Clonal differences in log end splitting in Eucalyptus grandis in relation to age, parent performance, growth rate and wood density in two even-aged trials in Mpumalanga, South Africa

Abstract

This paper discusses the juvenile–mature correlation of log end splitting among Eucalyptus grandis clones from two trials and how differences in splitting relate to differences in wood density, pith-to-bark gradient and growth rate. Two approximately 20-year-old Eucalyptus grandis clonal trials at Bergvliet plantation were sampled. The first trial (Trial A) consisted of clones of eight ortets (genotypes) specifically selected for their widely different propensities to develop end splitting. The second trial (Trial B) consisted of 80 clones and one seedling control. From the latter trial 15 clones were selected in such a manner that they constituted three groups of clones representing a combination of high- and low-volume producers and, among the high-volume producers, clones exhibiting high and low propensities towards end splitting. The fourth group represented the seedling control. Trial A showed highly significant clonal differences in splitting, basic wood density, as well as its pith-to-bark gradient. No relationship of splitting with differences in growth rate, density or density gradient was found. The clones derived from the low- and high-splitting ortets maintained their general statuses as low and high splitters over a considerable period of time, but some of the individual clones did not reflect the performance of the ortets from which they were derived. Trial B revealed that clones classified as good-volume producers at the age of 65 months still outperformed the clones classified as low-volume producers approximately 15 years later. No significant differences in average wood density between the groups of clones studied was found, irrespective of differences in growth rate and degree of splitting, but trees from the slow-growing group of clones were significantly more variable across the radius in average wood density. The outcome of both trials confirmed results of many previous studies on this species, in that rapid and uniform growth has no significant detrimental effect on wood density or its rate of change across the radius of the tree. It may, in fact, be associated with increased uniformity from pith to bark with increasing age, sometimes resulting in slightly increased overall wood density.