No abstract

Permafrost underlying forested north-facing slopes and seasonally frozen ground underlying mountain steppes on south-facing slopes co-exist within a small mountain basin that represents the most general landscape type in northern central Mongolia. A 5-year time series of hydro-meteorological parameters on these slopes is presented in order to identify the factors controlling ground temperature regimes. A thick organic layer (0.2-0.4 m) beneath the forest on a north-facing slope impedes the effects of summer air temperature on the ground, and the forest canopy strongly blocks downward shortwave radiation during summer. Active layer thickness was determined by summer warmth. The mountain steppe on a dry south-facing slope receives a large amount of downward shortwave radiation compared to an adjacent forested slope, and therefore the surface temperature exceeds air temperature during summer, leading to a warm soil profile. In winter, snow cover was the main factor controlling interannual variations in the thickness of seasonally frozen ground. The onset of soil thawing in the forested area was later than in the mountain steppe, even though soil freezing began simultaneously in both areas. Overall, the forest cover keeps the ground cool and allows permafrost to persist in this region.

In the mountainous regions of northern Mongolia, there are peculiar different patches of the landscape "ecotone" with north-facing forest slope and south-facing grassland slope. The slopes are distinguished as marked contrast of permafrost distribution as well. The present study revealed ecohydrological processes in terms of evapotranspiration and water balance based on continuous observation since 2004 in the ecotone area of Shiljiree river watershed, upper part of Tuul River in Khentii Mountains. Soil moisture amounts following rainfall during early growing season have primary importance on interannual variations in transpiration from larch forest slope. In addition, timings of snow disappearance and thawing active layer affect subsequent foliation of trees and their transpiration. The estimation of water balance during summer 2006 demonstrated that the larch forest slope with underlying permafrost suppressed the evapotranspiration with half of amounts in grassland slope. It means that precipitation and soil water in forest slope can be partitioned to evapotranspiration and river runoff. On the other hand, grassland at south-facing slope with no underlying permafrost has remarkably large amounts of evapotranspiration and therefore both precipitation and soil water was consumed entirely to evapotranspiration. The results depicted that the coexistence between permafrost and forest in mountain slope has an important role in sustaining water resource.

This study presents thermokarst lake changes at seven different sites in the continuous and isolated permafrost zones in Mongolia. Lakes larger than 0.1 ha were analyzed using Corona KH-4, KH-4A and KH-4B (1962-1968), Landsat ETM + (1999-2001), and ALOS/AVNIR-2 (2006-2007) satellite imagery. Between 1962 and 2007, the total number and area of lakes increased by +21% (347 to 420), and +7% (3680 ha to 3936 ha) in the continuous permafrost zone, respectively. These changes correspond to the appearance of 85 new lakes (166 ha) during the last 45 years. In contrast, lakes in the isolated permafrost zone have decreased by −42% (118 to 68) in number and −12% (422 ha to 371 ha) in area from 1962 to 2007. The changes in lake area and number are likely attributed to shifts in climate regimes and local permafrost conditions. Since 1962, the mean annual air temperature and potential evapotranspiration have increased significantly in the northern continuous permafrost zone compared to the southern isolated permafrost zone. Due to ongoing atmospheric warming without any significant trend in annual precipitation, patches of ice-rich subsurface have thawed, and the number and area of lakes have accordingly developed in the continuous permafrost zone. Shrinking of thermokarst lakes in the isolated permafrost zone may be due to disappearing permafrost, deepening of the active layer, and increased water loss through surface evaporation and subsurface drainage.