The effects of anthropogenic impact on plant and soil cover in Mongolia.
Under the prevailing variable natural conditions nomadic pastoralism was the most suitable form of land use in Mongolia and the neighboring countries in the past. Furthermore, small areas were used for agriculture in some regions. Therefore, anthropogenic influence was present throughout history. In the forest steppe zone this led to the disappearance of forest sites which then gave way to meadow steppe. The severe impact on the natural pastures by continuous increase of livestock herding and other human activities was perceptible during the socialist period. But since the 1990s, after the transition to market economy, this impact has increased greatly and a great number of degradation features and irreversible damages in the plant and soil cover can be observed now. Especially in the vicinity of the Mongolian capital Ulaanbaatar, along the north-south road corridor from Russia to the Chinese border, but also in some western regions of the country herders concentrate their livestock in search for better marketing opportunities. Especially there, the vegetation in vast areas is overgrazed by biting and trampling to such an extent that its regeneration is endangered. Degraded and devastated pasture types with a decrease of valuable forage plants and a high increase of ruderal elements can be found in these grasslands. Characteristic sequences of pasture degradation are presented here. The degradation effects steppes and desert steppes as well as reed, mire, and meadow vegetation in the river valleys and lowlands. In the desert regions saxaul vegetation is especially threatened. In some cases after degradation of the plant cover these degradation processes effect soil cover, too. Impacts on soil cover caused by ploughing of virgin land, by ploughing of long term used arable land, and by irrigation are the main reasons for soil degradation. Also recreational activities in the vicinity of settlement centers and traffic along the tracks lead to detrimental vegetation and soil cover changes. Unsystematic forest clearing goes on at a high level and often results in irreversible soil degradation.
Germination ecology of Central Asian Stipa spp: differences among species, seed provenances, and the importance of field studies
Stipa-species are wide-spread in Central Asia, but sexual reproduction in the dry steppes is rare. To facilitate conservation and restoration of these important rangelands, we studied germination characteristics of three common Mongolian Stipa-species under field- and lab conditions. Seeds of Stipa krylovii, Stipa gobica and Stipa glareosa were sown at the study site in Southern Mongolia over two consecutive years during which period tests were carried out to ascertain whether competition or herbivory are the main constraints of seedling establishment. In addition, we tested germination and seed viability in the laboratory under two different temperature regimes (20/10°C and 8/4°C), as well as the effect of cold-stratification. The lab experiments also included S. krylovii seeds originating from three climatically different provenances. None of the three Stipa-species seedlings emerged during the first 2 years of the field study. However, after an unusually intense rain event in the third year, 3% of S. krylovii, 0.6% of S. glareosa and 0.1% of S. gobica seeds germinated in the study plots. The factors ‘sowing-year’ and ‘vegetation’ significantly affected seedling emergence, whereas grazing had no effect at all. Under laboratory conditions a high percentage of viable seeds of S. gobica and S. glareosa germinated at both incubation temperatures, and cold-stratification had no effect on germination or viability. Germination of S. krylovii seeds required warmer temperatures and cold-stratification had a positive effect. Such evidence for dormancy was more pronounced in seeds from the moister, northern provenances. Germination of Stipa-species in the field is rare and only possible under exceptionally moist conditions. Conservation should thus concentrate on steppe conservation rather than on restoration. Where artificial reseeding is necessary, differences among species and also among different seed provenances should be taken into account.
Comparative study of in vitro flow characteristics between a human aortic valve and a designed aortic valve and six corresponding types of prosthetic heart valves.
A model of a pulsatile cardiovascular system is described including an electronic control, a hydraulic driving unit, a volume storage, a peripheral resistance and measuring equipment. Six different artificial heart valves, a designed aortic valve (Aachen pockett valve II) and a human aortic valve have been tested in the outflow position of the pump. Three of them are presently being used in clinical applications. Measurements of pressure and volume flow have been obtained for different frequencies n, pump volumes Vp and time ratios between pressure and suction phase. From the experimental results a comparative evaluation was deducted for different working conditions. In comparison to natural human aortic valve (NK) large volume flow is verified by the Aachen pocket valve II (AT), the Gott-Dagget value (GD), the Starr-Edwards valve (SE) and the Lillehei-Kaster valve (LK). The Gott-Daggett valve has the smallest back flow volume. The Smeloff-Cutter and in particular the Wada-Cutter valve have the largest regurgitation. This is to be expected since the Cutter valves are purposely so designed that they do not close tightly in order to avoid blood sedimentation. The human aortic valve, the Gott-Daggett valve and the Aachen pocket valve demonstrate the most favourable opening-closing characteristics. The human aortic valve achieves a negligible pressure loss, followed by the Björk-Shiley and the Lillehei-Kaster valve.