Longleaf Pine Inner Bark and Outer Bark Thicknesses: Measurement and Relevance

Measurements of bark thickness generally ignore the fact that bark is comprised of both living inner bark (phloem) and essentially dead outer bark (rhytidome). Discerning between them has ramifications for the utility of bark as a byproduct of timber harvesting and its functionality on a living tree. Inner bark and outer bark thicknesses for longleaf pine (Pinus palustris Mill.) were investigated using disks collected from trees harvested on a 70-year-old plantation. Inner bark thickness was relatively constant up the bole of each tree whereas outer bark thickness rapidly declined from its thickest point at stump height; at relative heights above 20%, the decrease in outer bark thickness was more gradual. The proportion of inner bark, therefore, increased up the bole, from an average of 15% at stump height to above 40% toward the top of the tree. Since inner bark is a richer source of extractives than old outer bark, tree tops may be preferable in terms of bark abundance and quality as feedstock for extractive-based products. Reductions in the inner and outer bark thicknesses on disk drying, with averages of roughly 20 and 10%, respectively, differed when the data were pooled by cardinal direction. Thus, variability in bark thickness around the circumference of a standing tree may actually be a manifestation of differences in bark moisture content.