Effect of physiological age and site on microfibril angle in Pinus radiata

The effect of physiological age (shoot age at propagation) and site on microfibril angle was examined for seedlings (physiological age = 0 years) and cuttings (physiological age = 5-16 years) of Pinus radiata D. Don. Two trials were examined by measuring microfibril angle in alternate growth rings on breast height discs. In the first trial, two sites were compared for 11-year-old trees propagated from seedlings, and cuttings of comparable genotype, at 0 and 5 years physiological age, respectively. In the second trial, a single site was examined comparing 25-year-old trees propagated from open pollinated seedlings, and cuttings physiologically aged by 12-16 years, originating from 10 seed-orchard clones. In each trial there was a significant effect of physiological age for microfibril angle in the first 9 growth rings with a greater effect in the trees of greater physiological age. Physiological aging produced a significant decrease in microfibril angles in the juvenile wood, on average reducing microfibril angle to values below 35 degrees in trees aged by 12-16 years. Juvenile wood size, as indicated by the point at which microfibril angle gradient changes, was reduced by an average of two rings in both sets of aged cuttings examined. There was no effect of site in the material examined. Differences were consistent among seedling/ramet pairs of similar genotype. The use of aged cuttings rather than seedlings should result in increased stiffness of the juvenile wood and reduced longitudinal shrinkage. However, other changes associated with physiological aging, such as reduced basic density and growth rate, may affect the practicality of using highly aged planting stock as part of general forestry practice in New Zealand.