Cambium

The cambium is a thin layer of actively dividing living cells just beneath a tree's bark, responsible for producing all new wood growth (inward) and new bark (outward) each growing season. It's the layer that generates every growth ring a tree ever adds, and the reason girdling a tree -- cutting through the cambium all the way around the trunk -- kills it, by severing the pathway that moves nutrients between roots and canopy.

For woodworkers, the cambium mostly matters at the edges of live-edge slabs, where the thin cambium layer sits directly between the bark and the first true wood -- it's often removed or treated separately during slab prep since it doesn't finish or hold together the same way the solid wood does.