Modeling environmental factors affecting the growth of eucalypt clones.
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Tree growth is influenced by environment and genetic factors. The same tree growing in different areas will have different growth patterns. Trees with different genetic material, e.g. pine and Eucalyptus trees, growing under the same environmental conditions have different growth patterns. Plantation trees in South Africa are mainly used for pulp and paper production. Growth is an important economic factor in the pulp and paper industry. Plantations with fast growth will be available for processing earlier compared to a slow growth plantation. Consequently, it is important to understand the role played by environmental factors, especially climatic factors, on tree growth. This thesis investigated the climatic effects on the radial growth of two Eucalyptus clones using growth data collected daily over five years by Sappi. The general linear model and the time series models were used to assess the effects of climate on radial growth of the two clones. It was found that the two clones have similar overall growth patterns over time, but differ in growth rates. The growth pattern of the two clones appears to be characterized by substantial jumps/changes in growth rates over time. The times at which the jumps/changes in growth rate occur are referred to as the “breakpoints”. The piecewise linear regression model was used to estimate when the breakpoints occur. After estimating the breakpoints, the climatic effects associated with these breakpoints were investigated. The linear and time series modeling results indicated that the contribution of climatic factors on radial growth of Eucalyptus clones was small. Most of the variation in radial growth was explained by the age of the trees. Consequently, this thesis also investigated the appropriate functional relationship between radial growth and age. In particular, this nonlinear growth models were used to model the radial growth process. The investigated growth curve models were those which included the maximum radius and the age at which the radial growth rate is largest as some of the parameters. The maximum growth rate was calculated from the estimated model of each clone. The results indicated that the two clones reach the maximum growth rate at different times. In particular, the two clones reach the maximum growth rates at around 368 and 376 days, respectively. Furthermore, the maximum radius was found to be different for the two clones.