Thaw Depth
The thickness of the active layer, known as the thaw depth, is determined by how far the frost front is forced to retreat due to heating during the summer.
Thus, the primary determinant of active layer thickness is the maximum temperature attained during the summer. If it is only a little above 0°C, the active layer can be very thin (only 10 cm on Ellesmere Island), whilst if it is quite warm, it is much thicker (about 2.5 m at Yakutsk), and if the permafrost is discontinuous and soil begins thawing earlier, it can be still thicker (5 meters at Yellowknife). The parent material of the soil is also important: active layers in soils made of sandy or gravelly parent materials can be up to five times deeper than those made from loam- or clay-rich parent material. This is because the coarser material allows for much greater conductivity of heat down into the soil.
This is important because roots of plants cannot penetrate beyond the active layer and are restricted by its thickness. Thus, in a continuous permafrost environment plants must have shallow roots, which restricts tree growth to specialised species such as Larix. In areas of discontinuous permafrost, most conifers are able to grow easily.
Read more about this topic: Active Layer
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