The Arctic was exposed to dramatic environmental changes during the Quaternary. It is possible to detect the biological effects of these changes in terrestrial arctic ecosystems using bioindication by vascular plants. The results of studies about changes of structure and dynamic of plant communities in arctic Siberia during the Late Quaternary are introduced. We reconstructed the composition of plant communities in different time slots from botanical macrofossil assemblages preserved in permafrost sequences in Northern Siberia.The studies have been carried out at the Bykovsky Peninsula southeast of the Lena-Delta. The study area is situated directly at the Laptev-Sea coast today. But the location was far inland during the last glaciation of the northern hemisphere due to the marine regression. The Laptev-Sea level fell down to 120-135 m beneath its present-day level. Its coast shifted consequently northward several hundred km.The permafrost sequence investigated is outcroped along a cliff line, which is up to 40 m high. It was made accessible for sampling by thermal erosion and wave action. The outcrop consists of several subprofils. These so called thermokarst mounds represent the sediments, which were accumulated in the former polygonal depressions after the thawing of the ice wedges walling them. The sediment-blocks are very rich in fossil seeds. Due to the freezing of the plant remains simultaneous with their accumulation they are normally very well preserved. The Thermokarst mounds were sampled for several studies, which were carried out within the multidisciplinary project, named Palaeoclimatic signals from ice rich Permafrost. A detailed age model was constituted based on radiocarbon data both conventional and AMS. According to this the investigated permafrost sequence was deposited continuously since about 60 000 years bp and is hence an excellent archive of the Late Quaternary history of vegetation and climate. 18 samples from different thermokarst mounds were sieved. Plant remains were picked and identified to the lowest taxonomical level possible and counted. The palaeoecological interpretation of the species spectra obtained from each sample based on the ecological requirements of the species, which are represented by habitat descriptions on the one hand and by a classification into a plant community on the other hand.The composition of late Pleistocene species spectra was significantly different from the Holocene one. Thus it was possible to distinguish two vegetation complexes. The late Pleistocene vegetation complex includes 6 different plant communities, which are introduced in order of their spatial appearance, beginning in the depressions and ending at the topof slopes.Freshwater aquatic vegetation: All aquatic species found in the section occur at shallow freshwater sites, which are characterized firstly by frequent disturbances and secondly by fluctuating salt concentrations. Both phenomena appear at impermanent waters, frequently drying out during the summer under arid climate conditions. The most important indication of the aquatics concerning the palaeoclimate is made by their temperature requirements. The northernmost distribution of most species listed here is situated within the boreal zone today. They thus require relatively high water temperature.The littoral vegetation consists of pioneer species, which colonize moist soils in the range of fluctuating water levels at shores of shallow, contracting lakes and depressions in the landscape, uncovered by perennial plants in arid climates. Its occurrence indicates temporary running dry of the small, shallow polygonal melting water pools in the course of summer. Some of the proven species are more or less salt-tolerant. Salt accumulation at the surface due to groundwater evaporation under arid conditions is therefore indicated.Saline meadows occur in floodplains of river valleys and lake basins with a strongly changing groundwater table and salt accumulation at the surface due to the evaporation of the groundwater. This community develops under regularly recurring grazing in Mongolia and Central Siberia. In these regions the highly productive community is one of the most important pastures especially in mountain regions. The frequently proven Carex duriuscula is indicative of a degraded facies of that community. It leads over to steppe vegetation where it is also representative of the degradation facies of northeast Asian steppes. In Mongolia this rather species-poor facies indicates degradation of steppe by overgrazing or trampling.Kobresia meadows represent the perennial vegetation at dry, exposed, snow less sites in kontinental regions of the Arctic and the alpine belt of mountains. They are extremely frost resistant and are adapted to extreme seasonal temperature fluctuations.The most species proved in the late Pleistocene part of the sequence belong to communities of open pioneer communities of the Arctic and the alpine belt of mountains on dry, immature mineral soils. They occur at sites, which are characterized by frequent disturbances caused by deflation. This plant grouping appears under dry conditions and a strong exposition in winter due to a lacking snow cover. The northernmost vascular plants occurring in arctic deserts today are species of this class.The following plant communities dominated the Holocene vegetation. Vegetation of bogs can be divided into two different plant sociological classes. The arctic bog communities, dominated by sedges appear at mineral soils, those dominated by dwarf shrubs occur after peat accumulation and acidification of the substrate. The latter one is therefore characteristic of the stage of humidification and acidification, characteristic of the 2nd half of Holocene. Both constitute the perennial vegetation of permanently wet habitats in swamps and at the shores of waterbodies in the Arctic.Only a few vascular plants are able to tolerate the environmental conditions close to snow accumulation sites. Species of the class Salicitea herbaceae are adapted to a shortened growing season due to a late beginning of snow-melt and a continuous supply of melting water. The soil-temperature has distinctly reduced by melting water. The soil is waterlogged causing a reductive environment.When we compare both vegetation complexes, at first the relatively high number of different plant communities occurring during the late Pleistocene is conspicuous, reflecting a high diversity of local site conditions. Holocene vegetation in contrast is rather uniform. A differentiating environmental gradient is scarcely distinct. Seasonal fluctuation of groundwater is indicated by the vegetation of Late Pleistocene, proven by the pioneer character of aquatic and littoral vegetation as well as by the evidence of several salt indicators. This result indicates a high evaporation under arid climatic conditions, what is confirmed by the finding of steppe elements and arctic dryness indicators as well. Plant communities of permanently wet habitats like bog communities occurred only sporadicly during Late Pleistocene. Species of Kobresia meadows and arctic pioneer communities show very low winter temperatures. In contrast steppe plants and boreal aquatics indicate relatively high summer temperatures. Thus, a very distinct seasonal temperature gradient and a low precipitation is reflected by the vegetation of Late Pleistocene. Both phenomena are characteristic of continental climates.Indicators of alternating moisture and xerophilous plants disappeared widely in the course of Holocene. Species of permanently wet habitats predominated the vegetation, plants characteristic of snow accumulation sites occurred. They show the existence of a thick snow cover. Humid conditions hence established during the Holocene. Thermophilous plants disappeared and indicators of extreme coldness in winter decreased conspicuously. The seasonal temperature gradient hence decreased distinctly in Holocene. A low seasonal gradient of air temperature as well as humid conditions are characteristic of maritime climate.Is it possible to specify the extent of temperature during the growing season in late Pleistocene? We tried to make quantitative reconstructions by means of the indicator species approach. According to the palaeobotanical record the mean July temperatures during some stages of Pleistocene were temporarily higher than today especially during the middle Weichselian interstadial. As we could see the arctic environment depends on the general character of climate, continental or maritime, which is influenced by intruding marine air masses rather than on global trends of temperature changes. The influence of moisture is stronger than changes of global annual mean temperature. This is explainable as follows: Moisture in the form of clouds hinders radiation to hit the ground directly. The Albedo of clouds is up to 80% of the incoming radiation. Clouds cause a loss of energy by reflection of radiation back to the universe. Consequently soil and air near the ground are heated insufficiently. This problem is aggravated by increased precipitation and water logging of the soil. The greatest part of heat supply is used to melt the snow and evaporate the water. In consequence of a thicker snow cover the growing season is shortened by prolonged snow melt.