Effect of Fact Growth Rate on the Mechanical Properties of Loblolly Pine

The static bending, compression parallel to the grain and toughness properties of loblolly pine (Pinus taeda L.) grown at different rates were determined to examine the influence of fast growth rate. The tests were done on clear wood of butt logs from 143 trees of similar diameters from three populations of loblolly pine from the same locality in South Carolina; namely, a naturally regenerated forest 41 years old, a plantation of ordinary commercial stock 25 years old, and a research plantation of 15-year-old trees from parents selected. for fast growth rate and good form. In generally, the younger the tree, the lower its mechanical properties. For example, the modulus of rupture of the inner and outer. wood of the 15-year-old trees was on average 48 and 76 percent, respectively of that of the outer wood of the 41-year-old trees. The inner wood from all populations was lower in density and mechanical properties than the wood from the outer zones. Over 40 percent of the cross-section of the 15-year-old trees was low density juvenile wood compared with about 6 percent for the 41-year-old trees. Consequently, lumber from fast-grown pines may be expected to have mechanical properties significantly different from those of slow-grown trees of similar diameter. The most significant differences among the mechanical properties of clear wood from the three age groups related to the differences in density of the wood.

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