I've found Ilford MGWT fiber based paper to be very responsive to toning. When used in combination with a warm tone developer, ID-78 in this case, the selenium seems to take really well.
Hey Daniel, I don't know how many times I've tried what you did, without the same level of success. Usually the exposed roots are really bright in color and the dirt (or rock in this case) behind really dark. And then to get it to work together this well is really impressive! Great stuff!