Thesis (MScFor)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Individual tree models, as opposed to stand models, have the potential to greatly improve sensitivity
of forest growth models to changing conditions such as silvicultural amendments, irregular stand
structures, etc. It was the purpose of this study to extend two sub-components of a European
individual tree growth model to introduce individual tree growth modelling concepts in South Africa
using Pinus elliottii as a study species. Two main objectives were established: Modelling the
potential height of stands across different site qualities and modelling diameter increment using a
potential modifier approach with a combination of competition indices that change in importance
according to the edaphic conditions of the site.
Potential height modelling used three steps in order to achieve this objective. The first was to
compare site index models based on different model fitting techniques, namely nonlinear least
squares, generalised nonlinear least squares and nonlinear mixed effects models. The nonlinear
mixed effects model proved to be superior in terms of achieving the principles of regression
assumptions and model fit for the data range observed. The second step was to fit potential height
using nonlinear quantile regression on observed spacing trial height measurements. This proved to
be a robust technique able to capture potentials according to the defined Chapman-Richards model
structure. The final step was to use the predicted site index as a site classification variable in order to
predict potential height. While some small deviation occurred, potential height seems to be well
correlated to site index and validation on selected sites suggested that site index can be used to
model potential height until a more sophisticated site classification model is used for future
improvement of the model. Diameter increment modelling followed six major steps in order to apply the full parameterisation
methodology of an age-independent diameter increment model dependent on tree diameter and
competition. Diameter increment potentials were fit using site index as a predictor of the potential
height curves. Multiple competition indices were tested on two sites to obtain a combination of two
indices, which can capture overtopping and local crowding effects. Principle components analysis
and 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 Area
competition indices. Changing importance of the two indices were observed on the two sites tested
indicating a shift in the mode of competition according to a water gradient.
These were combined in a deterministic potential modifier model, which mimicked competitive
stages over age; however the validation showed a skewed distribution, which was not sensitive to
stand density gradients. A stochastic model was constructed to model variance from observed
residual plots using linear quantile regression to determine bounds for a truncated normal
distribution which generates random deviates for a predicted increment. The stochastic element
significantly improved the performance and sensitivity of the model, however the model was still
not sensitive enough at very high and very low spacing densities. All in all two key models for an
adaptation of an individual tree growth simulator to South African conditions were successfully
demonstrated. The two main objectives were achieved; however some indicated improvements
could be made, especially for the competition indices where the sensitivity of competition to
changing 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 individual
tree growth model. / AFRIKAANSE OPSOMMING: In vergelyking met vakgroeimodelle, het individuele-boomgroeimodelle die potensiaal om die
sensitiwiteit van plantasiegroeimodelle vir veranderende omstandighede soos aanpassings in
boskultuur, onreëlmatige vakstrukture, ensovoorts, drasties te verbeter. Die doel van hierdie studie
was om twee subkomponente van ‘n Europese individuele-boomgroeimodel uit te brei om
sodoende individuele-boomgroei modelleringskonsepte in Suid-Afrika bekend te stel. Pinus elliottii is
gebruik as studiespesie. Twee hoofdoelstellings is bepaal. Eerstens, die modellering van hoogtegroei
potensiaal van opstande oor verskeie vlakke van groeiplek kwaliteit. Tweedens, die modellering van
deursnee-aanwas deur gebruik te maak van ‘n potensiaal matigingsbenadering “potential modifier
approach“ met ‘n kombinasie van kompetisie-indekse waarvan die belangrikheid verander volgens
die edafiese toestande van die groeiplek.
Die hoogtepotensiaalmodellering bestaan uit drie stappe. Tydens die eerste stap word groeiplek
bonniteitsmodelle vergelyk op grond van verskillende modelpassingstegnieke, naamlik nie-lineêre
minimum kwadrate, algemene nie-lineêre minimum kwadrate en nie-lineêre gemengde effek
modelle. Laasgenoemde het die beste gevaar in terme van die beginsels van regressiemodelle asook
die mate waarin die model die waargeneemde data pas. Tweedens is hoogtegroei potensiaal
gemodelleer deur nie-lineêre kwantielregressie op waargeneemde hoogtes van
spasiëringseksperimente toe te pas. Die metode is robuust en in staat om potensiale volgens die
gedefinieerde Chapman Richards modelstruktuur vas te vang. Laastens is die voorspelde bonniteits
indeks as ‘n groeiplek klassifasie veranderlike gebruik om sodoende die hoogtegroei potensiaal te
voorspel. Alhoewel klein afwykings voorgekom het, blyk hoogtegroei potensiaal goed gekorreleer te
wees met bonniteits indeks. Uit validasie op geselekteerde groieplekke blyk dit dat bonniteits indeks
gebruik kan word om hoogtegroei potensiaal te modelleer totdat ‘n meer gesofistikeerde groeiplek
klassifikasiemodel beskikbaar is wat die model verder sal kan vebeter. Die volledige parametriseringsmetodiek van ‘n ouderdoms-onafhanklike deursnee-aanwas model
wat afhanklik is van boomdeursnee en kompetisie bestaan uit ses hoof prosesse. Nie-lineêre
kwantielregressie is gebruik om deursnee-aanwaspotensiale te pas vir verskeie groeiplekke. Dié is
gekombineer met ‘n bonniteits indeks om ‘n nuwe model te vorm waarmee hoogtegroeipotensiaal
kurwes voorpel kon word. Daar is met veelvuldige kompetisie-indekse op twee groeiplekke
geëksperimenteer om ‘n kombinasie van slegs twee indekse te vind wat die effekte van
oorskaduwing en plaaslike verdringing kan vasvang, te vind. Hoof komponent analise “Principle
components analysis” en variansie inflasie faktore berekening “variance inflation factors calculation”
is gebruik om vir kollineariteit tussen die indekse te toets. Gepaste indekskombinasies is getoets. ‘n
Kombinasie van die KKL en plaaslike basale oppervlakte “Local Basal Area” kompetisie-indekse het
die beste resultate gelewer. Die twee indekse is as volg geselekteer. Veranderings in die
belangrikheid van elk van die indekse is waargeneem op die twee toetspersele. Dit dui op ‘n
verskuiwing in die modus van kompetisie afhangend van ‘n watergradiënt.
Die twee indekse is gekombineer in ‘n deterministiese potensiaal matigings model wat die
kompeterende stadiums oor ouderdom naboots. Validasie het egter ‘n skewe verdeling wat nie
sensitief vir opstandsdigtheidsgradiënte is nie, gewys. ‘n Stogastiese model is ontwikkel om variansie
in die residuele grafieke te modelleer. Lineêre kwantielregressie is gebruik om grense vir ‘n afgestompte normaalverdeling wat ewekansige afwykings vir ‘n voorspelde aanwas te bepaal. Die
stogastiese element het die prestasie van die deterministiese model merkbaar verbeter. Selfs met
die stogastiese element, is die model egter steeds nie sensitief genoeg vir baie hoë en baie lae
opstandsdigthede nie.
Ter opsomming is twee modelle vir ‘n aanpassing van ‘n individuele-boomgroeisimuleerder vir Suid-
Afrikaanse toestande suksesvol gedemonstreer. Die twee hoofdoelstellings is bereik. Daar is egter
steeds ‘n paar aangeduide verbeterings wat aangebring kan word. Die sensitiwiteit van die
kompetisie-indekse op hulpbronbeperkings wat verander op grond van die ruimtelike en temporale
skale moet veral verder bestudeer word. Verder moet die volle stel modelle wat benodig word om
individuele-boomgroei te modelleer nog toegepas word. As ‘n metodologiese benadering, het die
studie probleme uitgewys en toekomstige verbeterings aangedui, nuwe konsepte bekendgestel en
kan dus dien as ‘n riglyn vir toekomstige parametrisering van individuele-boomgroeimodelle.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/86621 |
| Date | 04 1900 |
| Creators | Lindner, Gerard Eckard |
| Contributors | Seifert, Thomas, Pretzsch, Hans, Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 136 p. : ill., map |
| Rights | Stellenbosch University |
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