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. Soil formation in the active layer Cryoturbation is the dominant force operating in the active layer, and tends to make it generally uniform in composition throughout. However, variation in the composition of soils due to differences in parent rock are very marked in permafrost regions due to the low rate of weathering in the very cold climate. The slow rate of decomposition of organic material means gelisols (permafrost soils) are very important as a sink for carbon dioxide. This carbon dioxide and other greenhouse gases (chiefly methane) forms from the very slow decomposition of the excess organic matter that remains in most gelisols and is mixed down into the pereletok layer during relatively hot summers and below that layer during warmer periods about 5000 to 6000 years ago. This storage of carbon means thawing of permafrost may accelerate global warming - some suggest the difference could become very significant especially if the carbon has been stored since before the recent glacial maxima. See also References Pedology Soil Permafrost