1.Since you are satisfied with the rest of the image, add a composite Layer (Cmd+Opt+Shift+E) to the top of the stack and duplicate the composite.
2. Then choose the top composite layer, Edit > Fill with your choice of Brown. Set its Layer Blending Mode to Color.
3. Add a Mask to the top composite that is white in the area of trunk and black in the remainder of the mask.
4. Apply a curve to the top composite image (not the mask) to arrive at the desired density, contrast and modeling of the trunk.
I think I have it. For step 4 I added a Curves adjustment layer using the previous layer as a clipping mask.
Thanks again.