Additive mixed models to study the effect of tree age and climatic factors on stem radial growth of Eucalyptus trees

Abstract

The effect of tree age and climatic variables on stem radial growth of two hybrid clones of Eucalyptus was determined using longitudinal data from eastern South Africa. The stem radius of was measured weekly as the response variable. In addition to tree age, average weekly temperature, solar radiation, relative humidity and wind speed were simultaneously recorded with total rainfall at the site. An additive mixed effects model that incorporates a non-parametric smooth function was used. The results of the analysis indicate that the relationship between stem radius and each of the covariates can be explained by nonlinear functions. Models that account for the effect of clone and season together with their interaction in the parametric part of the additive mixed model were also fitted. The interaction between clone and season was not significant in all cases. For analyzing the joint effect all the covariates, additive mixed models that included two or more covariates were fitted. A significant effect of tree age was found in all cases. Although tree age was the key determinant of stem radial growth, weather variables also had a significant effect that was dependent on season.

Keywords

Additive mixed effects / Dendrometer trial / Parametric modelling / Penalized splines / Weather variables

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