Abstract:
Reduction in the rotation ages of softwood saw log plantations in South Africa is
causing increased proportions of low stiffness sawn lumber at final harvest. It has
been shown for some species that the microfibril angle (MFA) of the S2 layer of
tracheids is strongly related to the modulus of elasticity (MOE) of wood, even more
so than wood density, especially in wood formed during juvenile growth. The
objectives of this study were to describe the variation in MFA in young Pinus patula trees and to determine the relationship between MFA and the dynamic MOE of sawn
Pinus patula lumber. Thirty 16-20 year old trees from six compartments from the
Mpumalanga escarpment were processed into discs and lumber. MFA, density and
ring width were measured at two height levels using Silviscan 3. The average annual
ring MFA varied between 7 o and 29o; the pattern of variation depending mainly on
height level and the ring number from the pith. The MFA in P. patula followed the
same within-tree variation trends as in New Zealand-grown Pinus radiata but the
average MFA was lower in absolute terms and differences between height levels
were less pronounced. MFA and density exhibited highly significant Pearson
correlations of 0.73 and 0.70 respectively with board dynamic MOE. A multiple
regression model, which included MFA, density and ring width, explained 71% of the
variation in the dynamic MOE of boards. A sensitivity analysis on the model showed
that MFA and density had approximately similar influences on predicting the dynamic
MOE of Pinus patula boards.
