The influence of the environment on the volume growth, stem form and disease tolerance of Eucalyptus grandis clones in the summer rainfall areas of South Africa

ENGLISH ABSTRACT: A thesis undertaken to quantify genotype-by-environment interaction within Eucalyptus grandis clones growing in the eastern portion of South Africa. Thirty one sites were selected to represent the "traditional" E. grandis growing areas of South Africa. Eleven common macro- site variables and twelve common micro- site soil variables were recorded at each site. Twenty seven E. grandis clones and four E. grandis hybrid clones were then evaluated over these 31 sites. An incomplete latin square design was used to evaluate the 31 test clones, and five E. grandis controls were incorporated into the trial design to link the 31 sites. Volume production, stem form, stem defects and survival were assessed at two and five years, as well as the disease infestation of three stem cankers at five years. The analytical methods which were used to evaluate and quantify the GEl portion of the study are the analysis of variance (ANOV A), correlation analysis, and joint regression analysis (IRA) together with the analysis of co-variance (ACOV AR). The growth-site association for volume production, stem form and Endothia disease infestation were investigated using factor analysis (FA), and equations derived for the species and for the individual clones using a stepwise multiple regression approach. GEl, as evaluated through JRA, revealed that an increase in site productivity lead to a positive linear response in productivity on a clonal level, and that there was a diverging or fanning pattern among the regression lines of the clones. This tendency was also observed for both the stem form and the Endothia infestation. Hence, no significant changes in the rankings of the clones were found, and only relevant differences between the clones were found to change significantly. Juvenile-mature genetic correlations for volume production and the stem form showed moderate (rg = 0,66 and rg = 0,70) correlations between the two and the five year assessments. On a species level, rainfall was the main environmental factor responsible for volume production, while latitude was the main influence on stem form and Endothia infestation. On an individual clone basis, some micro-site soil factor interaction within the clones was found for the growth-site response models. Keywords: Eucalyptus grandis, genotype environment interaction, clones, site factors, growth-site response, ANOV A, ACOV AR, GEl, FA, JRA,