Permafrost and Forest Degradation after Wet Climate Years in Eastern Siberian Boreal Forest

Abstract:

Unusual precipitation increase during summer through winter had continued since 2004 in the central Lena river basin, eastern Siberia. The precipitation increase led to deepening active layer (permafrost thawing near the surface) accompanying with remarkable increase in soil moisture. The perennially waterlogged conditions had exacerbated the boreal forest habitat; that is, larch trees had widely withered and died in this region. The present study clarified spatial extent of permafrost and forest degradation due to the unexpected hydro-climate-driven damages. We have attempted to extract the degraded boreal forest based on satellite image analyses, along with expansion of the perennially waterlogged surface area. We used ALOS-PALSAR and AVNIR-2 images taken from 2006 to 2009. Classification of waterlogged area was performed by PALSAR images with supervised classification based on a microwave backscattering coefficient. Then, we compared the distribution of the waterlogged area between multi-years. Additional supervised classification of boreal forest change was conducted using AVNIR-2 images. Both classifications produced the multi-years change in degraded boreal forest at the intensive observational sites in both left and right bank of Lena River near Yakutsk. In the right bank area, most of alas lakes expanded and boreal forest on the periphery of lakes turned to waterlogged surface. In the left bank area, in contrast, waterlogged surface expanded at concaved terrain and along valleys in conjunction with forest degradation. Field survey supported that humidified and deepening active layer along slope and near alas lakes correspond with the gradient of forest degradation and enhanced thermokarst activity. Both of increasing precipitation and thawing ice in permafrost might cause the degradation. In brief, the method combining ALOS satellite images has possibility to detect permafrost and forest degradation caused by wet climate in boreal forest.