The opening up of the Russian Far North is connected with strong natural ecosystems changes. Nowadays tundra vegetation in impact regions is changed very much. In natural Arctic terrestrial ecosystems vegetation has a particular role, because it controls the permafrost and ground ice condition in tundra soils. Disturbance of vegetation carpet leads to erosion processes (thermokarst etc.) progress which can become catastrophic for the whole ecosystem. The objective of presented research was to find out the influence of vegetation peculiarities, some relief and substrate conditions at thickness of permafrost active layer in the region with widespread thermokarst processes (Kotelny Island of New Siberian Islands archipelago). The active layer data were recorded in natural thermokarst habitats - on baidzharakh hillocks and in narrows between them.

On the Kotelny Island baidzharakhs (Yakutian), which are relic mounds enclosing polygonal ice blocks,are common and important component of landscape. Their genesis is connected with thermokarst processes. The height of baidzharakhs on Kotelny Island varies from 0,5 to 5m, and their diameter - from 3 to 12m. The width of depressions between them is 3-7m. The vegetation on tops,slopes, footslopes of baidzharakh hillocks and in hollows is presented by different plant communities.

The measurements of permafrost thaw depth were done in different plant communities: on top of hillocks (more than 250 samples),in depressions (more than 250 samples),and in surrounded spotty tundra (40 samples,beneath spots and moss carpet). The data were collected in one day in the end of summer -the 6 August.Three days before the wheater was very warm, so, we can suppose that the obtained thicknesses of active layer were maximum.

The mean thaw depth in spotty tundra under spots was 36,5cm, and under moss cover-32 cm.The mean active layer on baidzharakh hillocks was 35 cm deep, and in depressions -34cm. But in reality the thaw depth varied very much: from 9,5 cm to 52 cm on top of baidzharakh, and from 25 to 45 cm in narrow. (Here and below all presented data are means.) The main obtained results are:

1. The maximum and minimum thickness of active layer was noted on hillocks.The minimum depth (9,5cm) was in hillocks formed by peat, with total plant cover only 20% (lichens covered 20%),vegetation was sparse and poor.On baidzharakhs formed by sundy loam, with eroded cone topes,where graminoids and forbs dominated(total plant cover was about 25%), the maximum thaw depth was noted (45-52 cm).
2. On baidzharakh hillocks, where mosses, Salix polaris, and graminoids were abundant, the thaw depth was connected with plant cover density: 95% (total plant cover) - from 26 to 32 cm deep,50-70% - from 36 to 38 cm, 35% - 47 cm.
3. On hillocks with epigeous lichens, graminoids, Salix polaris and forbs, total plant cover was 50-70%, and the ative layer ranged from 24 to 50 cm. Its thickness depended of substrate: on peat mounds it was 24-35 cm, on loam ones -33-50cm.
4. In depressions the active layer varied less than on hillocks: it ranged from 25 to 45 cm. The hollows had mostly close moss cover (total plant cover was 80-100%, mosses- 60-100%). The thaw depth varied in communitie with Dupontia fischeri from 25 to 33 cm, in communities with Alopecurus alpinus and Juncus biglumis - from 27 to 40 cm, in communities with Salix polaris and graminoids- from 32 to 41cm.
5. In narrows, where epigeous lichens, graminoids and forbs were abundant, the active layer was 38-45 cm.
6. The minimum thicknesses of active layer were noted on hillocks and depressions with peat substrate, and the maximum one - on sundy loam eroded mounds with sparse plant cover.
7. The mean permafrost thaw depths were always more in baidzharakh hillock-hollow complexes with loam or sundy loam (not peat) substrate than in surrounded tundras.

Olga I. Sumina
Department of Biology and Soil Sciences
St.Peterburg State University
Universitetskaya amb.,7/9
St. Petersburg, 199034
RUSSIA
phone: (812)218-14-72
fax: (812) 218-08-52
e-mail: osumina@gs.bio.pu.ru