Let's make sure I understand the layers of a tree
The live portion of a tree is the outside layers, the Phloem, Cambium and Xylum.
It is these layers which are likely to become infected when I prune a tree with dirty tools, while the green is still showing after a deer comes by and rubs, or while bugs are digging in. Essentially the foliage and these layers are what Round-up can travel in and affect also.
The inner layer, the heartwood, is there for structural support but does nothing to keep the tree green.