Application of Linear Mixed Model: The Effect of Climatic Factors on the Wood Anatomy of Two Eucalypt Clones

Eucalypt trees are one of tree species used for the manufacturing of papers in South Africa. The manufacturing of paper consists of cooking the wood with chemicals until obtaining a pulp. The wood is made of different cells. The shape and structure of these cells, called wood anatomical characteristics are important for the quality of paper. In addition, the anatomical characteristics of wood are influenced by environmental factors like climatic factors, soil compositions etc…. Therefore, in this study we investigated the effect of climatic factors on wood anatomical characteristics of two Eucalyptus clones. In the experiment, two sets of data were recorded daily, the climatic parameters and the tree growth. After cutting the trees, the anatomical properties of the wood were measured using microscope and image analysis. The longitudinal linear mixed model with age, season, temperature, rainfall, solar radiation, relative humidity and wind speed as the fixed effects factors and tree as random effect factor was fitted to the data. Lagged effects climatic variables were identified and included in the model. To account for the physical characteristics of the trees we included the effect of diameter at breast height (DBH), stem radius, daily radial increment, and the suppression or dominance of the tree in the model. It was found that wood anatomical characteristics of the two clones were more affected by climatic variables when the tree was on juvenile stage as compared to mature stage.