Site and stand analysis for growth prediction of Eucalyptus grandis on the Zululand coastal plain: scientific paper
M. Du Plessis
The integration of site information with that of tree growth is of special importance in Zululand, where sustainable supply of timber is essential for local processing and export commitments. Site prediction growth models need to be based on easily attainable input variables that are suitable for operational implementation by planners and for deployment of advanced silvicultural technology. Recently concluded growth studies based on permanent sampling plots established across Eucalyptus grandis plantations yielded useful information for revising the current knowledge on site-growth relationships in the region. The Chapman-Richards model was used to define the height growth curves over a range of sites. Standspecific site indices were calculated for trees of five years of age and regressed against a range of site variables. The multiple regression analysis showed that a large portion of the variation (r2=0, 63) in the site index could be explained by topsoil organic carbon and clay content in the subsoil. The soil data in the routine survey format and modelled climatic data in a grid pattern did not contribute significantly to the models. It is clear that the variables capturing the nutrient status of the soil and the soil's ability to store and make water available to the trees are the most important ones on the Zululand coastal plain when site prediction growth modelling is performed. Future research on site productivity modelling should include site variables specifically designed for that purpose and should further be enhanced with studies including their influence on wood quality.
Key words: Eucalyptus grandis, Site quality prediction model, Site index, Organic carbon content, Clay content