After an hour of sanding, details of the tree's wood growth patterns emerged (photo at left). The first thing to notice is the dark area marked "R". This is an area of wood rot that developed after a limb had been pruned off several years ago. Above the pruning wound the area of wood marked "C" is the callus tissue that formed to seal off the pruning wound. At the point this large limb was removed from the tree, callus had almost completely covered over the pruning wound.
There are two more interesting things to note in this cross section of pecan wood. The fine brown line marked "B" is a specially hardened layer of wood cells that forms in response to the pruning wound. This boundary layer prevents wood rotting fungi found in the pruning wound from attacking the newer, outside layers of wood growth. The swirly pattern in the wood grain marked "CW" is what wood workers call "crotch wood". Furniture makers often feature crotch wood on cabinet doors to add extra interest to their work. In terms of tree structure, crotch wood is actually layer upon layer of new branch collar that develops as the tree increases in diameter. Note that the branch collar was small (narrow) when the branch was small (lower portion) and then got increasingly larger (wider) as the branch grew in diameter (upper portion). Once the limb was pruned off, the branch collar turns into callus tissue to cover the wound and the formation of crotch wood stopped.