Fig. 5.6 (Above) Location of the northern limits to the boreal tundra forest and West Siberian Lowlands, as recorded by a Normalized Difference Vegetation Index (NDVI) satellite image recorded in May 1998 while the tundra was covered in snow. The snow-covered region does not register on the NDVI scale as there is no photosynthetically active visible vegetation. The evergreen forest emerging above the snow gives positive readings, shown by various shades ofgreen with the more intense colours indicating more positive values. Note the southern depression of the evergreen treeline in the region of the Siberian Lowlands where forest has been replaced by the development over the past 6000 years by a very extensive bog, creating probably the largest example in the world of paludification of former forest. (Opposite) Detail of the transition zone between forest and tundra as seen in the normalized vegetation index recorded in May 1998 (1 km resolution). Colourscale: blue = 0; blue-green = 0.11—0.25; dark-green 0.26-0.40; bright green = 0.41—0.64. It is considered (see text) that this mosaic represents a self-renewing cyclic process taking place over hundreds of years as patches offorest develop on land that dries out after being raised by frost-heave and then reverts again to bog as tree cover cools the underlying ground. (Images prepared from 8 km resolution Pathfinder data set — US Geological Survey, EROS Data Center, Sioux Falls, South Dakota.)
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