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Variation in strength, stiffness and related wood properties in young South African-grown Pinus patula

The objective of this study was to examine the variation in and intercorrelation among wood properties determining the suitability of 16- to 20-year-old South African-grown Pinus patula trees for structural timber. A total of 1 112 sawn boards from 340 logs, 170 trees and 17 different compartments were examined. Sawlogs were taken from two height levels from each tree. The mean modulus of elasticity measured on edge (MOEedge) was far below, and the mean twist higher than the limits set for structural grade softwood timber in South Africa. All the desirable properties for structural timber improved with distance from the pith with the exception of the fifth percentile value for modulus of rupture (MOR), which was higher at the pith than for the boards processed adjacent to the pith. Boards processed from the lower part of the stem were superior in most of the important properties compared to the properties higher up in the stem. The correlation between the dynamic modulus of elasticity (MOEdyn) and MOR of boards processed from the logs taken higher up in the stem was much weaker than in the case of the boards processed from the log taken from the lower part in the stem, suggesting that indirect (non-destructive) prediction of MOR decreased in reliability with increase in height in trees. A relatively strong negative correlation was found between the mean growth ring widths of the pith boards and the mean MOEdyn values per compartment, suggesting that slower initial growth in a compartment resulted in increased mean stiffness of boards from that compartment.