STANDPAK EVALUATION OF THE EFFECTS OF SITE, SILVICULTURE, AND GENETICS ON MEAN LOG AGE AND THE PROPORTION OF JUVENILE WOOD

Recent developments in the stand modelling system STANDPAK have allowed the prediction of Mean Log Age, growth rings per small-end diameter (s.e.d.), juvenile wood volume, and the proportion of juvenile wood. This capability was used in a theoretical exercise with Pinus radiata D.Don to examine the influence of a number of site, silvicultural, and genetic factors on Mean Log Age and the proportion of juvenile wood in New Zealand domestic log grades. Sensitivity analysis of individual factors and/or comparisons between simulations of major forestry management practices indicated that Mean Log Age was affected by felling age, tree stocking, and site quality. The proportion of juvenile wood in merchantable log volumes was found to be affected most by felling age, the number of growth rings containing juvenile wood, and site quality. Silvicultural factors (pruning severity, timing of thinning, and tree stocking) had a relatively small effect. Genetic improvement expressed as a Growth and Form (GF) rating, was indicated in this study to have little effect on the proportion of juvenile wood. An examination of historic changes in silvicultural regime, felling age, and site quality of P. radiata forests in the central North Island indicated increases in the proportion of juvenile wood resulting from modern practices that may explain the current concerns about wood quality. Inclusion of log age or number of rings/s.e.d. as additional criteria in the current system of log grading, should better segregate logs by the proportion of juvenile wood and wood density.