At present, the thawing depth in permafrost-affected soils is measured from the surface of the moss-lichen cover; if the latter is absent, measurements are performed from the surface of the litter layer. In the first case, the thickness of the root zone is less than the thawing depth by the thickness of the moss-lichen layer; in the second case, the thickness of the root zone is virtually equal to the thawing depth. Thus, upon equal thawing depths, the soil temperature regime in the root zone under the moss-lichen cover differs from that in the soil without this cover. In order to avoid this discrepancy, it is suggested that the thawing depth should be measured from the upper boundary of the root zone.