Modelling the Effect of Tree Age and Climatic Factors on the Stem Radial Growth of Juvenile Eucalypt Clones

Abstract

Tree growth is influenced by environmental and genetic factors. Trees with different genetic materials growing under the same environmental conditions have different growth patterns. Adequate management requires a good understanding of factors affecting tree growth. The aim of this study is to determine the factors that influence stem radial growth of juvenile Eucalyptus hybrids clones. The longitudinal data used in this study is obtained from Sappi land holdings in coastal Zululand of eastern South Africa. For the first two years of growth, measurements of stem radius were repeatedly obtained using a dendrometer attached to 18 trees. Weekly measures of climatic data (temperature, solar radiation, relative humidity, and wind speed) were simultaneously recorded with total rainfall from the site. A fractional polynomial model under linear mixed modeling framework, that incorporates the covariance structure into statistical model, was used. The results of the analysis indicate that the relationship between tree age and stem radius can be explained by a second-degree fractional polynomial model. Subsequently, the model was extended to account for the effect of climatic variables. A significant difference was observed between the growths of the two clones. The results indicate that the effect of weather variables on stem radial growth depends on season. It is found that some weather variables (like temperature and solar radiation) that have a positive effect in one season might have a negative effect in another season. In conclusion, although tree age is the primary determinant of stem radial growth, weather variables are also found to have a significant effect.