Characterization of the Strength of Juvenile Wood of Loblolly Pine (Pinus Taeda L.)

The specific gravity, flexural properties, and longitudinal shrinkage of juvenile wood, i.e., wood from within about 10 growth rings of the pith, and of mature wood were determined from a sample of 19 trees of loblolly pine from North Carolina. The specimens were cut from 5-foot-long bolts taken at average heights above the ground of 3, 13, 23, 33, or 43 feet, eight trees supplying two or more bolts. A total of 294 specimens of final size 1 by 1 by 16 inches were accepted as sufficiently clear and square to the growth rings for testing, but an additional 49 specimens were satisfactory for inclusion in the density study. Juvenile wood was in general less dense than mature wood at all heights, the respective averages being 0.42 and 0.49 for butt logs and 0.40 and 0.46 for all the higher logs. However, the variability was such that the ranges of specific gravity of both types of wood overlapped considerably. Consequently, although juvenile wood showed the lowest values of density, and also of strength, rejection of any piece solely on the grounds that it contained juvenile wood would not be an adequate nor economical method of eliminating low-density, low-strength material. Juvenile and mature wood were part of the same population as far as modulus of rupture and modulus of elasticity were concerned, differences in these properties being mainly related to differences in specific gravity rather than to position in the tree. There was a high correlation between the flexural properties and specific gravity, providing the butt log specimens were excluded. The latter had properties which were significantly less than those for material of the same density from higher in the tree. The anomaly was not apparently related to the amount of compression wood or extractives. About 75 percent of the specimens elongated on drying from the initial green state to 12 percent moisture content, although on the whole, juvenile wood tended to shrink. The changes in length were generally slight except for material from butt logs. For upper logs, the average movement was 0.001 percent shrinkage for juvenile wood and 0.013 percent elongation for mature wood. For butt logs, the average movement was 0.096 percent shrinkage for juvenile wood and 0.007 percent elongation for mature wood.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member