Pith
Pith is the soft, spongy core at the very center of a tree trunk, representing the original sapling's growth. In sawn lumber, boards containing pith are prone to splitting, checking, and warping because the wood immediately surrounding the pith has unstable juvenile characteristics. Pith is generally considered a defect.
In-Depth Explanation
The pith is the remnant of the tree's earliest growth and consists of parenchyma cells with thin walls and little structural strength. The wood within the first few growth rings surrounding the pith — called juvenile wood — has shorter fibers, higher microfibril angles, and different shrinkage characteristics than mature wood farther from the center. This means lumber containing pith is inherently less stable: it tends to check, split, and warp more severely during drying and in service. In timber framing with large beams, boxed heart (pith enclosed within the member) is common and expected, with surface checking being a normal and usually structural insignificant characteristic. In smaller-dimension lumber and hardwood boards, pith is avoided when possible. Sawyers optimize their cutting patterns to keep pith in one piece rather than splitting it between two boards, as boards sawn through the pith are the most prone to splitting.
Usage Example
The sawyer adjusted the log placement to box the pith in a single 6x6 timber rather than exposing it across two weaker boards.