformation was made for root:shoot ratios before the test.The hypothesis was that the populations do not differ in any of these characteristics from each other during the experiment.If a test rejected the hypothesis, Tukey's HSD-test was used for pairwise comparisons.
The linear relationships of sapwood cross sectional area, transport root mass and height to needle mass was tested with analysis of regression.In addition, it was also tested, using analysis of covariance, whether the regression coefficients differed among the populations.

Biomass Growth and Structural Properties
Biomass growth during the experiment is presented in Fig. 3.There was statistically significant variation in all biomass compartments among the populations (Table 5).In case of needles, stem and transport roots, the northern HB had more biomass than the two northeastern populations (P < 0.05) while KC and S differed in fine root biomass (P < 0.01).No statistically significant interactions (population × time) were found in the biomass compartments.
There was no statistically significant variation in root:shoot ratio or in taproot length among the populations (Table 6).At the end of the experiment, mean root:shoot ratio varied from 0.43 to 0.48, and mean taproot length from 39.0 to 40.6 cm.The formation of deep taproot was distinct in all populations already at the first harvesting time when mean tap root length was 34.0-35.8cm.Statistically significant variation in nitrogen use efficiency (NUE), height and stem diameter was found among the populations (Table 6).KC and HB differed in NUE and height (P < 0.05), and HB and the other two populations in stem diameter (P < 0.05).At the end of the experiment, mean NUE ranged from 68.7 to 72.9 g DM g -1 N, height from 7.4 to 8.3 cm, and stem diameter from 3.2 to 4.2 mm among the populations.
There were clear linear relationships between sapwood cross sectional area and needle mass, and between transport root mass and needle mass (Table 7).The linear regression models resulted high proportions of explained variance (r 2 = 0.91 or higher) for both relationships in all populations.Sapwood cross sectional area:needle mass ratio, ε s , varied from 3.47 to 3.97 mm 2 g -1 DM, and transport root mass:needle mass ratio, ε tr , varied from 0.47 to 0.51 mm 2 g -1 DM.No statistically significant variation in ε s or ε tr was found among the populations.
The linear relationship between height and needle mass was not as clear as the above mentioned relationships (Table 7).In this case, the linear regression model resulted in considerably smaller proportions of explained variance (r 2 = 0.32-0.63).The height growth parameter b varied from 19.5 to 22.8 mm g -1 DM among populations but the variation was insignificant.

Carbon and Nitrogen Concentrations
Total carbon concentrations in needles and stems were rather constant throughout the experiment whereas the concentrations in roots showed variation (Fig. 4).Mean carbon concentration in needles varied between 46.0-48.2% of dry mass, and the differences were not statistically significant among the populations.At the age of 35 weeks, carbon concentration in needles was higher than at the first two harvesting time (P < 0.01).In stems, mean carbon concentration ranged from 45.8 to 47.6 % and statistically significant variation among populations (P < 0.01) and among harvesting times (P < 0.001) was found.However, only KC and HB had different carbon concentration in their stems.At the age of 29 weeks, carbon concentration in stem was lower than six weeks later, and at the end of the experiment.Total carbon concentrations in transport and fine roots were considerably lower than in the above-ground biomass.Mean carbon concentrations in transport roots ranged from 24.8 and 34.5 %, and the differences were not statistically significant among the populations.Time had significant effect on carbon concentration in transport roots (P < 0.001).In fine roots, mean carbon concentration varied from 16.1 to 33.1 %, and the variation among populations was statistically significant (P < 0.01).S had lower concentration than other populations (P < 0.05).Time also had significant effect on the carbon concentration in fine roots (P < 0.001).
Mean nitrogen concentration in needles ranged from 1.6 to 2.2 %, and variation in the concen-tration was statistically significant among populations (P < 0.01).HB had significantly higher concentration than KC (P < 0.01), and there was also significant interaction (population × time) (P < 0.05).Mean nitrogen concentration in stem was 0.6-1.3%, and no statistically significant variation was found among the populations.Time had significant effect on nitrogen concentration in stem (P < 0.001).
In transport roots, mean nitrogen concentration varied between 0.5-0.8%, and both population (P < 0.01) and time (P < 0.05) had statistically significant effect on it.Only HB and S differed significantly (P < 0.01).The interaction was also significant (P < 0.05).In fine roots, mean nitrogen concentration ranged from 0.4-1.1 %.Both population and time had significant effect on the nitrogen concentration in fine roots (P < 0.01).HB had higher concentration than S (P < 0.01).The interaction also had significant effect on the nitrogen concentration in fine roots (P < 0.001).

Sensitivity Analysis of the Growth Model
The results of the Monte Carlo simulations are presented in Table 8, and the fit of the simulated to measured biomass growth during the experiment is presented in Fig. 3 (using mean parameter values from Table 8).Using the given a priori distributions, the two most important parameters determining model performance were within-shoot shading (s) and specific nitrogen uptake rate (σ N ).After these, with varying ranking among the populations, were daily utilisation of the carbon pool (b), daily photosynthesis of unshaded needles (P d ), maintenance respiration of transport roots (r tr ), and transport root mass:needle mass ratio (ε tr ).The ranking was calculated as a ratio between the initial and final range of a given parameter.The rest of the parameters were not important in determining performance of the model within the given parameter ranges.It should be kept in mind, however, that the results of the sensitivity analysis depend on the selected a priori distributions of the parameters.The Monte Carlo simulations with 500 runs were repeated four times in KC, three in S, and six in HB before no differences were observed between the accepted and rejected subsets of the parameters.The simulated needles, stem, and transport roots often fulfilled the performance criteria for acceptance, while the simulated fine roots caused a rejection.Thus, the final parameter ranges in the last simulation resulted in rather low proportions of accepted runs; 19.2, 8.6 and 18.8 % in KC, S and HB, respectively.The considerable low proportion of accepted runs in case of S was due to a slightly higher variation in the measured fine root biomass as compared with the other populations.
There was obvious negative correlation in the final parameter sets of all populations between shading (s) and daily photosynthesis of unshaded needles (P d ) (Spearman rank correlation coefficients were -0.59, -0.64 and -0.76 for KC, S and HB, respectively), while the correlations between other parameters were less significant.Linear regression analyses showed that P d explained ca.34, 42 and 56 % of the variation in s.When the correlations were included into the stochastic parameter input, the acceptance percentages increased to 27.4, 12.8, and 43.2 % in KC, S and HB, respectively.

Discussion
The presented process-based growth model for the grass stage pine seedlings was calibrated for three population of P. merkusii grown under controlled environment.With given sets of parameter values, the simulated growth of the biomass compartments fitted rather well the observed biomass growth during the experiment.The simulated height development, however, did not fit the observed one as well as biomass development.This was because the linear regression model did not explain well enough the relationship between height and needle mass.Thus this relationship requires more attention while improving and testing the model with field data.
The results indicate that the two most critical parameters for acceptable model performance were within-shoot shading and specific nitrogen uptake rate of fine roots.The two parameters have major effects on simulated photosynthesis and nitrogen uptake; processes that largely determine the ratio between soluble nitrogen and carbon pools which regulate carbon allocation according to the present model structure.Hence Monte Carlo simulations identified the most critical component in the model structure affecting acceptable model behaviour.Excess water and nitrogen supply allowed seedlings to exhibit steady-state growth during the experiment.Therefore, the accepted parameter combinations, especially shading and specific nitrogen uptake rate, were those which kept the N p :C p ratio high enough.
One of the major problems in calibrating models is the imbalance between the complexity of the model and the availability of the data (Janssen and Heuberger 1995).In the present study, the complexity of the model was low and the experiment was purposely planned to produce relevant data to enable the sensitivity analysis with quantitative misfit measures.The analysis as applied here is more concerned with parameter estimation than testing the model structure (e.g.Fedra et al. 1981).Therefore, it remains to be tested how well the model performance will fit biomass data under low nitrogen availability.In that case, the amount of simulated fine roots would increase at the expense of needles, subsequently decreasing carbon allocation to stem and transport roots.This is realistic behaviour according to the present knowledge of tree growth dynamics.
The most stringent subcriteria for an acceptable model output during the sensitivity analysis was the fit of the simulated fine root growth to observed growth.This highlights the importance of the selection of the performance criteria so that they are relevant from a model structure point of view.In addition, data for a performance criterion should be accurately measurable.In case of fine roots, more inaccuracy will always remain in measured data than in other biomass compartments, even in a laboratory study.This should be taken into account when selecting the performance criteria.
In the present model, nitrogen acquisition, and ultimately the N p :C p ratio, depends not only specific uptake rate, but also the amount of fine roots.Thus it is logical that calibration of the growth model is highly dependent on the fit between simulated and measured fine roots.If only the above-ground biomass growth had been selected as a performance criterion, lower values of specific nitrogen uptake rate would have been accepted provided that enough carbon was available for more intensive fine root growth.
Under natural conditions in northern Thailand, the grass stage P. merkusii seedlings have been observed to form a deep taproot (Koskela et al. 1995).Despite the excess water availability, the formation of the deep taproot was also distinct in this study.All seedlings allocated most of their biomass into needles and transport roots while less biomass was allocated into stem and fine roots.In an another grass stage pine, P. palustris, Prior et al. (1997) found that low nitrogen availability increased root:shoot ratio due to increased allocation to taproots and fine roots, whereas water stress had little effect on the ratio.The same authors concluded that soil nitrogen availability was the overall controlling resource concerning the growth of P. palustris seedlings.This statement may also be true in P. merkusii since both species are adapted to frequent fire occurrence and low nitrogen availability in their natural environment.The deep taproot growth habit of the grass stage pine seedlings seems to be independent of water availability, and thus it is obviously under a genetic control.
It is likely that fine root growth was slow due to high nitrogen availability during the experiment.The rather low values of NUE (cf.Sheppard and Cannell, 1985) also indicate this since NUE decreases as nitrogen availability increases (Birk and Vitousek, 1986).High nitrogen availability also explains why the seedlings allocated a rather low proportion of total biomass into the stem.No thick secondary cortex, characteristic of field-grown seedlings, was formed during the experiment.This suggests that no large carbohydrate storage was accumulated into the stem since there was sufficiently nitrogen available for structural growth.Sirikul (1990) reported that P. merkusii populations from northern Thailand exhibited slower shoot development during the grass stage than the northeastern ones.In this study, however, the northern population (HB) did not exhibit the slowest height growth of all populations.Thus the classification of the mainland Southeast Asian P. merkusii populations into short-and longlasting grass stage populations solely based on altitude or geographical distribution may not always hold.
Genotypic variation was observed in biomass growth, NUE, height, stem diameter, total carbon concentrations of stem and fine roots, and total nitrogen concentrations of needles, transport roots and fine roots among the populations during the experiment.The northern HB population had more biomass and larger stem diameter than the two northeastern populations.In case of other characteristics, the variation was not consistent with the geographical distribution of the populations.Considering the parameter distributions, the final ranges of the parameter b and σ N were somewhat narrower in HB than in other populations after Monte Carlo simulations.Thus 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 growth may result from variation in internal carbon and nitrogen dynamics among the populations.
In conclusion, first-year simulated biomass development, produced by the presented processbased growth model with certain parameter sets, fitted rather well with the observed biomass development in three P. merkusii populations.Mon-te Carlo simulations revealed that the most important parameters affecting model behaviour were within-shoot shading and specific nitrogen uptake rate of fine roots.The observed genotypic variation in seedling biomass and stem diameter among P. merkusii populations was consistent with the geographical distribution of the populations while the variation in the rest of the measured characteristics was not.ble carbon pool and structural carbon remains the same, this decrease in total needle nitrogen concentration would decrease the concentration of soluble nitrogen from 0.17 % to 0.04 %.It was further assumed that the concentration of soluble nitrogen in other biomass compartments decreases from 0.08 % to 0.05 % and that C p is more or less constant.This gives a value of 0.015 for the N p :C p ratio when z = 1.When the N p :C p ratio varies between 0.015 and 0.04, z was assumed to depend linearly from the ratio, i.e. increasing constantly with the decreasing N p :C p ratio (Fig. 2).

Fig. 1 .
Fig. 1.A schematic presentation of the structure of the growth model.For the sake of clarity only state variables (boxes), material flows (solid lines), and selected information flows (dotted lines) are presented.P and R denote daily photosynthesis and respiration, P d and s are daily photosynthesis of unshaded needles and within-shoot shading, N d and σ N are daily nitrogen uptake and specific nitrogen uptake rates, C p and N p are soluble carbon and nitrogen pools, and G i and n i G i are carbon and nitrogen used for structural growth of different biomass compartments (i = needles, stem, transport roots and fine roots).

Fig. 2 .
Fig. 2. Dependence of multiplier z on N p :C p ratio which controls carbon allocation between needles and fine roots.

Fig. 3 .
Fig. 3. Measured (G needles, stem, I transport roots, fine roots) and simulated (continuous lines) biomass of three populations of Pinus merkusii seedlings as a function of time during the experiment.Mean parameter values for each population (Table8) were used in the simulations.

Fig. 4 .
Fig. 4. Carbon (filled symbols) and nitrogen (open symbols) concentrations of biomass compartments (G needles, N stem, I transport roots and L fine roots) as a function of time during the experiment in three populations of Pinus merkusii.

Table 1 .
Seed origin of the three populations of Pinus merkusii from Thailand used in the study.

Table 2 .
List of symbols used in the growth model.

Table 3 .
Parameters of the model which were varied in the Monte Carlo simulations.

Table 4 .
Parameters of the model which were assigned constant values in the Monte Carlo simulations.

Table 5 .
Analysis of variance for biomass compartments.

Table 6 .
Analysis of variance for structural properties.

Table 7 .
Regression equations for relatioship between sapwood cross sectional area [mm 2 ] and needle mass [g], transport root mass [g] and needle mass, and height [mm] and needle mass in three populations of Pinus merkusii.

Table 8 .
Results of the Monte Carlo simulations (see Table2for the definitions of the parameters).