Development of potential height growth and diameter increment models for the parameterisation of an individual tree growth model for Pinus elliottii plantations in South Africa

[摘要] ENGLISH ABSTRACT: Individual tree models, as opposed to stand models, have the potential to greatly improve sensitivityof forest growth models to changing conditions such as silvicultural amendments, irregular standstructures, etc. It was the purpose of this study to extend two sub-components of a Europeanindividual tree growth model to introduce individual tree growth modelling concepts in South Africausing Pinus elliottii as a study species. Two main objectives were established: Modelling thepotential height of stands across different site qualities and modelling diameter increment using apotential modifier approach with a combination of competition indices that change in importanceaccording to the edaphic conditions of the site.Potential height modelling used three steps in order to achieve this objective. The first was tocompare site index models based on different model fitting techniques, namely nonlinear leastsquares, generalised nonlinear least squares and nonlinear mixed effects models. The nonlinearmixed effects model proved to be superior in terms of achieving the principles of regressionassumptions and model fit for the data range observed. The second step was to fit potential heightusing nonlinear quantile regression on observed spacing trial height measurements. This proved tobe a robust technique able to capture potentials according to the defined Chapman-Richards modelstructure. The final step was to use the predicted site index as a site classification variable in order topredict potential height. While some small deviation occurred, potential height seems to be wellcorrelated to site index and validation on selected sites suggested that site index can be used tomodel potential height until a more sophisticated site classification model is used for futureimprovement of the model.Diameter increment modelling followed six major steps in order to apply the full parameterisationmethodology of an age-independent diameter increment model dependent on tree diameter andcompetition. Diameter increment potentials were fit using site index as a predictor of the potentialheight curves. Multiple competition indices were tested on two sites to obtain a combination of twoindices, which can capture overtopping and local crowding effects. Principle components analysisand variance inflation factors calculation were applied to test for collinearity between indices.Suitable combinations were tested resulting in a combination of the KKL and Local Basal Areacompetition indices. Changing importance of the two indices were observed on the two sites testedindicating a shift in the mode of competition according to a water gradient.These were combined in a deterministic potential modifier model, which mimicked competitivestages over age; however the validation showed a skewed distribution, which was not sensitive tostand density gradients. A stochastic model was constructed to model variance from observedresidual plots using linear quantile regression to determine bounds for a truncated normaldistribution which generates random deviates for a predicted increment. The stochastic elementsignificantly improved the performance and sensitivity of the model, however the model was stillnot sensitive enough at very high and very low spacing densities. All in all two key models for anadaptation of an individual tree growth simulator to South African conditions were successfullydemonstrated. The two main objectives were achieved; however some indicated improvementscould be made, especially for the competition indices where the sensitivity of competition tochanging resource limitation according to site and temporal scales needs to be further investigated.Furthermore, the full set of models for simulating individual tree growth still needs to be applied.Overall, as a methodological approach, the study outlined problems and future improvements,introduced new concepts and can serve as a guideline for future parameterisation of an individualtree growth model.

[发布机构] Stellenbosch University