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Systems theory can provide a useful conceptual framework for the development of forest growth models. Modelling approaches are examined and compared, based on the state-space description of dynamic systems. This viewpoint is used also to discuss the principles behind height-based site indices and possible extensions, and problems of statistical parameter estimation. Four main kinds of estimation methods for dynamic systems can be distinguished. A multidimensional generalization of the Richards model illustrates many of the ideas.

We used stem-analysis data from 60 to 100 years-old second-growth stands, and single-point observations in 260-460 years-old trees, to study and model height development of Nothofagus obliqua ("roble"). The data covers a much wider range of ages than previous work on this species. A stochastic differential equation approach facilitated the estimation and comparison of alternative model variants based on the Richards equation. A number of methodological issues in the application of stemanalysis techniques were examined. It was found that breast-height age can improve the reliability of site quality assessment and growth projections, compared to the total or stump age currently used. Best results were obtained with a polymorphic model fitted to the stem-analysis observations, conditioning the asymptote to be compatible with the old-growth tree measurements.

The trembling aspen (Populus tremuloides Michx.) -white spruce (Picea glauca (Moench) Voss) mixedwoods near Fort Nelson are distinguished by the large size of individual trees, longevity, and the low occurrence of internal decay in trembling aspen. The development of these forest ecosystems has had limited documentation and may be significantly different than those described in other portions of the boreal forest. At five study stands, stem analysis techniques were used to examine the patterns of height and radial growth over time according to species and structure type. There were two patterns of species establishment that were consistent with the stand structure. In codominant stands, recruitment periods for trembling aspen and white spruce overlapped. The stratified stands were consistently associated with a 29-to 58-year lag in white spruce recruitment. Spruce that were codominant with aspen at the time of sampling had sustained periods of rapid height and diameter growth. White spruce that were later to establish on site had slower rates of height and diameter growth. White spruce ages indicated that a dominant recruitment episode was more common than continuous recruitment. Height and diameter growth of trembling aspen were similar in both stand types. The differences in trembling aspen growth patterns between stands were due to site quality. The white spruce in codominant stands did not appear to go through a period of suppression and then release associated with stand-level trembling aspen mortality, as commonly described for other boreal mixedwoods. The vigor and longevity of trembling aspen in Fort Nelson appear to prolong the period of trembling aspen domination of mixed stands well beyond the time periods observed in other boreal ecosystems.Résumé : Les peuplements mélangés de peuplier faux-tremble (Populus tremuloides Michx.) et d'épinette blanche (Picea glauca (Moench) Voss) situés près de Fort Nelson se distinguent par la forte dimension et la longévité des arbres ainsi que la faible présence de carie de coeur chez le peuplier faux-tremble. Le développement de ces écosystèmes forestiers est peu documenté et pourrait être très différent de ceux qui ont été décrits dans d'autres parties de la forêt boréale. L'analyse de tiges a été utilisée pour étudier l'évolution de la croissance radiale et en hauteur dans le temps dans cinq peuplements, en fonction de l'espèce et du type de structure. Deux patrons d'établissement des espèces consistants avec la structure du peuplement ont été observés. Dans les peuplements où ils sont codominants, les périodes de recrutement du peuplier faux-tremble et de l'épinette blanche se recoupent. Les peuplements stratifiés sont toujours associés à un délai de 29 à 58 ans dans le recrutement de l'épinette blanche. Les épinettes blanches qui étaient codominantes avec le peuplier au moment de l'échantillonnage ont connu des périodes de croissance rapide en hauteur et en diamètre. Les épinettes blanches qui se sont établies plus tard sur le site ont un taux de croissance en ha...

Conventional top height estimates are biased if the area of the sample plot differs from that on which the definition is based. Sources of bias include a sampling selection effect and spatial autocorrelation. The problem was studied in relation to the use of data sets with varying spatial detail for modelling Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation growth. Improved top height estimators, developed taking into account the selection effect, eliminated the bias. Bias was reduced, but not eliminated completely, when the estimators were tested using more highly autocorrelated eucalypt data.

Height growth was modelled for spruce-dominated, even-aged stands in the Sub-Boreal Spruce biogeoclimatic zone of British Columbia, Canada, using both stem analysis (SA) and permanent sample plot (PSP) data. The model is based on a stochastic differential equation (SDE) formulation of the Bertalanffy–Richards growth equation. The SDE approach accounts for serial correlation and heterogeneous variance and makes hypothesis testing possible. Statistically significant differences in height–age trends between SA and PSP data were found that may be attributed to bias caused by dominance changes in SA trees. Error structure in SA and PSPs was also significantly different. Combining both data sources in a way that respects these different error structures reduced bias and increased precision. Four parametrizations were tested; the best was a polymorphic version. The final model fit the data well with no appreciable bias over the full range of ages and site qualities. The currently used spruce site-index model was found to underestimate growth and overestimate site index in young stands. The new model can be recommended for height prediction and site-quality assessment in interior spruce.

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A biologically inspired whole-stand growth and yield model was developed for even-aged thinned or unthinned stands dominated by trembling aspen (Populus tremuloides Michx.). The estimation used permanent sample plot data from British Columbia, Alberta, Saskatchewan, and Manitoba, supplemented by published site index and young stand information. An ingrowth imputation procedure was devised to facilitate the use of plot measurements where small trees are not measured. Two published site index models were closely approximated by a simple age-base invariant equation. Good parameter estimates for mortality and basal area growth were obtained without using age observations, which were unreliable or missing. Four differential equations describe the dynamics of top height, trees per hectare, basal area, and a site occupancy factor. Current values of these variables are used to estimate total and merchantable volumes up to any diameter limit and diameter distribution parameters. When an independent source of site quality estimates is available, the final model does not require stand age knowledge for making growth and yield predictions.